Bulk viscous cosmology with causal transport theory

TL;DR

This paper explores whether a single bulk viscous fluid with causal transport theories can unify dark matter and dark energy, analyzing its dynamical behavior and comparing it with the standard LambdaCDM model.

Contribution

It investigates the effects of causal versus non-causal viscous fluid theories on cosmological dynamics and identifies parameter ranges where the models resemble LambdaCDM.

Findings

Eckart's and full causal theories are disfavoured by data.

Truncated causal theory yields results similar to LambdaCDM.

Bulk viscous speed squared c_b^2 between 10^{-11} and 10^{-8} fits observations.

Abstract

We consider cosmological scenarios originating from a single imperfect fluid with bulk viscosity and apply Eckart's and both the full and the truncated M\"uller-Israel-Stewart's theories as descriptions of the non-equilibrium processes. Our principal objective is to investigate if the dynamical properties of Dark Matter and Dark Energy can be described by a single viscous fluid and how such description changes when a causal theory (M\"uller-Israel-Stewart's, both in its full and truncated forms) is taken into account instead of Eckart's non-causal theory. To this purpose, we find numerical solutions for the gravitational potential and compare its behaviour with the corresponding LambdaCDM case. Eckart's and the full causal theory seem to be disfavoured, whereas the truncated theory leads to results similar to those of the LambdaCDM model for a bulk viscous speed in the interval 10^{-11} << c_b^2 < 10^{-8}. Tentatively relating such value to a square propagation velocity of the order of T/m of perturbations in a non-relativistic gas of particles with mass m at the epoch of matter-radiation equality, this may be compatible with a mass range 0.1 GeV < m << 100 GeV.