### Conveners

#### Status of the H_0 and Sigma_8 Tensions: Theoretical Models and Model-Independent Constraints: Block 1

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

#### Status of the H_0 and Sigma_8 Tensions: Theoretical Models and Model-Independent Constraints: Block 2

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

#### Status of the H_0 and Sigma_8 Tensions: Theoretical Models and Model-Independent Constraints: Block 3

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

### Description

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.

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.

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...

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...

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,...

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...

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 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,...

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...

In the talk, I review a string-inspired running vacuum model (RVM) of Cosmology, and its potential connection with the dark sector of the (observable) Universe. Specifically, I consider a gravitational model of the early Universe that is inspired by the low-energy effective actions of string theory. I assume that only gravitational degrees of freedom appear as external fields at early eras. I...

The discordance can be due to a wide range of non-standard cosmological or astrophysical processes as well as from some particular systematics. Here, without considering any particular astrophysical process or extension to the standard model at the background level, we look to project the effect of these differences in the values of the key cosmological parameters on to the shape of the...

The standard Lambda CDM cosmological model now seems to face some puzzles. One of the most serious problems is the so-called Hubble tension; the values of the Hubble constant obtained by local measurements look inconsistent with that inferred from CMB. Although introducing extra energy components such as the extra radiation or early dark energy appears to be promising, such extra components...

I will explain the limitations of resolutions to Hubble tension within Einstein gravity and the FLRW paradigm. I will then leverage persistent discrepancies in the cosmic radio dipole to argue that we must move beyond FLRW. I will provide hints across various cosmological probes that the Hubble constant is higher in the hemisphere aligned with the CMB dipole.

We study Brans-Dicke gravity with a cosmological constant and cold dark matter (BD-$\Lambda$CDM hereafter). This theory is the first historical attempt to extend Einstein's General Relativity by promoting the Newtonian coupling constant $G_N$ to a dynamical one $G(t)$. We present the background and the perturbation equations, which allows us to test the theoretical predictions with a complete...

Several observations using electromagnetic signal have led to a paradigm shift in our understanding of the Universe, with the realization that two unknown quantities - namely dark matter and dark energy - constitute about 95% of the Universe, even though their existence could not be explained by the known laws of physics and fundamental particles discovered until now. Moreover, measurements of...

The cosmological constant (CC) term, $\Lambda$, in Einstein's equations has been for some three decades a

fundamental building block of the concordance or standard $\Lambda$CDM model of cosmology. Even though the model is not free of fundamental problems, they have not been circumvented by any alternative dark energy

proposal either.

However, an interesting alternative is that the vacuum...

$\Lambda CDM$ is increasingly challenged by observations in the late-time Universe. Here we consider unparticle cosmology for its potential to alleviate some of these issues. Unparticles offer a scale invariant contribution by an extra parameter $\delta$, here studied for $\delta \in [-6,1]$ (corresponding to scaling dimension $d_u \in [-2,3/2]$). For most values of $\delta$, the model...

I will present an end-to-end exploration of the simplest modified gravitational theory in Jordan-Brans-Dicke (JBD) gravity, from an analytical and numerical description of the background expansion and linear perturbations, to the nonlinear regime captured with a hybrid suite of $N$-body simulations, to the parameter constraints from existing cosmological probes. In the analysis, the nonlinear...

In this talk I will consider a very popular scenario where the dark energy is a dynamical fluid whose energy density can be transferred to the dark matter, and vice versa, via a coupling function proportional to the energy density of the dark energy. In particular, I will discuss this model’s ability to address the $H_0$ and $S_8$ tensions showing that considering data from Planck, BAO and...

I will discuss recent and ongoing work focused on attempts to restore concordance amongst cosmological data sets in the context of the H0 and S8 tensions. Particular attention will be paid to models invoking new physics at or prior to recombination, including small-scale baryon-clumping models (e.g., due to primordial magnetic fields) and quasi-accelerating early dark energy models.

We report how to alleviate both the H0 and sigma8 tensions simultaneously within

torsional gravity from the perspective of effective field theory. Following these observations, we construct concrete models of Lagrangians of torsional gravity. Specifically, we consider a novel f(T) parametrization where two out of the three parameters are independent. This modified gravity model can...

In this talk, we discuss cosmological constraints inferred from detection of galaxy clusters in the mm-wavelengths (through the thermal Sunyaev-Zeldovich effect), using Planck observations.

We focus in particular on the well known sigma8 tension. We provide a novel analysis of Planck clusters (also in combination with the power spectrum of the total tSZ signal), showing that constraints are...

We propose a late time gravitational transition at low redshifts $z_t<0.1$ as a possible solution of both the Hubble and growth tensions. Such a transition would naturally lead to a transition of the intrinsic SnIa luminosity and absolute magnitude $M$ at $z_t$ and could also be accompanied by a transition in the dark energy equation of state parameter $w$. Thus we would have a late $w - M$...

First proposed in 1964 by Sjur Refsdal, gravitational lensing provides a straightforward and elegant geometrical way of estimating the Hubble constant from cosmologically distant variable sources. The method relies on observationally determined time delays between light arriving through different multiple images, and the mass models of the lens, which are constrained by observed image...

Magnetic fields, if present in the plasma prior to last scattering, would induce baryon inhomogeneities and speed up the recombination process. As a consequence, the sound horizon at last scattering would be smaller, helping to relieve the Hubble tension and the S8 tension. Intriguingly, the strength of the magnetic field required to alleviate the Hubble tension happens to be of the right...

We present a fully relativistic framework to evaluate the impact of stochastic inhomogeneities on the prediction of the Hubble-Lemaitre diagram. To this aim, we relate the fluctuations of the luminosity distance-redshift relation in the Cosmic Concordance model to the intrinsic uncertainty associated to the estimation of cosmological parameters from high-redshift surveys (up to z = 4). We...