Nonlinear density evolution from an improved spherical collapse model

TL;DR

This paper develops an improved spherical collapse model incorporating deviations from spherical symmetry, using a Taylor series expansion to better understand non-linear density evolution and structure formation.

Contribution

It introduces a physically motivated closure condition and a Taylor series approach to model non-linear density evolution beyond the standard spherical collapse model.

Findings

Stable structures form with collapse halted near the virial radius.

The model connects individual overdense regions with non-linear scaling relations.

The modified equations effectively describe the evolution of density perturbations over a large range of δ.

Abstract

We investigate the evolution of non-linear density perturbations by taking into account the effects of deviations from spherical symmetry of a system. Starting from the standard spherical top hat model in which these effects are ignored, we introduce a physically motivated closure condition which specifies the dependence of the additional terms on the density contrast, $\delta$. The modified equation can be used to model the behaviour of an overdense region over a sufficiently large range of $\delta$. The key new idea is a Taylor series expansion in ($1/\delta$) to model the non-linear epoch. We show that the modified equations quite generically lead to the formation of stable structures in which the gravitational collapse is halted at around the virial radius. The analysis also allows us to connect up the behaviour of individual overdense regions with the non-linear scaling relations satisfied by the two point correlation function.