Spherical Collapse in Chameleon Models

TL;DR

This paper investigates how a chameleon scalar field influences the gravitational collapse of matter overdensities, revealing that thin shell effects can shield modifications to gravity and alter collapse thresholds.

Contribution

It introduces a modified spherical collapse model accounting for chameleon fields and analyzes the impact of thin shells on gravitational collapse in cosmology.

Findings

Chameleon fields can significantly modify gravitational collapse at small scales.

Thin shell effects can shield the influence of the chameleon field, restoring standard gravity.

The critical density for collapse depends on the initial size of the inhomogeneity.

Abstract

We study the gravitational collapse of an overdensity of nonrelativistic matter under the action of gravity and a chameleon scalar field. We show that the spherical collapse model is modified by the presence of a chameleon field. In particular, we find that even though the chameleon effects can be potentially large at small scales, for a large enough initial size of the inhomogeneity the collapsing region possesses a thin shell that shields the modification of gravity induced by the chameleon field, recovering the standard gravity results. We analyse the behaviour of a collapsing shell in a cosmological setting in the presence of a thin shell and find that, in contrast to the usual case, the critical density for collapse depends on the initial comoving size of the inhomogeneity.