Cosmic no-hair theorems for viscous contracting Universes

TL;DR

This paper proves that all initially contracting, homogeneous Bianchi Universes with shear viscosity tend to a flat, isotropic state, revealing a new isotropisation mechanism independent of ekpyrotic scenarios.

Contribution

It introduces a cosmic no-hair theorem for viscous contracting Universes, demonstrating a novel isotropisation process without relying on ultra-stiff equations of state.

Findings

All such Universes asymptote to flat, isotropic states

Shear viscosity induces isotropisation in contracting Universes

Provides a new mechanism distinct from ekpyrotic models

Abstract

A cosmic no-hair theorem for all initially contracting, spatially homogeneous, orthogonal Bianchi Cosmologies is derived - which shows that all such Universes asymptote to a spatially flat, isotropic Universe with the inclusion of a shear viscous stress. This establishes a new mechanism of isotropisation in a contracting Universe, which does not take recourse to an ekpyrosis-like mechanism using an effective ultra-stiff equation of state fluid, that is, one in which the pressure is much greater than the energy density.