### Conveners

#### Theories of Gravity: Alternatives to the Cosmological and Particle Standard Models: Block 1

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

#### Theories of Gravity: Alternatives to the Cosmological and Particle Standard Models: Block 2

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

#### Theories of Gravity: Alternatives to the Cosmological and Particle Standard Models: Block 3

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

#### Theories of Gravity: Alternatives to the Cosmological and Particle Standard Models: Block 4

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

### Description

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.

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...

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.

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...

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...

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...

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...

The quasi-static approximation (QSA) is a useful tool to get a quick and clear physical understanding of the phenomenology of modified gravity which is encoded in two functions (of scale and time): the effective gravitational constant (describing the modified evolution of matter perturbations) and the slip (parametrizing the relations between the two gravitational potentials). This...

We consider the possibility that the Milky Way’s dark matter halo possesses a non-vanishing equation of state. Consequently, we evaluate the contribution due to the speed of sound, assuming that the dark matter content of the galaxy behaves like a fluid with pressure. In particular, in the galactic core we compare two scenarios: a supermassive black hole in vacuum and a dark...

Scalar-tensor theories leaving significant modifications of gravity at cosmological scales rely on screening mechanisms to recover General Relativity (GR) in high-density regions and pass stringent tests with astrophysical objects. Much focus has been placed on the signatures of such modifications of gravity on the propagation of gravitational waves through cosmological distances while...

Quasinormal modes of massless test scalar field in the background of gravitational field for a non-extremal dilatonic dyonic black hole are explored. The dyon-like black hole solution is considered in the gravitational $4d$ model involving two scalar fields and two 2-forms. It is governed by two 2-dimensional dilatonic coupling vectors $\vec{\lambda}_i$ obeying $\vec{\lambda}_i...

The Post Newtonian (PN) expansion of General Relativity (GR) yields a series of potentials that accurately describe the trajectories of compact binaries to a very high degree of accuracy. In the mathematical treatment of PN expansion of GR, a relation is established between the ratio of the orbital velocity to the speed of light and the compactness-to-closeness ratio: the ratio between average...

Geometrothermodynamics (GTD) can be used in relativistic cosmology to generate models of barotropic fluids. As an alternative approach, we use the GTD formalism to find the most general thermodynamic fundamental equations that describe the barotropic fluids of the Lambda-CDM model. This allows us to investigate the thermodynamic properties of these barotropic fluids from the point of view of...

We study the entanglement production for Dirac and Klein-Gordon fields in an expanding spacetime characterized by the presence of torsion. Torsion is here considered according to the Einstein-Cartan theory with a conformally flat Friedmann-Robertson-Walker spacetime. In this framework, torsion is seen as an external field, fulfilling precise constraints directly got from the...

We explore a geometrical mechanism of cancellation that heals the cosmological constant problem. To do so, during the primordial universe we assume quantum fluctuations to hold and the effective cosmological constant built up in terms of its bare and quantum contributions. We thus notice that if we assume a discontinuity of the Friedmann-Robertson-Walker metric, a corresponding phase of energy...

Existence of the Dark Energy became now a commonly-accepted paradigm of cosmology, but the physical essence of this quantity remains absolutely unknown and its numerical values are drastically different in the early and modern Universe. In fact, the Dark Energy is usually introduced in literature either by postulating some additional terms in the Lagrangians or by employing the empirical...

We present a comparative analysis of current observational constraints on three recently discussed alternative models for explaining the low-redshift acceleration of the universe: the generalized coupling model by Feng and Carloni, the scale invariant model by Maeder (an example of a broader class first proposed by Canuto et al., which we also study), and the so-called steady-state torsion...

I will talk about a recent work, where we have investigated the interaction between electromagnetic, gravitational, and plasma related perturbations on homogeneous and hypersurface orthogonal LRS class II spacetimes. By using these spacetimes, which allow for the inclusion of a non-zero magnetic field, as backgrounds in a perturbative approach, we are able to see interactions between the...

Einstein’s general relativity predicts that a gravitational wave is allowed to have two polarizations called tensor-mode: plus and cross modes. On the other hand, the general metric theory of gravity predicts that a gravitational wave is allowed to have up to six polarizations: two scalar and two vector modes in addition to tensor modes. In case the number of laser-interferometric...

The accelerated expansion of the universe demands presence of an exotic matter, namely the dark energy. Though the cosmological constant fits this role very well, a scalar field minimally coupled to gravity, or quintessence, can also be considered as a viable alternative for the cosmological constant. We study $f(R)$ gravity models which can lead to an effective description of dark energy...

We check the dynamical and observational features of four typologies of logotropic dark energy models, leading to a thermodynamic cosmic speed up fueled by a single fluid that unifies dark energy and dark matter. We first present two principal Anton-Schmidt fluids where the Gruneisen parameter is free to vary and then fixed to the special value We also investigate the pure logotropic...

We investigate radiation from asymptotic zero acceleration motion where a horizon is formed and subsequently detected by an outside witness.

CGHS black holes have rightfully garnered much attention over the last few decades as the models are simplified (1+1)-dimensional versions of black hole evaporation. Their solubility has lead to tractable physical insights into the radiative process. Concurrently, moving mirrors are well-known simplified (1+1)-dimensional models for black hole evaporation. We synthesize the two by finding an...

Testing gravity on low and high energy domains is essential to reconcile primordial and late times. Focusing on local and cosmic scales, red and blue shift measures between two observers plays a crucial role. We revise their use in two distinct contexts, making use of de Sitter–Schwarzschild solution and q-metric to describe low and higher energy regimes. First, we assume de...

We introduce a novel model of affine gravity, which implements the no-scale scenario. Namely, in our model the Planck mass and Hubble constant emerge dynamically, through the mechanism of spontaneous breaking of scale invariance. This naturally gives rise to the inflation, thus introducing a new inflationary mechanism. Moreover, the time direction and non-degenerate metric emerge dynamically...