On the viability of the truncated Israel-Stewart theory in cosmology

TL;DR

This paper examines the differences between the full and truncated Israel-Stewart theories in cosmological models, revealing that the truncated version can produce unphysical solutions and that shear viscosity can drive the system far from equilibrium.

Contribution

It provides a comparative analysis of the full and truncated Israel-Stewart theories in anisotropic cosmological models, highlighting potential issues with the truncated approach.

Findings

Truncated Israel-Stewart theory yields solutions different from the full theory.

Truncated theory can produce unphysical properties.

Shear viscosity can significantly drive the system away from equilibrium.

Abstract

We apply the causal Israel-Stewart theory of irreversible thermodynamics to model the matter content of the universe as a dissipative fluid with bulk and shear viscosity. Along with the full transport equations we consider their widely used truncated version. By implementing a dynamical systems approach to Bianchi type IV and V cosmological models with and without cosmological constant, we determine the future asymptotic states of such universes and show that the truncated Israel-Stewart theory leads to solutions essentially different from the full theory. The solutions of the truncated theory may also manifest unphysical properties. Finally, we find that in the full theory shear viscosity can give a substantial rise to dissipative fluxes, driving the fluid extremely far from equilibrium, where the linear Israel-Stewart theory ceases to be valid.