Dark matter perturbations and viscosity: a causal approach

TL;DR

This paper investigates how causal bulk viscosity affects dark matter density perturbations and structure formation, revealing conditions under which clustering can be enhanced or resemble standard cosmological models.

Contribution

It introduces a causal Israel-Stewart framework for dark matter viscosity, deriving a third-order evolution equation and exploring its impact on cosmic structure growth.

Findings

Certain parameters cause the density contrast to mimic $ m extLambda$CDM at high redshift.

Some parameter sets lead to increased clustering contrary to expectations.

The causal approach differs significantly from non-causal theories in perturbation evolution.

Abstract

The inclusion of dissipative effects in cosmic fluids modifies their clustering properties and could have observable effects on the formation of large scale structures. We analyse the evolution of density perturbations of cold dark matter endowed with causal bulk viscosity. The perturbative analysis is carried out in the Newtonian approximation and the bulk viscosity is described by the causal Israel-Stewart (IS) theory. In contrast to the non-causal Eckart theory, we obtain a third order evolution equation for the density contrast that depends on three free parameters. For certain parameter values, the density contrast and growth factor in IS mimic their behaviour in $\Lambda$CDM when $z \geq 1$. Interestingly, and contrary to intuition, certain sets of parameters lead to an increase of the clustering.