CMB in the river frame and gauge invariance at second order

TL;DR

This paper proves the gauge invariance of the second-order Sachs-Wolfe formula for CMB anisotropies and introduces the river frame, a new cosmological frame that simplifies the analysis of perturbations and their geometric interpretation.

Contribution

It establishes the gauge invariance of the second-order CMB temperature anisotropy formula and introduces the river frame for clearer geometric analysis of cosmological perturbations.

Findings

Proved gauge invariance of second-order Sachs-Wolfe formula.

Introduced the river frame for cosmological analysis.

Provided compact expressions for gauge transformations.

Abstract

GAUGE INVARIANCE: The Sachs-Wolfe formula describing the Cosmic Microwave Background (CMB) temperature anisotropies is one of the most important relations in cosmology. Despite its importance, the gauge invariance of this formula has only been discussed at first order. Here we discuss the subtle issue of second-order gauge transformations on the CMB. By introducing two rules (needed to handle the subtle issues), we prove the gauge invariance of the second-order Sachs-Wolfe formula and provide several compact expressions which can be useful for the study of gauge transformations on cosmology. Our results go beyond a simple technicality: we discuss from a physical point of view several aspects that improve our understanding of the CMB. We also elucidate how crucial it is to understand gauge transformations on the CMB in order to avoid errors and/or misconceptions as occurred in the past. THE RIVER FRAME: we introduce a cosmological frame which we call the river frame. In this frame, photons and any object can be thought as fishes swimming in the river and relations are easily expressed in either the metric or the covariant formalism then ensuring a transparent geometric meaning. Finally, our results show that the river frame is useful to make perturbative and non-perturbative analysis. In particular, it was already used to obtain the fully nonlinear generalization of the Sachs-Wolfe formula and is used here to describe second-order perturbations.