Spherical collapse in modified gravity with the Birkhoff-theorem

TL;DR

This paper investigates how a modified gravity model affects structure formation, showing that it lowers the spherical collapse threshold and enhances galaxy cluster formation, with implications for observational constraints.

Contribution

It introduces a generalized spherical collapse model in modified gravity using the Birkhoff-theorem, providing new insights into cluster formation and observational signatures.

Findings

Lowered delta_c(z) compared to LambdaCDM

Enhanced cluster number densities and merging rates

Potential to distinguish models with SZ surveys

Abstract

We study structure formation in a phenomenological model of modified gravity which interpolates between LambdaCDM and phenomenological DGP-gravity. Generalisation of spherical collapse by using the Birkhoff-theorem along with the modified growth equation shows that the overdensity for spherical collapse delta_c in these models is significantly lowered compared to LambdaCDM, leading to enhanced number densities of massive clusters and enhanced cluster merging rates. We find that delta_c(z) is well fitted by a function of the form delta_c(z) = a - b\exp(-cz). We examine the sensitivity of PLANCK's and SPT's Sunyaev-Zel'dovich survey to constrain the modified gravity parameterisation and find that these experiments can easily distinguish between models with a cosmological constant and modified gravity, if prior constraints from CMB temperature and polarisation anisotropies are included.