Averaging anisotropic cosmologies

TL;DR

This paper explores how spatial inhomogeneities influence anisotropic cosmological models by averaging their properties, revealing potential effects on singularities and cosmic acceleration within the framework of general relativity.

Contribution

It introduces a novel approach to averaging anisotropic cosmologies using the Buchert scheme, including a propagation formula for shear, and analyzes backreaction effects on singularities and acceleration.

Findings

Backreaction can alter Kasner-like singularities.

Nonzero average shear constrains accelerated expansion.

Backreaction effects may suppress Mixmaster oscillations.

Abstract

We examine the effects of spatial inhomogeneities on irrotational anisotropic cosmologies by looking at the average properties of anisotropic pressure-free models. Adopting the Buchert scheme, we recast the averaged scalar equations in Bianchi-type form and close the standard system by introducing a propagation formula for the average shear magnitude. We then investigate the evolution of anisotropic average vacuum models and those filled with pressureless matter. In the latter case we show that the backreaction effects can modify the familiar Kasner-like singularity and potentially remove Mixmaster-type oscillations. The presence of nonzero average shear in our equations also allows us to examine the constraints that a phase of backreaction-driven accelerated expansion might put on the anisotropy of the averaged domain. We close by assessing the status of these and other attempts to define and calculate `average' spacetime behaviour in general relativity.