A novel approach to cosmological non-linearities as an effective fluid

TL;DR

This paper introduces a two-parameter extension of the standard cosmological model that accounts for matter inhomogeneities through effective fluids, potentially resolving key tensions in cosmology.

Contribution

It proposes a novel effective fluid framework for matter inhomogeneities, extending the DM model and fitting observational data to address existing cosmological tensions.

Findings

The model can potentially resolve the Hubble tension.

It can address the  tension by incorporating backreaction effects.

Fits observational data well, supporting the model's viability.

Abstract

We propose a two parameters extension of the flat $\Lambda$CDM model to capture the impact of matter inhomogeneities on our cosmological inference. Non virialized but non-linearly evolving overdense and underdense regions, whose abundance is quantified using the Press-Schechter formalism, are collectively described by two effective perfect fluids $\rho_{\rm{c}},\rho_{\rm{v}}$ with non vanishing equation of state parameters $w_{\rm{c,v}}\neq 0$. These fluids are coupled to the pressureless dust, akin to an interacting DM-DE scenario. The resulting phenomenology is very rich, and could potentially address a number of inconsistencies of the standard model, including a simultaneous resolution of the Hubble and $\sigma_8$ tensions. To assess the viability of the model, we set initial conditions compatible to the Planck 2018 best fit $\Lambda$CDM cosmology and fit its additional parameters using SN~Ia observations from DESY5, BAO distances from DESI DR2 and a sample of uncorrelated $f\sigma_8$ measurements. Our findings show that backreaction effects from the cosmic web could restore the concordance between early and late Universe cosmological probes.