### Conveners

#### Dark Energy and the Accelerating Universe: Block 1

- Alexei Starobinsky (Landau Institute for Theoretical Physics RAS)
- David Polarski (Université de Montpellier)

#### Dark Energy and the Accelerating Universe: Block 2

- Alexei Starobinsky (Landau Institute for Theoretical Physics RAS)
- David Polarski (Université de Montpellier)

#### Dark Energy and the Accelerating Universe: Block 3

- Alexei Starobinsky (Landau Institute for Theoretical Physics RAS)
- David Polarski (Université de Montpellier)

### Description

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.

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...

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...

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...

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...

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....

"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...

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...

We place observational constraints on two models within a class of scenarios featuring an elastic interaction between

dark energy and dark matter that only produces momentum exchange up to first order in cosmological perturbations. The first one corresponds to a perfect-fluid model of the dark components with an explicit interacting Lagrangian, where dark energy acts as a dark radiation at...

We discuss a generalised form of an emergent dark energy model with one degree of freedom for the dark energy sector that has the flexibility to include both ΛCDM model as well as the Phenomenologically Emergent Dark Energy (PEDE) model proposed by Li & Shafieloo (2019) as two of its special limits. The free parameter for the dark energy sector, namely Δ, has the value of 0 for the case of the...

In this talk I will review recent progress that the SDSS-IV / eBOSS collaboration has made in constraining cosmology from the clustering of galaxies, quasars and the Lyman-alpha forest. The SDSS-IV / eBOSS collaboration has measured the baryon acoustic oscillation (BAO) and redshift space distortion (RSD) features in the correlation function in redshift bins from z~0.15 to z~2.33. These...

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...

The consistency between the cosmic expansion and growth may hold clues about the nature of the acceleration of the Universe. Using model-independent methods, we reconstruct the growth history from redshift-space distortion and deduce the corresponding expansion history, which we test against supernovae data. Motivated by these results, we then introduce a model of two-component dark energy...

We study here phantom models of dark energy represented by a scalar field and with tracker properties. By means of a change of polar-like of variables, we study a general class of models classified in terms of a set of three free parameters. Upon comparison of the models with observations, and considering Bayesian evidence, our results suggest a preference for phantom-like dark energy and...

We analyze the Joint Light-curve Analysis (JLA) Type Ia supernovae (SN Ia) compilation implementing the non-parametric iterative smoothing method. We explore the SN Ia light-curve hyperparameter space and find no dark energy model dependence nor redshift evolution of the hyperparameters. We also analyze the more recent Pantheon SN Ia compilation to search for possible deviations from the...

Various observations have shown that dark energy accounts for nearly two-thirds of the energy density of the Universe.

The simplest model to explain the nature of dark energy is the cosmological constant ($\Lambda$CDM) model. Although Planck observations supports using $\Lambda$CDM model as the base cosmological model, there exist some inconsistencies in parameter estimates when compared with...

Standard cosmology is based on the assumption that the dark fluids behave as standard, hard matter.

On the other hand, soft matter is a well studied field in condensed matter physics. We investigate the possibility of ``soft cosmology'', namely the appearance (intrinsically or effectively) of soft-matter properties in the dark sectors. We propose a novel parametrization introducing the...

We explore the ability of future cosmological surveys to put string theory under pressure through the Swampland program. It is well known that constructing consistent string theory solutions in a de Sitter background is tremendously difficult. This led to speculative constraints on the shape of the potential felt by the dark energy scalar field. This is known as the "de-Sitter conjecture" and...

Nowadays one of the greatest mysteries of science is to find out why the universe has an accelerated expansion. In this talk we show that by considering the quantum nature of the gravitational field, we can associate an effective Compton mass to the wavelength of the graviton. The Compton Mass Dark Energy (CMaDE) model proposes that this mass can be interpreted as dark energy, with a

Compton...

As an alternative to the popular parametrisations of the dark energy equation of state, we construct a quintessence model where the scalar field has a linear dependence on the number of e-folds. Constraints on more complex models are typically limited by the degeneracies that increase with the number of parameters. The proposed parametrisation conveniently constrains the dark energy...

We present a phenomenological but thorough analysis of current observational constraints on classes of FLRW cosmological models in which the matter side of Einstein's equations includes, in addition to the canonical term, a term proportional to some function of the energy-momentum tensor ($T^2\equiv T_{\alpha\beta}T^{\alpha\beta}=\rho^2+3p^2$), or of its trace ($T=\rho-3p$). Qualitatively, we...

Baryon Acoustic Oscillations (BAO) involve measuring the spatial distribution of galaxies to determine the growth rate of cosmic structure. We derive constraints on cosmological parameters from 17 uncorrelated BAO measurements that were collected from 333 published data points in the effective redshift range 0.106≤z≤2.36. We test the correlation of the subset using random covariance matrix....

We investigate the influence of the chimney topology TxTxR of the Universe on the

gravitational potential and force that are generated by point-like massive bodies. We obtain three distinct expressions for the solutions. One follows from Fourier expansion of delta functions into series using periodicity in two toroidal dimensions. The second one is the summation of solutions of the Helmholtz...

The Hubble constant (H0) tension between Type Ia Supernovae (SNe Ia) and Planck measurements ranges from 4 to 6 σ. To investigate it, we estimate H0 in the ΛCDM and w0waCDM models by dividing the Pantheon sample, a collection of 1048 SNe Ia, into 3, 4, 20, and 40 bins. A preliminary consistency check is performed, considering the compatibility of contours for 3 and 4 bins with the ones of the...

In this talk we focus on weak inhomogeneous Universe models at low redshift, described by the Lemaître-Tolman-Bondi (LTB) metric within the framework of f(R) modified gravity theories. The principal aim of this study was to compare the evolution of inhomogeneous perturbations in the ΛCDM and alternative f(R) cosmological models, assuming a flat Friedmann-Lemaître-Robertson-Walker (FLRW) metric...