Viable Gauge Choices in Cosmologies with Non-Linear Structures

TL;DR

This paper investigates gauge choices in cosmological models with non-linear structures, revealing that many common gauges are unviable in such regimes, while longitudinal and Newtonian motion gauges remain suitable.

Contribution

It demonstrates the limitations of traditional gauges in non-linear cosmology and identifies viable gauge choices for modeling large density contrasts.

Findings

Most standard gauges are unviable with non-linear structures.

Longitudinal and Newtonian motion gauges are viable in both linear and non-linear regimes.

Guidelines for gauge selection in complex cosmological simulations.

Abstract

A variety of gauges are used in cosmological perturbation theory. These are often chosen in order to attribute physical properties to a particular choice of coordinates, or otherwise to simplify the form of the resultant equations. Calculations are then performed with the understanding that they could have been done in any gauge, and that transformations between different gauges can be made at will. We show that this logic can be extended to the domain of large density contrasts, where different types of perturbative expansion are required, but that the way in which gauges can be chosen in the presence of such structures is severely constrained. In particular, most gauges that are commonly considered in the cosmology literature are found to be unviable in the presence of non-linear structures. This includes spatially flat gauge, synchronous gauge, comoving orthogonal gauge, total matter gauge, N-body gauge, and the uniform density gauge. In contrast, we find that the longitudinal gauge and the Newtonian motion gauge are both viable choices in both standard cosmological perturbation theory, and in the post-Newtonian perturbative expansions that are required in order to model non-linear structures.