Testing cosmic homogeneity and isotropy using galaxy correlations

TL;DR

This paper presents a straightforward method to test the cosmological principle by comparing galaxy distributions in equal volumes at the same redshift, finding consistency with isotropy and homogeneity within 5%.

Contribution

It introduces a new direct comparison method of galaxy correlations at fixed redshift to test cosmic homogeneity and isotropy, reducing systematic biases present in previous approaches.

Findings

Galaxy correlation radii are consistent within 5% across regions separated by 2 Gpc.

Galaxy number densities show no significant variation across different regions.

Results support the cosmological principle of homogeneity and isotropy.

Abstract

Despite its fundamental importance in cosmology, there have been very few straight-forward tests of the cosmological principle. Such tests are especially timely because of the hemispherical asymmetry in the cosmic microwave background recently observed by the Planck collaboration. Most tests to date looked at the redshift dependence of cosmological parameters. These are subject to large systematic effects that require modeling and bias corrections. Unlike previous tests, the tests described here compare galaxy distributions in equal volumes at the same redshift z. This allows a straight-forward test and z-dependent biases are not a problem. Using ~10^6 galaxies from the SDSS DR7 survey, I show that re- gions of space separated by ~2 Gpc have the same average galaxy correlation radii, amplitudes, and number density to within approx. 5%, which is consistent with standard model expectations.