Galaxy correlations and the BAO in a void universe: structure formation as a test of the Copernican Principle

TL;DR

This paper investigates how large-scale inhomogeneities in void universe models distort galaxy correlation functions, providing a potential observational test to challenge the Copernican Principle and distinguish these models from standard cosmology.

Contribution

It computes the radial and transverse galaxy correlation functions in void models, revealing distinctive features that can test the Copernican Principle.

Findings

Radial and transverse correlation functions differ significantly from the concordance model.

Void models can be distinguished from standard cosmology using correlation function features.

These features persist even when assuming the same average BAO scale.

Abstract

A suggested solution to the dark energy problem is the void model, where accelerated expansion is replaced by Hubble-scale inhomogeneity. In these models, density perturbations grow on a radially inhomogeneous background. This large scale inhomogeneity distorts the spherical Baryon Acoustic Oscillation feature into an ellipsoid which implies that the bump in the galaxy correlation function occurs at different scales in the radial and transverse correlation functions. We compute these for the first time, under the approximation that curvature gradients do not couple the scalar modes to vector and tensor modes. The radial and transverse correlation functions are very different from those of the concordance model, even when the models have the same average BAO scale. This implies that if void models are fine-tuned to satisfy average BAO data, there is enough extra information in the correlation functions to distinguish a void model from the concordance model. We expect these new features to remain when the full perturbation equations are solved, which means that the radial and transverse galaxy correlation functions can be used as a powerful test of the Copernican Principle.