Cosmological Backreaction from Perturbations

TL;DR

This paper reformulates the averaged Einstein equations for linear perturbations in Newtonian gauge, quantifies their effects on cosmological evolution, and finds these effects are very small, around 10^-5 of the matter density, across different cosmologies.

Contribution

It introduces a new formulation of averaged Einstein equations suitable for linear perturbation theory and quantifies their minimal impact on large-scale cosmological evolution.

Findings

Effective energy density from linear perturbations is about 10^-5 of matter density.

Deviations from standard cosmology remain small, around 10^-5, even at quasilinear scales.

The effective equation of state from perturbations is approximately -1/19.

Abstract

We reformulate the averaged Einstein equations in a form suitable for use with Newtonian gauge linear perturbation theory and track the size of the modifications to standard Robertson-Walker evolution on the largest scales as a function of redshift for both Einstein de-Sitter and Lambda CDM cosmologies. In both cases the effective energy density arising from linear perturbations is of the order of 10^-5 the matter density, as would be expected, with an effective equation of state w ~ -1/19. Employing a modified Halofit code to extend our results to quasilinear scales, we find that, while larger, the deviations from Robertson-Walker behaviour remain of the order of 10^-5.