Do we need soft cosmology?

TL;DR

This paper explores the concept of 'soft cosmology', where small deviations in the properties of dark sectors could influence cosmic evolution and structure formation, potentially addressing current tensions in cosmological data.

Contribution

It introduces the idea of softness parameters for dark sectors, showing how slight deviations can impact clustering and potentially resolve existing cosmological tensions.

Findings

Softness parameters can improve clustering behavior.

Small deviations can alleviate the $f\sigma_8$ tension.

Background evolution remains unaffected by softness.

Abstract

We examine the possibility of "soft cosmology", namely small deviations from the usual framework due to the effective appearance of soft-matter properties in the Universe sectors. One effect of such a case would be the dark energy to exhibit a different equation-of-state parameter at large scales (which determine the universe expansion) and at intermediate scales (which determine the sub-horizon clustering and the large scale structure formation). Concerning soft dark matter, we show that it can effectively arise due to the dark-energy clustering, even if dark energy is not soft. We propose a novel parametrization introducing the "softness parameters" of the dark sectors. As we see, although the background evolution remains unaffected, due to the extreme sensitivity and significant effects on the global properties even a slightly non-trivial softness parameter can improve the clustering behavior and alleviate e.g. the $f\sigma_8$ tension. Lastly, an extension of the cosmological perturbation theory and a detailed statistical mechanical analysis, in order to incorporate complexity and estimate the scale-dependent behavior from first principles, is necessary and would provide a robust argumentation in favour of soft cosmology.