Spherical Collapse and Cluster Counts in Modified Gravity Models

TL;DR

This paper investigates how modifications to gravity, specifically Yukawa-type potentials, influence the formation and abundance of cosmic structures, providing insights into constraints on alternative gravity models through cluster counts.

Contribution

It introduces a method to estimate cluster abundances in modified gravity models without assuming Birkhoff's theorem, and compares predictions with simulations to understand structure formation.

Findings

Weaker large-scale gravity results in fewer massive halos when normalized to early fluctuations.

Cluster abundance constraints can differentiate between gravity models normalized to the same present-day power spectrum.

Low-mass object formation remains unaffected by modifications to gravity.

Abstract

Modifications to the gravitational potential affect the nonlinear gravitational evolution of large scale structures in the Universe. To illustrate some generic features of such changes, we study the evolution of spherically symmetric perturbations when the modification is of Yukawa type; this is non-trivial, because we should not and do not assume that Birkhoff's theorem applies. We then show how to estimate the abundance of virialized objects in such models. Comparison with numerical simulations shows reasonable agreement: When normalized to have the same fluctuations at early times, weaker large scale gravity produces fewer massive halos. However, the opposite can be true for models that are normalized to have the same linear theory power spectrum today, so the abundance of rich clusters potentially places interesting constraints on such models. Our analysis also indicates that the formation histories and abundances of sufficiently low mass objects are unchanged from standard gravity. This explains why simulations have found that the nonlinear power-spectrum at large k is unaffected by such modifications to the gravitational potential. In addition, the most massive objects in CMB-normalized models with weaker gravity are expected to be similar to the high-redshift progenitors of the most massive objects in models with stronger gravity. Thus, the difference between the cluster and field galaxy populations is expected to be larger in models with stronger large-scale gravity.