Comments on gauge-invariance in cosmology

TL;DR

This paper examines the gauge-invariance problem in cosmological perturbation theory, exploring its connection to covariance in general relativity and comparing covariant and metric approaches.

Contribution

It provides a comparative analysis of covariant and metric approaches to gauge-invariance in cosmology, clarifying their similarities and differences.

Findings

Highlights the relation between gauge-invariance and covariance in GR

Compares covariant and metric perturbation methods

Clarifies conceptual differences in gauge treatments

Abstract

We revisit the gauge issue in cosmological perturbation theory, and highlight its relation to the notion of covariance in general relativity. We also discuss the similarities and differences of the covariant approach in perturbation theory to the Bardeen or metric approach in a non-technical fashion.