Modified Averaging Processes in Cosmology and the Structured FRW model

TL;DR

This paper examines the limitations of volume averaging in inhomogeneous cosmology, proposes modifications, and finds that backreaction effects mimic dark matter rather than dark energy, with gauge dependence.

Contribution

It introduces modifications to averaging methods in cosmology and demonstrates that backreaction effects resemble dark matter, challenging previous claims.

Findings

Backreaction behaves like dark matter, not dark energy.

Backreaction density is about 10^{-5} of matter density.

Averaging results are gauge dependent.

Abstract

We study the volume averaging of inhomogeneous metrics within GR and discuss its shortcomings such as gauge dependence, singular behavior as a result of caustics, and causality violations. To remedy these shortcomings, we suggest some modifications to this method. As a case study we focus on the inhomogeneous structured FRW model based on a flat LTB metric. The effect of averaging is then studied in terms of an effective backreaction fluid. It is shown that, contrary to the claims in the literature, the backreaction fluid behaves like a dark matter component, instead of dark energy, having a density of the order of 10^{-5} times the matter density, and most importantly, it is gauge dependent.