Bulk Viscous Matter-dominated Universes: Asymptotic Properties

TL;DR

This study uses observational data and dynamical systems analysis to evaluate bulk viscous matter models, concluding they are incompatible with the standard cosmological paradigm due to their phase space structure and thermodynamic inconsistencies.

Contribution

It provides a comprehensive dynamical systems analysis of bulk viscous matter models, demonstrating their failure to reproduce standard cosmological evolution stages.

Findings

Bulk viscous models lack radiation and matter-dominated critical points.

Such models require unstable initial conditions to mimic standard cosmology.

Negative bulk viscosity at early times conflicts with thermodynamics.

Abstract

By means of a combined use of the type Ia supernovae and H(z) data tests, together with the study of the asymptotic properties in the equivalent phase space - through the use of the dynamical systems tools - we demonstrate that the bulk viscous matter-dominated scenario is not a good model to explain the accepted cosmological paradigm, at least, under the parametrization of bulk viscosity considered in this paper. The main objection against such scenarios is the absence of conventional radiation and matter-dominated critical points in the phase space of the model. This entails that radiation and matter dominance are not generic solutions of the cosmological equations, so that these stages can be implemented only by means of unique and very specific initial conditions, i. e., of very unstable particular solutions. Such a behavior is in marked contradiction with the accepted cosmological paradigm which requires of an earlier stage dominated by relativistic species, followed by a period of conventional non-relativistic matter domination, during which the cosmic structure we see was formed. Also, we found that the bulk viscosity is positive just until very late times in the cosmic evolution, around z < 1. For earlier epochs it is negative, been in tension with the local second law of thermodynamics.