Mass function of galaxy clusters in relativistic inhomogeneous cosmology

TL;DR

This paper explores how averaging Einstein's equations in inhomogeneous cosmology affects the predicted mass function of galaxy clusters, addressing the backreaction problem in relativistic cosmology.

Contribution

It introduces a method for averaging scalar Einstein equations and applies it to derive the galaxy cluster mass function in relativistic inhomogeneous cosmology.

Findings

Demonstrates the impact of inhomogeneities on cluster mass predictions

Provides a new approach to account for backreaction effects

Suggests modifications to standard cosmological mass functions

Abstract

The current cosmological model ($\Lambda$CDM) with the underlying FLRW metric relies on the assumption of local isotropy, hence homogeneity of the Universe. Difficulties arise when one attempts to justify this model as an average description of the Universe from first principles of general relativity, since in general, the Einstein tensor built from the averaged metric is not equal to the averaged stress--energy tensor. In this context, the discrepancy between these quantities is called "cosmological backreaction" and has been the subject of scientific debate among cosmologists and relativists for more than $20$ years. Here we present one of the methods to tackle this problem, i.e. averaging the scalar parts of the Einstein equations, together with its application, the cosmological mass function of galaxy clusters.