Evolution of a universe filled with a causal viscous fluid

TL;DR

This paper investigates the dynamics of a universe filled with a causal viscous fluid, deriving exact solutions and analyzing stability, revealing how relaxation processes influence the universe's past evolution and energy condition violations.

Contribution

It provides new exact solutions for Einstein equations with causal viscous fluids and analyzes their stability and impact on cosmic evolution.

Findings

Relaxation processes alter the universe's past behavior.

Solutions can violate energy conditions, leading to bounces.

Asymptotic future behavior remains unaffected by viscosity.

Abstract

The behaviour of solutions to the Einstein equations with a causal viscous fluid source is investigated. In this model we consider a spatially flat Robertson-Walker metric, the bulk viscosity coefficient is related to the energy density as $\zeta = \alpha \rho^{m}$, and the relaxation time is given by $\zeta/\rho$. In the case $m = 1/2$ we find the exact solutions and we verify whether they satisfy the energy conditions. Besides, we study analytically the asymptotic stability of several families of solutions for arbitrary $m$. We find that the qualitative asymptotic behaviour in the far future is not altered by relaxation processes, but they change the behaviour in the past, introducing singular instead of deflationary evolutions or making the Universe bounce due to the violation of the energy conditions.