Accelerated and decelerated expansion in a causal dissipative cosmology

TL;DR

This paper investigates a cosmological model with a dissipative fluid using Israel-Stewart theory, revealing conditions for accelerated or decelerated expansion, thermodynamic constraints, and stability of solutions.

Contribution

It introduces a new exact solution for a dissipative universe with a barotropic EoS within the full causal thermodynamic framework, analyzing its physical and thermodynamic properties.

Findings

Accelerated expansion driven by viscosity can mimic quintessence behavior.

Decelerated solutions satisfy thermodynamic growth and convexity conditions.

Dissipative effects are negligible near stiff matter EoS, leading to constant entropy.

Abstract

In this work we explore a new cosmological solution for an universe filled with one dissipative fluid, described by a barotropic EoS $p = \omega \rho$, in the framework of the full Israel-Stewart theory. The form of the bulk viscosity has been assumed of the form $\xi = \xi_{0}\rho^{1/2}$. The relaxation time is taken to be a function of the EoS, the bulk viscosity and the speed of bulk viscous perturbations, $c_{b}$. The solution presents an initial singularity, where the curvature scalar diverges as the scale factor goes to zero. Depending on the values for $\omega$, $\xi_{0}$, $c_{b}$ accelerated and decelerated cosmic expansion can be obtained. In the case of accelerated expansion, the viscosity drives the effective EoS to be of quintessence type, for the single fluid with positive pressure. Nevertheless, we show that only the solution with decelerated expansion satisfies the thermodynamics conditions $dS/dt > 0$ (growth of the entropy) and $d^{2}S/dt^{2} < 0$ (convexity condition). We show that an exact stiff matter EoS is not allowed in the framework of the full causal thermodynamic approach; and in the case of a EoS very close to the stiff matter regime, we found that dissipative effects becomes negligible so the entropy remains constant. Finally, we show numerically that the solution is stable under small perturbations.