# Cosmological Backreaction in Spherical and Plane Symmetric Dust-Filled Space-Times

## TL;DR

This paper compares Buchert's and Green & Wald's averaging formalisms in symmetric dust-filled cosmological models, finding Buchert's approach effectively captures large-scale expansion, while Green & Wald's formalism depends on background choice and shows negligible back-reaction effects.

## Contribution

It provides a detailed comparison of two averaging formalisms in symmetric cosmological models, highlighting Buchert's effectiveness and Green & Wald's limitations in assessing large-scale expansion.

## Key findings

- Buchert's averaging accurately reflects large-scale expansion.
- Green & Wald's formalism depends on background choice.
- Back-reaction effects are negligible in studied models.

## Abstract

We examine the implementation of Buchert's and Green & Wald's averaging formalisms in exact spherically symmetric and plane symmetric dust-filled cosmological models. We find that, given a cosmological space-time, Buchert's averaging scheme gives a faithful way of interpreting the large-scale expansion of space, and explicit terms that precisely quantify deviations from the behaviour expected from the Friedmann equations of homogeneous and isotropic cosmological models. The Green & Wald formalism, on the other hand, does not appear to yield any information about the large-scale properties of a given inhomogeneous space-time. Instead, this formalism is designed to calculate the back-reaction effects of short-wavelength fluctuations around a given "background" geometry. We find that the inferred expansion of space in this approach is entirely dependent on the choice of this background, which is not uniquely specified for any given inhomogeneous space-time, and that the "back-reaction" from small-scale structures vanishes in every case we study. This would appear to limit the applicability of Green & Wald's formalism to the study of large-scale expansion in the real Universe, which also has no pre-defined background. Further study is required to enhance the evaluation and comparison of these averaging formalisms, and determine whether the same difficulties exist, in less idealized space-time geometries.