LTB solutions in Newtonian gauge: from strong to weak fields

TL;DR

This paper demonstrates that LTB solutions can be accurately represented in Newtonian gauge, showing that nonlinear effects in inhomogeneous cosmologies are gauge artefacts and validating the Newtonian approximation in nonlinear regimes.

Contribution

It provides explicit gauge transformations from synchronous to Newtonian gauge for LTB solutions, clarifies the gauge artefact nature of large deviations, and discusses implications for backreaction.

Findings

LTB solutions can be expressed in Newtonian gauge as weakly perturbed FLRW metrics.

The Newtonian approximation remains valid even in nonlinear regimes.

Gauge artefacts explain large deviations in synchronous gauge for realistic inhomogeneities.

Abstract

Lemaitre-Tolman-Bondi (LTB) solutions are used frequently to describe the collapse or expansion of spherically symmetric inhomogeneous mass distributions in the Universe. These exact solutions are obtained in the synchronous gauge where nonlinear dynamics (with respect to the FLRW background) induce large deviations from the FLRW metric. In this paper we show explicitly that this is a gauge artefact (for realistic sub-horizon inhomogeneities). We write down the nonlinear gauge transformation from synchronous to Newtonian gauge for a general LTB solution using the fact that the peculiar velocities are small. In the latter gauge we recover the solution in the form of a weakly perturbed FLRW metric that is assumed in standard cosmology. Furthermore we show how to obtain the LTB solutions directly in Newtonian gauge and illustrate how the Newtonian approximation remains valid in the nonlinear regime where cosmological perturbation theory breaks down. Finally we discuss the implications of our results for the backreaction scenario.