The cosmological backreaction: gauge (in)dependence, observers and scalars

TL;DR

This paper examines the gauge dependence of cosmological backreaction averages, proposing a gauge-invariant method to define observers and showing residual gauge effects at finite volumes are minimal.

Contribution

It introduces a gauge-invariant averaging procedure for cosmological backreaction and clarifies the role of observers associated with scalar fields.

Findings

Gauge dependence vanishes in large averaging volumes.

Different observers can be characterized by scalar fields.

Residual gauge dependence at finite volumes is suppressed.

Abstract

We discuss several issues related to a recent proposal for defining classical spatial averages to be used in the so-called cosmological backreaction problem. In the large averaging-volume limit all gauge dependence disappears and different averages can be univocally characterized by the observers associated with different scalar fields. The relation between such averaging procedure and the standard one is emphasized and a gauge invariant way to select different observers is presented. For finite averaging volumes we show that, within our proposal, a residual gauge dependence is left, but is suppressed by several effects.