Sixteenth Marcel Grossmann Meeting

Jul 5, 2021, 6:30 AM → Jul 10, 2021, 7:30 PM Europe/Rome

Remo Ruffini (ICRANet), Robert Jantzen (Villanova University), Gregory Vereshchagin (ICRANet)

Since 1975, the Marcel Grossmann  Meetings (on Recent Developments in Theoretical and Experimental General Relativity, Gravitation, and Relativistic Field Theories) have been organized in order to provide opportunities for discussing recent advances in gravitation, general relativity and relativistic field theories, emphasizing mathematical foundations, physical predictions and experimental tests. The objective of these meetings is to elicit exchange among scientists that may deepen our understanding of spacetime structures as well as to review the status of ongoing experiments aimed at testing Einstein's theory of gravitation either from the ground or from space. Previous meetings have been held in Trieste (MG1: 1975) and (MG2: 1979), Shanghai (MG3: 1982), Rome (MG4: 1985, MG9: 2000), Perth (MG5: 1988), Kyoto (MG6: 1991), Stanford (MG7: 1994), Jerusalem (MG8: 1997), Rio (MG10: 2003), Berlin (MG11: 2006), Paris (MG12: 2009), Stockholm (MG13: 2012),  MG14 in 2015 and MG15 in 2018 both in Rome.

Booklet MG16 Awards.pdf

International Coordinating Committee

International Organizing Committee

Local Organizing Committee

MG16 official poster.pdf

MG16 official website

MG16 special poster.pdf

Mon, July 5

8:00 AM → 8:30 AM Opening remarks: Remo Ruffini

YouTube Link

8:30 AM → 9:30 AM MG16 Awards ceremony (part 1): Awards presented by Prof. Roy P. Kerr

YouTube Link

9:30 AM → 12:00 PM Monday Plenary Session: Block 1 - Events in Relativistics Astrophysics

Monday plenary session

Convener: Prof. Remo Ruffini (ICRANet)

YouTube Link

9:30 AM SRG Orbital Observatory: X-Ray map of the Universe with a million accreting supermassive black holes

SRG with German (eRosita) and Russian (ART-XC) X-Ray telescopes was launched by RosKosmos on July 13th of 2019 from Baikonur. During the flight to the L2 point of the Sun-Earth system, SRG performed calibrations and long duration Performance Verification (PV) observations of a dozen of targets and deep fields. Starting in the middle of December 2019, the SRG scanned the whole sky three times. During these scans, SRG discovered two and a half million point X-Ray sources: mainly AGNs and QSOs, stars with hot and bright coronae, and 40 thousand clusters of galaxies. There is a competition and synergy with the search for clusters of galaxies by Atacama Cosmology and the South Pole Telescopes sensitive in the microwave spectral band. We see X-Rays from hundreds of stars accompanied by exoplanets. SRG provided the X-Ray map of the whole sky in hard and soft bands, the last is now the best among existing. It reveals a lot of information about the distribution of absorbing cold gas in the Milky Way and provides a beautiful image of the North Polar Spur and similar bright emitting eRosita Bubble on the Southern side from the Central Part of the Galaxy. I will describe the Observatory plans for the future and demonstrate several results from the PV phase observations. The huge samples of the X-ray selected quasars at the redshifts up to z=6.2 and clusters of galaxies will be used for well-known cosmological tests and detailed study of the growth of the large scale structure of the Universe during and after reionization. SRG/eRosita is discovering every day several extragalactic objects which increased or decreased their brightness more than 10 times during half of the year after the previous scan of the same strip on the sky. A significant part of these objects has observational properties similar to the Tidal Disruption Events. ART-XC discovered a lot of bright galactic and extragalactic transients.

Speaker: Prof. Rashid Sunyaev

10:05 AM New results from testing relativistic gravity with radio pulsars

We experience a golden era in testing and exploring relativistic gravity. Whether it is results from gravitational wave detectors, satellite or lab experiments, radio astronomy plays an important complementary role. Here one can mention the cosmic microwave background, black hole imaging and, obviously, binary pulsars. This talk will concentrate on the latter and new results from studies of strongly self-gravitating bodies with unrivalled precision. I compare the results to other methods, discuss implications for other areas of relativistic astrophysics and will give an outlook of what we can expect from new instruments in the near future.

Speaker: Prof. Michael Kramer (Max-Planck-Institut fuer Radioastronomie)

10:40 AM A 21-solar mass black hole in the X-ray binary system Cygnus X-1

The fate of massive stars is influenced by the mass lost to stellar winds over their lifetimes, which limit the masses of the stellar remnants that they eventually produce. In this talk I will discuss our recent redetermination of the black hole mass in the X-ray binary system Cygnus X-1. At 21 solar masses, our measurement makes this the most massive dynamically-confirmed stellar-mass black hole yet detected without the use of gravitational wave facilities. With the system having been formed in an environment with close to solar metallicity, this measurement challenges existing estimates of wind mass loss rates from massive stars. I will present the new astrometric measurements that resolved the discrepancy between radio and optical parallax values, and outline how this enabled us to refine the measured black hole mass. Finally, I will briefly discuss the implications of this result for massive star evolution.

Speaker: Prof. James Miller-Jones (International Centre for Radio Astronomy Research - Curtin University)

11:15 AM Black Holes in the 21cm signal of HI from cosmic dawn

A deep absorption in the 21-cm line of atomic hydrogen (HI), redshifted to the epoch of cosmic dawn (z ~ 20), was reported by the EDGES experiment. To explain that absorption trough it has been proposed that either an additional exotic cooling mechanism, or a brighter radio background emission previously unaccounted for is needed. Here we discuss the possibility that the required cosmic radio background could be produced by non-thermal emission from a prolific population of black holes formed at cosmic dawn. We conclude that unless black holes formed at that epoch are radically different from those observed in the local Universe, the radio emission is orders of magnitude below the required levels.

Speaker: Prof. Felix Mirabel

12:00 PM → 12:30 PM Public lecture: Diverse Production Sites of Cosmic and Gamma-Rays Discerned Through Selected Observations of MAGIC

MAGIC is the pioneering imaging air Cherenkov telescopes (IACT) instrument, which started performing high-sensitivity measurements in the sub-200 GeV energy range, down to few 10s of GeVs. Since 2009 MAGIC is operating as a double system of 17m diameter IACTs for performing astrophysical measurements in the very high energy range 30GeV – 100TeV. In recent years, by using novel observation techniques, we further enhanced the sensitivity of MAGIC both at the lowest energies of 20-30 GeV as well as at the highest energy of ~100TeV. The very high sensitivity is allowing us to perform original studies, deepening our understanding of selected important aspects of the Universe. In this report we want to show selected observational results of MAGIC of both galactic and extragalactic origins. These, supported by multi-messenger and multi-wavelength measurements, help us to discern and learn important details about the possible production sites of cosmic and gamma rays.

Speaker: Razmik Mirzoyan (Max-Planck-Institute for Physics)

YouTube Link

1:00 PM → 1:30 PM Public lecture: The Development of General Relativity

The development of the Einstein Field Equations is traced and it is argued that Einstein probably had included the cosmological constant in his field equations but then dropped it at first and later re-inserted it when he needed it for a static cosmological solution of his field equations. His initial derivation would have been geometrical, rather than field theoretic. The significance of this fact for the interpretation of the cosmological constant and its role as "dark energy" is discussed.

Speaker: Asghar Qadir (None)

YouTube Link

1:30 PM → 2:00 PM Public lecture: Ulugh Beg’s Scientific School in Samarkand

Ulugh Beg was the grandson of Tamerlane who conquered a vast area in Transoxania and Iran around 1400. Mohammad Taraghay, best known as Ulugh Beg (lit. “Grand prince”) was born in 1394 in Sultaniya (Zanjan, Iran). In 1409, he became the ruler of Samarkand where he founded a school in 1420 which is still well preserved there. Astronomy was the major subject taught in the school and Ulugh Beg gathered a group of astronomers there. He also founded an observatory in 1424 which was designed by the Iranian scholar Jamshid Kashani (al-Kashi) who upon Ulugh Beg’s request, supervised the construction and operation of the observations made there. After Ulugh Beg’s tragic murder arranged by his son in 1449, the observatory was destroyed and forgotten. Its remnants were rediscovered in 1908 near Samarkand. The main part of the observatory was a huge stone sextant more than 40 meters long. It measured the meridian transit of celestial bodies from which the declination of the ecliptic, the equinoxes and the geographical latitude of the locality could be determined accurately. The results of the observations were composed in a Persian treatise called Zij Ulugh Beg. Zijis a Persian word used for a collection of astronomical tables with explanations for using them in astronomical calculations. Several commentaries are written on this work5and selections of it are translated into Arabic, English, French, Russian and Turkish. Ulugh Beg also devised a method for finding the sine of one degree for which he solved a cubic equation by an iterative method.

Speaker: Dr Mohammad Bagheri (Institute for the History of Science, University of Tehran)

YouTube Link

2:00 PM → 3:00 PM MG16 Awards ceremony (part 2): Awards presented by Prof. Roy P. Kerr

YouTube Link

4:30 PM → 7:30 PM Accretion Discs and Jets: Block 1

Accretion disks are the systems that most closely approach compact objects and are ideal systems to explore the non-linear and strong gravity regime. The guiding theme of this parallel session is expected, but are not limited to: theoretical and numerical modelling of accretion process in the strong gravitational field and related phenomena, relativistic turbulence and viscosity, accretion disks and jets, the evolution of accretion disks, and modelling of accretion disks in various gravity theories.

Conveners: Audrey Trova (ZARM, Univeersity of Bremen), Shokoufe Faraji (ZARM)

YouTube Link - July 5-7

4:30 PM Relativistic Jets from Spinning Black Holes

Many accreting black holes in Nature are observed to have relativistic jets, and it has been suggested that the jets receive much of their power from the spin energy of the central black hole. There is considerable evidence in support of this idea from computer simulations of accretion flows. The talk will review some recent progress in this field.

Speaker: Prof. Ramesh Narayan (Harvard University)

5:10 PM Simulations of black hole accretion and jets

In this talk I will review the recent insights into the physics of black hole accretion and jets enabled by the advances in general relativistic numerical simulations. In particular, I will discuss how the jets form, collimate, accelerate, and interact with the ambient medium.

Speaker: Alexander Tchekhovskoy (Northwestern University)

5:50 PM The Nature of the X-ray Emission and Innermost Accretion Regions of Typical Radio-Loud Quasars

Radio-loud quasars (RLQs) are typically more X-ray luminous, by a factor of 2-20, than matched radio-quiet quasars (RQQs). This excess X-ray emission has generally been attributed to small-scale jets. To determine the nature of this excess X-ray emission, we have constructed a large, uniform sample of 729 optically selected RLQs with high fractions of X-ray detections and radio-slope measurements. We investigate correlations between their X-ray, optical/UV, and radio luminosities, as well as their X-ray spectral and variability properties. Strikingly, we find that steep-spectrum RLQs (SSRQs) follow a quantitatively similar relation between X-ray vs. optical luminosities as RQQs, suggesting a common accretion-disk corona origin for the X-ray emission of both classes. Formal statistical model selection supports these conclusions, as does consideration of analogies with black-hole X-ray binaries. However, the relation's intercept for SSRQs is larger than that for RQQs and increases with radio loudness, suggesting a connection between the radio jets and the accretion-flow configuration. Flat-spectrum RLQs also generally appear to have corona-dominated X-ray emission, though in some cases jets make large contributions. Our spectral measurements of X-ray continuum shapes and (average) reflection signatures confirm these conclusions, as do our inter-observation measurements of X-ray variability on timescales of weeks-years. Our results indicate the corona-jet, disk-corona, and disk-jet connections of RLQs are likely driven by independent physical processes. Moreover, the observed corona-jet connection implies that small-scale processes in the vicinity of black holes, probably associated with the magnetic flux/topology instead of black-hole spin, are controlling quasar radio loudness.

Speaker: William Brandt (Penn State University)

6:10 PM Measurement of Black Hole Spin with the Event Horizon Telescope: Theory of Radiative Echoes and VLBI Observations

Observational measurement of the black-hole spacetime is one of the essential topics in modern physics and astrophysics, since it will lead to a critical test of the theory of general relativity. In general relativity, the spacetime around is uniquely determined by its mass and spin parameter. The mass can be accurately measured by observing orbits of stars or gas dynamics inside the sphere of its gravitational influence. On the other hand, it is not easy to extract the spin information, which depends on the complexity of accretion flow properties and spacetime effects in the vicinity of the black hole. 

In this presentation, we simulate Event Horizon Telescope (EHT) observations for a gas cloud intermittently falling onto a black hole and construct a method for spin measurement based on its relativistic flux variation. We investigate the spin’s signature by calculating an infalling gas cloud’s motion and photon trajectories in the Kerr spacetime by the general relativistic ray-tracing method. The light curve of the infalling gas cloud is composed of peaks formed by photons that directly reach a distant observer and by secondary ones reaching the observer after more than one rotation around the black hole. The time interval between the peaks is determined by a period of photon rotation near the photon circular orbit, which uniquely depends on the spin. 

To optimize our new spin measurement method, we perform synthetic EHT observations for the supermassive black hole at the Galactic center (Sgr A) under a more realistic situation by performing three-dimensional general relativistic magnetohydrodynamics simulations. Even for the realistic situation, the black hole spin’s signature is detectable in correlated flux densities, which are accurately calibrated by baselines between sites with redundant stations. The synthetic observations indicate that our methodology can be applied to EHT observations of Sgr A from April 2017-2022.

Speaker: Kotaro Moriyama (Massachusetts Institute of Technology)

6:30 PM Accretion of the relativistic Vlasov gas onto a moving Schwarzschild black hole

I will discuss recent analytic results on the stationary accretion of the relativistic collisionless Vlasov gas onto a moving Schwarzschild black hole. The model assumes that the gas obeys the Maxwell-Juttner distribution at infinity. The Vlasov equation is solved formally in terms of suitable action-angle variables in the framework proposed originally by Rioseco and Sarbach. Depending on the asymptotic temperature, the results interpolate between two regimes: In the limit of infinite asymptotic temperature of the gas, we recover the qualitative picture known form the relativistic Bondi-Hoyle-Lyttleton accretion of the perfect gas with the ultra-hard equation of state, in which the mass accretion rate is proportional to the Lorentz factor associated with the black hole velocity. For low asymptotic temperatures, the mass accretion rate is not a monotonic function of the velocity of the black hole. The model can be applied in situations where the gas is not likely to be in thermal equilibrium in the vicinity of the black hole, for instance in the context of dark matter accretion. The talk is based on two papers written jointly with Andrzej Odrzywolek: Phys. Rev. Lett. 126, 101104 (2021) and Phys. Rev. D 103, 024044 (2021).

Speaker: Patryk Mach (Jagiellonian University)

6:50 PM Properties of accretion disk models with quadrupole

There is an alternative generalization of the $\rm q$-metric Weyl's procedure describes a deformed compact object in the presence of an external distribution of matter via exercising quadrupole moments. This metric may associate the observable effects to these parameters as dynamical degrees of freedom in the system. The talk will describe this metric and exploring the properties of analytical accretion disk models in this background.

Speaker: Shokoufe Faraji (ZARM)

7:10 PM Limiting effects in tori clusters

We consider agglomerates of misaligned tori orbiting a supermassive black hole. The aggregate of tilted tori is modelled as a single orbiting configuration by introducing a leading function governing the distribution of toroids (and maximum pressure points inside the disks) around the black hole attractor. The orbiting clusters are composed by geometrically thick, pressure supported, perfect fluid tori.
This analysis places constraints on the existence and properties of tilted tori and more general aggregates of orbiting disks.
We study the constraints on the tori collision emergence and the instability of the agglomerates of tori with general relative inclination angles, the possible effects of the tori geometrical thickness and on the oscillatory phenomena.
Some notes are discussed on the orbiting ringed structure in dependence of the dimensionless parameter $\xi$ representing
the (total) BH rotational energy extractable versus the mass of the BH, associating $\xi$ to the characteristics of the accretion processes.

Speaker: Daniela Pugliese

4:30 PM → 7:30 PM The Black Hole in M87: Block 1

The Event Horizon Telescope (EHT) has heightened interest in the black hole in M87. This sessions aims to explore the origin of the annulus of emission at 230 GHz that was detected by EHT. Particular interest is given to non-MHD, low density modelling of the region adjacent to the event horizon. Subjects of primary interest are magnetic reconnection and possible accretion in this limit, as well as jet launching. The high energy emission from this region is an important possibility. The session will also invite discussion of the most recent high resolution images of the jet (at 3mm wavelength).

Conveners: Brian Punsly (ICRANet), Jorge Armando Rueda Hernandez (ICRANet)

YouTube Link - July 5

4:30 PM What do the EHT observations tell us about the source?

The Event Horizon Telescope collaboration has released 1.3mm interferometric observations of the core of the galaxy M87. I will review the observations and the general physical principles involved in their interpretation. After describing the basic heuristics needed to understand the effect of a black hole on the observational appearance of nearby emission, I will emphasize that gravitational lensing is largely irrelevant at EHT resolution. Instead, the observational appearance of a given source is determined entirely by the emission profile and redshift effects. I will comment on the specific interpretation of the public 2017 observations and discuss what we can expect going forward.

Speaker: Sam Gralla (University of Arizona)

4:50 PM What do EHT observations tell us about the source

Sam Gralla (continued)

5:10 PM Is there a Narrowness Tension in M87*?

The 2017 Event Horizon Telescope (EHT) observations of the core of the galaxy M87 are the first electromagnetic observations probing event horizon scales of a black hole. The data strongly favor an observational appearance dominated by a ring of approximately 40 micro-arcseconds in diameter. However, many interesting questions remain about the appearance of the source. In particular, the thickness of the ring is much less certain. I will argue that the most likely parameter region is in tension with theoretical expectations - the observed ring is too narrow - and explore whether this tension can be resolved by alternative data analysis methods.
First, I will report on our independent verification of a subset of the EHT collaboration’s geometric modeling results, using a new code built from scratch. Second, I will discuss some subtleties in the choice of likelihood function used in model-fitting, and test the sensitivity of the results on the choice of method. We find that the choice of likelihood function does in fact bias the results for ring width in particular, but not enough to completely remove the tension.

Speaker: Will Lockhart (University of Arizona)

5:30 PM Orientation of the crescent image of M87*

The first image of the black hole (BH) M87 obtained by the Event Horizon Telescope (EHT) has the shape of a crescent extending from the E to WSW position angles, with a possibly distinct bright hotspot in the ESE sector. We have explored highly simplified toy models for geometric distribution and kinematics of emitting regions in the Kerr metric, assuming that the BH spin vector is fixed to the jet axis and that the emitting regions are stationary and symmetric with respect to the BH spin. Since the observed direction of the large-scale jet is WNW, emission from the crescent sector between SSE and WSW can be readily explained in terms of an equatorial ring with either circular or plunging geodesic flows, regardless of the value of BH spin. We have also considered plane-symmetric polar caps with plunging geodesic flows, in which case the dominant image is that of the cap located behind the BH. Within the constraints of our model, we have not found a viable explanation for the ESE hotspot. Most likely, it has been produced by a non-stationary localised perturbation in the inner accretion flow. The recent polarimetric EHT image of M87 shows that the ESE hotspot is essentially unpolarized, which seems to support its distinct origin. Possible causes for this apparent depolarization will be discussed.

Speaker: Krzysztof Nalewajko (Nicolaus Copernicus Astronomical Center, Polish Academy of Sciences)

5:50 PM Discussion of the EHT images

Round Table Discussion of the EHT Images

6:10 PM Magnetic Reconnection in Jet-Accretion disk Systems

Magnetic Reconnection is currently regarded as a rather important process in magnetically dominated regions of galactic and extragalactic sources like the surrounds of black holes and relativistic jets. In this contribution, we discuss briefly the theory of fast magnetic reconnection, especially when driven by turbulence which is very frequent in astrophysical flows, and its implications for relativistic particle acceleration. Then we discuss these processes in the context of the sources above, showing recent analytical and multidimensional numerical MHD studies that indicate that fast reconnection can be a powerful process to accelerate particles to relativistic velocities, produce the associated high energy non-thermal emission, and account for efficient conversion of magnetic into kinetic energy in these flows.

Speaker: Chandra Bahadur Singh

CBS_MG16.pdf

6:30 PM A kinetic view at black hole magnetospheres

Spinning black holes have long been suspected to be involved in some of the most extreme astrophysical phenomena such as AGN and their relativistic jets for supermassive black holes, and gamma-ray bursts for stellar-mass black holes. The activity of black holes is often associated with the creation and the launching of a relativistic magnetized plasma jet accompanied by efficient particle acceleration and non-thermal radiation. Horizon-scale observations of supermassive black holes reveal that these processes occur in the closest vicinity to the black-hole horizon: the magnetosphere, the inner parts of the accretion flow and the jet. Yet, the underlying physical mechanisms are still poorly understood because they result from a complex interplay between general relativity, electrodynamics and plasma physics. I will review our current efforts to model black hole magnetospheres from first principles with the help of general relativistic radiative particle-in-cell simulations. These numerical methods can capture plasma processes at a microscopic kinetic level where particle acceleration takes place, and therefore they may hold the key to bridge the gap between theoretical models and horizon-scale observations of black holes.

Speaker: Benoît Cerutti (CNRS & Univ. Grenoble Alpes)

6:50 PM Synthetic observables from simulations of black-hole magnetospheres

GRMHD simulations have been very successful in interpreting observations from M87*. However, they are unable to account for several important features, such as the plasma loading of the jet or the details of non-thermal radiation, from first principles. Kinetic simulations, on the other hand, are well suited to the task. In this talk, I will review what we have learned from these kinetic simulations. Including radiative processes allows modeling plasma supply realistically, proving that the Blandford-Znajek mechanism can be activated self-consistently. I will also highlight the role of current sheets in the extraction and conversion of energy from the black hole. Finally, I will put the emphasis on extracting synthetic observables from these simulations, such as gamma-ray lightcurves and millimeter images. That allows us to model accurately the non-thermal radiation emitted from the innermost regions of black-hole magnetospheres, which can be directly compared to the EHT observations, for example.

Speaker: Benjamin Crinquand (IPAG)

7:10 PM Black hole flares: Accretion-driven accumulation and reconnection-driven ejection of magnetic flux

Magnetic reconnection in current sheets is conjectured to power bright TeV flares from the black hole in the center of the M87 Galaxy. It is unclear how, where, and when current sheets form in black-hole accretion flows. We show extreme resolution 3D general-relativistic magnetohydrodynamics and 2D general-relativistic particle-in-cell simulations to model reconnection and plasmoid formation in black hole magnetospheres. Plasmoids can form in thin current sheets In the inner 15 Schwarzschild radii from the event horizon, after which they can merge, grow to macroscopic hot spots of the order of a few Schwarzschild radii and escape the gravitational pull of the black hole. Large plasmoids are energized to relativistic temperatures via magnetic reconnection near the event horizon and they significantly heat the jet, contributing to its limb-brightening. We find that only hot plasmoids forming in magnetically dominated plasmas can potentially explain the energetics of flares. The flare period is determined by the reconnection rate, which we find to be consistent with studies of reconnection in isolated Harris-type current sheets.

Speaker: Bart Ripperda (Princeton University/Flatiron Institute)

4:30 PM → 7:30 PM Mathematical Problems of Relativistic Physics: Classical and Quantum: Block 1

We will survey recent advances in mathematical analysis of relativistic and semi-relativistic phenomena, including:
1. Joint classical and quantum evolution of charged point particles and fields in special and general relativity;
2. Dirac's equation on electromagnetic background spacetimes;
3. Schroedinger-Newton equation and bosonic stars;
4. Interacting photon-electron systems in Dirac's multi-time formalism;
5. The ground state of Positronium as an ultralight spin-zero boson and its application to the dark matter puzzle;
6. Divison-algebraic underpinnings of the Standard Model of Elementary Particles.

Conveners: A. Shadi Tahvildar-Zadeh (Rutgers University), Michael Kiessling (Rutgers University, Dept. of Mathematics)

YouTube Link - July 5

YouTube Link - July 8

4:30 PM The point spectrum of the Dirac Hamiltonian on the zero-gravity Kerr-Newmann spacetime

In relativistic quantum mechanics, the point spectrum of the Dirac Hamiltonian with Coulomb potential famously agrees with Sommerfeld's fine structure formula for Hydrogen. In the Coulomb approximation, the proton is assumed to only have an electric charge. However, the physical proton also appears to have a magnetic moment. The resulting hyperfine structure of Hydrogen is computed perturbatively. Aiming towards a non-perturbative approach, Pekeris in 1987 proposed taking the Kerr-Newmann spacetime with its ring singularity as a source for the proton's electric charge and magnetic moment. Given the proton's mass and electric charge, the resulting Kerr-Newmann spacetime lies well within the naked singularity sector which possess closed timelike loops. In 2014 Tahvildar-Zadeh showed that the zero-gravity limit of the Kerr-Newmann spacetime (zGKN) produces a topologically nontrivial flat spacetime which is no longer plagued by closed timelike loops. In 2015 Tahvildar-Zadeh and Kiessling studied the Hydrogen problem with Dirac’s equation on the zGKN spacetime and found that the Hamiltonian is essentially self-adjoint and contains a nonempty point spectrum. In this talk, we show how some of their ideas can be extended to classify the point spectrum.

Speaker: Eric Ling (Rutgers University)

4:50 PM The second Bianchi identity for spacetimes with timelike singularities

The second Bianchi identity is a well-known and fundamental differential identity which holds on any smooth (semi-)Riemannian manifold. In general relativity, due to the relation of the curvature tesnor and the energy-momentum tensor via the Einstein equations, this identity then naturally implies energy and momentum conservation for matter fields. What happens in situations where curvature singularities associated with timelike singularities occur and the classical Bianchi identity no longer makes sense? In this talk we establish a distributional version of the contracted Bianchi identity, and investigate for which matter fields this identity holds. Surprisingly, the well-known Reissner-Weyl-Nordström spacetime of a single point charge does not belong to this class, but other electromagnetic theories and certain perfect fluids with one-dimensional timelike singularities satisfy the second Bianchi identity weakly. Joint work with Michael Kiessling and Shadi Tahvildar-Zadeh.

Speaker: Annegret Burtscher (Radboud University Nijmegen)

preprint

slides

5:10 PM Times of arrival and gauge invariance

Arrival-time operators (or observables) describing time-of-flight experiments are naturally constrained by gauge invariance requirements. Surveying the literature on time operators, including POVMs, I will show that a natural generalization of Aharonov-Bohm-Kijowski's arrival-time distribution (referred to as the ``standard arrival-time distribution'' by some authors) fails to be gauge invariant. In particular, this undermines the associated time-energy uncertainty relations. A direct comparison to the quantum flux distribution, which does not exhibit this flaw, and which does not correspond to a quantum observable (or POVM), will be drawn (its acknowledged drawback concerning the quantum backflow effect notwithstanding). Ref: S. Das and M. Nöth, Proc. R. Soc. A. 477 (2021)

Speaker: Siddhant Das (Ludwig-Maximilians-Universitat Muenchen)

5:30 PM Causal fermion systems: Classical gravity and beyond

The theory of causal fermion systems is an approach to fundamental physics. It gives quantum mechanics, general relativity and quantum field theory as limiting cases and is therefore a candidate for a unified physical theory. The dynamics of causal fermion systems is described by a variational principle called the causal action principle (for more details see https://causal-fermion-system.com).

In the talk, I will outline how and in which sense the causal action principle gives rise to classical gravity. Moreover, I will explain in various examples how to go beyond classical gravity:
- The general definition of the total mass of a static causal fermion system
- A general connection between area change and matter flux
- Geometric structures giving a setting of Lorentzan quantum geometry
We conclude with an outlook on quantum gravity.

Speaker: Felix Finster (Universität Regensburg)

5:50 PM Division algebraic symmetry breaking

Can the 32C-dimensional algebra R(x)C(x)H(x)O offer anything new for particle physics? Indeed it can. Here we identify a sequence of complex structures within R(x)C(x)H(x)O which induces a cascade of breaking symmetries: Spin(10) -> Pati-Salam -> Left-Right symmetric -> Standard model + B-L (both pre- and post-Higgs-mechanism). These complex structures derive from the octonions, then from the quaternions, then from the complex numbers.

Speaker: Dr Nichol Furey (Humboldt-Universität zu Berlin)

6:10 PM Essential Self-Adjointness of Dirac operators under the influence of general-relativistic gravity

Physical reasoning give expressions for the Hamiltonian of a system. These Hamiltonians are differential operators that are mostly symmetric in a densely defined domain.
However, to study the dynamics of the unitary group corresponding to a Hamiltonian, it is
required that the Hamiltonian be self-adjoint or essentially self-adjoint. I will present our study
on how the static non-linear electromagnetic-vacuum space-time of a point nucleus affects the
self-adjointness of the general- relativistic Dirac Hamiltonian for a test electron.

Speaker: Dr Ebru Toprak (Rutgers University)

6:30 PM Spacetime Harmonic Functions and the Mass of 3-Dimensional Asymptotically Flat Initial Data for the Einstein Equations

We give a lower bound for the ADM mass of 3-dimensional asymptotically flat initial data sets for the Einstein equations. The bound is given in terms of linear growth `spacetime harmonic functions' in addition to the energy-momentum density of matter fields, and is valid regardless of whether the dominant energy condition holds or whether the data possess a boundary. A corollary is a new proof of the spacetime positive mass theorem for complete initial data or those with weakly trapped surface boundary. The proof has analogy with both the Witten spinorial approach as well as the marginally outer trapped surface (MOTS) method of Eichmair, Huang, Lee, and Schoen. This is joint work with Sven Hirsch and Marcus Khuri.

Speaker: Demetre Kazaras (Duke University)

6:50 PM On recent developments in the theory of dissipative relativistic fluids

My goal in this talk is to address some of the fundamental mathematical questions in the field of relativistic dissipative fluid dynamics. This is an area that has witnessed progress within the physics community but for which many foundational mathematical questions remain open. Some of these problems, such as the study of causality, local well-posedness and breakdown of solutions, are crucial for for establishing solid theoretical foundations for the understanding of the quark-gluon plasma, a state of matter found in the very early universe. The talk is based on joint work with J. Noronha, F. Bemfica, M. Disconzi and M. Radosz.

Speaker: Vu Hoang

7:10 PM The Maxwell-Bopp-Lande-Thomas-Podolsky-Einstein system for a static point source

In this talk, we discuss the existence of a static, spherically symmetric spacetime that is the solution of the Einstein field equations coupled with an electric field obeying the equations of electromagnetism of Maxwell-Bopp-Lande-Thomas-Podolsky for a static point charge. Contrary to what happens with the Reissner-Nordstrom spacetime, it is shown that the electric field energy is finite, just as for this same theory on a background flat spacetime.

Speaker: Erik De Amorim (Universität zu Köln)

4:30 PM → 7:30 PM Theories of Gravity: Alternatives to the Cosmological and Particle Standard Models: Block 1

In the standard model of cosmology, the ΛCDM model based on Einstein's General Relativity, dark energy is introduced completely ad hoc in order to explain the present acceleration of the universe. The model requires also the introduction of dark matter dominating (by far) ordinary baryonic matter but yet undetected in the laboratory, and it suffers from astrophysical problems. Modifying gravity is a possible alternative, and many such proposals have been presented in recent years. Likewise, the standard model of particle physics is unable to incorporate all the current particle phenomenology and proposed dark matter candidates.
Cosmology and particle physics come together in the early universe and, surprisingly, also in theories and models of the present, accelerating universe. This session is formulated in a wide framework to include several topics related to these problems, and spanning alternative theories of gravity and cosmology, alternatives to the ΛCDM model, quantum field theory applied to gravity, extensions of the standard model of particle physics, and dark energy and dark matter from a particle physics point of view. This session represents the interplay between, and the efforts to match, particle physics and cosmology, giving particular emphasis to the role played by particle quantum field theory in the early and the late universe.

Conveners: Orlando Luongo (University of Camerino, Physics Division), Stefano Bellucci

YouTube Link - July 5

YouTube Link - July 6

YouTube Link - July 7

YouTube Link - July 9

4:30 PM A model independent approach to the study of f(R) cosmologies with expansion histories close to ΛCDM

We propose a new framework for studying the cosmology of f(R) gravity which completely avoids using the reconstruction programme. This allows us to easily obtain a qualitative feel of how much the ΛCDM model differs from other f(R) theories of gravity at the level of linear perturbation theory for theories that share the same background dynamics. This is achieved by using the standard model independent cosmographic parameters to develop a new dynamical system formulation of f(R) gravity which is free from the limitation of having to first specify the functional form of f(R). By considering a set of representative trajectories, which are indistinguishable from ΛCDM, we use purely qualitative arguments to determine the extent to which these models deviate from the standard model by including an analysis of the linear growth rate of density fluctuations and also whether or not they suffer from the Dolgov-Kawasaki instability. We find that if one demands that a late time f(R) cosmology is observationally close to the ΛCDM model, there is a higher risk that it suffers from a Dolgov-Kawasaki instability. Conversely, the more one tries to construct a physically viable late time f(R) cosmology, the more likely it is observationally different from the ΛCDM model.

Speaker: Prof. Peter Dunsby (University of Cape Town)

4:50 PM Black hole mimickers and the limits of GR

We consider a class of exact solutions of Einstein's equations that describe a black hole mimicker for which the relativistic description would fail close to the horizon scale. We investigate how such an hypothetical object may be distinguished from a black hole via observations.

Speaker: Daniele Malafarina (Nazarbayev University)

5:10 PM Cosmology in scalar-tensor f(R,T) gravity

We apply cosmological reconstruction methods to f(R,T) modified gravity, in its recently developed scalar-tensor representation. We do this analysis assuming a perfect fluid in a Friedmann-Lemaı̂tre-Robsertson-Walker (FLRW) universe. Solutions with general scale factor, curvature parameter and equation of state are found for the energy density, pressure, and one of the dynamical fields of the scalar-tensor representation. We then apply three particular forms of the scale factor: an exponential expansion (in analogy with the de Sitter solution); and two types of power-law expansion (radiation domination and matter domination). This allows us to find, in each particular case, a complete solution. We do so for each of the three values of the curvature parameter, and with three different values of the equation of state corresponding, in general relativity, to the equation of state of a cosmological constant, of matter and of radiation.

Speaker: Mr Tiago B. Gonçalves (Faculty of Sciences of the University of Lisbon)

Cosmo_fRT_MG16_TGoncalves.pdf

5:30 PM BREAK

6:00 PM Thermodynamics of scalar-tensor gravity: a new approach

We propose a new approach to the thermodynamics of scalar-tensor gravity and its possible diffusion'' toward general relativity, previously regarded as an equilibrium state in spacetime thermodynamics. The main idea is describing scalar-tensor gravity as an effective dissipative fluid and applying Eckart’s first order thermodynamics to it. This gives explicit effective quantities: heat current density,temperature of gravity”, viscosity coefficients, entropy density, plus an equation describing the “diffusion” to Einstein gravity. These quantities, otherwise missing in spacetime thermodynamics, pop out with minimal assumptions.

Speaker: Prof. Valerio Faraoni (Bishop's University)

FaraoniAT4.pdf

6:20 PM Do gamma-ray burst measurements provide a useful test of cosmological models?

Context. We study eight different gamma-ray burst (GRB) data sets to examine whether current GRB measurements — that probe a largely unexplored part of cosmological redshift (z) space — can be used to reliably constrain cosmological model parameters.
Aims. We use three Amati-correlation samples and five Combo-correlation samples to simultaneously derive correlation and cosmolog- ical model parameter constraints. The intrinsic dispersion of each GRB data set is taken as a goodness measurement. We examine the consistency between the cosmological bounds from GRBs with those determined from better-established cosmological probes, such as baryonic acoustic oscillation (BAO) and Hubble parameter H(z) measurements.
Methods. We use the Markov chain Monte Carlo method implemented in MontePython to find best-fit correlation and cosmological parameters, in six different cosmological models, for the eight GRB samples, alone or in conjunction with BAO and H(z) data. Results. For the Amati correlation case, we compile a data set of 118 bursts, the A118 sample, which is the largest — about half of the total Amati-correlation GRBs — current collection of GRBs suitable for constraining cosmological parameters. This updated GRB compilation has the smallest intrinsic dispersion of the three Amati-correlation GRB data sets we examined. We are unable to define a collection of reliable bursts for current Combo-correlation GRB data.
Conclusions. Cosmological constraints determined from the A118 sample are consistent with — but significantly weaker than — those from BAO and H(z) data. They also are consistent with the spatially-flat ΛCDM model, in which dark energy is the cosmological constant Λ, as well as with dynamical dark energy models and non-spatially-flat models. Since GRBs probe a largely unexplored region of z, it is well worth acquiring more and better-quality burst data which will give a more definitive answer to the question of the title.

Speaker: Mr Narayan Khadka (Kansas State University)

6:40 PM Unifying baryogenesis with dark matter production

According to several observational evidences, the Hot Big Bang Model is the best framework in which to explain the origin and the evolution of the universe. By the way, it is still not the definitive model. Among its weaknesses, we have to count the lack of a satisfying explanation of how baryons and dark matter formed. In this article we attempt to describe these phenomena through a new interpretation of the model itself. We propose baryogenesis can occur as the environment field, associated with universe's expansion, couples to effective quark and lepton fields. Consequently, we propose how to unify the baryogenesis with dark matter production during reheating and evaluate the corresponding densities. Soon after dark matter's born, we justify how to cancel out vacuum energy degrees of freedom through a mechanism that counterbalance vacuum energy with dark matter pressures. We thus predict both dark matter and baryon densities, showing which dark matter constituent is expected to guarantee the mechanism above described. Here, for simplicity, we do not consider strong interactions, so a generalization of this work will be necessary in order to obtain a complete and realistic physical model.

Speaker: Nicola Marcantognini (University of Camerino)

4:30 PM → 7:30 PM Ghost-Free Models of Modified Gravity: Massive Gravity, Horndeski and DHOST Theories, Other Related Models; Their Properties and Solutions.: Block 1

This section will focus on theoretical aspects of various models of modified gravity which are free from unphysical ghostly degrees of freedom with a negative norm. The well known examples are the theories of ghost-free massive gravity and bigravity, which are free from the Boulware-Deser ghost. Related to them are the Galileon models and, more generally, Horndeski and DHOST theories free from the Ostrogradsky ghost. Other examples are provided by the theory of non-local gravity, etc. All of these theories are interesting from the purely theoretical viewpoint and may provide a description of the Dark Energy and/or Dark Matter. Their various aspects may be discussed, as for example their Hamiltonian formulation and constraints; the causal structure and Cauchy problem; disformal dualities and solutions — cosmologies, black holes, ultracompact objects; observational constraints; quantum aspects; etc.

Conveners: Dmitry Galtsov, Mikhail Volkov (University of Tours, France)

YouTube Link - July 5

YouTube Link - July 8

4:30 PM DHOST theories

The last few years have witnessed a great enthusiasm for modified theories of gravity and particularly for scalar-tensor theories. The motivations to modify gravity are to test the limits of general relativity on the one hand and also to propose "answers" to open
questions of cosmology and astrophysics. In this context, many theories have emerged and a very complex landscape of theories has appeared in the literature. In this talk, I will show how we can classify some of these theories and how we can construct the most general tensor-scalar theories (aka DHOST theories) that are physically viable (in a precise sense that I will give). Finally, we will show how these modified theories can be applied to cosmology (to account for dark energy) and in astrophysics.

Speaker: Karim Noui (IDP Tours & APC Paris)

5:15 PM Anisotropic cosmological models in Horndeski gravity

It was found recently that the anisotropies in the homogeneous Bianchi~I cosmology considered within the context of a specific Horndeski theory are damped near the initial singularity instead of being amplified. In this work we extend the analysis of this phenomenon to cover the whole of the Horndeski family. We find that the phenomenon is absent in the K-essence and/or Kinetic Gravity Braiding theories, where the anisotropies grow as one approaches the singularity. The anisotropies are damped at early times only in more general Horndeski models whose Lagrangian includes terms quadratic and cubic in second derivatives of the scalar field. Such theories are often considered as being inconsistent with the observations because they predict a non-constant speed of gravitational waves. However, the predicted value of the speed {\it at present} can be close to the speed of light with any required precision, hence the theories actually agree with the present time observations. We consider two different examples of such theories, both characterized by a late self-acceleration and an early inflation driven by the non-minimal coupling. Their anisotropies are maximal at intermediate times and approach zero at early and late times. The early inflationary stage exhibits an instability with respect to inhomogeneous perturbations, suggesting that the initial state of the universe should be inhomogeneous. However, more general Horndeski models may probably be stable.

Speaker: Prof. Mikhail Volkov (University of Tours, France)

5:45 PM Analytic infinite derivative gravity as the quantum gravity probe

I will discuss the general aspects of the Analytic Infinite Derivative (AID) gravity theories. It will be shown in details why an infinite number of derivatives is required to eradicate ghosts. Explicit ghost-free construction will be presented. Then it will be explained how unitarity is maintained in this non-local setup upon accounting loop corrections. Observational aspects will be briefly touched.

Speaker: alexey koshelev (universidade da beira interior)

6:15 PM Non-local R^2-like inflation, Gravitational Waves and Non-Gaussianities

The emergence of $R^2$ (Starobinsky) inflation from the semi-classical modification of gravity due to matter quantum fields clearly points out the importance of fundamental physics and the first principles in the construction of successful cosmological models. Along with the observational success, $R^2$ gravity is also an important step beyond general relativity (GR) towards quantum gravity. Furthermore, several approaches of quantum gravity to date are strongly indicating the presence of non-locality at small time and length scales. In this regard, ultraviolet (UV) completion of $R^2$ inflation has been recently studied in a string theory-inspired ghost-free analytic non-local gravity. We discuss the promising theoretical predictions of non-local $R^2$-like inflation with respect to the key observables such as tensor-to-scalar ratio, tensor tilt which tell us about the spectrum of primordial gravitational waves, and scalar Non-Gaussianities which tell us about the three-point correlations in the CMB fluctuations. Any signature of non-local physics in the early Universe will significantly improve our understanding of fundamental physics at UV energy scales and quantum gravity.

Speaker: KORUMILLI SRAVAN KUMAR (Tokyo Institute of Technology, Tokyo, Japan)

6:40 PM Exact gravitational waves in non-local gravity

We study exact solutions of infinite derivative gravity within the class of so-called almost universal spacetimes. For such an ansatz, the field equations reduce to a single non-local but linear equation which is exactly solvable with the ghost-free choice $\exp(-\ell^2 \Box)$ of the non-local form factor by eigenfunction expansion or using the heat kernel method. This procedure allows us to obtain non-local analogues of Aichelburg--Sexl and Hotta--Tanaka solutions which represent gravitational waves generated by null sources propagating in Minkowski, de Sitter or anti-de Sitter backgrounds. We discuss properties of these non-local solutions and also point out that the non-locality regularizes curvature singularities at the locations of the sources.

Speaker: Tomáš Málek (Institute of Mathematics, Czech Academy of Sciences)

7:05 PM Ultrarelativistic spinning objects in nonlocal ghost-free gravity

We study the gravitational field of ultrarelativistic spinning objects (gyratons) in a modified gravity theory with higher derivatives. In particular, we focus on a special class of such theories with an infinite number of derivatives known as “ghost-free gravity” that include a nonlocal form factor such as exp(-\Box\ell^2), where \ell is the scale of nonlocality. First, we obtain solutions of the linearized ghost-free equations for stationary spinning objects. To obtain gyraton solutions we boost these metrics and take their Penrose limit. This approach allows us to perform calculations for any number of spacetime dimensions. All solutions are regular at the gyraton axis. In four dimensions, when the scale nonlocality \ell tends to zero, the obtained gyraton solutions correctly reproduce the Aichelburg–Sexl metric and its generalization to spinning sources found earlier by Bonnor. We also study the properties of the obtained four-dimensional and higher-dimensional ghost-free gyraton metrics and briefly discuss their possible applications.

Speaker: Jens Boos (William & Mary)

4:30 PM → 7:30 PM Black Hole Thermodynamics: Block 1

This parallel session will be devoted to physical and mathematical aspects of black hole thermodynamics. Topics of interest include, but are not limited to, different definitions of entropy, fundamental equations, thermodynamic laws and variables, phase transitions, extended phase space, stability properties, and critical coefficients of black holes in any dimension. The session will cover also the development and application of different analytical and geometric methods in the study of black hole thermodynamics.

Convener: Hernando Quevedo (National Autonomous University of Mexico)

YouTube Link - July 5

YouTube Link - July 6

4:30 PM Thermodynamics of AdS Black Holes: Central Charge Criticality

We reconsider the thermodynamics of AdS black holes in the context of gauge-gravity duality. In this new setting where both the cosmological constant $\Lambda$ and the gravitational Newton constant $G$ are varied in the bulk, we rewrite the first law in a new form containing both $\Lambda$ (associated with thermodynamic pressure) and the central charge $C$ of the dual CFT theory and their conjugate variables. We obtain a novel thermodynamic volume, in turn leading to a new understanding of the Van der Waals behavior of the charged AdS black holes, in which phase changes are governed by the degrees of freedom in the CFT. Compared to the "old" $P-V$ criticality, this new criticality is "universal" (independent of the bulk pressure) and directly relates to the thermodynamics of the dual field theory and its central charge.

Speaker: Dr David Kubiznak (Perimeter Institute)

5:05 PM Superentropic black hole with Immirzi hair

In this talk I will report an analytic solution describing asymptotically anti–de Sitter black holes with hyperbolic horizon, derived in the context of f(R) generalizations to the Holst action, endowed with a dynamical Immirzi field. These black holes exhibit scalar hair of the second kind, which ultimately depends on the Immirzi field radial behavior. In particular, the latter is reponsible for modifications to the usual entropy law associated to the black hole and it boils down to a constant value in the asymptotic region, thus restoring the standard loop quantum gravity picture. I will then discuss the black hole thermodynamics in the extended phase space approach, proving the violation of the reverse isoperimetric inequality, which results in the superentropic nature of the black hole, and discussing the thermodynamic stability of the solution.

Speaker: Simon Boudet

5:30 PM Information recovery from evaporating black holes

We show that the apparent horizon and the region near r=0 of an evaporating charged, rotating black hole are timelike. It then follows that for black holes in nature, which invariably have some rotation, have a channel, via which classical or quantum information can escape to the outside, while the black hole shrinks in size. We discuss implications for the information loss problem.

Speaker: Dr Zhiwei Wang

5:55 PM Stress-energy Tensor for a Quantized Scalar Field When a Black Hole in Four Dimensions Forms From the Collapse of a Null Shell

A method will be presented which allows for the numerical computation of the stress-energy tensor for a quantized massless minimally coupled scalar field in the region outside the event horizon of a 4D Schwarzschild black hole that forms from the collapse of a null shell. This method involves taking the difference between the stress-energy tensor for the $in$ state in the collapsing null shell spacetime and that for the Unruh state in Schwarzschild spacetime. The construction of the modes for the $in$ vacuum state and the Unruh state will be discussed. Applying the method, the renormalized stress-energy tensor in the 2D case has been computed numerically and shown to be in agreement with the known analytic solution. In 4D, the presence of an effective potential in the mode equation causes scattering effects that make the the construction of the $in$ modes more complicated. The numerical computation of the $in$ modes in this case will be presented.

Speaker: Shohreh Gholizadeh Siahmazgi (Wake Forest University)

6:20 PM Linear growth of the two-point function for the Unruh state in 1+1 dimensional black holes

The two-point function for a massless minimally coupled scalar field in the Unruh state is computed for various examples of 1+1 dimensional black holes. It is found that for spacelike separations of the points the two-point function grows linearly in terms of a time coordinate that is well-defined on the future black hole horizon, and for Schwarzschild-de Sitter black holes is also well-defined on the future cosmological horizon. The two-point function for a massive scalar field in Schwarzschild-de Sitter spacetime is also discussed.

Speaker: Paul Anderson (Wake Forest University)

6:45 PM Why the black hole information problem is a false problem.

I review the arguments supporting the idea that there is an information puzzle in black holes physics. Namely that unitarity is conflicting with local quantum field theory and the equivalence principle. I show that these arguments rely on speculative extra assumptions, justified only by faith in specific hypothesis on quantum gravity. Therefore the black hole information puzzle a problem only for these peculiar approaches to quantum gravity. Distinguishing thermodynamical entropy from von Neumann entropy and event horizons from apparent horizons shows that the black hole information problem is, by itself, a false problem.

Speaker: Carlo Rovelli (AMU University)

4:30 PM → 7:30 PM Radio Astronomy from Space: Block 1

Over the past century, radio astronomy has played a central role in experimental studies of General Relativity. Key milestones include measurement of the Shapiro time delay, discovery of cosmic background radiation, detection of gravitational waves using binary pulsars, discovery of superluminal motion in quasars, and the first image of a black hole using the EHT. Radio astronomy in space offers key advantages: WMAP and Planck revolutionized modern observational cosmology, while VSOP and RadioAstron achieved the sharpest resolution in the history of astronomy.
This parallel session will be devoted to recent results and plans for future space missions that target breakthroughs in experimental relativity using radio observations. The plans include high-resolution studies of the supermassive black holes Sgr A and M87 using a space-enhanced EHT, and cosmological studies using Lunar ultra-long wavelength radio telescopes.

Conveners: Michael Johnson (Center for Astrophysics | Harvard & Smithsonian), Yuri Kovalev (Lebedev Physical Institute)

YouTube Link - July 5

YouTube Link - July 8

4:30 PM Event Horizon Telescope Paper VII: imaging the polarized emission around M 87*

In 2017, the Event Horizon Telescope (EHT) observed the supermassive black hole M 87* at the center of the giant elliptical galaxy Messier 87 using very-long baseline interferometry between a global network of radio telescopes. Operating at a high radio frequency of 230 GHz, EHT enables imaging of the optically thin emission region in the immediate vicinity of the event horizon of M 87*, achieving resolution of ~3 Schwarzschild radii. Recently, the first images of the linearly polarized emission component were published. They indicate that only a part of the M 87* ring is significantly polarized. The resolved fractional linear polarization has a maximum located in the southwest part of the ring, where it rises to the level of ~15%. The polarization position angles are arranged in a nearly azimuthal pattern. Properties of the compact emission were characterized and evidence for the temporal evolution of the polarized source structure over one week of EHT observations was found. I will present the challenges of polarimetric calibration and imaging and strategies to mitigate them with a variety of analysis tools. Then I will discuss the morphology of the polarimetric images of the M 87* and derived quantities characterizing these images, which enabled the theoretical interpretation of these results.

Speaker: Maciek Wielgus (Black Hole Initiative (Harvard University))

4:50 PM Event Horizon Telescope Paper VIII: Dynamically Important Magnetic Fields at the M 87* Horizon

In 2017, the Event Horizon Telescope (EHT) observed the black hole at the center of the giant elliptical galaxy, Messier 87 using very-long baseline interferometry between a global network of radio telescopes. The resulting linearly polarized images of the accretion flow near the horizon of the black hole (M 87*) show resolved polarized structure with a spiral pattern in the electric vector position angle. I will present the implications of these images for our understanding of accretion flows around supermassive black holes. In particular, I will present the theoretical analysis recently published by the EHT, in which the EHT image reconstructions were compared to ray traced images of general relativistic magnetohydrodynamic (GRMHD) simulations of M 87* according to five metrics: average linear polarization fraction, net linear polarization, net circular polarization, and the amplitude and phase of a complex coefficient corresponding to azimuthal structure in linear polarization, $\beta_2$. Regardless of the details of the scoring procedure used, only simulations with dynamically important fields, so-called magnetically arrested disks, yield images consistent with EHT observations while producing a jet of sufficient power. The polarized image constraints refine the previous EHT estimates of the M 87* accretion rate by an order of magnitude to the narrower range of $(3 - 20)\times 10^{-4} M_{\odot} {\rm yr}^{-1}$.

Speaker: Daniel Palumbo (Center for Astrophysics | Harvard & Smithsonian)

5:10 PM The Black Hole Photon Ring

The photon ring is a narrow ring-shaped feature, predicted by General Relativity but not yet observed, that appears on images of sources near a black hole. It is caused by extreme bending of light within a few Schwarzschild radii of the event horizon and provides a direct probe of the unstable bound photon orbits of the Kerr geometry. The precise shape of the observable photon ring is remarkably insensitive to the astronomical source profile and can therefore be used as a stringent test of strong-field General Relativity. A space-based interferometry experiment targeting the photon ring of M87* could test the Kerr nature of the source to the sub-sub-percent level.

Speaker: Alex Lupsasca (Princeton University)

5:40 PM Linear and Circular Polarization Images near Black Holes: Imprints of Magnetic Fields Structure

The first imaging of the super massive black hole in M87 by the Event Horizon Telescope (EHT) has marked the beginning of a new era in black hole research that explores the properties through direct image observations. In particular, polarimetric images of the vicinity of black holes have attracted much attention because they reflect the magnetic field structure, which plays a key role in the formation of the jets. In this study, we calculatedd a general relativistic radiation transfer that takes into account the synchrotron emission, self-absorption, and Faraday effects for the Stokes parameters $(I,Q,U,V)$, and predicted the polarimetric images with future EHT observations in mind.

First, we present a polarization image of M87* under the parameters consistent with the results of the black hole shadow published in 2019 April. We found that high black hole spin is favored and the linear polarization (LP) vectors undergo strong Faraday rotation. Furthermore, we suggest that the circular polarization (CP) components can be significantly detected in ring shape, due to the Faraday "conversion" from the LP components by the ordered magnetic field structure.

Secondly, for another target of EHT, Sgr A*, we predicted the polarimetric images using models with high disk temperature. The images obtained were found to be ring-shaped when the accretion disk was viewed from nearly face-on observer, and three-forked when it was viewed from nearly edge-on. As for the polarization components, we proposed a scenario in which the LP and CP components complementarily provide information on the magnetic fields configuration and plasma properties.

Speaker: Yuh Tsunetoe (Kyoto University)

6:00 PM Imaging of M87 and Sgr A* with the Millimetron Space Observatory.

Making a high resolution image of a supermassive black hole shadow is a direct method to verify the theory of general relativity at extreme gravity conditions. Very Long Baseline Interferometry (VLBI) observations at millimeter/sub-millimeter wavelengths can provide just provide angular resolution sufficient to start resolving supermassive black holes, located in Sgr A and M87. Recent VLBI observations of M87 with the Event Horizon Telescope (EHT) has demonstrated such capability. The maximum obtainable spatial resolution of ground based VLBI is limited by Earth diameter and by atmospheric transmission and phase variations at the short wavelengths. In order to improve on space resolution a much larger Space-Earth baselines are required. In the cm wavelengths this has been successfully demonstrated by Radioastron Space mission. Millimetron is a next space mission with VLBI capabilities that operates at mm/submm wavelengths. It will have a cooled 10m diameter main dish. The base orbit of Millimetron will be located near the Anti-solar Lagrangian point - L2. In the later phase Millimetron can be pushed on to elongated elliptical orbit to optimize VLBI U-V coverage. We report simulation results of imaging capabilities of Space Earth VLBI consisting of Millimetron and EHT. We used General-relativistic magneto dynamic models (GRMHD) for back holes environment of Sgr A and M87 for dynamic and static imaging simulations. The impact of atmospheric phase fluctuations is evaluated. ETH-Millimetron observation will significantly improve spatial resolution for static images both from L2 and elliptical orbits. A short integration time snapshot images of Sgr A* from elliptical orbit may allow studying dynamical behavior at smaller timescales.

Speaker: Prof. Andrey Baryshev (Kapteyn Astronomical Institute)

6:30 PM Space VLBI studies of AGN: Extreme physics at extreme baselines.

Very long baseline interferometry (VLBI) probes cosmic phenomena at the highest angular resolution in astronomy, with the present record set at about 10 microsecond of arc. This record is achieved in space VLBI (SVLBI) observations of the Russian-led RadioAstron mission which combined a worldwide array of radio telescopes with a 10-m antenna in orbit around the Earth. Continuing on the path of SVLBI studies set off by the TDRSS experiments in the USA and the Japanese mission VSOP, RadioAstron provided the most detailed account of the inner jet regions and the highly energetic processes governing them. Results from RadioAstron Key Science Programs on AGN imaging have revealed an intricate structure and extreme brightness temperature of the jet plasma in these regions, probing the physical processes which govern formation and acceleration of jets. A brief summary of these results and some prospects for future space VLBI missions will be presented here.

Speaker: Andrei Lobanov (Max-Planck-Institut für Radioastronomie)

4:30 PM → 7:30 PM Explosive Events Associated with Compact-Object Binary Mergers: Block 1

Double compact object mergers involve the densest objects in Universe; neutron stars (NS) and black holes (BH). Their electromagnetic (EM) radiation is routinely observed in short gamma-ray bursts, in the X-rays, as well as in the optical/IR via their associated kilonovas. Recently, BH-BH/BH-NS/NS-NS mergers are also routinely detected in gravitational-waves by LIGO/Virgo.

This session will be dedicated to any phenomena that may produce observable, either EM or non-EM, signals to merging (in any configurations) neutron stars and black holes. Along standard mechanisms (like ones operating in short gamma-ray bursts or kilonovas), we encourage discussion of other challenging and/or exotic proposals for detection of these sources by other not-yet-considered means, including possible future observational missions able to detect them.

Additional related questions are encouraged to be discussed in this session. Do BH-BH mergers produce any EM counterparts? Is this population of BH-BH merging binaries compatible with our previously gained astrophysical knowledge? Are BH-NS mergers expected to be accompanied by kilonovas? Have we observed any BH-NS merger in short gamma-ray bursts? Do we have any chance to detect neutrinos from NS-NS/BH-NS mergers? What are formation sites of compact object merging binaries?

Conveners: Jorge Armando Rueda Hernandez (ICRANet), Krzysztof Belczynski

YouTube Link - July 5

YouTube Link - July 9

4:30 PM Selected topics on EM counterparts of gravitational waves

I will discuss two less-discussed, yet physically-motivated channels for EM counterparts of gravitational wave events: brief FRB-like signals from charged CBCs (especially binary black hole mergers and plunging neutron star - black hole mergers) and short-GRB-less X-ray transients. I will also discuss the physical processes that contribute to the delay timescale between CBC signals and their associated GRBs and how future observations will help to reveal the jet launching and dissipation mechanisms from neutron star mergers.

Speaker: Bing Zhang (University of Nevada, Las Vegas)

4:55 PM Uncertainties in Modeling Kilonova Emission

Neutron star mergers have long been believed to drive short-duration gamma-ray bursts, one of the most powerful explosions in the universe. They have also long been believed to be a promising source of the r-process isotopes observed in the Milky Way. These two theories were violently validated in the observation of the first neutron star merger in gravitational waves. The electromagnetic follow-up of this event proved that mergers could both produce relativistic jets and heavy r-process isotopes. But determining the exact composition of from the electromagnetic emission requires detailed physics and current models are currently forced to approximate this physics. I will discuss the uncertainties in these physical assumptions and their effect on the emission from neutron star mergers and our inference of the ejecta properties from events like the merger producing GW170817.

Speaker: Chris Fryer (Los Alamos National Laboratory)

5:20 PM Multi-messenger astrophysics in the Einstein Telescope era

The next decade of Universe exploration is expected to undergo a revolution for the transient astrophysics. The third generation of gravitational-wave (GW) observatories, such as Einstein Telescope (ET) and Cosmic Explorer (CE) will allow us for the first time to observe GWs along the cosmic history back to the cosmological dark ages. These observatories will be an unprecedented resource to address open questions of fundamental physics, astrophysics and cosmology. They will operate in synergy with a new generation of innovative electromagnetic (EM) observatories, such as CTA, Athena, the Vera Rubin Observatory, JWST, ELT, SKA and the mission concepts THESEUS and HERMES. This network of observatories will probe the formation, evolution and physics of binary systems of compact object in connection with kilonovae and short gamma-ray bursts along with the star formation history and chemical evolution of the Universe. The talk will summarize the multi-messenger science case for ET and the perspectives for the next decade.

Speaker: Marica Branchesi (Gran Sasso Science Institute/INFN)

5:45 PM Kilonova signals in NS-BH mergers

A kilonova signal is generally expected after a Black Hole - Neutron Star merger. The strength of the signal is related to the Equation of State of neutron star matter and it increases with the stiffness of the latter. The recent results obtained by NICER suggest a rather stiff Equation of State and the expected kilonova signal is therefore strong, at least if the mass of the Black Hole does not exceed $\sim 10 M_\odot$. We compare the predictions obtained by considering Equations of State of neutron star matter satisfying the most recent observations with the results predicted in the two-families scenario. In the latter a soft hadronic Equation of State produces very compact stellar objects while a rather stiff quark matter Equation of State produces massive strange quark stars, satisfying NICER results. The expected kilonova signal in the two-families scenario is very weak: the Strange Quark Star - Black Hole merger does not produce a kilonova signal because, according to simulations, the amount of mass ejected is negligible and the Hadronic Star - Black Hole merger produces a signal much weaker than in the one-family case because the hadronic Equation of State is very soft. This prediction will be easily tested with the new generation of detectors.

Speaker: Alessandro Drago (University of Ferrara)

KN signals in NS-BH mergers.pdf

6:10 PM No Detectable Kilonova Counterpart is expected for O3 neutron star-black hole candidates

As the new era of GW-led multi-messenger astronomy is ushered in, one may especially expect to catch GW signals from neutron star-black hole (NSBH) mergers and search for associated as-yet undiscovered NSBH kilonova emissions. However, in spite of many efforts for follow-up searches of potential NSBH candidates during the third run (O3) of LIGO/Virgo Collaboration (LVC), no surely EM counterpart candidate was identified. In this talk, I will show our simulated NSBH kilonova luminosity function based on our NSBH kilonova models and analyze the detectability of kilonova emissions from cosmological NSBH populations for present and future follow-up telescopes. Furthermore, I will analyze the tidal disruption probability of potential NSBH merger GW events detected during the O3 of LVC and the detectability of kilonova emissions in connection with these events. Plausible explanations for the lack of NSBH associated kilonova detection during O3 will be given.

Speaker: Jinping Zhu (Peking University)

6:35 PM Searching for joint gravitational-wave and high energy neutrino events with LLAMA

Multi-messenger detections allow us to learn more about the astrophysical sources by probing different physics and also by guiding the astronomers more precisely with low latency follow-ups. We will present the statistically optimal methods for multi-messenger searches and summarize the joint gravitational-wave and high energy neutrino event searches' results of Low Latency Algorithm for Multi-messenger Astrophysics (LLAMA) with IceCube's neutrinos and LIGO/Virgo's public detections and announcements.

Speaker: Doğa Veske (Columbia University)

7:00 PM Probing the progenitors of spinning binary black-hole mergers with long gamma-ray bursts

Long gamma-ray bursts are associated with the core-collapse of massive, rapidly spinning stars. However, the believed efficient angular momentum transport in stellar interiors leads to predominantly slowly-spinning stellar cores. In this talk, I will report on binary stellar evolution and population synthesis calculations, showing that tidal interactions in close binaries not only can explain the observed sub-population of spinning, merging binary black holes, but also lead to long gamma-ray bursts at the time of black-hole formation, with rates matching the empirical ones. We find that ~10% of the GWTC-2 reported binary black holes had a long gamma-ray burst associated with their formation, with GW190517 and GW190719 having the highest probability of being among them.

Speaker: Tassos Fragos (University of Geneva)

4:30 PM → 7:30 PM Scalar Fields in Cosmology: Block 1

In the last years, the scalar field is becoming an interesting field of study in Cosmology and Astrophysics. It appears in the formulation of many phenomena in gravitational theories. Scalar fields occur throughout physics, as spin--zero quantum fields. A scalar field is always present in the context of Dirac's large number hypothesis and also in all unified field theories; it appears as a possible type of matter, i.e., as dilatons and as inflatons in the early periods of the Universe, as a candidate to describe the dark matter nature, and as a possible Bose-Einstein condensates. The purpose of this session is to discuss different bosonic systems, scalar fields, appearing in Cosmology and Astrophysics.

Conveners: Alfredo Macias (Physics Department. Universidad Autonoma Metropolitana-Iztapalapa), Carlos Herdeiro (Aveiro University, Portugal)

YouTube Link - July 5

YouTube Link - July 8

4:30 PM Light in the dark: GW190521 as Proca star merger

The detections of gravitational waves are opening a new window to the Universe. The nature of black holes and neutron stars may now be unveiled, but gravitational radiation may also lead to exciting discoveries of new exotic compact objects, oblivious to electromagnetic waves. In particular, Advanced LIGO-Virgo recently reported a short gravitational-wave signal (GW190521) interpreted as a quasi-circular merger of black holes, one at least populating the pair-instability supernova gap. We found that GW190521 is also consistent with numerically simulated signals from head-on collisions of two (equal mass and spin) horizonless vector boson stars (aka Proca stars). This provides the first demonstration of close degeneracy between these two
theoretical models, for a real gravitational-wave event.

Speaker: Dr Nicolas Sanchis-Gual (University of Aveiro)

5:30 PM Testing the hypothesis of a bosonic star at Sgr A*

The GRAVITY collaboration has recently a detected continuous circular relativistic motion during infrared flares of Sgr A*, which has been interpreted as orbital motion near the event horizon of a black-hole. In this work, we use the ray-tracing code GYOTO to analyze the possibility of these observations being consistent with a central bosonic star instead of a black-hole. Our model consists of an isotropically emitting hot-spot orbiting a central boson or Proca star. Images of the orbit at different times and the integrated flux were obtained for both models and compared with the case of a Schwarzschild black-hole. Although the overall qualitative picture is comparable, the bosonic star models present an extra image when the emitting hot-spot passes behind the central object caused by photons travelling through the interior of the star. Furthermore, there are also measurable differences in the angles of deflection, orbital periods, and centroid of the flux, which can potentially be detected.

Speaker: João Rosa (University of Tartu)

6:00 PM Primordial Black Holes in a scalar field dominated universe.

I review in this talk the mechanism of Primordial Black Hole (PBH) formation at the end of inflation from an oscillating scalar field. I will first present solutions to the Klein Gordon and Einstein equations in this regime for linear perturbations, as well as long-wavelength nonlinear solutions. I argue that these are indicators of the collapse of inhomogeneities onto PBHs. The tiny black holes produced in these models quickly evaporate and may produce Planck mass relics. I will show that these relics can be abundant enough to constitute all of dark matter, and present the constraints that this brings on the models of complex scalar field reheating.

Speaker: Carlos Hidalgo (ICF-UNAM)

6:30 PM The imitation game: Proca stars that can mimic the Schwarzschild shadow

Can a dynamically robust bosonic star (BS) produce an (effective) shadow that mimics that of a black hole (BH)? The BH shadow is linked to the existence of light rings (LRs). For free bosonic fields, yielding mini-BSs, it is known that these stars can become ultra-compact - i.e., possess LRs - but only for perturbatively unstable solutions. We show this remains the case even when different self-interactions are considered. However, an effective shadow can arise in a different way: if BSs reproduce the existence of an innermost stable circular orbit (ISCO) for timelike geodesics (located at rISCO=6M for a Schwarzschild BH of mass M), the accretion flow morphology around BHs is mimicked and an effective shadow arises in an astrophysical environment. Even though spherical BSs may accommodate stable timelike circular orbits all the way down to their centre, we show the angular velocity along such orbits may have a maximum away from the origin, at RΩ; this scale was recently observed to mimic the BH's ISCO in some scenarios of accretion flow. Then: (i) for free scalar fields or with quartic self-interactions, RΩ≠0 only for perturbatively unstable BSs; (ii) for higher scalar self-interactions, e.g. axionic, RΩ≠0 is possible for perturbatively stable BSs, but no solution with RΩ=6M was found in the parameter space explored; (iii) but for free vector fields, yielding Proca stars (PSs), perturbatively stable solutions with RΩ≠0 exist, and indeed RΩ=6M for a particular solution. Thus, dynamically robust spherical PSs can mimic the shadow of a (near-)equilibrium Schwarzschild BH with the same M, in an astrophysical environment, despite the absence of a LR, at least under some observation conditions, as we confirm by comparing the lensing of such PSs and Schwarzschild BHs.

Speaker: Alexandre Pombo (Aveiro University)

7:00 PM A stabilization mechanism for excited fermion-boson stars

Gravitationally bound structures composed by fermions and scalar particles known as fermion-boson stars are regular and static configurations obtained by solving the coupled Einstein-Klein-Gordon-Euler (EKGE) system. As it happens for boson stars, there are different families of solutions labelled by the number of nodes in the radial profile of the scalar field; the ground state solutions have zero nodes in the radial profile, while excited states have 1 or more nodes. We study one possible scenario through which these fermion-boson stars may form by solving numerically the EKGE system under the simplifying assumption of spherical symmetry. Our initial models assume an already existing neutron star surrounded by an accreting cloud of a massive and complex scalar field. We considered an initial Gaussian radial profile for the cloud of scalar field. Our results show that from this generic initial data, we could form both ground and excited fermion-boson stars. Prompted by this finding we construct equilibrium configurations of excited fermion-boson stars and study their stability properties using numerical-relativity simulations. Contrary to purely boson stars in the excited state, which are known to be generically unstable, our study reveals the appearance of a cooperative stabilization mechanism between the fermionic and bosonic constituents of those excited-state mixed stars.

Speaker: Fabrizio Di Giovanni (Universitat de Valencia - Departamento de Astronomia y Astrofisica)

Talk_MG16_Di_Giovanni.pdf

4:30 PM → 7:30 PM Cosmic Backgrounds from Radio to Far-IR: Block 1

This parallel session will focus on the interpretation and perspectives for cosmology and astrophysics coming from cosmic backgrounds from radio to far-IR, both in temperature and in polarization.
The latest results from the Planck mission have been recently presented, while new sub-orbital experiments are investigating CMB polarization anisotropies and searching for primordial gravitational waves, and future CMB missions of different scales are foreseen or under study. The sub-mm / far-IR domain, crucial for high-frequency foreground mitigation, allows to study a number of astrophysical cosmology topics, including the early stages of star and galaxy formation. In parallel, on-going and future radio projects promise to shed light on the dawn age and on the reionization epoch and to provide 3D images of the Universe's evolution.
The authors of both invited and contributed talks are encouraged to underline the connection between astrophysical and cosmological results.

Convener: Dr Carlo Burigana (INAF-Istituto di Radioastronomia)

YouTube Link - July 5

4:30 PM Cosmology with Radio Astrophysics

In this talk we will consider several ways to use background radio radiation to learn about the large scale features of our universe as well as fundamental physics. In particular, we will highlight the power in cataloguing and understand a large number of Fast Radio Bursts and their background environments to learn about cosmology.

Speaker: Amanda Weltman (University of Cape Town)

4:49 PM High-redshift universe with redshifted 21 cm line

The redshifted 21 cm line of neutral hydrogen is one of the most useful probes of the early universe. Several experiments are ongoing and are being planned to detect the signal from high redshifts. Detection of the signal will help in understanding the first stars in the Universe, the formation and evolution of galaxies and also constraining cosmological parameters. In this talk, we will discuss some of the most interesting problems in cosmology and the high-redshift universe that can be studied using the 21 cm line, highlighting possible synergies with observations in other wave bands.

Speaker: Tirthankar Roy Choudhury (National Centre for Radio Astrophysics, Tata Institute of Fundamental Research)

5:08 PM The Planck Legacy Archive, present and future

The Planck Legacy Archive (PLA) hosts the products from the European Space Agency mission to study the Cosmic Microwave Background (CMB). The PLA web interface (https://pla.esac.esa.int) directs the users to a wide variety of Planck products, e.g., time ordered data, CMB maps, frequency and astrophysical components maps (Dust, Synchrotron, Free-Free, CIB,..), source catalogues and other products needed for cosmological studies (angular power spectra, likelihood, lensing maps, simulations). Advance Search panels are available to extensively query the PLA database, in addition to embedded links to the Planck Explanatory Supplement documentation, multiple data download options, and Helpdesk support.

Three major releases of Planck products took place in 2013, 2015, and 2018 and a selection of products have been tagged as "Legacy" to identify the version of each product most appropriate for general scientific use. In 2021 a new release of products will take place with a joint reprocessing of LFI+HFI time ordered data that includes additional information not used in previous releases. In addition, EU funded projects reprocessing Planck data, or combining it with other experiments, are expected to the deliver to the PLA higher level data products of interest to the CMB Community.

The PLA also offers specialized tools that facilitate the processing of Planck products. These tools are mainly designed to help users who are not familiar with some of the particularities of the Planck products, and can be categorized into distinct groups: map operations including component subtraction, unit conversion, colour correction, bandpass transformation, and masking of map-cutouts/full-sky maps; component separation codes, map-making codes and effective beam-averaging. In addition, the PLA includes an interface to the latest version of the Planck Sky Model simulation tool, with a simple user interface that allows users to simulate the microwave/sub-millimetre sky with Planck, as well as future CMB experiments and custom-defined instruments.

Speaker: Marcos López-Caniego (Aurora Technology for ESA)

5:27 PM High angular resolution Sunyaev Zel'dovich observations: the case of MISTRAL

The interaction of the Cosmic Microwave Background (CMB) photons with hot electron gas in Galaxy Clusters and surrounding medium can be detected through the Sunyaev Zel'dovich effect. When this effect is detected with high enough angular resolution (~10'') it allows astrophysicists to study the physics of galaxy clusters, relaxed and non-relaxed clusters, and detect filamentary structures which could give the possibility to detect the Cosmic Web. These observations are one of the main targets of the MISTRAL instrument together with a long list of scientific targets spanning from extragalactic astrophysics to solar system science. MISTRAL (MIllimetric Sardinia radio Telescope Receiver based on Array of Lumped elements kids) is a millimetric camera working in the W–band (77–103 GHz) which will be hosted on the Sardinia Radio Telescope (SRT), the Italian 64-m radio telescope located near Cagliari at 600m above the sea level, managed by INAF. It is being built as a facility instrument by the Sapienza University for INAF, funded by a PON contract for the upgrade of the SRT at high frequency. It will consist of a compact cryostat hosting the re–imaging optics, cooled at 4 K, and a 408–pixel array of photon–noise limited lumped element kinetic inductance detectors, manufactured at CNR-IFN, and cooled at a base temperature lower than 300 mK.

Speaker: Elia Stefano Battistelli (Spaienza University of Rome)

5:46 PM New Planck tSZ map and its cosmological analysis

The thermal Sunyaev-Zeldovich (tSZ) effect is produced by the inverse Compton scattering of cosmic microwave background (CMB) photons by hot electrons, particularly in galaxies clusters. It has been used as a powerful probe to constrain the cosmological parameters, given its particular sensitivity to sigma8 and omega_m.

We present a new all-sky tSZ map constructed from the latest Planck PR4 data released in 2020 with the MILCA algorithm. We will review the obtained improvements in this tSZ map in terms of signal-to-noise and resolution with respect to the map produced by the Planck collaboration in 2015. We will also present the results of the cosmological analysis with this new tSZ map.

Speakers: Hideki Tanimura (L'Institut d'Astrophysique Spatiale (IAS)), Dr Nabila Aghanim (Institut d'Astrophysique spatiale), Dr Marian Douspis (Institut d'Astrophysique spatiale)

6:00 PM The CMB Dipole: Eppur Si Muove

The largest temperature anisotropy in the cosmic microwave background (CMB) is the dipole. The simplest interpretation of the dipole is that it is due to our motion with respect to the rest frame of the CMB (with debate over the possibility of alternative explanations). As well as creating the $\ell=1$ mode of the CMB sky, this motion affects all astrophysical observations by modulating and aberrating sources across the sky. It can be seen in galaxy clustering, as well as in principle through a dipole-shaped acceleration pattern in quasar positions. Additionally, the dipole modulates the CMB temperature anisotropies with the same frequency dependence as the thermal Sunyaev-Zeldovich (tSZ) effect, and so these modulated CMB anisotropies can be extracted from the tSZ maps produced by Planck. I will discuss this novel way of measuring our motion with respect to the CMB frame, as well as discussing other signatures that may be possible to measure in future.

Speaker: Ms Raelyn Sullivan (University of British Columbia)

6:19 PM The impact of the Lorentz symmetry violation on the CMB polarization

In the standard cosmological scenario, no circular polarization is predicted for Cosmic Microwave Background (CMB) radiation. However, in the frame of moving particle, Lorentz symmetry can violate and lead to circular polarization for CMB radiation. We estimate the circular polarization power spectrum $C_l^{V(S)}$ in CMB radiation due to Compton scattering in presence of the Lorentz symmetry violation. We show that the V-mode power spectrum can be obtained in terms of linear polarization power spectrum at the last scattering surface.

Speaker: seddigheh tizchang (School of Particles and Accelerators, Institute for Research in Fundamental Sciences (IPM)

6:33 PM New gravitational degrees of freedom as a solution to the dark matter problem.

Cosmological and astronomical observations indicate that the majority
of mass and energy density of fields in the universe are in a form
which interacts extremely weakly, if at all, with light. The standard
interpretation is the existence of dark matter, commonly thought to be
in the form of particles not part of the standard model of particle
physics. At present a firm detection of such a particle is lacking,
and moreover, all these observations concern a mismatch between the
observed dynamics of visible matter with its gravitational influence.
Hence, a less explored interpretation is that the underlying theory of
gravity may not be General Relativity. A hint that this may be the
case is the observation by Milgrom that discrepancies concerning
galaxies are controlled by a single, seemingly universal, acceleration
scale.

In this talk, I will discuss this possibility and focus on a
particular relativistic realization constructed to reproduce Milgrom’s
Modified Newtonian Dynamics law at the scale of galaxies. I will show
that this proposal leads to (i) correct gravitational lensing on
galactic scales, (ii) tensor modes propagating at the speed of light,
and (iii) cosmological evolution in line with observations of the
Cosmic Microwave Background anisotropies and the large-scale structure
power spectrum.

Speaker: Constantinos Skordis (CEICO, Institute of Physics of the Czech Academy of Sciences)

6:52 PM Planck constraints on the tensor-to-scalar ratio

I will present constraints on the tensor-to-scalar ratio r using Planck data as described in [Tristram et al., A&A, 647, A128 (2021)].
In this paper, we use the latest release of Planck maps (PR4), processed with the NPIPE code, which produces calibrated frequency maps in temperature and polarisation for all Planck channels from 30 GHz to 857 GHz using the same pipeline. We computed constraints on r using the BB angular power spectrum, and we also discuss constraints coming from the TT spectrum. Given Planck’s noise level, the TT spectrum gives constraints on r that are cosmic-variance limited (withσr =0.093),butweshowthatthemarginalisedposteriorpeakstowardsnegativevaluesofrataboutthe1.2σlevel.Wederived Planck constraints using the BB power spectrum at both large angular scales (the ‘reionisation bump’) and intermediate angular scales (the ‘recombination bump’) from l = 2 to 150 and find a stronger constraint than that from TT, with σr = 0.069. The Planck BB spectrum shows no systematic bias and is compatible with zero, given both the statistical noise and the systematic uncertainties. The likelihood analysis using B modes yields the constraint r < 0.158 at 95 % confidence using more than 50 % of the sky. This upper limit tightens to r < 0.069 when Planck EE, BB, and EB power spectra are combined consistently, and it tightens further to r < 0.056 when the Planck T T power spectrum is included in the combination. Finally, combining Planck with BICEP2/Keck 2015 data yields an upper limit of r < 0.044.

Speaker: Matthieu Tristram (IJClab, CNRS, France)

7:11 PM Cosmological and astrophysical results exploiting magnification bias with high-z submillimeter galaxies

The high-z submillimeter galaxies (SMGs) can be used as background sample for gravitational lensing studies thanks to their magnification bias (Gonzalez-Nuevo et al. 2017), which can manifest itself through a non-negligible measurement of the cross-correlation function between a background and a foreground source sample with non-overlapping redshift distributions. In particular, the choice of SMGs as background sample enhances the cross-correlation signal so as to provide an alternative and independent observable for cosmological studies regarding the probing of mass distribution.
In particular (Bonavera et al. 2020), the magnification bias can be exploited in order to constrain the free astrophysical parameters of a Halo Occupation Distribution (HOD) model and some of the main cosmological parameters. Urged by the improvements obtained when adopting a pseudo-tomographic analysis (Gonzalez-Nuevo et al. 2021), we adopt a tomographic set-up to explore not only a $\Lambda$CDM scenario, but also the possible time evolution of the dark energy density in the $\omega_0$CDM and $\omega_0\omega_a$CDM frameworks (Bonavera et al. tbs).
The results that have been obtained so far by our group will be discussed.

Speaker: Laura Bonavera (University of Oviedo)

4:30 PM → 7:30 PM Status of the H_0 and Sigma_8 Tensions: Theoretical Models and Model-Independent Constraints: Block 1

This session will be devoted to study the ability of the LCDM model (the "concordance model" of cosmology) to describe the modern cosmological observations and compare with model-independent analyses as well as with a variety of alternative theoretical frameworks which have been proposed to describe the same set of observations. Among the hot subjects that should be discussed in this session we have e.g.
i) The discordant measurements between the Hubble parameter determination from CMB data (under the assumption of the LCDM) and the (cosmology-independent) distance ladder determinations. Also the time-delay measurements from strongly lensed quasars and their current status;
ii) The long standing mismatch between the background and structure formation data, in particular the sigma_8 and S_8 tensions, both being quantities whose values in the LCDM are predicted to be larger than what is needed to improve the adjustment of the structure formation data obtained from galaxy clustering and weak lensing surveys;
iii) Possible solutions to the aforesaid tensions coming from theoretical models of different kinds; and
iv) The need to analyze data in a model independent way.
Discussions are also necessary concerning possible unaccounted systematic effects in the data.

Conveners: Adrià Gómez-Valent (Institut für Theoretische Physik Heidelberg), Joan Solà Peracaula (Universitat de Barcelona)

YouTube Link - July 5

YouTube Link - July 7

YouTube Link - July 8

4:30 PM Measuring H(z) in a cosmology-independent way

I present a method to estimate H(z)/H_0 without assuming a cosmological model. The method employs the clustering of standard candles from future surveys like LSST. We find that LSST can constrain H(z)/H_0 up to z=0.7 with uncertainties, in the best cases, around 5%. The method can be further improved by including large galaxy surveys.

Speaker: Luca Amendola

4:52 PM Interacting vacuum cosmology and observational constraints

An interacting vacuum, with fixed equation of state w=-1, provides a simple model for dark energy in our Universe today, distinct from models with a varying equation of state. I will review the phenomenology of simple models where the vacuum can exchange energy and momentum with dark matter and consider the observational bounds on the interaction coming from the cosmic microwave background and large-scale structure. Such models introduce a degeneracy between the Hubble constant and the interaction strength, which determines the evolution of the dark matter density. I will present some recent work modelling structure formation in this model and perturbations in the vacuum energy. I will discuss breaking the Hubble degeneracy in this model and the implications for current tensions in cosmology.

Speaker: David Wands (University of Portsmouth)

Wands-MG16.pdf

5:14 PM Cosmological tensions: hints for a new concordance model?

The Cosmic Microwave Background temperature and polarization anisotropy measurements have provided strong confirmation of the LCDM model of structure formation. Even if this model can explain incredibly well the observations in a vast range of scales and epochs, with the increase of the experimental sensitivity, a few interesting tensions between the cosmological probes, and anomalies in the CMB data, have emerged with different statistical significance. While some portion of these discrepancies may be due to systematic errors, their persistence across probes strongly hints at cracks in the standard LCDM cosmological scenario. The most statistically significant is the Hubble constant puzzle and I will show a couple of interesting extended cosmological scenarios that can alleviate it.

Speaker: Eleonora Di Valentino (Durham University)

divalentino.pdf

5:36 PM Hubble speed from first principles

We introduce a novel way of measuring H0 from a combination of independent geometrical datasets, namely Supernovae, Baryon Acoustic Oscillations and Cosmic Chronometers, without the need of calibration nor of the choice of a cosmological model. Our method builds on the distance duality relation which sets the ratio of luminosity and angular diameter distances to a fixed scaling with redshift, for any metric theory of gravity with standard photon propagation. In our analysis of
the data we employ Gaussian Process algorithms to obtain constraints that are independent from the underlying cosmological model. We find H0 = 69.5?+/- 1.7 Km/s/Mpc,showing that it is possible to constrain H0 with an accuracy of 2% with minimal assumptions.

Speaker: Alessandra Silvestri

h0_from_first_principles.pdf

5:58 PM Let us bury the prehistoric h: arguments against using Mpc/h units in observational cosmology

It is common to express cosmological measurements in units of Mpc/h. Here, I review some of the complications that originate from this practice. A crucial problem caused by these units is related to the normalization of the matter power spectrum, which is commonly characterized in terms of the linear-theory rms mass fluctuation in spheres of radius 8 Mpc/h, σ8. This parameter does not correctly capture the impact of h on the amplitude of density fluctuations. I show how the use of σ8 has caused critical misconceptions for both the so-called σ8 tension regarding the consistency between low-redshift probes and cosmic microwave background data and the way in which growth-rate estimates inferred from redshift-space distortions are commonly expressed. We propose to abandon the use of Mpc/h units in cosmology and to characterize the amplitude of the matter power spectrum in terms of σ12, defined as the mass fluctuation in spheres of radius 12 Mpc, whose value is similar to the standard σ8 for h ~ 0.67.

Speaker: Ariel G. Sanchez (Max Planck Institute for Extraterrestrial Physics)

6:20 PM Quantifying the S8 tension with the Redshift Space Distortion data set

One problem of the ΛCDM model is the tension between the S8 found in Cosmic Microwave Background (CMB) experiments and the smaller one obtained from large-scale observations in the late
Universe. The σ8 quantifies the relatively high level of clustering. Bayesian Analysis of the Redshift
Space Distortion (RSD) selected data set yields: S8 = 0.700+0.038
−0.037. The fit has 3σ tension with the
Planck 2018 results. With Gaussian processes method a model-independent reconstructions of the
growth history of matter in-homogeneity is studied. The fit yields S8 = 0.707+0.085
−0.085, 0.701+0.089
−0.089,
and 0.731+0.063
−0.062 for different kernels. The tension reduces and being smaller then 1.5 σ. With future measurements the tension may be reduced, but the possibility the tension is real is a plausible
situation.

Speaker: David Benisty (University of Cambridge)

Quantifying the S_8 tension with the.pdf

6:42 PM Measuring Hubble's Constant with the Inverse Distance Ladder

The precise value of Hubble's constant has become one of the most interesting cosmological tensions in recent years. Measurements of H_0 with Type Ia supernovae, in a series of papers by Reiss et al., use a distance ladder of parallax and Cepheid variable stars, and find a value of H_0 which is significantly higher than expected in a LCDM cosmology with Planck CMB parameters. In this work, we use an 'inverse distance ladder' method, using distance measurements from Baryon Acoustic Oscillations to calibrate the intrinsic magnitude of the SNe. We study 207 SNe from the Dark Energy Survey, at redshift 0.018 < z < 0.85, with existing measurements of 122 low redshift (z < 0.07) SNe. We find a value of H_0 = 67.8 +/- 1.3 km s-1 Mpc-1, which is consistent with the Planck + LCDM value. Our measurement makes minimal assumptions about the underlying cosmological model, and our analysis was blinded to reduce confirmation bias.

Speaker: Ed Macaulay

7:01 PM Early and not so early dark energy. What do cosmological observations tell us about them?

Dark energy might be in charge of the late-time acceleration of the universe, but not only so. Many quintessence models possess scaling or attractor solutions where the fraction of dark energy follows the one of the dominant component in previous epochs of the universe’s expansion. Hence, they could play a role in some physical processes at redshifts z>>O(1). For instance, the presence of a non-negligible early dark energy (EDE) component around the matter-radiation equality time teq has raised as an interesting mechanism of loosening the famous H0 tension. In this work we constrain the fraction of EDE using some simple fluid parametrizations and also a non-parametric approach based on the binning of the EDE density. The latter allows us to reconstruct the shape of Ωde(z) not only before the decoupling of CMB photons, but also after it. We have employed the CMB temperature, polarization and lensing data from Planck 2018, the Pantheon compilation of supernovae of Type Ia (SNIa), data on galaxy clustering from several surveys, the prior on the absolute magnitude of SNIa obtained from the first steps of the cosmic distance ladder by SH0ES, and weak lensing data from KiDS+VIKING-450 and DES-Y1. We update previous constraints on the constant fraction of EDE in the radiation- and matter-dominated epochs, and show that with such a simple shape EDE has a negligible impact on the cosmological tensions. We reconfirm that for more complicated forms of Ωde(z), with a significant value around teq, EDE can alleviate the H0 tension at the expense of enhancing the large-scale structure (LSS) formation processes in the universe with respect to the standard ΛCDM model. This holds not only when we employ σ8 and S8 as our LSS estimators, but also when we use the recently proposed σ12 and S12 parameters. This issue can be alleviated through the presence of EDE during the post-recombination era.

Speaker: Adrià Gómez-Valent (Institut für Theoretische Physik Heidelberg)

Adrià Gómez-Valent. EDE [CM3].pdf

4:30 PM → 7:30 PM From Cosmic Strings to Superstrings: Block 1

Cosmic strings arise naturally in many proposed theories of new physics beyond the standard model, including superstring inspired inflation models. In the latter case, fundamental superstrings may have stretched to macroscopic scales, known as cosmic superstrings. If observed, these objects provide a unique window into the early universe. Recent observational progress highlights how some of these scenarios could be constrained, but they also show a bottleneck in the lack of accurate high-resolution network simulations that can be used as templates for robust statistical analysis. Additionally, most numerical simulations and analytic modeling so far are for the simplest cosmic strings, while realistic ones might have nontrivial internal structure, implying that current constraints are unreliable for these scenarios. This session will discuss recent progress in numerical simulations and analytic modeling, with a view to obtaining a more reliable assessment of the cosmological roles of these networks.

Conveners: Carlos Martins (CAUP), Ivan Rybak (Instituto de Astrofísica e Ciências do Espaço (IA), Porto)

YouTube Link - July 5

4:30 PM High-resolution calibration of string modelling

The canonical velocity-dependent one-scale (VOS) model for cosmic string evolution contains a number of free parameters which cannot be obtained ab initio. Therefore it must be calibrated using high resolution numerical simulations. We exploit our state of the art graphically accelerated implementation of the evolution of local Abelian-Higgs string networks to provide a statistically robust calibration of this model. In order to do so, we will make use of the largest set of high resolution simulations carried out to date, for a variety of cosmological expansion rates, and explore the impact of key numerical choices on model calibration, including the dynamic range, lattice spacing, and the choice of numerical estimators for the mean string velocity. This sensitivy exploration will show that certain numerical choices will indeed have consequences for observationally crucial parameters, such as the loop chopping parameter. To conclude, we will also briefly illustrate how our results impact observational constraints on cosmic strings.

Speaker: José Ricardo Correia (Instituto de Astrofísica / Faculdade de Ciências da Unviersidade do Porto)

4:50 PM Loop decay in Abelian-Higgs string networks

The evolution of cosmic strings, in particular cosmic string loops, has been an open question for a number of years. The dynamics observed by field theory lattice simulations and by the Nambu-goto approximation do not agree, giving big differences in the lifetimes of loops, which for example affects their gravitational wave production.

In this talk we will discuss the results obtained from lattice field theory loop evolution simulations, focusing on loops produced during the evolution of an actual realistic cosmic string network. We show that those loops decay proportional to $L$, but with a larger proportionality constant than the decay by GW. We see no dependency on the behaviour on the string decay on the string length. Moreover, motivated by recent results that show $L^{2}$ decay for loops created by artificially setting up string configurations, we propose another method that confirms the $L^{2}$ decay. This shows that the decay proportional to $L$ is intrinsic to network loops, and requires further investigation.

Speaker: Joanes Lizarraga (University of the Basque Country)

5:10 PM Large number of cusps in high-harmonic loops

There has been significant progress in recent years on modelling the evolution of cosmic string and cosmic superstring networks. As we are targeting gravitational wave signals from strings, attention is shifting to the closed string (loop) component of those networks. The predicted signal depends on a number of parameters, some of which are assumed/argued to be of order unity. I will focus on one of these parameters, namely the number of cusps per period of oscillation of the loop and will present evidence, based on the study of high-harmonic loops, that this can be significantly larger than unity. This could potentially lead to an enhancement of the predicted gravitational wave signal; to quantify this effect one needs to model the loop distribution in the string network.

Speaker: Dr Anastasios Avgoustidis (University of Nottingham)

5:30 PM Scaling solutions of wiggly cosmic strings

Cosmic strings may have formed in the early universe due to the Kibble mechanism. While string networks are usually modeled as being of Nambu-Goto type, this description is understood to be a convenient approximation, which neglects the typically expected presence of additional degrees of freedom on the string worldsheet. Previous simulations of cosmic strings in expanding universes have established beyond doubt the existence of a significant amount of short-wavelength propagation modes (commonly called wiggles) on the strings, and a wiggly string extension of the canonical velocity-dependent one-scale model has been recently developed. Here we improve the physical interpretation of this model, by studying the possible asymptotic scaling solutions of this model, and in particular how they are affected by the expansion of the universe and the available energy loss or transfer mechanisms—e.g., the production of loops and wiggles. In addition to the Nambu-Goto solution, to which the wiggly model reduces in the appropriate limit, we find that there are also solutions where the amount of wiggliness can grow as the network evolves or, for specific expansion rates, become a constant. Our results show that full scaling of the network, including the wiggliness, is much more likely in the matter era than in the radiation era, which is in agreement with numerical simulation results.

Speaker: Ana Almeida

5:50 PM Break

6:00 PM Electroweak axion string and superconductivity

We study the axion strings with the electroweak gauge flux in the DFSZ axion model and show that these strings, called the electroweak axion strings, can exhibit superconductivity without fermionic zero modes. We construct three types of electroweak axion string solutions. Among them, the string with W-flux can be lightest in some parameter space, which leads to a stable superconducting cosmic string. We also show that a large electric current can flow along the string due to the Peccei-Quinn scale much higher than the electroweak scale. This large current induces a net attractive force between the axion strings with the same topological charge, which opens a novel possibility that the axion strings form Y-junctions in the early universe.

Speaker: Dr Yu Hamada (KEK)

6:20 PM Approach to scaling in axion string networks

In the QCD axion dark matter scenario with post-inflationary Peccei-Quinn symmetry breaking, the number density of axions, and hence the dark matter density, depends on the length of string per unit volume at cosmic time $t$, by convention written $\zeta/t^2$. The expectation has been that the dimensionless parameter $\zeta$ tends to a constant $\zeta_0$, a feature of a string network known as scaling. It has recently been claimed that in larger numerical simulations $\zeta$ shows a logarithmic increase with time. This case would result in a large enhancement of the string density at the QCD transition, and a substantial revision to the axion mass required for the axion to constitute all of the dark matter. With a set of new simulations of global strings we compare the standard scaling (constant-$\zeta$) model to the logarithmic growth. We also study the approach to scaling, through measuring the root-mean-square velocity $v$ as well as the scaled mean string separation $x$. We find good evidence for a fixed point in the phase-space analysis in the variables $(x,v)$, providing a strong indication that standard scaling is taking place. We show that the approach to scaling can be well described by a two parameter velocity-one-scale (VOS) model, and show that the values of the parameters are insensitive to the initial state of the network. We conclude that the apparent corrections to $\zeta$ are artifacts of the initial conditions, rather than a property of the scaling network.

Speaker: Asier Lopez Eiguren (Tufts University)

6:40 PM Radiation from Global Cosmic Strings using Adaptive Mesh Refinement

We present results from adaptive mesh refinement (AMR) simulations of global cosmic strings. Using the public code, GRChombo, we perform a quantitative investigation of the dynamics of single sinusoidally displaced string configurations. We study a wide range of string energy densities $\mu \propto \ln{\lambda}$, defined by the string width parameter $\lambda$ over two orders of magnitude. We investigate the resulting massless (Goldstone boson or axion) and massive (Higgs) radiation signals, using quantitative diagnostic tools to determine the eigenmode decomposition. Given analytic radiation predictions for global Nambu-Goto strings, we compare the oscillating string decay with a backreaction model accounting for radiation energy losses, finding excellent agreement. We establish that backreaction decay is accurately characterised by the inverse square of the amplitude being proportional to the inverse tension $\mu$ for $3 \leq \lambda \leq 100$. The investigation of massive radiation at small to intermediate amplitudes finds evidence that it is suppressed exponentially relative to the preferred massless channel with a $\sqrt{\lambda}$ dependence in the exponent. We conclude that analytic radiation modelling in the thin-string (Nambu-Goto) limit provides the appropriate cosmological limit for global strings.

Speaker: Amelia Drew (University of Cambridge)

7:00 PM Discussion

4:30 PM → 7:30 PM Dark Energy and the Accelerating Universe: Block 1

This parallel session will be devoted to the study of the nature and the physical properties of Dark Energy producing the observed accelerated expansion of the present Universe. It will cover the phenomenological reconstruction of dark energy properties from observations, as well as consideration of a wide variety of theoretical models and approaches aimed to explain existing observational data, including modified gravity models, interacting dark energy and other extensions.

Conveners: Alexei Starobinsky (Landau Institute for Theoretical Physics RAS), David Polarski (Université de Montpellier)

YouTube Link - July 5

YouTube Link - July 6

YouTube Link - July 8

4:30 PM Cosmological reconstruction of gravity

We use the CMB, BAO, SN and galaxy weak lensing data to jointly reconstruct the effective dark energy density and the two phenomenological functions (mu and Sigma) describing possible modified gravity effects in the evolution of large scale structure. I will focus on the dependence of such reconstructions on the underlying assumptions (priors) and their implications for dark energy and modified gravity theories.

Speaker: Levon Pogosian (Simon Fraser University)

4:50 PM Probing modified gravity with cosmology and solutions to the Hubble tension

The late time cosmic acceleration is one of the most puzzling phenomena in modern cosmology. Its modeling within General Relativity (GR) through the cosmological constant (L) results in the LCDM scenario. Although the latter gives a precise description of the Universe, it is known that it still contains a number of unresolved problems. These lead researchers to look for modified gravity models, for example by including additional degrees of freedom.  In this talk I will present the phenomenology and the cosmological bounds of theories consistent with the gravitational-wave event GW170817. In particular I will discuss models which solve the Hubble tension between Planck and local measurements and for which data show a statistically significant preference over LCDM.

Speaker: Noemi Frusciante (IA/FCUL, Lisbon (PT))

5:10 PM Testing the equivalence principle on cosmological scales

The weak equivalence principle is one of the cornerstone of general relativity. Its validity has been tested with impressive precision in the Solar System, with experiments involving baryonic matter and light. However, on cosmological scales and when dark matter is concerned, the validity of this principle is still unknown. In this talk I will show how relativistic effects in the large-scale structure can be used to test whether dark matter obeys the weak equivalence principle. I will present forecasts for this new test of gravity for future surveys like DESI and the SKA, showing that deviations from the equivalence principle can be constrained with a precision of order 10 percent.

Speaker: Camille Bonvin (University of Geneva)

5:30 PM Non-linear perturbations in the Galileon Ghost Condensate model

Although the LCDM model is very successful in explaining current cosmological observations, in light of numerous tensions between data and theory, it is worth investigating the evolution of perturbations in alternative models, especially in the non-linear regime, where future surveys will provide a wealth of data. In this talk I will derive the relevant equations necessary to describe matter perturbations within the spherical collapse model for the Galileon Ghost Condensate, which extends the well known cubic covariant Galileon. I will show how the mass function is affected by the different evolution of perturbations and present a simple recipe which maps the linear matter power spectrum to the non-linear one. I will also extend the analysis to discuss the lensing convergence.

Speaker: Dr Francesco Pace (Dipartimento di Fisica ed Astronomia, Università di Bologna)

5:50 PM Testing F(Q) gravity with redshift space distortions

A cosmological model with Symmetric Teleparallel Gravity where gravity is non-metrical is constrained with redshidt space distortions data. The cosmological background for the model mimics a ΛCDM evolution but differences arise in the perturbations. The linear matter fluctuations are numerically evolved and the study of the growth rate of structures is analysed in this cosmological setting. The best fit parameters reveal that the σ8 tension between Planck and Large Scale Structure data can be alleviated within this framework.

Speaker: Tiago Barreiro (Instituto de Astrofísica e de Ciências do Espaço, FCUL)

6:10 PM Observational Constraints of Dark D-Brane Cosmology

"Dark energy", a matter/energy source whose nature is still not well understood, is widely assumed as an explanation for the observed accelerated expansion of the Universe. The standard model of cosmology, the ΛCDM model, consists of the simplest scenarios in which dark energy is a cosmological constant. Even though it provides an impressive fit to the available cosmic background radiation and large-scale distribution of galaxies observational data, this model is still hunted by conceptual problems and observational tensions. 
To tackle some of these issues, it is common to take generalisations of the cosmological constant, such as promoting it to a dynamical scalar field, with the possibility of having interaction with the matter sector. 
In this talk I will give a brief overview of interacting dark energy models, with particular focus on disformal couplings and its cosmological implications. More concretely, I will focus on the general Dark D-Brane scenario, that predicts a natural interaction in the dark sector related with the induced metric on a moving brane. Furthermore, I will present the main cosmological predictions of this setting, obtained through a numerical study, together with a statistical data analysis to produce observational constraints.

Speaker: Elsa Teixeira (University of Sheffield)

6:30 PM Simulation-wise Comparison of Yukawa and Newtonian Gravitational Forces

In a cubic cosmological simulation box with three-dimensional periodicity, we determine the gravitational potential and force generated by a single particle. Using both the Newtonian approximation and Yukawa law of gravity within the cosmic screening approach [1,2], we zoom into the regions in the box where the distinction among them becomes significant. Extending the analysis to corresponding physical distances today as well as at late stages of matter domination, we show how employing Newtonian approximation over Yukawa gravity affects simulations of structure formation in terms of force computation. Additionally, we compare the plain Yukawa (non-periodic) and Yukawa-Ewald (periodic) forces in the box to study the impacts of periodic boundaries.

[1] M. Eingorn, First-order cosmological perturbations engendered by point-like masses. ApJ 825, 84 (2016). arXiv:1509.03835.
[2] E. Canay, M. Eingorn, Duel of cosmological screening lengths. Phys. Dark Univ. 29, 100565 (2020). arXiv:2002.00437.

Speaker: Ezgi Canay (Istanbul Technical University)

4:30 PM → 7:30 PM Cosmography with Gravitational Lensing: Block 1

Gravitational lensing has tremendously contributed to our understanding of the Universe in the past thirty years. Progress in this field has been extremely rapid, thanks to major advances in both observation and theory. Several recent studies have shown that gravitational lensing can provide accurate and precise estimates, independent from and complementary to those of other probes, of the Hubble constant and of the geometry of the Universe. The wealth of data from current and future surveys will transform gravitational lensing into a fundamental alternative tool for measuring some of the most relevant cosmological quantities. Analyses will no longer be statistically limited and they might point to exciting new physics. A concerted effort between observers and theorists will be needed to control systematics and reap the rewards from the large gravitational lensing datasets.

Conveners: Claudio Grillo (University of Milan), Prof. Mimoza Hafizi

YouTube Link - July 5

4:30 PM H_0 measurement from time-delay cosmography

The Hubble constant ($H_{0}$) is one of the most important parameters in
cosmology. Its value directly sets the age, the size, and the critical
density of the Universe. Despite the success of the flat $\Lambda$CDM model, the
derived Hubble constant from Planck data under the assumption of a flat
LCDM model has 4.4-$\sigma$ tension with the direct measurements. If this
tension is not due to the systematics, it may indicate the new physics
beyond the standard cosmological model. $H_0$ from time-delay lensing is
a powerful independent tool for addressing the $H_0$ tension since it is
independent of both Planck and the distance ladder. One way to do this
is to increase the number of high-quality lens systems since this allows
us to look for correlations and other effects due to systematics, and to do
hierarchical approaches to assess known systematic effects.
Keck AO data is not only the key component to increase the precision of $H_0$
measurement but also provides systematic checks with the $H_0$ results
based on HST imaging. In this talk, I will present the view of the current $H_0$
measurement, the systematic checks, and the future prospects of
TDCOSMO collaboration.

Speaker: Geoff Chih-Fan Chen (UCLA)

4:55 PM Time-delay cosmography in the age of JWST

Our cosmological discourse is currently dominated by the discrepancy between early and late-time cosmological probes. This tension, if confirmed, can only be resolved by yet unknown physics or by our lack of accounting for systematic uncertainties in the methods. Given the drastic implications of the former, the latter has been of great interest lately. In the context of time-delay strong lensing (TDSL), which has been established as one of the few powerful and independent probes of H0, the prominent mass-sheet degeneracy (MSD) is commonly cited as being a significant source of systematics. This degeneracy is tightly linked to our lack of understanding of the inner mass density profiles of galaxies and introduces a full degeneracy with H0 in the models. Additional tracers of the underlying gravitational potential are needed to break the degeneracy. Yet, current observational facilities fall short in obtaining the required data. In my talk, I will show how JWST will help us to finally break the MSD, by tightly constraining the amount of mass-sheet that is physically associated with the lens. Based on detailed simulations with JWST-like stellar kinematics, we find that uncertainties of the time-delay distance and the lens angular diameter distance can be limited to better than 10%, without assumptions on the background cosmological model. These distance constraints would translate to a < 4% precision measurement on H0 in flat LCDM cosmology from a single lens. Based on these forecasts, TDSL will regain much of its precision while still allowing for models which are maximally degenerate with H0. This will enable us to obtain a < 2% precision measurement on H0 by means of only a few lens systems and potentially provide a smoking gun evidence to address the H0 tension within JWST’s first few years of operation.

Speaker: Akin Yildirim (Max Planck Institute for Astrophysics)

5:15 PM A tale of two double quasars: Hubble constant tension or biases?

For a flat $\Lambda$CDM (standard) cosmology, a small sample of gravitationally lensed quasars with measured time delays has recently provided a value of the Hubble constant $H_0$ in agreement with data from SNe, but in tension with the Planck flat $\Lambda$CDM result. Identifying biases in some methods may solve this tension, avoiding hasty rejection of the standard cosmological model. As a case study, we use two double quasars of the GLENDAMA sample (SBS 0909+532 and SDSS J1339+1310) to discuss the $H_0$ value in a standard cosmology. Our preliminary analysis focus on the role of several parameters: astrometry for the lens system, time delay between images, external convergence and mass model for the main lens galaxy

Speaker: Luis J. Goicoechea (Universidad de Cantabria)

5:35 PM Cosmography with space-based strong lensing observations

In this talk, I will present my work on cosmography with strong-lensing in galaxy-clusters observed with the Hubble Space Telescope. I will detail some particular aspects of the analysis, in preparation for future surveys like Euclid and CSST.

Speaker: Eric JULLO (Aix-Marseille Univ/LAM)

6:00 PM Cosmology and Stellar Physics with Strong Lensing

Strongly lensed supernovae (SNe) are emerging as a new probe of cosmology and SN progenitors. The time delays between the multiple images of a lensed SN can be used to determine the Hubble constant (H0) that sets the expansion rate of the Universe. An independent determination of H0 is important to ascertain the possible need of new physics beyond the standard cosmological model, given the tension in current H0 measurements. I would like to present investigations of SN Refsdal, the first strongly lensed SN with multiple spatially-resolved SN images. While strongly lensed SNe are very rare with currently only 2 known systems, future surveys, particularly the Rubin Observatory Legacy Survey of Space and Time, are expected to yield hundreds of such exciting events. I present a new program aimed to find and study lensed SNe for cosmology and stellar physics.

Speaker: Prof. Sherry Suyu (Max Planck Institute for Astrophysics / Technical University of Munich)

6:25 PM Cluster strong lensing cosmography: robust cosmological constraints from a sample of galaxy clusters

Galaxy cluster strong lensing has numerous applications in cosmology. Thanks to the wealth of multi-wavelength observations of clusters using state-of-the-art observatories, such as the Hubble Space Telescope and the Very Large Telescope, this field is providing significant contributions to the understanding of our Universe. One of the main points that are still not fully understood is the nature of the components of the Universe. The upcoming generation of observational surveys of the cosmos were designed to probe Dark Matter and Energy as one main scientific goals.

One cosmological probe not yet fully explored is the cluster strong lensing cosmography. Although the main contribution to the light deflection of lensed galaxies is the gravitational potential of foreground galaxies, the background geometry of the Universe has a non-negligible effect. Thanks to the high accuracy of current strong lens models, we can probe this ``secondary'' quantity with unprecedented precision. In the $\Lambda$CDM framework, these quantities are Dark Matter and Energy densities, and the Dark Energy equation of state.

In this talk, I will present the combined strong lensing constraints on the quantities above from a sample of galaxy clusters, in contrast to what was done in single systems until today. I will show that the combined constraints are powerful in probing the background geometry of the Universe, and are also nicely complementary to other probes such as the cosmic microwave background, Supernovae-Ia and Baryonic Acoustic Oscillations. Hence, cluster strong lensing will be a competitive cosmological probe and paramount in the observations of the next generation of surveys such as the Rubin Observatory Legacy Survey of Space and Time (LSST) and Euclid space telescope.

Speaker: Gabriel Bartosch Caminha (Max Planck Institute for Astrophysics)

6:45 PM A strong Lensing Model of SDSS J1029+2623: prospects for cosmography

In the last years, thanks to the increased precision of the measurements of the Hubble constant, H0, some tension has emerged between measurements from local and early-Universe probes. Strong gravitational (SL) lenses with measured time delays between the multiple images are yielding a competitive approach to estimate H0, that is independent and complementary to other techniques. Such studies are extremely timely since upcoming surveys, like LSST, are expected to discover hundreds of variable sources multiply lensed by galaxy clusters.
I will present a new SL analysis of the galaxy cluster SDSS J1029+2623 at a redshift of z=0.588, which is one of the few known lens galaxy clusters with multiple images of a background (z=2.1992) quasar, with a measured time delay. I have used archival HST multicolour imaging and MUSE IFS to identify cluster members, measure the stellar velocity dispersions for the brightest of them, and spectroscopically confirm lensed sources. With the newly acquired data, we are able to build a detailed parametric lens mass model, that can shed new light on the known flux ratio anomaly between the quasar images, and give some prospects on the use of this cluster lens to constrain cosmological parameters.

Speaker: Dr Ana Acebron (Università degli Studi di Milano)

7:05 PM Search for lensing signatures in the gravitational-wave observations from the first half of LIGO-Virgo's third observing run

The Advanced LIGO and Advanced Virgo detectors are now observing large numbers of gravitational-wave signals from compact binary coalescences, with 50 entries in the latest transient catalogue GWTC-2. The next detector upgrades will continue bringing rapidly growing event rates and redshift range, so our chances become better both to detect rare astrophysical effects on these novel cosmic messengers and to employ them as cosmological probes. Gravitational lensing, with its long and productive history in electromagnetic astronomy, holds particularly great potential for the future of GW astrophysics and cosmology. This presentation covers the first LIGO-Virgo collaboration search for signatures of gravitational lensing in data from O3a, the first half of the third advanced detector observing run. We study: 1) the expected rate of lensing at current detector sensitivity and the implications of a non-observation of strong lensing or a stochastic gravitational-wave background on the merger-rate density at high redshift; 2) how the interpretation of individual high-mass events would change if they were found to be lensed; 3) the possibility of multiple images due to strong lensing by galaxies or galaxy clusters; and 4) possible wave-optics effects due to point-mass microlenses. Overall, we find no compelling evidence for lensing in the observed GW signals from any of these analyses on current data, but we also highlight the future prospects of lensed GWs with the current detector network at design sensitivity and future detectors.

Speaker: David Keitel (Universitat de les Illes Balears)

4:30 PM → 7:30 PM Interacting Dark Matter: Block 1

The session is devoted to the physics of all possible aspects of interacting dark matter, including self interactions of dark matter and their cosmological consequences, or interactions of dark matter with ordinary matter and their consequences. Various types of dark matter in various models, including supersymmetry, are considered. Dark matter searches also constitute part of the session's subject.

Convener: Nikolaos Mavromatos (King's College London, Physics Department)

YouTube Link - July 5

YouTube Link - July 6

4:30 PM Primordial Black Holes

I will review the present status of massive and clustered PBH that may constitute all of the Dark Matter in the Universe.

Speaker: Juan Garcia-Bellido (Universidad Autonoma de Madrid)

MG16_5Jul2021_JGB.pdf

4:53 PM Fuzzy Dark Matter

Fuzzy dark matter (FDM) is a general term for the lightest possible dark matter particle. FDM is distinct from CDM in manifesting wavelike effects on cosmic scales, which lead to a vast array of methods to probe this model. Across more than 20 orders of magnitude, only two windows windows remain where FDM can constitute the entirety of the dark matter. I will discuss how these windows are arrived at using astrophysical and cosmological observables, including galaxy weak lensing, the dynamics of star clusters, and the properties of black holes. I will further derive precision bounds from the CMB and galaxy clustering that probe sub-dominant FDM populations at the 1% level. Improving these bounds with intensity mapping could test the possible connection between FDM and the grand unified scale. Finally, I will discuss how black hole superradiance caused by FDM can be used to test the topological invariants of string theory compactifications.

Speaker: David Marsh (King's College London)

fuzzy_dm_marsh.pdf

fuzzy_dm_marsh.pptx

5:16 PM DM halos from the maximum entropy principle and its link to particle physics

We discuss the problem of formation of dark matter (DM) halos from the principle of maximum (coarse-grained) entropy, when including for the quantum nature of the DM particles. In the case of DM fermions, such a formation mechanism involves for (long-range) particle-particle interactions, and can lead to a most-likely phase-space distribution accounting for the Pauli-principle as well as particle escape effects. We show the full family of DM profiles which can be built out of the above mechanism for self-gravitating fermions, and analyze which solutions are stable, long-lived and of astrophysical interest. We emphasize on a novel kind of ‘core – halo’ DM profiles, where the compact and dense core of DM can work as an alternative to the supermassive BH scenario, while the extended halo can explain the "flateness" of the rotation curves. Finally, we show the possibility to model such DM fermions within minimial extensions of the SM of particle physics which include for right handed (keV-ish) neutrinos.

Speaker: Carlos Raúl Argüelles (Universidad Nacional de La Plata)

5:39 PM Interacting dark sector: mapping fields and fluids, and observational signatures

In this talk, I will discuss a cosmological model with dark energy – dark matter interaction. Demanding that the interaction strength $Q_{\nu}$ in the dark sector must have a field theory description, a unique form of interaction strength can be obtained. I will show the equivalence between the fields and fluids for the $f(R,\chi)$ model where $f$ is an arbitrary, smooth function of $R$ and classical scalar field $\chi$, which represents dark matter. Up to first order in perturbations, there is a one-to-one mapping between the classical field theory description and the phenomenological fluid description of interacting dark energy and dark matter, which exists only for this unique form of interaction. Different formulations of interacting dark energy models in the literature can be classified into two categories based on the field-theoretic description. Then I will discuss the quantifying tools to distinguish between the interacting and non-interacting dark sector scenarios. I will focus on the variation of the scalar metric perturbed quantities as a function of redshift related to structure formation, weak gravitational lensing, and the integrated Sachs-Wolfe effect and show that the difference in the evolution becomes significant for lower redshifts (z<20), for all length scales. (Based on arXiv: 2006.04618)

Speaker: JOSEPH P JOHNSON (IIT Bomaby)

mg16_talk_ver1.pdf

5:59 PM Break

6:04 PM LHC experiments for long-lived particles of the dark sector

Dark matter scenarios are being tested at the LHC in the general-purpose experiments through promptly decaying states. In parallel, new dedicated detectors have been proposed for the LHC to probe dark matter portal theories predicting long-lived particles that decay away from the interaction point: MoEDAL-MAPP, MoEDAL-MALL, FASER, CODEX-b, MATHUSLA, AL3X, ANUBIS, milliQan. In addition, the SHiP beam-dump experiment is planned to operate with the SPS beam to extend the discovery reach for such particles. The detector design and expected physics sensitivity of these experiments will be presented with emphasis on scenarios explaining the nature of dark matter.

Speaker: Vasiliki Mitsou (IFIC - Univ. of Valencia and CSIC (ES))

Mitsou-DM-LLP-MG16.pdf

6:27 PM The MoEDAL-MAPP Experiment - Extending the Physics Reach of the LHC

MoEDAL-MAPP is a pioneering experiment designed to search for highly ionizing (HIP), feebly interacting (mQP) and long-lived particle (LLP) avatars of new physics in p-p and heavy-ions collisions at the LHC. The MoEDAL baseline detector first took data at LHC’s Run-2. This detector was dedicated to the search for HIPs, such as magnetic monopoles or massive (pseudo-)stable charged particles, that are predicted to existing a plethora of models beyond the Standard Model. The MoEDAL-MAPP experiment, including the MALL detector, is designed to extend the search for new physics to include mQPs and LLPs for LHC’s Run-3. MoEDAL’s ground breaking physics program defines a number of scenarios that yield potentially revolutionary insights into such foundational questions as: are there extra dimensions or new symmetries; what is the mechanism for the generation of mass; does magnetic charge exist; and what is the nature of dark matter. The current results from Run-2, the status of the MoEDAL-MAPP detector for Run-3 and the physics program for Run-3, will be discussed.

Speaker: Prof. James Pinfold (University of Alberta)

Marcel Grossmann 2021.pdf

6:50 PM Constraining the dark matter self-interaction cross section in low-mass halos with quadruple image strong gravitational lenses

Collapsed structures, or halos, formed in models with self-interacting dark matter (SIDM) have unique properties that distinguish them from structures formed in cold dark matter (CDM). In particular, momentum and energy exchange inside SIDM halos drives the formation of a central core that may eventually undergo core collapse, such that the halo becomes extremely centrally concentrated. We demonstrate that the flux ratios in quadruply imaged quasar strong lens systems (quads) provide an avenue to statistically constrain the unique features of SIDM in halos with masses below $10^9 M_{\odot}$, providing a new, purely gravitational probe of SIDM structure on sub-galactic scales. In the low-mass halos probed by lensing, particles move at relative velocities below 30 km/sec, and thus an analysis of quads can provide a new window on the self-interaction cross section below the velocity scales accessible with galaxies or galaxy clusters. To determine how a sample of quads can constrain SIDM models, we implement a structure formation model that predicts the properties of cored and core collapsed halos given an interaction cross section, and show that SIDM structure produces flux ratio perturbations distinct from those arising in CDM. We then forecast, with simulated datasets, that a sample of 30-50 quads, a sample size attainable in the next few years, can place stringent constraints on the amplitude and velocity dependence of the cross section, potentially ruling out certain SIDM models, or falsifying CDM.

Speaker: Daniel Gilman (University of Toronto)

Gilman_MG2021.pdf

7:10 PM Growth of Linear Perturbations in a Universe with Superfluid Dark Matter

The Lambda-Cold Dark Matter ($\Lambda$CDM) model agrees with most of the cosmological observations, but has some hindrances from observed data at smaller scales such as galaxies. Recently, Berezhiani and Khoury proposed a new theory involving interacting superfluid dark matter with three model parameters in \cite{khoury2015}, which explains galactic dynamics with great accuracy. In the present work, we study the cosmological behaviour of this model in the linear regime of cosmological perturbations. In particular, we compute both analytically and numerically the matter linear growth factor and obtain new bounds for the model parameters which are significantly stronger than previously found. These new constraints come from the fact that structures within the superfluid dark matter framework grow quicker than in $\Lambda$CDM, and quite rapidly when the DM-baryon interactions are strong.
Link to the paper- https://doi.org/10.1088/1475-7516/2020/07/034

Speaker: Shreya Banerjee (Friedrich Alexander University)

Shreya Banerjee-MG16.pdf

4:30 PM → 7:30 PM Quantum Fields: Block 1

This session is dedicated to all aspects of the theory of quantum fields. Special interest we will paid to the quantum fields in curved space-time and to any results having applications in General Relativity.

Convener: Andrei Lebed (Department of Physics, University of Arizona)

YouTube Link - July 5

YouTube Link - July 8

4:30 PM Breakdown of the Equivalence Principle for a composite quantum body

We investigate simplest composite quantum body – hydrogen atom – in a weak external gravitational field. Using the local Lorentz invariance of spacetime in general relativity, we calculate electron gravitational mass taking into account both kinetic and potential energies of electron in the atom. In addition to the expected change of electron mass due to total energy, we obtain the unexpected virial term, which is doubled kinetic energy plus potential one. The appearance of this term breaks the Einstein’s Equivalence Principle both at microscopic and macroscopic levels. Indeed, if we perform the quantum measurement of gravitational mass of an individual hydrogen atom, it can be not equal to the expected value E/c2. As to macroscopic level, we conclude that for macroscopic ensembles of the stationary quantum states the Equivalence Principle survives. Nevertheless, for special quantum macroscopic ensembles – coherent macroscopic ensembles of the quantum superpositions of stationary states – the Equivalence Principle is strongly broken due to the virial term [1,2]. We discuss possible experiments in the Earth’s laboratories, where the above mentioned phenomenon can be discovered.
[1] A.G. Lebed, Int. J. Mod. Phys. D, v. 28, 1930020 (2019); [2] A.G. Lebed, Mod. Phys. Lett. A, v. 35, 2030010 (2020).

Speaker: Prof. Andrei Lebed (Department of Physics, University of Arizona)

4:50 PM Hydrodynamic representation for fermion particles in curved space-times.

We study the hydrodynamic representation of the Dirac equation in arbitrary curved space-times coupled to an electromagnetic field. Using a generalized Madelung transformation we derive an integral of the corresponding Bernoulli equation for ferminos and show the corresponding Bernoulli equation. Using the comparison of the Dirac and the Klein-Gordon equations we derive the balance equations for fermion particles.

Speaker: Omar Gallegos (Cinvestav-IPN)

hydrodynamic_Dirac_MGM16.pdf

Hydrodynamic representation and Energy Balance for the Dirac and Weyl fermions in curved space-times

5:10 PM Extended DeWitt-Schwinger subtraction scheme, heavy fields and decoupling

We will present the extended DeWitt-Schwinger subtraction scheme [1] in order to consistently remove the divergent pieces of the one loop effective action for a scalar field in curved spacetime. This scheme includes a $\mu$ dependence that results in the running of the coupling constants. We will prove that this scheme is compatible with the decoupling of heavy massive fields in the low energy limit as stated by the Appelquist-Carazzone theorem for flat spacetime. We will also use this scheme to construct an effective field theory that avoids the obstacles associated with the cosmological constant problem.

[1] A. Ferreiro and J. Navarro-Salas, Phys. Rev. D 102, 045021 (2020).

Speaker: Antonio Ferreiro (University of Valencia & IFIC)

5:30 PM Chiral vortical effect for free fermions on anti-de Sitter space

According to the axial vortical effect, an axial current $J^\mu_A$ is produced in a fluid undergoing a macroscopic vortical motion, which is equal to the local kinematic vorticity $\omega^\mu$ multiplied by the axial vortical conductivity $\sigma^\omega_A$. We probe the curvature corrections to $\sigma^\omega_A$ by computing the thermal expectation value of $J^\mu_A$ with respect to a rigidly-rotating quantum state at finite temperature. The calculation is computed in the real time formalism using a novel KMS relation which includes the effect of rotation, being based on an exact expression for the fermion vacuum two-point function (the analysis is restricted to subcritical rotations when no speed of light surface forms, such that the rotating and stationary vacua are identical). Our results confirm the Minkowski expression for $\sigma^\omega_A$, revealing a novel contribution proportional to the Ricci scalar. At vanishing mass, the conservation of $J^\mu_A$ implies a non-vanishing flux through the adS boundary, while at non-vanishing mass, the flux of $J^\mu_A$ is completely converted into a volumetric density of pseudoscalar condensate $-i {\bar \psi} \gamma^5 \psi$.

Speaker: Victor E. Ambrus (Goethe University)

Ambrus-MG16-adS.pdf

5:50 PM Renormalization, running couplings and decoupling for the Yukawa model in curved spacetime

Following the method presented in the talk "Extended DeWitt-Schwinger subtraction scheme, heavy fields and decoupling [1]", we consider the renormalization of the one loop effective action for the Yukawa interaction with a background scalar field in curved spacetime [2]. We compute the beta functions and discuss the decoupling in the running of the coupling constants. For the case of a quantized scalar field, all the beta function are compatible with the decoupling of heavy massive fields, including also the gravitational ones. For a quantized Dirac field, decoupling appears for all the beta functions except for the anomalous result of the mass of the background scalar field.

[1]A. Ferreiro and J. Navarro-Salas, Phys. Rev. D 102, 045021 (2020).
[2] A. Ferreiro, S. Nadal-Gisbert and J. Navarro-Salas. arXiv:2104.14318 (2021)

Speaker: Mr Sergi Nadal-Gisbert (Universitat de València-IFIC (CSIC))

6:10 PM Black hole induced false vacuum decay from first principles

We provide a method to calculate the rate of false vacuum decay induced by a black hole. The method uses complex tunnelling solutions and consistently takes into account the structure of different quantum vacua in the black hole metric via boundary conditions. We illustrate the technique on a two-dimensional toy model of a scalar field with inverted Liouville potential in an external background of a dilaton black hole.

Speaker: Andrey Shkerin (University of Minnesota)

6:30 PM Evaporation of four-dimensional dynamical black holes sourced by the quantum trace anomaly

A first approximation to describe the interplay between quantum matter and gravity can be obtained in the quantum field theory on curved spacetimes by studying the back-reaction of a quantum field on the spacetime geometry, using the so-called semiclassical Einstein equation. In this framework, the evaporation of four-dimensional spherically symmetric dynamical black holes can be explained by the appearance of a negative ingoing energy flux at the apparent horizon, which induces a negative variation of the black hole mass. This negative flux can be sourced by the trace anomaly of the quantum stress-energy tensor in case of a free massless conformally-coupled scalar field, once a certain averaged energy condition is valid outside the horizon. This condition holds assuming that the semiclassical Einstein equation is fulfilled outside the horizon by the background geometry which is sourced by classical collapsing matter. As an example, both the negative flux and the rate of evaporation can be explicitly evaluated in the Vaidya spacetime, which describes the exterior geometry of a null radiating star. The talk is based on a joint work with N. Pinamonti, S. Roncallo and N. Zanghì (arXiv:2103.02057 [gr-qc]).

Speaker: Paolo Meda (University of Genoa - INFN)

MEDA_PAOLO_Marcel_Grossmann.pdf

4:30 PM → 7:30 PM Topological Methods, Global Existence Problems, and Spacetime Singularities: Block 1

This is session is about global causal structure problems, topological methods for spacetime structure and evolution, global existence and stability problems, nature and classification of singularities, general theory of black holes, character of singularities in geometric extensions of GR and string theories.

Convener: Spiros Cotsakis (Univ. Aegean, Greece; IGC, RUDN Univ., Moscow, Russia)

YouTube Link - July 5

4:30 PM Asymptotic synchronization of Mixmaster spatial points

We consider the problem of asymptotic synchronization of different spatial points coupled to each other in inhomogeneous spacetime and undergoing chaotic Mixmaster oscillations towards the singularity. We demonstrate that for couplings larger than some threshold value, two Mixmaster spatial points $A,B$, with $A$ in the past of $B$, synchronize and thereby proceed in perfect unison towards the initial singularity. We further show that there is a Lyapunov function for the synchronization dynamics that makes different spatial points able to synchronize exponentially fast in the past direction. We provide an elementary proof of how an arbitrary spatial point responds to the mean field created by the oscillators, leading to their direct interaction through spontaneous synchronization. These results ascribe a clear physical meaning of early-time synchronization, the two BKL maps corresponding to two distinct oscillating spatial points converge to each other and become indistinguishable at the end of synchronization, and suggest that the universe organizes itself gradually through simpler, synchronized, states as it approaches the initial singularity. A discussion of further implications of early-time inhomogeneous Mixmaster synchronization for the horizon problem and the behavior of entropy is also provided.

Speaker: Spiros Cotsakis (Univ. Aegean, Greece; IGC, RUDN Univ., Moscow, Russia)

talkMG16-COTSAKIS.pdf

5:00 PM On the scattering laws of bouncing universes

I will present recent developments on the geometric analysis of Einstein's field equations for spacetimes containing singularity hypersurfaces, which represent gravitational waves, shock waves, or phase interfaces. I will explain the formulation and classification of scattering laws and junction conditions at singularities, and will discuss bouncing cosmologies (big bang, big crunch). I will then apply this formalism to the resolution of the global evolution problem for the Einstein equations when two gravitational plane-symmetric waves collide and generate a cyclic spacetime. This is a research project in collaboration with B. Le Floch (ENS, Paris) and G. Veneziano (CERN, Geneva).

Speaker: Prof. Philippe G. LeFloch (Sorbonne University)

5:30 PM Brane-world singularities in a fluid bulk

We present new results on the singularity structure and asymptotic analysis of a brane-world that consists of a flat 3-brane embedded in a five-dimensional bulk. The bulk matter is modelled by a fluid that satisfies a non-linear equation of state of the form $p=\gamma\rho^{\lambda}$, where p is the ‘pressure’ and $\rho$ is the ‘density’ of the fluid. We show that for appropriate ranges of the parameters $\gamma$ and $\lambda$, it is possible to construct a regular solution, compatible with energy conditions, that successfully localizes gravity on the brane. These results improve significantly previous findings of the study of a bulk fluid with a linear equation of state.

Speaker: Ifigeneia Klaoudatou (University of the Aegean)

6:00 PM Strong cosmic censorship theorem in Bakry-Emery spacetimes

A class of naked strong curvature singularities is ruled out in Bakry-Emery spacetimes by using techniques of differential topology in Lorentzian manifolds.
These spacetimes adimit a Bakry-Emery-Ricci tensor which is a generalization of the Ricci tensor. This result supports to validity of Penrose's strong cosmic censorship conjecture in scalar-tensor gravitational theories, which include dilaton gravity and Brans-Dicke theory.

Speaker: Makoto Narita

6:30 PM Geodesics near a curvature singularity of stationary and axially symmetric space-times

In this work we study the local behavior of geodesics in the neighborhood of a curvature singularity contained in stationary and axially symmetric space-times. Apart from these properties, the metrics we shall focus on will also be required to admit a quadratic first integral for their geodesics. In particular, we search for the conditions on the geometry of the space-time for which null and time-like geodesics can reach the singularity. These conditions are determined by the equations of motion of a freely-falling particle. We also analyze the possible existence of geodesics that do not become incomplete when encountering the singularity in their path. The results are stated as criteria that depend on the inverse metric tensor along with conserved quantities such as energy and angular momentum. As an example, the derived criteria are applied to the Plebanski-Demianski class of space-times. Lastly, we propose a line element that describes a wormhole whose curvature singularities are, according to our results, inaccessible to causal geodesics.

Speaker: Juan Carlos Del Águila Rodríguez (Centro de Investigación y de Estudios Avanzados del I.P.N.)

7:00 PM Open problems in the nature of singularities in relativistic spacetimes

I will discuss the status of our understanding of singularities in general relativistic spacetimes. I will cover briefly their definition, location, and existence, while focusing on their classical and quantum nature. I will emphasize what we know, and what we do not know about the effect of test particles and waves on a zoo of singularities, from quasiregular to nonscalar curvature to scalar curvature in both localized and cosmological scenarios.

Speaker: Deborah Konkowski (U S Naval Academy)

4:30 PM → 7:50 PM What Can We Learn from a Growing Sample of Fast Radio Bursts?: Block 1

This parallel sessions will cover Fast Radio Bursts (FRBs), a recently identified cosmic phenomenon consisting of few-millisecond radio bursts arriving from far outside the Milky Way, even from cosmological distances. The origins of FRBs are currently unknown. These two parallel sessions will cover the current observational status of FRBs, including results of recent and ongoing FRB surveys, current theoretical models of FRBs, as well as observational multi-wavelength follow-up of FRBs currently underway with the goal of constraining FRB models and exploiting FRBs as novel cosmic probes.

Conveners: Bing Zhang (University of Nevada, Las Vegas), Prof. Duncan Lorimer (WVU), Victoria Kaspi (McGill University)

YouTube Link - July 5

YouTube Link - July 8

4:30 PM Searching for fast radio bursts with MeerKAT

Searching for fast radio bursts with MeerKAT will be discussed.

Speaker: Manisha Caleb

4:50 PM Fast radio bursts with the Australian SKA Pathfinder

Fast radio bursts (FRBs) are amongst the most energetic objects in our Universe, but despite a number of plausible models, their origin remains a mystery. Thanks to recent advances using the Australian Square Kilometre Array Pathfinder (ASKAP) radio telescope we can now routinely localise FRBs to the galaxies they originate form, and in some cases even pinpoint the burst to a region within the galaxy. Deep optical and radio follow-up observations enable us to study the type of galaxies and environments that FRBs live in, providing some of the strongest constraints on progenitor models. In addition, localising FRBs to their host galaxies also allows us to use them as probes to trace the ionised gas in galaxy haloes, large-scale structure and the inter-galactic medium. In this talk, I will discuss the latest results from ASKAP and the ongoing efforts to improve the FRB detection rates. I will also present the current status of the UTMOST-2D project for FRB localisation.

Speaker: Dr Shivani Bhandari (CSIRO astronomy and space science)

5:10 PM The First CHIME/FRB Catalog

Over the past decade, population studies of fast radio bursts (FRBs) have been challenging to undertake due to the small number of known sources detected with different telescopes and detection pipelines. However, the Canadian Hydrogen Intensity Mapping Experiment Fast Radio Burst (CHIME/FRB) project has now detected a large sample of FRBs which is well suited for such studies. The first CHIME/FRB catalog contains 474 non-repeating sources and 61 bursts from 18 previously reported repeating sources observed in the frequency range of 400-800 MHz. Detailed characterization of burst properties has revealed differences in morphology between repeating and non-repeating sources. Additionally, absolute calibration of selection effects has enabled measurements of the all-sky FRB rate and source-counts distribution and has provided evidence for a large fraction of the FRB population having scattering times greater than 10 ms (at 600 MHz). In this talk, I will present an overview of the catalog and discuss results from associated analyses. I will also present preliminary results from a population synthesis study assessing the astrophysical implications of the existence of a large population of highly scattered FRBs.

Speaker: Ms Pragya Chawla (McGill University)

5:30 PM Break

5:40 PM Fast Radio Burst detections and discoveries with Apertif, and LOFAR

Apertif, the wide-field receiver system currently operating on the Westerbork Synthesis Radio Telescope, offers an unprecedented combination of sensitivity and speed at 1.4 GHz. Its time-domain supercomputing back end (ARTS) performs real-time detection and localisation of Fast Radio Bursts (FRBs). In stand-alone mode, this SKA pathfinder is already the globally most productive 1.4 GHz FRB machine. It is, furthermore, directly connected to LOFAR. That unique combination of two world-class telescopes has recently allowed us to detect the same, repeating FRB over more than an order of magnitude in wavelength, down to 120 MHz, for the first time.

Fast Radio Bursts must be powered by uniquely energetic emission mechanisms. Identifying their physical nature arguably requires such good localisation of more detections, and broadband studies enabled by real-time alerting. We will describe ALERT, the Apertif FRB survey. It has discovered two dozen new FRBs so-far, each localised to 0.4-10 sq. arcmin. We will present our latest discoveries and detections of one-off and repeating FRBs. Four FRBs cut through the halos of M31 and M33. We demonstrate that Apertif can localise one-off FRBs with an accuracy that maps magneto-ionic material along such well defined lines of sight. The combination of detection rate and localisation accuracy from these Apertif/ARTS FRBs thus marks a new phase in which a growing number of bursts can be used to probe our Universe.

Using simultaneous Apertif and LOFAR multi-wavelength observing, we next showed that repeating FRB 20180916B emits down to 120 MHz, and that its activity window is both narrower and earlier at higher frequencies. Our detections establish that some FRBs live in clean environments that do not absorb or scatter low-frequency radiation, a prerequisite for future FRB applications to cosmology.

Speaker: Joeri van Leeuwen

6:00 PM Cosmology with high redshift FRBs

The quest for high redshift FRBs is ongoing with telescopes such as FAST and GBT looking for highly dispersed events. If FRB-producing systems exist at early times, such sources would provide new unique ways to probe Cosmic Dawn and Reionization. On one hand, FRB dispersion would allow us to probe the history and topology of Reionization. On the other hand, number counts of high redshift FRBs would indirectly probe galaxy formation at high-redshifts. In this talk I will focus on the prospects of advancing our understanding of the first billion years of cosmic history using high-redshift FRBs.

Speaker: Anastasia Fialkov (Cambridge University)

6:20 PM FRB emission mechanisms

FRB emission mechanisms will be discussed.

Speaker: Yuri Lyubarsky

6:40 PM Break

6:50 PM A search for periodicity in multi-component FRB profiles

A millisecond periodicity in the signal of fast radio bursts (FRBs) has long been searched for, as such a signal could be present if these sources are rapidly rotating neutron stars. Here we report a periodic separation of 218 ms at a 6-sigma significance in the single components of a 3-s long FRB detected by the CHIME/FRB experiment. With its nine or more single components, this FRB represents an outlier in the FRB population. In addition, CHIME/FRB has detected at least two other FRBs showing more than five separate components in their pulse profiles with hints of periodic separations, albeit not as significant as in the first case. I will present the results on these remarkable sources and discuss possible models to explain the observed signal.

Speaker: Daniele Michilli (McGill University)

7:02 PM Searching for Lensed Fast Radio Bursts with CHIME/FRB

Gravitational lensing of fast radio bursts (FRBs) on timescales of nanoseconds to milliseconds is sensitive to the presence of massive bodies up to $100 M_{\odot}$--including brown dwarves, rogue stars, and exotic objects like MACHOs or primordial black holes. The CHIME telescope, a widefield low-frequency radio interferometer operating over the frequency range of 400-800 MHz, detects several FRBs every day, and I will describe the status of our search for a lensed FRB. Our coherent time-domain search uses data from the CHIME/FRB baseband system and a procedure similar to geodetic VLBI cross-correlation. This allows us to resolve images with $10^{-8}$ to $10^{-1}$ second lensing delays, and disentangles intrinsic FRB morphology from genuine multipath propagation induced by a lens.

Speaker: Calvin Leung

7:14 PM Localising FRBs with CHIME/FRB Outriggers

The Canadian Hydrogen Intensity Mapping Experiment (CHIME) telescope has detected more than 1,000 fast radio bursts (FRBs) with its dedicated transient-search backend (CHIME/FRB). With the goal of localising 1,000 bursts to ~50mas precision in less than two years, CHIME/FRB is now expanding to include a dedicated very long baseline interferometry (VLBI) array of transcontinental outrigger stations. In this talk, I will motivate the Outrigger project and its goals, discuss how we are overcoming the challenges of low-frequency VLBI, and give a project update and timeline.

Speaker: Jane Kaczmarek (NRC)

4:30 PM → 7:30 PM Photospheric Emission in GRBs: Block 1

Time resolved spectra of many gamma-ray bursts present signatures of evolving thermal component in keV band, either in the prompt emission or in the early afterglow. In some bursts this component is dominant with respect to the non-thermal one, in others it is less pronounced. Such thermal component is associated with the photosphere of highly relativistic outflows launched by the central engine. In our session we will discuss basic radiation mechanisms producing observed spectra and light curves of gamma-ray bursts and their respective roles. Special attention will be given to theoretical and observational results aiming at discrimination between these mechanisms, in particular synchrotron and photospheric emission.

Conveners: Damien Begue (Bar Ilan University), Gregory Vereshchagin (ICRANet)

YouTube Link - July 5

YouTube Link - July 6

4:30 PM Understanding prompt emission: where do we stand ?

In recent years, there is a renewed debate about the origin of the observed prompt emission signal. Some authors found that synchrotron emission can dominate the spectra of several long bursts, and a recent analysis show that it may be possible to overcome the famous 'line of death' argument by a direct fitting procedure. On the other hand, several recent works showed that non-dissipative photosphere is preferred as the dominant emission model in at least 1/4 of long and 1/3 of short GRB population. In this talk I will critically review the arguments given as well as their physical consequences. I will then present some recent results that show a connection between the prompt spectra and the early afterglow emission, thereby argue for an independent method of discriminating the physical conditions that result in the different dominant radiative processes.

Speaker: Asaf Pe'er (Bar Ilan University)

5:10 PM Probing the jet launching mechanism from prompt emission of GRBs

Despite years of extensive research, the launching mechanism and the nature of relativistic jets remain open questions. Using 3D RMHD simulations of GRB jets with different initial magnetizations and engine modulation timescales, we calculate the resulting prompt emission light curves by considering photospheric emission and internal shocks, and compare them with observations. Our results show that in order to reconstruct the observed high variability and efficiency of GRB light curves, the jets' degree of magnetization has to be at least ~1% and the central engine intermittency operates on ~10 ms timescales.

Speaker: Ore Gottlieb (Tel Aviv University/CIERA)

5:40 PM Diffusive photospheres in gamma-ray bursts

Photospheric emission from relativistic outflows may originate in two different regimes: photon decoupling within the outflow or radiative diffusion. I will discuss observed thermal component in the early afterglows of gamma-ray bursts as emission from such diffusive photospheres. In addition, I will discuss implications of photon diffusion for dissipative models of GRBs.

Speaker: Gregory Vereshchagin (ICRANet)

6:10 PM Monte Carlo Simulations of Photospheric Emission in Gamma Ray Bursts

The study of Gamma Ray Bursts (GRBs) has the potential to improve our understanding of high energy astrophysical phenomena. In order to reliably use GRBs to this end, we first need to have a well-developed grasp of the mechanism that produces the radiation within GRB jets and how that relates to their structure. One model for the emission mechanism of GRBs invokes radiation produced deep in the jet which eventually escapes the jet at its photosphere. While this model has been able to explain a number of observed GRB characteristics, it is currently lacking in predictive power and in ability to fully reproduce GRB spectra. In order to address these shortcomings of the model, we have expanded the capabilities of the MCRaT code, a state of the art radiative transfer code that can now simulate optical to gamma ray radiation propagating in a hydrodynamically simulated GRB jet. Using the MCRaT code, we have constructed mock observed light curves, spectra, and polarization from optical to gamma ray energies for the simulated GRBs. Using these mock observables, we have compared our simulations of photospheric emission to observations and found much agreement between the two. Furthermore, the MCRaT calculations combined with the hydrodynamical simulations allow us to connect the mock observables to the structure of the simulated GRB jet in a way that was not previously possible. While there are a number of improvements that can be made to the analyses, the steps taken here begin to pave the way for us to fully understand the connection between the structure of a given GRB jet and the radiation that would be expected from it.

Speaker: Tyler Parsotan

6:40 PM Explaining GRB prompt emission with photospheric emission model

Although the observed GRB prompt emission spectrum is well constrained, the underlying radiation mechanism is not very well understood. We have explored photospheric emission in GRB jets by modelling the Comptonization of fast cooled synchrotron photons whilst the electrons and protons are accelerated to highly relativistic energies by repeated energy dissipation events as well as Coulomb collisions. In contrast to the previous simulations, we implemented realistic photon-to-particle number ratios of ~100,000 or higher, that are consistent with the observed radiation efficiency of relativistic jets. Using our Monte Carlo radiation transfer code, we can successfully model the prompt emission spectrum when electrons are momentarily accelerated to highly relativistic energies (LF~50-100) powered by ~40-50 episodic dissipation events, for baryonic outflows originating from moderate optical depth ~20-30. We have shown that the resultant shape of the photon spectrum is independent of the photon energy distribution and jet baryonic energy content.

Speaker: Dr Mukul Bhattacharya (Virginia Tech, Blacksburg)

4:30 PM → 7:30 PM High and Very High Energy Emission from Gamma-Ray Bursts: Block 1

GeV and TeV observations of gamma-ray bursts (GRBs) gamma-ray and ground based telescopes over the past decade have opened a new era in the study of GRBs. This session will discuss recent observations of GRBs at GeV and TeV energies and their relation to the prompt < ~ MeV emission and the long-lived afterglow emission. The theoretical implications of these observations will be also discussed, which range from the progenitor nature to the prompt GRB emission mechanism and outflow Lorentz factor and composition, through the GRB jet launching and acceleration mechanism, to particle acceleration in collisionless shocks or magnetic reconnection, constraints on Lorentz invariance violation and the Extragalactic Background light. The session will also discuss the prospects of GRB detection by the future MeV to TeV telescopes.

Conveners: Fabian Schüssler (IRFU / CEA Paris-Saclay), Francesco Longo (University of Trieste and INFN, Trieste)

YouTube Link - July 5

YouTube Link - July 7

4:30 PM MAGIC view of gamma-ray bursts at very high energies

The detection of gamma-ray bursts (GRBs) is one of the main scientific targets pursued by the MAGIC collaboration since almost 20 years. The MAGIC telescopes were specifically designed for this purpose: the main figures of merit are the fast slewing speed (7deg/s), the low energy threshold (~50 GeV at zenith) and the high sensitivity in the low energy regime. These features make MAGIC one of the most suitable instrument for the follow-up and detection of GRBs. After more than 15 years of dedicated searches, finally the first detection at teraelectronvolt energies of a GRB, namely GRB 190114C, was achieved by the MAGIC collaboration, revealing a new emission component in the afterglow phase. This discovery opened up a new era in field of GRB studies, which is now witnessing other detections, as demonstrated with the case of GRB 201216C. Furthermore, a hint of detection by MAGIC from the short and nearby GRB 160821B gives precious hints on the possible very high energy emission from this class of bursts, also in relation to searches of gravitation wave counterparts. Therefore, MAGIC is giving a crucial contribution to GRB physics, leading to a better understanding of the mechanisms underlying these peculiar objects. In this contribution I will introduce the MAGIC follow-up program, focusing on the aspects which led to the successfull detection of GRBs and highlighting some key results. Finally, I will present the future challenges in these observations, discussing how MAGIC can contribute even more to the field.

Speaker: Alessio Berti (Max Planck Institute for Physics)

berti_MAGIC_GRBs_MG16.pdf

4:50 PM Theoretical implications on the very high energy emission from GRB 190114C

Since their discovery in the late 1960s Gamma-Ray Burst (GRB) emission has been deeply investigated with the help of the huge amount of data collected covering the entire electromagnetic spectrum. This large and broadband dataset was essential to constitute a general picture describing the GRB physics, revealing the most credible underlying physical processes and environmental conditions ongoing at the GRB site. Huge leaps in the comprehension of the GRB physics have been achieved recently, thanks to the detection of the newly energetic component in the Very High Energy (VHE, E> 100 GeV) domain. The possible presence of a TeV spectral window in GRBs was predicted and theorized for several decades, but the first observational proofs of its existence were reached only in 2019 thanks to the discoveries claimed by the MAGIC and H.E.S.S. telescopes. GRB190114C was successfully detected in the TeV band by the MAGIC telescopes starting from around one minute after its trigger time and lasting for nearly 40 minutes. A successful follow-up campaign was performed and the multi-wavelength afterglow emission of the event was collected from 1 to about 2 × 10$^{17}$ GHz. Such very broad dataset allows to perform unique studies on the radiation mechanisms and on the physical properties of such event. In this contribution I will describe the main results and the theoretical interpretations that have been derived from the multi-wavelength dataset of GRB190114C. In particular, the description of the TeV component detected by the MAGIC telescopes as produced via the Synchrotron Self-Compton (SSC) mechanism and its connection with the emission at lower energy bands will be presented. Such studies are a fundamental starting point for the interpretation of the current and upcoming events that will be observed in the VHE domain.

Speaker: Davide Miceli (University of Udine & INFN Trieste)

Miceli_MG16.pdf

5:10 PM The H.E.S.S. gamma-ray burst program

In the last few years, gamma-ray bursts (GRBs) have been detected at Very High Energy (>100 GeV) gamma rays for the first time since their initial discovery half a century ago. This breakthrough occurred thanks to years of technical and strategic improvements (as well as a bit of good luck). In this talk, I will give an overview of the H.E.S.S. GRB program — how H.E.S.S. follows up GRBs, how this has evolved, where we are pushing further — and discuss some of the latest highlights.

Speaker: Sylvia Zhu (DESY)

sjzhu__HESS_GRB_Program.pdf

5:30 PM Observations of GRB 190829A at VHE with H.E.S.S.

Major advancements in the study of gamma-ray bursts (GRBs) have arisen in the last few years thanks to the recent detections at very high energy (VHE). In this contribution, the observation of GRB 190829A at VHEs with H.E.S.S. is presented. This GRB is one of the closest-ever detected with a redshift z~0.08, a characteristic that allowed an extended temporal detection from 4 hours to 56 hours after the GRB onset over a broad energy range of 0.18 to 3.3 TeV. This proximity opened the possibility to accurately measure the intrinsic spectra, provided a relatively small absorption of photons through their travel to Earth. The H.E.S.S. detection of the afterglow shows similar temporal and spectral characteristics when compared to the observations in the X-ray band with Swift-XRT. We will discuss how these characteristics challenge the standard framework for VHE afterglows in GRBs.

Speaker: Edna Ruiz-Velasco (MPIK)

GRB190829A_MarcelGrossmann.pdf

5:50 PM The VERITAS Gamma-Ray Burst Follow-up Program

Recent detections of gamma-ray bursts (GRBs) at energies above 100 GeV demonstrate that imaging atmospheric Cherenkov telescopes (IACT) operating in the very high energy range (VHE; E > 100 GeV) can provide insight into the physics of GRBs. By searching for the highest-energy photons emitted by GRBs, these telescopes can help answer questions about the particle acceleration and emission processes that occur during both the prompt and afterglow phases of GRBs. VERITAS is a very-high-energy IACT array located at the Whipple Observatory in southern Arizona, which has maintained an active GRB observing program since mid-2006. In this presentation, we will share some of the recent achievements of the VERITAS GRB follow-up program. We will discuss the development of analysis methods tailored to transient signals, and how the upper limits on the VHE emission obtained from observations of prominent bursts by VERITAS allowed us to constrain radiation mechanisms in the afterglow (e.g., for GRB 130427A) and constrain properties of the environment in which the burst took place (e.g., for GRB 150323A). Compact binary mergers that trigger short GRBs may also result in gravitational wave emission, so we will review both our follow up program from LIGO/Virgo triggers, and also the use of archival VERITAS data to search for short GRBs based on sub-threshold events from LIGO/Virgo. Lastly, based on the properties of the VHE-detected GRBs, we will discuss recent changes to our follow-up strategy to account for the Swift/XRT properties for optimal VERITAS observing sensitivity.

Speaker: Deivid Ribeiro (Columbia University)

MG16_GRB_Ribeiro_talk.pdf

6:10 PM Searching for Gamma-Ray Bursts with the High-Altitude Water Cherenkov (HAWC) Observatory

Satellites and imaging atmospheric Cherenkov telescopes (IACTs) have shown that gamma-ray bursts (GRBs) are capable of producing very-high-energy photons— most notably GRB 190114C, observed up to 1 TeV by the MAGIC telescopes approximately one minute after triggering the Fermi GBM and Swift BAT satellites. Particularly suited to such searches and follow-up studies is the High-Altitude Water Cherenkov (HAWC) Observatory, which monitors 1/6th of the sky at any one time, complementing the pointed observations of TeV telescopes. It covers 2/3 of the sky every day, with near continuous uptime. The HAWC GRB program comprises two dedicated analyses: a self-triggered all-sky search and a rapid response follow-up of GRBs reported by satellites. Both methods are performed in real time at the HAWC site and additionally repeated on archival data with improved calibration and reconstruction algorithms. Recent upgrades have HAWC poised for detection of the highest-energy gamma rays associated with GRBs, which are key to developing GRB emission models as well as constraining possible beyond-the-Standard-Model physics.

Speaker: Kristi Engel (University of Maryland, College Park)

MGXVI_GRBs_Engel-fin.pdf

6:30 PM Synchrotron and Synchrotron-Self-Compton emission components in GRBs detected at Very High Energies

Gamma-Ray Bursts (GRBs) are energetic transients originating in a violent explosion of a massive star or merger of two compact objects. These explosions create relativistic blastwave whose expansion leads to external shocks. The emission thus produced is the afterglow observed in
GRBs after the prompt emission. The properties of the emitting region i.e. non-thermal
particle spectrum, magnetic amplification, and microphysical parameters, etc can be probed by
monitoring and modelling the afterglow radiation. The recent detection of very high energy (VHE) gamma rays (> 100 GeV) from GRBs has opened a possibility to test theoretical models such as the synchrotron self-Compton (SSC) in GRBs till late times in the afterglow phase. In this work, we study few bright GRBs (Fermi-LAT detected GRB 130427A, MAGIC detected GRB 190114C and HESS detected GRB 180720B) using Synchrotron-Self-Compton model.

I will also discuss how early optical afterglows
and gamma ray giant flares can be useful to reveal the magnetic and baryonic nature of the jet composition in GRBs.

Speaker: Jagdish Joshi (School of Astronomy and Space Science, Nanjing University, Nanjing 210093, China)

GRB_afterglow_GM16_by_JCJoshi.pdf

6:50 PM Very-high-energy gamma-ray follow-up observations of gravitational waves

The precedented multi-messenger campaign launch by the gravitational wave (GW) signal GW170817 and the quasi-simultaneous gamma-ray burst GRB170817A, enabled the study of the various transient counterparts, over different energy bands and timescales, and confirmed for the first time, the hypothesis that binary neutron starts are the progenitor of at least a sub-sample of short GRBs, among many other implications. In this contribution, the different instruments currently observing the very-high-energy (VHE) gamma-ray sky and the main challenges these instruments face when performing GW follow-up observation will be described. An overview of the strategies and searches for VHE emission associated to GW follow-up observations by current gamma-ray experiments during LIGO-Virgo observing runs O1 to O3 will be presented. Finally, we will go through the implications and lessons learned from these observations and we will outline the prospects for the future generation of VHE gamma-ray instruments during the next LIGO-Virgo-KAGRA observing runs.

Speaker: Monica Seglar-Arroyo (L.A.P.P.)

SeglarArroyo_MG16_2021_vFinal1.pdf

4:30 PM → 7:30 PM Electromagnetic Counterparts of Compact Binary Mergers: Block 1

Electromagnetic (EM) counterparts of merging compact binaries containing neutron stars (two neutron stars or a neutron star and a black hole) can arise from different components of the merger ejecta. Examples include the prompt gamma-ray signal associated with emission from the relativistic jet or cocoon, the multi-wavelength afterglow associated with the interaction of the jet with the surrounding medium, the kilonova resulting from the r-process heated ejecta and the kilonova afterglow arising from the interaction of the latter ejecta with its environment. The first detection of a gravitational wave signal from a binary neutron star merger, GW 170817, has vividly confirmed three of these predicted EM counterparts, as its violent burst of gravitational waves was accompanied by the short GRB 170817A, a spectacular kilonova and a long-lived afterglow. This triple association has already significantly boosted our understanding of each of those components individually and helps us construct a more comprehensive picture of compact binary mergers in an astrophysical context. Furthermore, it has even enabled us to put significant constraints on topics of broad interest in physics from the neutron star equation of state to the expansion rate of the Universe. It is an exceptional demonstration of the power of multi-messenger astrophysics. Future compact binary mergers detected in gravitational waves and / or EM counterparts and further observations of the (still detectable) EM counterparts of GW 170817, therefore hold great promise to boost our understanding further. This session aims to explore the lessons learned from the observed counterparts and to prepare the community for future detections.

Conveners: Jonathan Granot (The Open University of Israel), Paz Beniamini (Caltech)

YouTube Link - July 5

4:30 PM Electromagnetic Counterparts of Compact Binary Mergers: the recent case of GW170817

I will provide a critical review of what we learned from the NS-NS merger GW170817 during year of electromagnetic follow up across the spectrum. Specifically, I will focus on recent developments from our coordinated radio-X-ray monitoring campaign that revealed the emergence of a new component of emission.

Speaker: Raffaella Margutti (UC Berkeley)

5:00 PM Diversity of Kilonova Emission from Binary Neutron Star Mergers

The outcome of a binary neutron star depends sensitively on the mass of the binary and the equation of state of dense nuclear matter. All else being equal, lower mass binaries tend to produce rapidly rotating magnetar remnants that survive longer (if not indefinitely) before collapsing into black holes. I will discuss some of the implications of the resulting diversity imprinted by a range of binary masses on the properties of the kilonova emission. A long-lived remnant can influence the kilonova properties in a number of ways, ranging from the impact of strong neutrino irradiation from the remnant on the composition of the ejecta (and hence the colors of the kilonova imprinted by the ejecta opacity) to contributing an additional source of luminosity from a rotationally-driven outflow in excess of that from radioactivity alone. Insofar as the properties of a putative relativistic jet would also influenced by the remnant lifetime, we should expect close connections between the non-thermal (e.g. afterglow) and thermal kilonova signatures of the merger.

Speaker: Brian Metzger (Columbia University/Flatiron Institute)

5:30 PM Emission from Structured GRB Jets: Theoretical Overview

The unprecedented coincident detection of a short gamma-ray burst (GRB) with gravitational waves from a binary neutron star (BNS) merger in GW170817/GRB170817A, followed by the long-lasting broadband afterglow, put our understanding of the structure of GRB jets to the test. GRB170817A turned out to be a particularly interesting event, due to its nearby distance (~40 Mpc) and emission from an off-axis jet, that gave us a range of new insights and confirmed some old ones. The most important of which is the unequivocal realization that GRB jets have angular structure. This has important implications for the detection and understanding of future such events. In this review talk, I will present the theory of emission from structured GRB jets, covering both prompt and afterglow emission. I will highlight the differences between off-axis emission from the simpler and often used top-hat jet model and the structured jets using the prompt and afterglow observations of GRB170817A. The full range of afterglow lightcurves that can be observed from an off-axis structured jet will be discussed. Important diagnostics, namely the afterglow flux centroid motion, image size, and polarization, that can be used to understand the outflow structure and properties of the post-shock magnetic field in future events will be discussed.

Speaker: Ramandeep Gill

6:00 PM Jet physics, ejecta properties and Hubble constant from the afterglows of neutron star mergers

Radio afterglows of neutron star mergers are excellent probes of the fast ejecta (relativistic jets and fast tail of the dynamical ejecta) and provide strong constraints on the inclination angle, ejecta morphology and energetics. This information is complementary to the ejecta mass and composition derived from the early-time UV-optical-infrared emission (called the kilonova/macronova). Radio observations of GW170817 revealed an energetic and narrowly-collimated jet, similar to those seen in gamma-ray bursts, surrounded by a wider angle outflow (together called the structured jet or jet-cocoon). Very long baseline interferometric observations were especially important in constraining the geometry, thereby providing a (standard siren) measurement of the Hubble constant. I will present the latest results from GW170817 and discuss the prospects for detecting radio afterglows of mergers in the upcoming LIGO-Virgo-KAGRA observing runs.

Speaker: Kunal Mooley

6:15 PM CALET search for Gamma-ray Counterparts of Gravitational Wave Events

The CALorimetric Electron Telescope (CALET) cosmic ray detector on the International Space Station (ISS) has been in operation since its launch in 2015.
The main instrument, the CALorimeter (CAL), is monitoring the gamma ray sky from ~1 GeV up to ~10 TeV with a field-of-view of about 2 sr for more than five years.
In this paper, we describe the analysis of gamma ray candidate events observed by CALET and report on a search for gamma-ray emission from gravitational wave event candidates announced by the LIGO/Virgo third observing run since 2019 April.

Speaker: Masaki Mori (Ritsumeikan University)

6:30 PM Constraints on Kilonova Emission from Two Short GRBs at z=0.5

The connection between binary neutron star mergers and short gamma-ray bursts (GRBs) was solidified by the simultaneous detection of GW170817 and GRB 170817A. These events were followed by bright kilonova emission arising from the radioactive decay of freshly synthesized r-process ejecta, which were expelled during the neutron star merger. Kilonova emission is a fundamental signature of neutron star mergers. The ability to distinguish kilonova emission from the GRB afterglow requires a well characterized multi-wavelength afterglow, and sensitive near-infrared (nIR) observations. The majority of short GRBs lack these features, and, therefore, no meaningful limits on the kilonova ejecta mass can be determined in most cases. As such, evidence for kilonova emission has only been identified in a handful of short GRBs. In this talk I will present a multi-wavelength study of two cosmological short GRBs, GRB 160624A at z = 0.483 and GRB 200522A at z = 0.554, targeted at constraining kilonova emission from these events. Although associated to a similar distance, these events display extremely different emission properties. The optical/nIR limits for GRB 160624A are among the most stringent for short GRBs, and strongly disfavor kilonova ejecta masses larger than 0.1 solar mass. Whereas GRB 200522A displays a bright, nIR emission component that can be explained either by a radioactively powered kilonova with large ejecta mass, ~ 0.1 solar mass, or by intrinsic extinction from its host galaxy. These observations further extend the small sample of short GRBs with nIR observations, and pave the way for future results from the James Webb Space Telescope.

Speaker: Brendan O'Connor (George Washington University)

6:45 PM Neutron Star properties and EM follow-up of Kilonovae

When two Neutron Stars (NSs) merge a multi-band electromagnetic (EM) emission, known as Kilonova (KN), follows. It is believed to be powered by the radioactive decay of ejecta products. In this contribution we discuss how future measurements of KN light curves and spectra could constrain some interesting features of the NSs in the coalescing binary. In particular we will focus on the impact and uncertainties of the current knowledge of the equation of state of dense matter on the mass, velocity and other subsequent observables in the KN ejecta.

Speaker: M. Angeles Perez-Garcia (Department of Fundamental Physics, University de Salamanca, Spain)

7:00 PM Kilonova signal in NS-BH merger

A kilonova signal is generally expected after a Black Hole - Neutron Star merger. The strength of the signal is related to the Equation of State of neutron star matter and it increases with the stiffness of the latter. The recent results obtained by NICER suggest a rather stiff Equation of State and the expected kilonova signal is therefore strong, at least if the mass of the Black Hole does not exceed $\sim 10 M_\odot$. We compare the predictions obtained by considering Equations of State of neutron star matter satisfying the most recent observations with the results predicted in the two-families scenario. In the latter a soft hadronic Equation of State produces very compact stellar objects while a rather stiff quark matter Equation of State produces massive strange quark stars, satisfying NICER results. The expected kilonova signal in the two-families scenario is very weak: the Strange Quark Star - Black Hole merger does not produce a kilonova signal because, according to simulations, the amount of mass ejected is negligible and the Hadronic Star - Black Hole merger produces a signal much weaker than in the one-family case because the hadronic Equation of State is very soft. This prediction will be easily tested with the new generation of detectors.

Speaker: Alessandro Drago (University of Ferrara)

KN signal in NS-BH mergers.pdf

7:10 PM The Best Case Scenario: Towards prompt arcminute localization of a GW source with targeted joint sub-threshold GRB searches, intelligent scheduling, and early warning response

The rich EM phenomenology in the first few hours after a compact object merger encodes the nature of the post-merger remnant, and a wide array of other compelling physics. Unfortunately, the requirement to find, and classify a counterpart within the large GW localization regions before followup with sensitive instruments can begin, excludes access to these first few hours, even for the most well localized GW sources. The ability to rapidly localize a GW source to within the field-of-view of a narrow field sensitive facility, would enable extraordinary science, and is uniquely enabled by GRB imagers with arcminute localization, like Swift/BAT. Such a prompt localization is the best case scenario. I will present the Swift/BAT-GUANO rapid spacecraft commanding and targeted sub-threshold GRB search pipeline, which allows significantly deeper searches for faint GRB 170817-like bursts, achieving the farthest detection range for such transients among current instruments. This pipeline has already increased the rate of arcmin localized GRBs by >15%. GW/GRB searches in the joint sub-threshold regime can also significantly extend the BNS detection horizon, and I will discuss methods and results from a joint search during LVC O3. The angular resolution of BAT allows good spatial discrimination and push to higher temporal FARs with the small spatial overlap, further increasing the sensitivity of joint searches. However, Swift/BAT's field of view (1/6 sky) decreases the expected detection yield compared to all-sky instruments, even with the increased horizon. I will discuss biased scheduling techniques that can increase the joint GRB/GW detection rate, and efforts to use GUANO-enabled rapid commanding capabilities to respond to early warning GW alerts and put the GW location within the BAT FoV at merger time. The combination of all of these will increase the chance of a best case scenario, and set the stage for next generation space telescope response.

Speaker: Aaron Tohuvavohu (University of Toronto)

7:20 PM Constraining and comparing short gamma-ray burst beam profiles using gravitational waves

GRB 170817A was markedly dissimilar to any other detected short gamma-ray burst as it was observed off-axis. This was further made evident by the information gained from the accompanying observation of GW170817. The event has since sparked discussion into the short gamma-ray burst beam profile and how it can link the observed luminosity of GRB 170817A with the rest of the observed on-axis short gamma-ray burst population. By assuming the short gamma-ray burst beam profile is universal across events, we use a fully Bayesian analysis to place constraints on beam profiles associated with cocoon, structured and simple top-hat jet models, as well as the binary neutron star merger rate. The beam profiles are constrained to reconcile the discrepancy between GRB 170817A and the rest of the on-axis population, given the distance and inclination information from GW170817 and the neutron star merger rate inferred from LIGO's first and second observing runs. We further show that these models can be distinguished from one another given a population of future gravitational wave detections of neutron star mergers with and without a counterpart, promised by the observations made by third-generation detectors.

Speaker: Fergus Hayes (University of Glasgow)

4:30 PM → 7:30 PM GRB 170817A and Binary Models: Block 1

Conveners: Giulia Stratta (INAF/OAS-Bologna), Marica Branchesi (Gran Sasso Science Institute/INFN)

YouTube Link - July 5

4:30 PM The robustness of the association of GW170817 and GRB 170817A

The joint observation of GW170817 and GRB 170817A has provided the long sought for conclusive evidence for the connection between binary neutron star mergers and short-hard gamma-ray bursts. Following an overview of the observation of GW170817 by the LIGO-Virgo Collaboration, and of the observations of GRB 170817A by Fermi-GBM and INTEGRAL SPI-ACS, this talk reviews the unambiguous association of GW170817 and GRB 170817A both within a Frequentist approach and a Bayesian approach.

Speaker: Francesco Pannarale (Sapienza – University of Rome & INFN)

Pannarale_MG16.pdf

4:55 PM Numerical relativity modeling of ejecta and kilonovae from binary neutron star mergers

This talk will report on recent progresses in the simulations of binary neutron star mergers in numerical general relativity with focus on the modeling of merger remnants and electromagnetic counterparts. Applications to the observations of GW170817 and AT2017gfo will be discussed.

Speaker: sebastiano bernuzzi

5:20 PM Search of local analogs of GRB 170817 with Swift

A local population of faint short gamma-ray bursts (GRBs) with late afterglow onset and bright optical kilonova was revealed by the discovery of the first binary neutron star merger GW170817/GRB170817A. In our work we investigate whether similar nearby (<200 Mpc) events were observed by NASA's Neil Gehrels Swift observatory. We selected all the events not associated to any X-ray or optical counterpart, finding 4 cases possibly associated with galaxies at distance <200 Mpc. Although affected by low statistics, this number is higher than the one expected for chance alignments to random galaxies, and possibly suggests a physical association between these bursts and nearby galaxies. We discuss the nature of these objects, and use them to constrain the rate of local SGRBs. By comparing our inferred rates with the most recent results from the Advanced LIGO and Virgo O3 run we derive information about the outflow collimation and its structure.

Speaker: Simone Dichiara (UMD/NASA-GSFC)

5:45 PM Kilonova emission observed so far: a comparison with AT2017gfo

AT2017gfo is the first kilonova (KN) that could be extensively monitored in time both photometrically and spectroscopically. Moreover, it is the first optical counterpart of a gravitational wave source and it is associated with the short gamma-ray burst GRB 170817A. Here I present our search for the fingerprints of AT2017gfo-like kilonova emissions in the optical/NIR light curves of 39 short GRBs with known redshift. Afterwards, I show how, for the first time, our results allow us to study separately the range of luminosity of the blue and red components of AT2017gfo-like kilonovae in short GRBs. With these results at hand, I show up to which redshift a KN can be followed up by some of the current and future observatories.

Speaker: Andrea Rossi (Istituto Nazionale di Astrofisica (INAF))

6:10 PM Kilonova AT2017gfo: which evidence of heavy r-process elements?

The spectra of the optical/near-IR counterpart of the GW2017 binary neutron star merger show broad absorption features overimposed onto the continuum, that were interpreted as due to heavy elements formed through r-process nucleosynthesis. However, it is very arduous to identify individually the atomic species, owing essentially to the enormous amount of atomic transitions and to substantial line blending. I will review our present understanding of the available spectroscopic information.

Speaker: Dr Elena Pian (INAF OAS, Bologna)

6:35 PM A new source of X-rays in GW170817 3.4 years after the merger

GW170817 was detected 3.4 years ago as the first object to have both a gravitational wave and an EM counterpart. It provided the first confirmation of the connection between short gamma-ray bursts and binary neutron star mergers. For almost 3 years, the broadband EM observations of GW170817 from radio to X-rays showed a very well-behaved simple power-law spectrum, with no spectral evolution. The non-thermal emission across multiple wavelengths was best explained by a model with a structured jet viewed off-axis. However, observations after 3.4 years narrate a story different from expectations. We have observed a statistically significant excess in X-rays compared to the predictions from a structured jet model at the current epoch, which was not accompanied by an excess in radio. We investigate several theoretical models that could lead to such an excess in X-rays only, including a plausible emergence of a kilonova afterglow, which if true, would make it the first-ever to be observed. We finally discuss the implications of these observations on the nature of the merger remnant.

Speaker: Aprajita Hajela (Northwestern University)

7:00 PM Extragalactic Fast X-ray Transient Candidates Discovered by Chandra

Fast X-ray Transients (FXRTs) are as-yet unexplained phenomena. They are energetic X-ray flares that last a few tens to a few thousand seconds. Over the past few years, $\sim$30 extragalactic FXRTs have been discovered in Chandra, XMM-Newton, Swift/XRT and eROSITA data. Numerous proposed explanations include a tidal disruption (TDE) of a white dwarf (WD) by an intermediate-mass black hole (IMBH), a supernova shock breakout (SBO), and a binary neutron star merger (BNS). So far, FXRTs lack multiwavelength counterparts, and hence we rely on their host properties to understand their nature. In this talk, I will present a new population of FXRTs serendipitous discovered from Chandra archival data (observation from the Chandra Data Release 2). This new sample of 14 FXRTs might have a mix of origins. We identify a sub-sample of FXRTs that show similar timing and spectral properties to CDF-S XT2 (a FXRT previously identified in the Chandra Deep Field South), and volumetric rate density, which suggest an association with BNSs. The improve in the detection of FXRTs by the current and future X-ray missions will open new opportunities to study and understand exotic astrophysics phenomena associated with FXRTs.

Speaker: Mr Jonathan Quirola Vasquez (Pontificia Universidad Católica de Chile)

4:30 PM → 7:30 PM Numerical Relativity and Gravitational Wave Observations: Block 1

The session is open for talks on all aspects of computation related to the calculation of gravitational waves that are potentially observable. This includes the development and applications of codes, as well as the development of relevant mathematical theory or computational methods.

Convener: Nigel Bishop (Rhodes University)

YouTube Link - July 5

YouTube Link - July 8

4:30 PM Hyperbolicity of General Relativity in null folliations

Characteristic formulations of General Relativity are based on a null folliation of the spacetime. When combined with the standard Cauchy evolution they can in principle provide highly accurate waveform modelling. During this modelling process it is typical that the full non-linear Einstein field equations are solved numerically. A numerical solution to a PDE problem can converge to the continuum one with increasing resolution only for well posed PDE problems. Well posedness of the initial value problem in the L2 norm is characterized by strong hyperbolicity of the PDE system. It was recently found that the PDE systems formed by Einstein's field equations in commonly used characteristic gauges are only weakly hyperbolic. I will review the basic features of the commonly used characteristic gauges of the Bondi family and argue that within this family a strongly hyperbolic PDE system from Einstein's field equations is not possible, if at most first derivatives of the metric are introduced as variables. I will further provide an example of how weak hyperbolicity may be demonstrated in numerical simulations.

Speaker: Thanasis Giannakopoulos (Instituto Superior Técnico, Lisbon)

4:50 PM Einstein Field Equations in Spherical Symmetry on Hyperboloidal Slices: Dual Foliation Formulation

One of the challenges in numerical relativity is to include future null infinity in the computational domain with a well-posed formulation. Success will not only enable us to evolve any system of astrophysical interest, e.g. binary black holes and extracting the gravitational wave signal at future null infinity, with any desired accuracy, but also help in studying various phenomena of fundamental interest. One proposal is to use hyperboloidal slices. In this talk, I will present our ongoing efforts for obtaining a well-posed formulation of the Einstein Field Equations on hyperboloidal slices, all in spherical symmetry. The natural extension will be to generalize these methods to full 3d.

Speaker: Shalabh Gautam (Inter-University Centre for Astronomy and Astrophysics, Pune, India)

5:10 PM Eccentric binary black hole surrogate models for the gravitational waveform andremnant properties: comparable mass, nonspinning case

We develop new strategies to build numerical relativity surrogate models for eccentric binary black hole systems, which are expected to play an increasingly important role in current and future gravitational-wave detectors. We introduce a new surrogate waveform model, NRSur2dq1Ecc, using 47 non-spinning, equal-mass waveforms with eccentricities up to 0.2 when measured at a reference time of 5500M before merger. This is the first waveform model that is directly trained on eccentric numerical relativity simulations and does not require that the binary circularizes before merger. The model includes the (2,2),(3,2), and (4,4) spin-weighted spherical harmonic modes. We also build a final black hole model,NRSur2dq1EccRemnant, which models the mass, and spin of the remnant black hole. We show that our waveform model can accurately predict numerical relativity waveforms with mismatches≈0.001, while the remnant model can recover the final mass and dimensionless spin with absolute errors smaller than ≈0.0005 and ≈0.002 respectively. We demonstrate that the waveform model can also recover subtle effects like mode-mixing in the ringdown signal without any special ad-hoc modeling steps. Finally, we show that despite being trained only on equal mass binaries, NRSur2dq1Ecc can be reasonably extended up to mass ratio q≈3 with mismatches of 0.01 for eccentricities smaller than ∼0.05 as measured at a reference time of 2000M before merger. The methods developed here should prove useful in the building of future eccentric surrogate models over larger regions of the parameter space.

Speaker: Tousif Islam (University of Massachusetts Dartmouth)

5:30 PM Matter shells modifying gravitational wave signals

As detections of mergers of compact bodies begin to flow in, and as we enter an era of precision GW measurements, our understanding of compact bodies, their physics and that of the surrounding astrophysical environment, will continue to grow and at times even be challenged. The need to revise the mass bounds of compact bodies such as BHs and NSs and the possibility of the existence of GW echoes are just some of consequences of the first few years of GW detection. In previous work, using linearised perturbation theory, we made the novel finding that a dust shell will cause a GW to be modified both in magnitude and phase, but without any energy being transferred to or from the dust. We extend our analysis to matter shells surrounding compact body mergers and to intervening matter in cosmology. Instead of only monochromatic GW sources, as we used in our initial investigation, we also consider burst-like GW sources. The thin density shell approach is modified to include thick shells by considering concentric thin shells and integrating. Solutions are then found for these burst-like GW sources using Fourier transforms. In the context of cosmology, apart from the gravitational redshift, the effects are too small to be measurable. We show that GW echoes that are claimed to be present in the LIGO data of certain events, could not have been caused by a matter shell. We do find, however, that matter shells surrounding BBH mergers, BNS mergers, and CCSNe could make modifications of order a few percent to a GW signal. These modifications are expected to be measurable in GW data with current detectors if the event is close enough and at a detectable frequency; or in future detectors with increased frequency range and amplitude sensitivity.

Speaker: M Naidoo (Rhodes University)

5:50 PM Gravitational-wave signatures of the hadron-quark phase transition in binary compact star mergers

The long-awaited detection of a gravitational wave from the merger of a binary neutron star in August 2017 (GW170817) marked the beginning of the new field of multi-messenger gravitational wave astronomy. Reaching densities a few times that of nuclear matter and temperatures up to 100 MeV, such mergers also represent potential sites for a phase transition from confined hadronic matter to deconfined quark matter (HQPT). Gravitational-wave signatures of the HQPT can be subdivided according to the phase in which they are generated. A strong HQPT can already be detected in the late inspiral phase if the equation of state gives rise to a twin star property in the mass-radius curve [1]. Depending on the properties of the HQPT, a signature can be created promptly after the merger or during the post-merger evolution. Especially during the postmerger evolution of the produced hypermassive/supramassive hybrid star the occurrence of a "delayed HQPT" might give a clear gravitational wave signature of the production of quark matter, if the HQPT is strong enough [2,3]. The appearance of a HQPT in the interior region of the remnant and its conjunction with the spectral properties of the emitted gravitational wave have been computed by fully general-relativistic hydrodynamic simulations.

[1] Gloria Montana, Matthias Hanauske, and Luciano Rezzolla. "Constraining twin stars with GW170817." Physical Review D 99.10 (2019): 103009.
[2] Lukas R. Weih, Matthias Hanauske, and Luciano Rezzolla. "Postmerger gravitational-wave signatures of phase transitions in binary mergers." Physical review letters 124.17 (2020): 171103.
[3] Hanauske, Matthias, Lukas R. Weih, Horst Stöcker, and Luciano Rezzolla. "Metastable hypermassive hybrid stars as neutron-star merger remnants." The European Physical Journal Special Topics (2021): 1-8.

Speaker: Matthias Hanauske (Goethe University Frankfurt, Institute for Theoretical Physics)

MG16_V1-2021-Hanauske-oa.pdf

6:10 PM Dynamics of Quadratic Gravity in Spherical Symmetry

We present the first numerically stable nonlinear evolution for the leading-order gravitational effective field theory (Quadratic Gravity) in the spherically-symmetric sector. The formulation relies on (i) harmonic gauge to cast the evolution system into quasi-linear form (ii) the Cartoon method to reduce to spherical symmetry in keeping with harmonic gauge, and (iii) order-reduction to 1st-order (in time) by means of introducing auxiliary variables. Well-posedness of the respective initial-value problem is numerically confirmed by evolving randomly perturbed flat-space and black-hole initial data. Our study serves as a proof-of-principle for the possibility of stable numerical evolution in the presence of higher derivatives. We also discuss physically (un)stable branches of black holes in quadratic gravity.

Speaker: Hyun Lim (Los Alamos National Laboratory)

6:30 PM Memory-like effects due to relative velocity and acceleration

A burst of gravitational waves creates a permanent change in separation between two initially comoving test particles; this is known as the gravitational wave memory effect. Near null infinity, two contributions to the memory effect arise: linear memory, which appears in linearized gravity and is due to changes in conserved quantities, and nonlinear memory, which arises due to the nonlinear nature of general relativity. Moreover, the nonlinear memory is expected to be the dominant contribution to the memory effect for binary black hole mergers, such as those detected by LIGO and Virgo. In this talk, we discuss the case where the particles have initial relative velocity and acceleration, and determine the contributions of each to the final separation. Each contribution provides additional memory-like effects, and we show that a similar linear vs. nonlinear split arises near null infinity.

Speaker: Alexander Grant (University of Virginia)

4:30 PM → 7:30 PM Compact Stars as Laboratories for Testing Strong Gravity: Block 1

There are reasons to believe the 21st century will be the best ever for astrophysics: the James Webb Space Telescope will extend nearly twenty times the present observational limit of visible light; neutrino massiveness opens a new window for exploration on dark energy and dark matter physics and is expected to provide insights into the fate of the Universe; the Higgs boson may allow for an understanding of the weakness of gravity; gravitational waves produced at the birth of the Universe and by compact stellar objects (supermassive black holes, black hole/neutron star mergers, gamma-ray bursts, white dwarf inspirals) have unveiled a new area of astronomy. Framed by this background, compact stars represent an unique astrophysical laboratories for probing the fabric of space-time and the building blocks of matter and their interactions at physical regimes not attainable in terrestrial laboratories. The strong gravitational fields of compact stars - black holes, pulsars, neutron, and exotic stars - provide this way an unique test ground for strong gravity and modified theories of gravity and can offer restrictions for extended theories of general relativity. The aim of this session is to bring together researchers from cosmology, particle physics, nuclear theory and astrophysics, working on these topics from different but complementary viewpoints.

Conveners: Aurora Maria Perez Martinez (Instituto de Cibernetica Matematica y Fisica ICIMAF), Cesar Augusto Zen Vasconcellos (ICRANet/UFRGS)

YouTube Link - July 5

4:30 PM Exotic thin-shell fluid stars with arbitrary compactess

We propose two models for constant density relativistic perfect-fluid spheres supported by thin shell configurations. These models are obtained from the Schwarzschild constant density star solution: the first via the collapse of the external layers of the fluid into a thin shell by performing a matching with the exterior Schwarzschild solution at a matching radius smaller than the star radius; and the second via the creation of a vacuum bubble inside the star by matching it with an interior Minkowski spacetime. Both models are shown to satisfy both the weak and the strong energy conditions (WEC and SEC) and can have a compactness arbitrarily close to that of a black-hole without developing singularities at the center, thus being exceptions to the Buchdahl limit. We compute the stability regimes of the models proposed and we show that there are combinations of the star radius $R$ and the matching radius $R_\Sigma$ for which the solutions are stable, the dominant energy condition (DEC) is satisfied, and the radius of the object is smaller than $3M$, implying that these models could be used as models for dark matter or exotic compact objects.

Speaker: João Rosa (University of Tartu)

4:50 PM Probing Strong Field Gravity and the Structure of Ultra-Dense Matter with the Thermal Evolution of Neutron Stars

Thermal evolution of neutron stars is studied in the $f(R)=R+\alpha R^{2}$ theory of gravity.
We first describe the equations of stellar structure and evolution for a spherically symmetric spacetime plus a perfect fluid at rest.
We then present numerical results for the structure of neutron stars using four dense matter equations of state and a series of gravity theories for
$\alpha$ ranging from zero, i.e., General Relativity, up to $\alpha \approx 10^{16}$ cm$^2$.
We emphasize properties of these neutron star models that are of relevance for their thermal evolution as the threshold masses for enhanced neutrino emission
by the direct Urca process, the proper volume of the stellar cores where this neutrino emission is allowed, the surface gravitational acceleration that impact the observable effective temperature, and the crust thickness.
Finally, we numerically solve the equations of thermal evolution using the public code \texttt{NSCool} and explicitly analyze the effects of altering gravity.
We find that uncertainties in the dense matter microphysics, as its chemical composition and superfluidity/superconductivity properties, as well as the
astrophysical uncertainties on the chemical composition of the surface layers, have a much stronger impact than possible modification of gravity
within the studied family of $f(R)$ theories.
We conclude that within this family of gravity theories, conclusions from previous studies of neutron star thermal evolution are not significantly
altered by alteration of gravity.
Conversely, this implies that neutron star cooling modeling may not be a useful tool to constrain deviations of gravity from Einstein theory
unless these are much more radical than in the $f(R)$ framework.

Speaker: Martin Javier Nava-Callejas (Instituto de Astronomia - UNAM)

5:10 PM Effects of modified theories of gravity on neutrino pair annihilation energy deposition near neutron stars

We present our studies on the neutrino pairs annihilation into electron-positron pairs ($\nu{\bar \nu}\to e^-e^+$) near the surface of a neutron star in the framework of extended theories of gravity. The latter modifies the maximum energy deposition rate near to the photonsphere and it might be several orders of magnitude greater than that computed in the framework of General Relativity. These results provide a rising in the Gamma-Ray Bursts energy emitted from a close binary neutron star system and might be a fingerprint of modified theories of gravity, changing our view of astrophysical phenomena.

Speaker: Leonardo Mastrototaro (Università degli Studi di Salerno - INFN, Sezione di Napoli, Gruppo collegato di Salerno)

Effects of modified theories of gravity on neutrino pair annihilation energy deposition near neutron stars_Leonardo Mastrototaro.pptx

5:30 PM Neutron Stars in Scalar-tensor Theories: Analytic Scalar Charges and Gravitational-wave Constraints

Neutron stars in scalar-tensor theories may undergo spontaneous scalarization, which is important for probing the theories with binary pulsar and gravitational wave observations. Since the effect is nonlinear, most studies of spontaneous scalarization were carried out numerically. In the first part of my talk, I explain how one can compute the effect of scalarization analytically based on a perturbative analysis and analytic modeling of neutron stars through the Tolman VII solution. I show that the analytic calculations match accurately with numerical ones. These findings improve our understanding of spontaneous scalarization and provide us quick and ready-to-use expressions of scalar charges. In the second part, I present current and future prospects of constraining scalar-tensor theories with gravitational waves from a mixed binary of a black hole and a neutron star. I show that future observations can significantly improve bounds on these theories.

Speaker: Kent Yagi (University of Virginia)

5:50 PM Probing modified gravitational-wave propagation through tidal measurements of binary neutron star mergers

Gravitational-wave(GW) sources can serve as standard sirens to probe cosmology by measuring their luminosity distance and redshift. Such standard sirens are also useful to probe theories beyond General Relativity with a modified GW propagation. Previous studies on the latter assume multi-messenger observations so that the luminosity distance can be measured with GWs while the redshift is obtained by identifying sources’ host galaxies from electromagnetic (EM) counterparts. Given that GW events of binary neutron star(BNS) coalescences with associated EM counterpart detections are expected to be rather rare, it is important to examine the possibility of using standard sirens to probe gravity with GW measurements alone. In this paper, we achieve this by extracting the redshift from the tidal measurement of BNSs(originally proposed within the context of GW cosmology). We also improve previous work by considering multi-band GW observations between ground-based (e.g.Einstein Telescope) and space-based (e.g.DECIGO) interferometers. We find that such multi-band observations with the tidal information can constrain a parametric non-Einsteinian deviation in the luminosity distance more stringently than the case with EM counterparts (due to a larger number of events) by a factor of a few.

Speaker: Ms Nan Jiang (University of Virginia)

6:10 PM Neutron Stars in Scalar-tensor Theories: Universal Relations and Analytic Investigations

Neutron stars are ideal astrophysical sources to probe general relativity due to their large compactnesses and strong gravitational fields. For example, binary pulsar and gravitational wave observations have placed stringent bounds on certain scalar-tensor theories in which a massless scalar field is coupled to the metric through matter. A remarkable phenomenon of neutron stars in such scalar-tensor theories is spontaneous scalarization, where a normalized scalar charge remains order unity even if the matter-scalar coupling vanishes asymptotically far from the neutron star. On the other hand, certain quasi-universal relations have been found for global quantities of neutron stars (such as the moment of inertia and quadrupole moment) that are insensitive to the underlying equations of state. We find a new quasi-universal relation in massless scalar-tensor theories between the scalar charge and stellar binding energy (related to stellar compactness). Although the above finding is based on numerical calculations, we give mathematical support for this universal relation by computing for the first time scalar charges analytically for both Tolman VII and constant density stars. Such analytic results provide ready-to-use expressions for scalar charges in massless scalar-tensor theories.

Speaker: Michael Stepniczka

6:30 PM Application of a hierarchical MCMC follow-up to Advanced LIGO continuous gravitational-wave candidates

We present the first application of a hierarchical Markov Chain Monte Carlo (MCMC) follow-up on continuous gravitational-wave candidates from real-data searches. The follow-up uses an MCMC sampler to draw parameter-space points following the F-statistic. As outliers are narrowed down, coherence time increases, imposing more restrictive phase-evolution templates. We introduce a novel Bayes factor to compare results from different stages: The signal hypothesis is derived from first principles, while the noise hypothesis uses extreme value theory to derive a background model. The effectiveness of our proposal is evaluated on fake Gaussian data and applied to a set of 30 outliers produced by different continuous wave searches on O2 Advanced LIGO data. The results of our analysis suggest all but three outliers are inconsistent with an astrophysical origin under the standard continuous wave signal model. We successfully ascribe two of the surviving outliers to an instrumental artifact and a strong hardware injection present in the data. The behavior of the third outlier suggests an instrumental origin as well, but we could not relate it to any known instrumental cause.

Speaker: Rodrigo Tenorio (Universitat de les Illes Balears)

6:45 PM Neutron stars in f (R, T ) gravity using realistic equations of state in the light of massive pulsars and GW170817

In this work we investigate neutron stars (NS) in f (R, T ) gravity
for the case R + 2λT , R is the Ricci scalar, and T the trace of the
energy-momentum tensor. The hydrostatic equilibrium equations are
solved considering realistic equations of state (EOS). The NS masses
and radii obtained are subject to a joint constrain from massive pulsars
and the event GW170817. The parameter λ needs to be negative as in
previous NS studies, however, we found a minimum value for it due to the existence
of the NS crust. The pressure in this modified theory of gravity depends on the
inverse of the sound velocity. Since, this velocity is lower in the crust, |λ| needs to be very small. We found that the increment in the star mass is less
than 1%, much smaller than previous ones obtained not considering the
realistic stellar structure, and the star radius cannot become larger, its
changes compared to GR is less than 3.6% in all cases. The finding that
using several relativistic and non-relativistic models the variation on
the NS mass and radius is almost the same for all the EsoS, manifests
that our results are insensitive to the high-density part of the EOS.
It confirms that stellar mass and radii obtained in f(R,T) depends only on the NS crust, where the EoS is essentially the same for all the models. Finally, we highlight that our results indicate that conclusions obtained from NS studies
done in modified theories of gravity without using realistic EsoS that
describe correctly the NS interior can be unreliable.

Speaker: Manuel Malheiro (Instituto Tecnológico de Aeronáutica)

7:00 PM Magnetic field in Neutron stars, vacuum birrefrigence consequences

A. Perez Martinez^1T, M. A. Perez-Garcia^1(∗), E. Rodriguez Querts² and A. Romero Jorge²


Vacuum in presence of magnetic field exhibits birrefrigence. We have obtained this effect from linear correction of dispersion relation of photon travelling perpendicular to the magnetic field valid even for magnetic fields close to B_c=10¹³ G.
Although this phenomenon has not been yet detected evidenceof this effect has been reported for neutron star RX J1856.5−3764 by Mignani et al.. In the light of this finding we analyze our results.
In this context we discussed possible experiments in lab with pulsanting laser beams.


¹ Departamento de Física Fundamental, Plaza de la Merced, Edificio Trilingue, Universidad de Salamanca, España
(∗) This research has been supported by Junta de Castilla y León SA096P20 and Spanish Ministry of Science PID2019-107778GB-100 projects.
^(Ͳ) On leave from Departamento de Física Teórica, Instituto de Cibernética Matemática y Física (ICIMAF), Calle E esq. 15 No. 309, La Habana, CP 10400, Cuba.
Departamento de Física Teórica, Instituto de Cibernética Matemática y Física (ICIMAF), Calle E esq. 15 No. 309, La Habana, CP 10400, Cuba.

Speaker: Aurora Perez Martinez (Instituto de Cibernetica Matematica y Fisica ICIMAF)

7:15 PM The Brach Cut Universe: from the origin of the universe to the formation of compact stars

In this contribution we identify two scenarios for the evolutionary branch cut universe. In the first scenario, the universe evolves continuously from the negative complex cosmological time sector, prior to a primordial singularity, to the positive one, circumventing continuously a branch cut, and no primordial singularity occurs in the imaginary sector, only branch points. In the second scenario, the branch cut and branch point disappear after the {\it realisation} of the imaginary component of the complex time by means of a Wick rotation, which is replaced by the thermal time. In the second scenario, the universe has its origin in the Big Bang, but the model contemplates simultaneously a mirrored parallel evolutionary universe going backwards in the cosmological thermal time negative sector. A quantum formulation based on the WDW equation is sketched and preliminary conclusions are drawn. Finally, a description of the evolutionary process of the branch cut universe, from its beginnings to the creation phase of compact stars is proposed.

Speaker: Prof. Cesar Augusto Zen Vasconcellos (ICRANet)

4:30 PM → 7:30 PM Variation of the Fundamental Constants, Tests of the Fundamental Symmetries and Probes of the Dark Sector: Block 1

This session is devoted to recent developments in the investigation of and high-precision searches for variations of the fundamental constants of nature and tests of the fundamental symmetries of nature, including application to searches for ultra-low-mass dark matter and related dark components, as well as dark forces.

Conveners: Victor Flambaum (University of New South Wales, School of Physics, Sydney 2052, Australia), Yevgeny Stadnik

YouTube Link - July 5

YouTube Link - July 6

YouTube Link - July 8

4:30 PM Precision isotope-shift spectroscopy in neutral Yb and joint Yb/Yb$^+$ King-plot analysis

The isotope shifts (IS) in the frequency of an atomic transition are approximately linearly correlated with the shifts in another transition. This linearity is reflected in the so-called King-plot analysis. It has been suggested to search for deviations from linearity as a way to probe beyond-Standard-Model interactions mediated by light bosons [1]. These searches require availability of precision IS data in a chain of isotopes of a given element. In a recent report on precision spectroscopy in a pair of Yb$^+$ transitions [2], a large nonlinearity was observed in the King-plot, that primarily arises due to the quadratic field shift [2], or the influence of the nuclear deformation on the field shift [3]. Further availability of precision IS data in the same element is crucial to check modeling of the cause of the nonlinearity [3], and potentially separate within Standard-Model effects from possible new physics contributions to the nonlinearity [4].
We will discuss an experiment involving precision spectroscopy of the $^1S_0-^1D_2$ optical transition in neutral Yb, in order to determine the IS in the naturally abundant, nuclear-spin zero Yb isotopes. We will present our preliminary experimental results, and show a joint King-plot of our data combined with those on Yb$^+$, that reveals an order of magnitude larger nonlinearity, compared to that of the Yb$^+$ work.

[1] J. C. Berengut, et al., Phys. Rev. Lett. 120,091801 (2018).
[2] I. Counts, J. Hur, D. P. L. Aude Craik, H. Jeon, C. Leung, J. C. Berengut, A. Geddes, A. Kawasaki, W. Jhe,and V. Vuletic, Phys. Rev. Lett. 125, 123002 (2020).
[3] Saleh O. Allehabi, V. A. Dzuba, V. V. Flambaum, and A. V. Afanasjev. Phys. Rev. A 103, L030801 (2021).
[4] J. C. Berengut, C. Delaunay, A. Geddes, and Y. Soreq, Phys. Rev. Research 2, 043444 (2020)

Speaker: Dr Dionysios Antypas (Johannes Gutenberg-Universität Mainz, Helmholtz-Institut Mainz, Mainz 55128, Germany)

D. Antypas MG16.pdf

5:00 PM Axion Quark Nuggets and Matter-Antimatter asymmetry as two sides of the same coin: theory, observations and future experimental searches

In this talk I want to discuss the (unorthodox) scenario when the baryogenesis is replaced by a charge segregation process in which the global baryon number of the Universe remains zero. In this, the so-called axion quark nugget (AQN) dark matter model the unobserved antibaryons come to comprise the dark matter in the form of dense nuggets. In this framework, both types of matter (dark and visible) have the same QCD origin, form at the same QCD epoch, and both proportional to one and the same fundamental dimensional parameter of the system, which explains how the two, naively distinct, problems could be intimately related, and could be solved simultaneously within the same framework. I specifically focus on several recent papers written with AMO (Atomic-Molecular-Optic), Nuclear physics and Astro-physics people to apply these generic ideas to several recent proposals: 1. on broadband strategy in the axion searches; 2. on daily modulations and amplifications generated by the AQN dark matter and how they can be studied; 3. on recently detected by Telescope Array the Mysterious Burst Events which are very distinct from conventional cosmic air showers.

The talk is based on several recent papers including:

D.Budker, V.V.Flambaum, X.Liang and A.Zhitnitsky,
``Axion Quark Nuggets and how a Global Network can discover them,''
Phys. Rev. D 101 no.4, 043012 (2020)
[arXiv:1909.09475 [hep-ph]].

A.~Zhitnitsky,
``The Mysterious Bursts observed by Telescope Array and Axion Quark Nuggets,''
Journal of physics G: Nuclear and Particle Physics (2021) [arXiv:2008.04325 [hep-ph]]

Speaker: Ariel Zhitnitsky (University of British Columbia)

Zhitnitsky-PT-3.pdf

5:30 PM Towards the test of local Lorentz invariance with $^{172}$Yb$^+$ ion Coulomb crystals

We report on our progress of an improved test of local Lorentz invariance (LLI) in the electron-photon sector using the highly sensitive meta-stable electronic $F$-state of the $^{172}$Yb$^{+}$ ion [1].

The Zeeman structure of the $F$-state contains two orthogonally oriented orbitals which gives us access to test LLI violation. To suppress the magnetic field noise during the measurement, we mix the Zeeman substates via dynamical decoupling [2]. This method allows us to profit from a long coherence time and high spatial homogeneity of the radio-frequency source used for interrogation, which enables easy up-scaling of the ion number.

In preparation of this measurement, we demonstrated the first coherent excitation to the $F$-state via the highly forbidden electric octupole (E3) transition with a reduced uncertainty of less than 10 Hz [3], improving on earlier measurements [4] by about 5 orders of magnitude. Recently, we observed a coherence time of 1.5 s when applying the dynamical decoupling sequence in the electronic ground state of Yb$^{+}$.

With these results, we are ready to perform the first test of LLI with a single Yb$^{+}$ ion, after which we will scale it up to $\approx$ 10 ions to improve on the current best upper bound [5].

[1] V.A. Dzuba et al., Nature Physics 12, 465-468 (2016).
[2] R. Shaniv et al., Phys. Rev. Lett. 120, 103202 (2018).
[3] H. A. Fürst et al., Phys. Rev. Lett. 125, 163001 (2020).
[4] M. Roberts et al., Phys. Rev. Lett. 78, 1876 (1997).
[5] C. Sanner et al., Nature 567, 204-208 (2019).

Speaker: Chih-Han Yeh (Physikalisch-Technische Bundesanstalt)

20210705_MG16.pdf

6:00 PM Constraining modified gravity with quantum optomechanics

In this talk, I will present some recent results on estimating the performance of quantum optomechanical sensors for searches of modified gravity. Specifically, I will show how we derive the best possible bounds that can be placed on Yukawa- and chameleon-like modifications to the Newtonian gravitational potential with a cavity optomechanical quantum sensor. We do so by modelling the effects from an oscillating spherical source on the optomechanical system from first-principles. To then estimate the sensitivity to chameleon-like modifications, we take into account the size of the optomechanical probe and quantify the resulting screening effect for the case when both the source and probe are spherical. Our results show that an optomechanical system in high vacuum could, in principle, further constrain the parameters of chameleon-like modifications to Newtonian gravity.

Speaker: Dr Sofia Qvarfort (Imperial College London & University College London)

6:30 PM Linking the lithium problem and the $H_0$ tension: the gravitational constant at BBN

The primordial abundance of lithium is still a subject of controversy, given the disagreement between numerical results and observational estimates. We show how this discrepancy can be undestood in the context of variation of fundamental constants at the epoch of Big Bang Nucleosynthesis. The variation of Newton's constant plays a crucial role. In particular, its interpretation in terms of additional relativistic degrees of freedom suggests an alleviation to the $H_0$ tension.

Speaker: Sebastián Franchino-Viñas (Universität Heidelberg - Universidad Nacional de La Plata)

franchino.pdf

7:00 PM Probing the TeV scale and beyond via particle electric dipole moments

Revolutionary progress is underway in the ability to detect CP-violating electric dipole moments (EDMs) of particles such as the electron and proton. I will describe recent searches for the electron EDM that are already sensitive to new physics at scales around 10 TeV. I will also discuss new techniques projected to soon enable orders of magnitude further improvement in the field.

Speaker: Prof. David DeMille (University of Chicago and Argonne National Laboratory)

4:30 PM → 7:30 PM Dragging is Never Draggy: MAss and CHarge Flows in GR: Block 1

To spin or not to spin? That is not the only question. In GR, inertia of a body is affected by every other mass-energy present in space-time, whether in sources or in geometry. Thus even “to be” is partially relative. Already before completing his theory, Einstein knew that a particle is heavier if inside a massive shell, and that it becomes dragged along if the shell starts to accelerate. Dragging is still not draggy almost 110 years later: it involves the magnetic component of the field, apparently more imaginative than the electric one, and it very probably drives some of the most exciting phenomena in the Universe, such as jets exhausted from accreting black holes. In this session, we shall be tasting some recent results in the field.

Convener: Oldrich Semerak (Charles University, Faculty of Mathematics and Physics, Institute of Theoretical Physics)

YouTube Link - July 5

4:31 PM Magnetized black holes: the role of rotation, boost, and accretion in twisting the field lines and accelerating particles

Combined influence of linear boost and rotation of a black hole can distort an ambient magnetic field to the extent that magnetic field lines develop a neutral point, where the magnetic intensity vanishes. This purely geometrical effect interacts with the accretion flow that can carry and distort the frozen-in magnetic lines, too. Near the event horizon, the magnetic null is threaded by a non-vanishing electric component; these are circumstances favourable for acceleration of electrically charged particles. We outline the mechanism which could operate in the magnetosphere of astrophysical black holes that rotate and move through diluted gaseous environment pervaded by an organized (super-horizon scale) magnetic field. This set-up may work as a pre-acceleration agent near the ergospheric boundary (cf. The Astrophysical Journal, Volume 900, id.119, 2020; https://arxiv.org/abs/2008.04630).

Speaker: Vladimír Karas

karas-MG16-talk-2021.pdf

4:49 PM Gravitomagnetic resonance and gravitational waves

Gravitational waves are usually described in terms of a transverse and traceless (TT) tensor, which allows to introduce the so-called TT coordinates. However, another possible approach is based on the use of a Fermi coordinates system, defined in the vicinity of the world-line of an observer arbitrarily moving in spacetime. In particular, Fermi coordinates have a direct operational meaning, since they are the coordinates an observer would use to perform space and time measurements; indeed, using these coordinates the metric tensor contains (up to the required approximation level) only quantities that are invariant under coordinate transformations internal to the reference frame. Using this approach it is simple to emphasise that what an observer measures depends both on the background field where he is moving and, also, on his kind of motion. This is quite similar to what happens when we study classical mechanics in non inertial frames: inertial forces appear, depending on the peculiar motion of the frame with respect to an inertial one. We show that using Fermi coordinates the effects of a plane gravitational wave can be described by gravitoelectromagnetic fields: in other words, the wave field is equivalent to the action of a gravitoelectric and a gravitomagnetic field, that are transverse to the propagation direction and orthogonal to each other. In particular, the gravito-magnetic field acts on spinning particles and we show that, due to the action of the gravitational wave field a gravitomagnetic resonance may appear. We give both a classical and a quantum description of this phenomenon and suggest that it can be used as the basis for a new type of gravitational wave detectors.

Speaker: Matteo Luca Ruggiero (Politecnico di Torino and INFN, LNL)

5:07 PM Progress towards waveforms for extreme mass ratio inspirals

Extreme mass ratio inspirals (EMRIs) are expected to be a key source of gravitational waves for the LISA mission. In order to extract the maximum amount of information from EMRI observations by LISA, it is important to have an accurate prediction of the expected waveforms. In particular, it will be necessary to have waveforms that incorporate effects that appear at second order in the mass ratio. In this talk we present the latest progress towards this goal, including recent results for the second-order gravitational-wave energy flux from black hole binaries.

Speaker: Barry Wardell (University College Dublin)

5:25 PM Testing the general relativistic nature of the Milky Way Rotation Curve with Gaia DR2

Gaia directly measures the kinematics of the stellar component of the Galaxy with the goal to create the largest, most precise three-dimensional map of the Milky Way (MW).
The very core of the Gaia data analysis and processing involves General Relativity (GR) to guarantee accurate scientific products. Nevertheless, any Galactic model should be developed consistently with the relativistic-compliant kinematics delivered by Gaia. In this respect, I will present the first test for a relativistic Galactic rotation curve (RC) with the Gaia second release (DR2) products (MNRAS, Issue 496, 2, 2020, M.Crosta at al.). Dark Matter (DM) is supposed to reside mostly in the Galactic halo. Both a GR model and a DM-based analogue were fit to the best-ever kinematics, derived exclusively from DR2 data, of a carefully selected homogenous sample of disk stars tracing the axisymmetric part of the Galactic potential.

The relativistic RC results statistically indistinguishable from its state-of-the-art DM-based analogue. This supports the ansatz that a gravitational dragging effect could drive the stellar velocities in the plane of our Galaxy far away from its center and mimic DM. Furthermore, one of Einstein’s equations provides the necessary baryonic matter density to close the observed gap with respect to the expected Newtonian velocities. Despite some inadequacies, the simplified GR model has proven also to be quite useful to estimate the (external) radial size of the Galactic bulge and the disc thickness at radial distances R>4 kpc.
These findings push on the fully use of Einstein’s theory and state the need to develop more complex relativistic galactic “geometries” that take into account the MW multi-structures in concomitance of the incoming and increasingly accurate Gaia data releases and with other Galactic observations targeting the Galactic center.

Speaker: Mariateresa Crosta (INAF-OATo)

CROSTA_PT5-MGM16.pdf

5:43 PM The measurement of the Lense-Thirring effect within the LARASE experiment

The LAser RAnged Satellites Experiment (LARASE), funded by the National Scientific Committee 2 (CSN2) of the Italian National Institute for Nuclear Physics (INFN) in the years 2013-2019, had among its main objectives that of verifying the gravitational interaction in the weak-field and slow-motion limit of General Relativity. Three geodynamic satellites: LAGEOS (NASA, 1976), LAGEOS II (ASI/NASA, 1992) and LARES (ASI, 2012) were taken as test masses of the experiment and their motions were carefully studied and compared with that of a timelike geodesic of General Relativity. Among the various measurements performed, the precession of the orbits of the satellites produced by the Earth's rotation, that is the precession induced by the angular momentum of our planet, has a particular consideration. This precession is generally known in the literature as Lense-Thirring effect or frame-dragging effect and proves that mass-energy currents affect the geometry of spacetime and, consequently, participate in the creation of its curvature. The results obtained in measuring the Lense-Thirring effect will be presented, highlighting the difficulties that must be overcome to the extent of a very small effect compared to the overall classical precession produced by the Earth's gravitational field and which acts on the same orbital elements subject to this relativistic precession. Emphasis will be given to the discussion of the systematic errors of the measurement, with special attention to gravitational perturbations.

Speaker: David Lucchesi (INAF/IAPS Tor Vergata and INFN ROMA2)

6:01 PM Spinning particle: Is Newton-Wigner the only way?

A rapidly spinning compact object couples to an ambient curved background via the so-called spin-curvature coupling. In expressing this, one has to deal with the ambiguity of the definition of the center of mass of the body. What is worse, in a Hamiltonian formalism, this choice corresponds to an unphysical "parasitic" degree of freedom in the dynamical system. A solution to this is to apply a Hamiltonian constraint on the system and to obtain a set of brackets where the center-of-mass degree of freedom is erased from the algebra. In this talk I will report on my progress in this procedure in the case of the so-called Tulczyjew-Dixon (or "covariant") supplementary spin condition and in my effort to cover the resulting phase space with canonical coordinates.

Speaker: Vojtěch Witzany (University College Dublin)

6:19 PM Gravitomagnetism in the Lewis cylindrical metrics

The Lewis solutions describe the exterior gravitational field produced by infinitely long rotating cylinders, and are useful models for global gravitational effects. When the metric parameters are real (Weyl class), the metrics of rotating and static cylinders are locally indistinguishable, but known to globally differ. The significance of this difference, both in terms of concrete physical effects and of the mathematical invariants where the rotation imprints itself, remained however an open problem. In this talk we will address these issues. We show that the Weyl class metric can be put into a 'canonical' form which depends explicitly only on three parameters with a clear physical significance, and reveals that the two settings differ only at the level of the gravitomagnetic vector potential which, for a rotating cylinder, cannot be eliminated by any global coordinate transformation. It manifests itself in frame-dragging effects such as the Sagnac and gravitomagnetic clock effects. This mirrors the electromagnetic field of a rotating charged cylinder, which likewise differs from the static case only in the vector potential, responsible for the Aharonov-Bohm effect (formally analogous to the Sagnac effect). The notions of local and global staticity are also revisited.

Speaker: Filipe Costa (CAMGSD - Instituto Superior Técnico, Universidade de Lisboa)

6:37 PM Quantum detection of inertial frame dragging

The Unruh De-Witt detector was introduced originally to give an operational meaning to particle detection in curved spacetimes. This simple two level quantum system interacts with the quantum field through a monopole type coupling, possibly exciting it to the excited state in the process. As the vacuum state of the field depends on global features of the background spacetime, the transition probability of a detector may be able to pick up these features too. As a result, UDW are better-than-classical-detectors. Due to inertial frame dragging, inertial observers inside of a spherical rotating shell are dragged into rotation with respect to distant stars. However to a classical observer inside the shell, the local surrounding spacetime is Minkowski — by performing local gravitational experiments, the observer cannot tell if the shell is rotating. In contrast, we shall see that the transition probability of a UDW detector is sensitive to the shell’s rotation. This is true even when the “switched-on” time of the detector is shorter than the time it takes for a signal to travel to the shell and back.

Speaker: wan cong (University of Waterloo)

6:55 PM Black Holes - Nature or Nurture?: The Roles of Rotation and Accretion in Powering Cosmic Sources

Black holes power many of the most powerful sources in the universe through their disks, jets and winds. They are powered by their rotational energy (Nature) and by the gravitational energy of accreting gas and stars (Nurture). The balance of these two modes and their implications, will be re-examined in the light of recent, remarkable observations of the nearby galaxy M87 by the Event Horizon Telescope as well as other developments. The importance of the dragging of inertial frames for rotational energy extraction, particle acceleration and imaging will be highlighted.

Speaker: Prof. Roger Blandford (KIPAC, Stanford University)

4:30 PM → 7:30 PM Loop Quantum Gravity: Block 1

Loop quantum gravity is a background independent, non-perturbative approach to quantum gravity. The focus of this session is on the structure of the theory, its computational techniques and applications to cosmology and black hole physics. We welcome talks reporting recent developments in canonical loop quantum gravity, spin-foam models, group field theory and related approaches to quantum gravity. The common theme is the background independent quantization of Einstein's gravity and the occurrence of quantum geometry. Loop quantum cosmology and reduced quantum models of black holes will be discussed in the separate session QG3.

Conveners: Jerzy Lewandowski (Uniwersytet Warszawski), Marcin Kisielowski (National Centre for Nuclear Research, Poland)

YouTube Link - July 5

YouTube Link - July 7

4:30 PM Causality in Spinfoams

Current Lorentzian Spinfoams are formulated in terms of a two-complex with spins on faces and intertwiners on edges. In this talk, I discuss how to add a causal structure on wedges. The EPRL model turns out to be given by a sum over these wedge-causal structures. I will show how this sum can be restricted to a single causal configuration and its relation to Engle's proper vertex. [Based on work in collaboration with Pierre Martin-Dussaud]

Speaker: Eugenio Bianchi (Penn State)

Bianchi-Causality-in-Spinfoams-MG16.pdf

5:00 PM Semiclassics of Spin-Foams with Generic Causal Character

The Lorentzian EPRL spin-foam model has been shown to asymptote in an appropriate regime to a Regge-like theory of gravity. Analogous results have recently been obtained for the Conrady-Hnybida (CH) extension of the model, but several questions regarding the amplitudes of time-like triangles remain open. In this talk I will present new progress on the asymptotic analysis of such amplitudes, in particular by proposing an alternative coherent-state parameterization of the theory and by generalizing to non-simplicial polyhedra. I will argue that, unlike for the other cases considered in the CH extension, the amplitude of time-like polygons is not exclusively dominated by Regge-like contributions. Finally I will discuss how the so-called "Cosine Problem" may naturally be avoided.

This talk reports on joint work with Sebastian Steinhaus, soon to be out on the Arxiv.

Speaker: José Diogo Simão (Theoretical Physics Institute Jena)

5:30 PM Spin foam framework for the black-to-white hole transition

Black holes formation and evolution have been extensively studied at the classical level. However, not much is known regarding the end of their lives, a phase that requires to consider the quantum nature of the gravitational field. A black-to-white hole transition can capture the physics of this phenomenon, in particular the physics of the residual small black holes at the end of the Hawking evaporation. In this talk I discuss how the spin foam formalism achieve to describe this non-perturbative phenomenon. I examine the three distinct regions of the black hole spacetime in which quantum effects cannot be neglected. I argue that the scenario in which the black hole geometry undergoes a quantum transition in a white hole geometry is natural and conservative. I study this quantum transition using the spin foam formalism, explicitly computing the resulting transition amplitude. The ongoing numerical analysis of this transition amplitude may provide an estimation of the back-to-white transition timescales and improve the understanding of its phenomenology.

Speaker: Farshid Soltani (Western U., Canada)

6:00 PM Numerical techniques in covariant LQG

The application of numerical techniques to covariant LQG may able to provide answers to many of the current open questions in theory. In this presentation, I first introduce the formalism currently used to implement numerical computations. I illustrate a recent application of numerical techniques concerning the study of divergences in the EPRL self-energy amplitude, on which so far there were
only analytical upper and lower bounds spanning more than 9 orders of magnitude.

Speaker: Pietropaolo Frisoni (Western University)

6:30 PM SL(2,R) Holonomies on the Light Cone

This talk describes how the Barbero--Immirzi parameter deforms the SL(2,R) symmetries on a null surface boundary. Our starting point is the definition of the action and its boundary terms. We introduce the covariant phase space and explain how the Holst term alters the symmetries on a null surface. This alteration only affects the algebra of the edge modes on a cross-section of the null surface boundar, whereas the algebra of the radiative modes is unchanged by the addition of the Barbero--Immirzi parameter. To compute the Poisson brackets explicitly, we work on an auxiliary phase space, where the SL(2,R) symmetries of the boundary fields are manifest. The physical phase space is obtained by imposing both first-class and second-class constraints. All gauge generators are at most quadratic in terms of the fundamental SL(2,R) variables. Finally, we discuss various strategies to quantise the system.

Speaker: Wolfgang Wieland (Institute for Quantum Optics and Quantum Information (IQOQI, Vienna))

7:00 PM Covariant phase space of standard GR and Palatini: metric vs tetrad formulation

Gravity admits several formulations. Some of the most well-known are standard GR and Palatini both in tetrad and metric formulation. In this talk, I will show the equivalence, in the covariant Phase Space, of all four formulations on a spacetime manifold with boundary. To this end, we will rely on the cohomological approach provided by the relative bicomplex framework.

Speaker: Juan Margalef (PSU)

4:30 PM → 7:30 PM Quantum Gravity Phenomenology: Block 1

This session will be devoted to discussion of phenomenological models aimed at probing and possibly leading to detection of various phenomena of quantum gravity origin. Such models are particularly timely now, at the down of the multi-messenger astronomy, which give us an access to new observations, possibly capable of detecting Planck-scale effects. The aim of the session will be to present recent developments in both theoretical investigations and current and near-future observational opportunities.

Conveners: Jerzy Kowalski-Glikman (Institute for Theoretical Physics, University of Wroclaw, Poland; National Centre for Nuclear Research, Warsaw, Poland), Giovanni Amelino-Camelia (Università Federico II di Napoli)

YouTube Link - July 5

YouTube Link - July 6

YouTube Link - July 7

4:30 PM Discrete symmetries and quantum gravity phenomenology

In my talk I will introduce kappa-deformation of discrete symmetries and I will discuss its phenomenological consequences.

Speaker: Prof. Jerzy Kowalski-Glikman (Institute for Theoretical Physics, University of Wroclaw, Poland; National Centre for Nuclear Research, Warsaw, Poland)

5:05 PM Poincaré invariance with a minimal length

A minimal length is generally expected to result in Lorentz-violating dispersion relations. I show how one can formulate a lattice theory that carries a representation of the Poincaré group in the Brillouin zone, and discuss how light cones arise for a subalgebra of observables. [Based on work in collaboration with Bekir Baytaş and Pietro Donà]

Speaker: Eugenio Bianchi (Penn State)

Bianchi-Lattice-Poincare-MG16.pdf

5:40 PM Spinfoams, $\gamma$-duality and Parity Violation in Primordial Gravitational Waves

There has been an expectation that the presence of the Barbero-Immirzi parameter ($\gamma$) in Loop Quantum Gravity (LQG) results in a quantum correction to the classical theory of gravity in the form of parity violation in primordial gravitational waves. In this paper, we show that a discreet symmetry of the Spinfoams action, $\gamma$-duality, constrains the form of the effective action for gravitational perturbations. As a consequence, tensor perturbations with different helicities evolve differently, and their circular polarization depends explicitly on $\gamma$. In this manner, the observation of primordial parity violation together with the mechanism that we propose would provide a way to set a bound on the value of the Barbero-Immirzi parameter, and therefore on the scale of discreetness of geometrical observables, such as the area and volume of a quantum chunk of space.

Speaker: Monica Rincon Ramirez

6:00 PM A model of polymer gravitational waves: theory and some possible observational consequences

We derive the effective polymer Hamiltonian of gravitational waves propagating on an FLRW background. We overcome the problem of polymerizing a time-dependent system by using a novel approach by using the extended phase space approach. Using the resulting Hamiltonian, we study some of the possible observational consequences of such a polymerized gravitational wave Hamiltonian.

Speaker: Saeed Rastgoo (York University, Canada)

6:30 PM Quantum Gravity Phenomenology in the Infrared

Quantum gravity effects are traditionally tied to short distances and high energies. In this talk I will argue that, perhaps surprisingly, quantum gravity may have important consequences for the phenomenology of the infrared. I will center my discussion around a conception of quantum gravity involving a notion of quantum spacetime that arises in metastring theory. This theory allows for an evolution of a cosmological Universe in which string-dual degrees of freedom decouple as the Universe ages. Importantly such an implementation of quantum gravity allows for the inclusion of a fundamental length scale without introducing the fundamental breaking of Lorentz symmetry. The mechanism seems to have potential for an entirely novel source for dark matter and dark energy. The simplest observational consequences of this scenario may very well be residual infrared modifications that emerge through the evolution of the Universe.

Speaker: Prof. Djordje Minic (Virginia Tech)

4:30 PM → 7:30 PM Loop Quantum Gravity: Cosmology and Black Holes: Block 2

Non-perturbative quantum gravity effects as understood from loop quantum gravity and related approaches play an important role in resolution of singularities of cosmological and black hole spacetimes, and leave potential signatures in the physics of early universe and black hole spacetimes. The goal of this session would be to highlight state of the art of various developments in this field with talks focused on physical implications.

Conveners: Jorge Pullin (Louisiana State University), Parampreet Singh (Louisiana State University)

YouTube Link - July 5

YouTube Link - July 8

4:30 PM Properties of a class of spherically symmetric polymer black holes

In this talk, we shall present our studies of a recently-proposed model of spherically symmetric polymer black/white holes by Bodendorfer, Mele
and M\"unch (BMM), which generically possesses five free parameters. However, we find that, out of these five parameters, only three independent combinations
of them are physical and uniquely
determine the local and global properties of the spacetimes. After exploring the whole 3-dimensional (3D) parameter space, we show that the model has very rich
physics, and depending on the choice of these parameters, various possibilities exist, including: (i) spacetimes that have the standard black/white hole structures, (ii) Spacetimes that have wormhole-like structures, and (iii) Spacetimes that still possess curvature singularities, which can be either hidden inside trapped regions or naked. However, such spacetimes correspond to only some limit cases. In particular, the necessary (but not sufficient)
condition is that at least one of the two polymerization" parameters vanishes. These results are not in conflict to the Hawking-Penrose singularity theorems, as the effective energy-momentum tensor, purely geometric and resulted from thepolymerization'' quantization, satisfies none of the three (weak, strong or dominant) energy
conditions in any of the two asymptotically flat regions for any choice of the three independent free parameters, although they can hold at the throat and/or at the two
horizons for some particular choices of them. In addition, it is true that quantum gravitational effects are mainly concentrated in the region near the throat, however,
in this model even for solar mass black/white holes, such effects can be still very large at the black/white hole horizons, again depending on the choice of the parameters.
Moreover, in principle the ratio of the two masses (for both of the black/white hole and wormhole spacetimes) can be arbitrarily large.

Speaker: Mr Wen-Cong Gan (Baylor University)

4:47 PM Infrared signature of quantum bounce in collapsing geometry

We study the mode decomposition of the unitarily evolving wave packet constructed for the quantum model of spherically symmetric dust collapsing in marginally bound Lemaître-Tolman-Bondi (LTB) model. We consider the model developed by Kiefer et al. [Phys.Rev.D 99 (2019) 12, 126010], where black hole singularity is replaced by a bounce from collapsing phase to expanding phase in the quantum dynamics of the dust cloud. We identify the observable depicting mode decomposition and using the freedom of operator ordering ambiguity wrote Hermitian extension of this operator alongside the Hermitian Hamiltonian. After identifying incoming and outgoing modes with this operator's eigenstates, we estimate their contributions. True to a quantum description, the expanding and contracting branches do not entirely comprise of outgoing and incoming radiation. For large wavenumber, the contribution of incoming and outgoing radiation is equal and very small. However, the infrared sector of this process shows salient features. Near-infrared modes are very sensitive to the dynamics of the dust cloud. Near the epoch of classical singularity, there is a significant contribution of incoming/outgoing modes of small wavenumber in the expanding/collapsing phase of the dust cloud. This contribution keeps on decreasing as one moves away from the singularity. Moreover for small wavenumber, the collapsing branch largely comprises of incoming modes, and the expanding branch comprises of outgoing modes. If one focuses on the infrared sector, the information of the bounce is carried over to the infrared modes, much before the information of the bounce comes about to any observer. A flip from largely incoming to largely outgoing radiation, as the evolution progressed from collapsing to expanding phase, is observed in the infrared regime. The information of the short scale physics is carried over to the longest wavelength in this quantum gravity model.

Speaker: Harkirat Singh Sahota (IISER Mohali)

QLTB_MG16.pdf

5:04 PM Closer look at white hole remnants

The idea that, after their evaporation, Planck-mass black holes might tunnel into metastable white holes has recently been intensively studied. Those relics have been considered as a dark matter candidate. We show that the model is severely constrained and underline some possible detection paths. We also investigate, in a more general setting, the way the initial black hole mass spectrum would be distorted by both the bouncing effect and the Hawking evaporation.
Based on Barrau, Renevey, Martineau, Ferdinand Phys. Rev. D 103 (2021) 4, 043532

Speaker: Aurélien Barrau (LPSC - CNRS)

5:29 PM Effect of loop quantization prescriptions on the physics of non-singular gravitational collapse

We study some consequences of the loop quantization of the outermost shell in the Lema\^itre–Tolman–Bondi (LTB) dust spacetime using different quantization strategies motivated by loop quantum gravity. Prior work has dealt with this loop quantization by employing holonomies and the triads, following the procedure in standard loop quantum cosmology. In this work we compare this quantization with the one in which holonomies and gauge-covariant fluxes are used. While both of the quantization schemes resolve the central singularity, they lead to different mass gaps at which a trapped surface forms. This trapped surface which is matched to an exterior generalized Vaidya spacetime disappears when the density of the dust shell is in the Planck regime. We find that the quantization based on holonomies and gauge-covariant fluxes generically results in an asymmetric evolution of the dust shell in which the mass associated with the ``white hole" is about 2/3 of the "black hole" for an external observer. Further, unlike the quantization using only holonomies, there can be situations in which only a black hole forms without its white hole twin. These turns out to be a distinct phenomenological signature distinguishing these two quantization prescriptions.

Speaker: Prof. Parampreet Singh (Louisiana State University)

5:54 PM Holonomy corrections in effective midisuperspace models

We develop a systematic approach to obtain spherically symmetric midisuperspace models with modifications inherited from loop quantum gravity. We obtain a family of effective constraints that satisfy Dirac's deformation algebra and show that (scale-dependent) holonomy corrections can be consistently implemented in the presence of matter with local degrees of freedom. These deformed Hamiltonians are expected to modify the dynamics of general relativity and to avoid the singularities predicted for gravitational collapsing models.

Speaker: Asier Alonso-Bardaji (Universidad del País Vasco/Euskal Herriko Unibertsitatea)

6:11 PM Loop quantum Schwarzschild interior and black hole remnant

The interior of a Schwarzschild black hole is quantized by the method of loop quantum gravity. The Hamiltonian constraint is solved and the physical Hilbert space is obtained in the model. The properties of a Dirac observable corresponding to the Arnowitt-Deser-Misner mass of the Schwarzschild black hole are studied by both analytical and numerical techniques. It turns out that zero is not in the discrete spectrum of this Dirac observable. This supports the existence of a stable remnant after the evaporation of a black hole. Our conclusion is valid for a general class of schemes adopted for loop quantization of the model.

Speaker: Cong Zhang (University of Warsaw)

6:28 PM Revisiting the Hamiltonian formalism of the Ashtekar-Olmedo-Singh black hole model

In this talk, I will present the main features of the solutions to a recently-derived set of dynamical equations that governs the effective dynamics of black holes in loop quantum cosmology which were obtained via a revision of the Hamiltonian calculation underlying the Ashtekar-Olmedo-Singh black hole model. I will analyze the possibility that certain quantum parameters are treated as Dirac observables and that the radial and angular sectors of phase space are not dynamically decoupled in general. I will show how to derive in this way the corresponding Hamiltonian equations. Finally, I will discuss the features of the resulting model, emphasizing how this apparently slight modification of the formalism might open a door to the alleviation of some of the criticisms that the Ashtekar-Olmedo-Singh model has received.

Speaker: Alejandro García-Quismondo (Instituto de Estructura de la Materia (IEM-CSIC))

Marcel Grossmann '21 talk.pdf

6:45 PM Effective black hole interior and the Raychadhuri equation

We show that loop quantization leads to the emergence of defocusing terms in the expansion and its rate of change, the Raychaudhuri equation. These terms are suppressed in the region far from the singularity but dominate close to that region and prevent both the expansion and its rate from diverging everywhere inside the black hole. This in turn signals the disappearance of the caustic points and the resolution of singularity in the interior of the black hole.

Speaker: Saeed Rastgoo (York University, Canada)

7:02 PM How classical are Gaussian states in loop quantum cosmology?

In traditional (Dirac quantized) quantum mechanics, Gaussian wave functions play an important role in understanding semi-classicality: they may be chosen to be as sharply-peaked around classical position coordinates and they saturate the uncertainty relation, thereby minimizing quantum fluctuations. Gaussian states may likewise be constructed on the kinematic volume Hilbert space of loop quantum cosmology (LQC), and are often viewed as good candidates for semi-classical quantum geometries. However, it is not obvious that they exhibit the same nice features as traditional Gaussian quantum states. In this talk, I show that contrary to common intuition, Gaussian states in LQC generally do not saturate their uncertainty relations, and indeed, that there exist LQC Gaussian states for which the fluctuations are arbitrarily large. It is shown, however, that the usual volume regularization procedure of LQC allows one to suppress these diverging fluctuations as much as one wishes, and so uncertainty minimization is obtained asymptotically as $V_0\to\infty$. It is further illustrated that the relationship between the fiducial volume $V_0$ and holonomy length $\lambda$ plays an important role in determining the fluctuations of the these states.

Speaker: Patrick Fraser (University of Toronto)

MG16_Presentation.pdf

4:30 PM → 7:30 PM White Dwarf Explosions: Block 1

Type Ia supernovae play a crucial role as standardizable candles for cosmology, and underpin measurements of both dark energy and the Hubble tension. Yet, the nature of the stellar progenitors and explosion mechanisms for type Ia supernovae remains an area of active research. This session will bring together members of the supernova cosmology community along with observers and theorists investigating the type Ia stellar progenitor problem and explosion mechanisms, and aims to achieve dialog on problems of interest linking both groups.
Particular areas of focus for the session include:
* Type Ia stellar progenitors and explosion mechanisms
* Multidimensional type Ia simulations
* Photometric and spectroscopic discriminants of type Ia progenitors, particularly at early and late times
* Evolutionary effects on SNe Ia supernova cosmology and their impact on dark energy measurements
* The absolute brightness of normal SNe Ia; the link of absolute brightness and the distance scale; views on the Hubble constant tension.

Conveners: Prof. Pilar Ruiz-Lapuente (ICCUB U. Barcelona and IFF (CSIC)), Robert Fisher (University of Massachusetts Dartmouth)

YouTube Link - July 5

YouTube Link - July 8

4:30 PM Type Ia Supernovae: Astrophysics and Cosmology

I will review current cosmological applications of Type Ia Supernovae (SN Ia) to measure the Hubble constant and constrain the nature of dark energy, with an emphasis on the limiting factors in these measurements. I will describe how near-infared observations of SN Ia provide an alternate path for future supernova cosmology. Astrophysical systematic uncertainties arise from our lack of detailed understanding of the progenitors and explosion physics of SN Ia, and I will explore their potential implications for current and future surveys.

Speaker: Saurabh Jha (Rutgers University)

4:50 PM Multidimensional simulations of Type Ia supernovae

Despite substantial progress in theoretical modeling and numerical simulations over the past years, our understanding of the physical mechanism of Type Ia supernovae remains incomplete. This has two main reasons. (i) The progenitor systems from which these explosions arise have not been identified, and therefore the initial conditions for the explosion simulations are uncertain. (ii) Modeling the explosion stage itself is a severe multi-scale multi-physics challenge and relies on assumptions and approximations.
Some of these approximations could be mitigated with multidimensional hydrodynamical simulations. They form a cornerstone of a consistent modelling pipeline that follows a progenitor model over explosion and nucleosynthesis to the formation of observables. By avoiding tuneable parameters, this approach facilitates a direct comparison of model predictions with astronomical data and conclusions on the validity of the assumed progenitor scenarios. I will describe the construction and the application of such a pipeline of multidimensional models and discuss achievements and shortcomings of current models of Type Ia supernovae.

Speaker: Friedrich Röpke (Heidelberg University / HITS)

5:10 PM Type Ia Supernovae from Double Detonations in Merging Double White Dwarf Systems: The D6 Scenario

Our theoretical understanding of the progenitors of Type Ia supernovae has undergone a revolution in the last decade, with sub-Chandrasekhar-mass scenarios quickly coming to the forefront of research. In this talk, I will focus on the "dynamically driven double-degenerate double-detonation" (D6) scenario, in which a double detonation on a sub-Chandrasekhar-mass white dwarf takes place during the merger of two white dwarfs. Our theoretical work shows that such explosions can accurately reproduce the entire observable range of Type Ia supernovae, from subluminous to overluminous. The scenario also predicts the possibility of a unique hypervelocity survivor. The discovery of three such stars makes this the only Type Ia supernova scenario that has been directly confirmed. This combination of theoretical and observational successes makes the D6 scenario an extremely promising channel to explain the bulk of Type Ia supernovae.

Speaker: Ken Shen (UC Berkeley)

5:30 PM Coffee Break

Brief Intermission

5:35 PM Type Ia Supernova Explosions and their Nucleosynthesis: Constraints on Progenitors

What the progenitors of Type Ia supernovae (SNe Ia) are, whether they are Chandrasekhar mass or sub-Chandrasekhar mass white dwarfs, has been matter of debate for decades. Various observational hints are supporting both models as the main progenitor. In this talk, I will review the explosion physics and their chemical abundance patterns of SNe Ia from these two classes of progenitors. I will discuss how the observational data of SNe Ia, their remnants, the Milky Way Galaxy and galactic clusters can help us to determine the essential features where numerical models of SNe Ia need to match.

Speaker: Shing-Chi Leung (TAPIR, Caltech)

5:55 PM Type Ia explosions and progenitors: insights from spectral time series”

The progenitor scenarios of Type Ia supernovae remain a mystery having a crippling effect on the many area that have strong connections to these explosive events (e.g. cosmology, chemical evolution of the Universe, stellar evolution, etc.). The current viable scenarios can be divided into two broad categories: 1) 1.4 M$_\odot$ white dwarves that are likely created in an accretion process and self-ignite due to high central pressure/density. 2) White dwarves of considerably lesser mass ($\approx1~\textrm{M}_\odot$) can be ignited in shell detonations or in the merging process that formed them. Both scenarios are able to correctly predict the bulk properties of Type Ia observations and thus determining which scenario is responsible for these events is difficult.

One difference in their scenarios is that the heavy white dwarf progenitor requires a deflagration with a subsequent detonation process, while the other reproduces the observations with a simple detonation. The former process is turbulent and results in a mixed ejecta, while the latter will result in a highly stratified remnant. Observed spectral time series have the power to distinguish between both of those burning modes through a technique known as supernova tomography.

In this talk, I will present the supernova tomography technique which uses observed spectral time series and radiative transfer code to reconstruct the explosion. I will focus on recent works that have shown how to infer supernova parameters by coupling traditional radiative transfer codes, Bayesian statistics, and machine learning to study observed spectral time series. The results can then quantitatively compared to explosion simulations to verify either scenario.

I will conclude with our study of Type Ia supernova SN 2002bo and will give an outlook of the next steps in determining the still mysterious origins of these events.

Speaker: Wolfgang Kerzendorf (Michigan State University)

6:15 PM The Sub-Chandrasekhar Mass Pathway to Type Ia Supernovae

Type Ia supernovae (SNe) are some of the most common cosmic transients, yet their progenitors are still not known. I will discuss the sub-Chandrasekhar mass pathway to these explosions, known as the double detonation scenario, where a White Dwarf (WD) is able to explode below the Chandrasekhar mass limit through the aid of an accreted helium shell. An ignition of this helium can send a shock wave into the center of the WD which, upon convergence, can ignite the core causing a thermonuclear runaway resulting in a Type Ia-like explosion. I will describe the hydrodynamic techniques I use to simulate these explosions as well as the radiation transport methods I use to translate the hydrodynamical output into synthetic light curves and spectra.  Using these methods, I have calculated some distinct observational signatures that should be exhibited by double detonation explosions in both the photospheric and nebular phase.  I will discuss the populations of SNe Type Ia which are consistent with these features. Lastly, I will present the first observed supernova, SN 2018byg, that exhibits the "smoking gun" signatures predicted, establishing the most direct evidence to date that there are multiple pathways through which WDs explode.

Speaker: Abigail Polin (Carnegie Observatories & Caltech)

6:35 PM Charged polarized white dwarfs with finite temperature as a possible source of type Ia supernovae

In this work, we investigate the structure of polarized charged white dwarfs (WDs) with finite temperature as a possible type Ia supernovae source. The WD is considered with an isothermal core and an envelope where there is a temperature distribution that depends on the density. Regarding the hot fluid, we assume that it is composed of nucleons and electrons with temperature contributions. The structure of the polarized charged white dwarfs is obtained by solving the Einstein-Maxwell equations with charge densities represented by two Gaussians, forming an electric dipole layer at the stellar surface. We obtain larger and more massive white dwarfs when polarized charge and the Gaussians distance are increased. We found that to appreciate effects in the white dwarf's structure, the electric polarized charge must be in the order of $5.0\times10^{20} \rm [C]$. We obtain a maximum white dwarf mass of around $2M_\odot$ for a polarized charge of $1.5\times10^{21} \rm[C]$. These results could indicate polarized charged white dwarfs as possible progenitors of superluminous type Ia supernovae. Furthermore, we show that the curves we obtain are very similar to the ones of strongly magnetized white dwarfs obtained recently.

Speaker: Sílvia Nunes (ITA)

6:40 PM Round Table Discussion

Roundtable on hydrodynamical models, pathways to explosions, stellar progenitors (sub-Chandrasekhar and Chandrasekhar mass white dwarf progenitors).

Tue, July 6

6:30 AM → 9:30 AM Extended Theories of Gravity and Quantum Cosmology: Block 1

The current understanding of gravitation is based on Albert Einstein's classical theory of General Relativity. While the study of gravitational waves in recent years have brought tremendous success to the classical General Relativity, this description is incomplete when describing several phenomena such as the singularity and event horizon of a black hole, the origin of the universe, fundamental understanding of dark energy, etc. Facing those conceptual issues in our universe, there is an increasing demand for the study on extended theories of gravity and quantum cosmology. In this parallel session, we invite researchers to pay attention to the aforementioned issues and welcome oral talks to report their progress on the latest study.

Convener: Yi-Fu Cai (University of Science and Technology of China)

YouTube Link - July 6

YouTube Link - July 7

YouTube Link - July 9

6:30 AM Quantum gravity phenomenology from thermodynamics of spacetime

On one hand, the formalism developed in thermodynamics of spacetime allows a derivation of Einstein equations from the proportionality of entropy to the area. On the other hand, low energy quantum gravity effects imply a modified entropy formula with an additional term logarithmic in the area. Combining both concepts, I will introduce the derivation of quantum modified gravitational dynamics from the modified entropy and discuss its main features. Moreover, I will show its physical implications on a simple cosmological model and show that it suggests the replacement of the Big Bang singularity by a regular bounce.

Speaker: Marek Liška (Institute of Theoretical Physics, Charles University, Prague)

6:50 AM Cosmic stringlike objects in hybrid metric-Palatini gravity

We consider static and cylindrically symmetric interior string type solutions in the scalar-tensor representation of the hybrid metric-Palatini modified theory of gravity. As a first step in our study,we obtain the gravitational field equations and further simplify the analysis by imposing Lorentz invariance along the t and z axes, which reduces the number of unknown metric tensor components to a single function $W^{2}(r)$. In this case, the general solution of the field equations can be obtained,for an arbitrary form of the scalar field potential, in an exact closed parametric form, with the scalar field $φ$ taken as a parameter. We consider in detail several exact solutions of the field equations, corresponding to a null and constant potential, and to a power-law potential of the form
$V (φ) = V_0φ^{3/4}$, in which the behaviours of the scalar field, of the metric tensor components and of the string tension can be described in a simple mathematical form. We also investigate the string models with exponential and Higgs type scalar field potentials by using numerical methods. In this way we obtain a large class of novel stable string-like solutions in the context of hybrid metric-Palatini gravity, in which the basic parameters, such as the scalar field, metric tensor components, and string tension, depend essentially on the initial values of the scalar field, and of its derivative,on the $r = 0$ circular axis.

Speaker: Hilberto Silva (Instituto Astrofísica e Ciências do Espaço)

PresentationMG.pdf

7:10 AM Gravitational waves in spacetime with extra dimensions

We study the effect of compact extra dimensions on the gravitational wave luminosity and waveform. We consider a toy model, with a compactified fifth dimension, and matter confined on a brane. We work in the context of five dimensional (5d) general relativity, though we do make connections with the corresponding Kaluza-Klein effective 4d theory. We show that the luminosity of gravitational waves emitted in 5d gravity by a binary with the same characteristics (same masses and separation distance) as a 4d binary is 20.8% less relative to the 4d case, to leading post-Newtonian order. The phase of the gravitational waveform differs by 26% relative to the 4d case, to leading post-Newtonian order. Such a correction arises mainly due to the coupling between matter and dilaton field in the effective 4d picture and agrees with previous calculations when we set black holes' scalar charges to be those computed from the Kaluza-Klein reduction. The above corrections to the waveform and the luminosity are inconsistent with the gravitational-wave and binary pulsar observations and thus they effectively rule out the possibility of such a simple compactified higher dimensions scenario. We also comment on how our results change if there are several compactified extra dimensions, and show that the discrepancy with 4d general relativity only increases.

Speaker: Yuchen Du (University of Virginia)

7:30 AM Primordial Black Holes and Stochastic Gravitational Waves in the Sound Speed Resonance Cosmology

Primordial black hole (PBH) is a kind of important Dark Matter candidate of cosmological origin. And it is also a potential seed of supermassive black holes. However, the formation and the astrophysical effects of PBH still remain unclear. From theoretical perspective, the speaker and his collaborators proposed sound speed resonance （SSR） mechanism as an efficient novel effect to produce PBH. The speaker will briefly review PBH and SSR mechanism and summarize what they have done in this topic. After that, he will introduce their recent work on the SSR mechanism of stochastic gravitational waves which might be a new probe for new physics in the early universe.

Speaker: Sheng-Feng Yan (INFN(Milano), USTC)

7:50 AM Effective f(R) actions for modified Loop Quantum Cosmologies

General Relativity is an extremely successful theory, at least for weak gravitational fields, however, it breaks down at very high energies, such as in correspondence of the initial singularity. Quantum Gravity is expected to provide more physical insights concerning this open question. Indeed, one alternative scenario to the Big Bang, that manages to completely avoid the singularity, is offered by Loop Quantum Cosmology (LQC), which predicts that the Universe undergoes a collapse to an expansion through a bounce. In this talk, I will discuss how, in a recent paper, we used metric f(R) gravity to reproduce the modified Friedmann equations which have been obtained in the context of modified loop quantum cosmologies. I will describe the order reduction method that was used and how this allowed us to obtain covariant effective actions that lead to a bounce, for specific models of modified LQC, considering matter as a scalar field.

Speaker: Ana Rita Ribeiro (Faculdade de Ciências da Universidade de Lisboa)

16th_Marcel_Grossmann_Meeting_Ana_Rita_Ribeiro.pdf

8:10 AM Quadratic curvature theories formulated as Covariant Canonical Gauge theories of Gravity

The Covariant Canonical Gauge theory of Gravity is generalized by including at the Lagrangian level all possible quadratic curvature invariants. In this approach, the covariant Hamiltonian principle and the canonical transformation framework are applied to derive a Palatini type gauge theory of gravity. The metric gµν, the
affine connection γλµν and their respective conjugate momenta, kµνσ and qαξβη
tensors, are the independent field components describing the gravity. The metric is the basic dynamical field, and the connection is the gauge field. The torsion-free and metricity-compatible version of the space-time Hamiltonian is built from all possible invariants of the qαξβη tensor components up to second order. These correspond in the Lagrangian picture to Riemann tensor invariants of the same order. We show that the quadratic tensor invariant is necessary for constructing the canonical momentum field from the gauge field derivatives, and hence for transforming between Hamiltonian and Lagrangian pictures. Moreover, the theory is extended by dropping metric compatibility and enforcing conformal invariance. This approach could be used for the quantization of the quadratic curvature theories, as for example in the case of conformal gravity