Spherical collapse in chameleon models

Abstract

We study the gravitational collapse of an overdensity of nonrelativistic matter under theaction of gravity and a chameleon scalar field. We show that the spherical collapse model is modified bythe 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 inhomogeneitythe collapsing region possesses a thin shell that shields the modification of gravity induced by thechameleon field, recovering the standard gravity results. We analysethe behaviour of a collapsingshell in a cosmological setting in the presence of a thin shell and find that, in contrast to the usual case, the critical density forcollapse in principle depends on the initial comoving size of the inhomogeneity.