Ultra-compact radio sources and the isotropy and homogeneity of the Universe

TL;DR

This study uses a large sample of ultra-compact radio sources to test the isotropy of the Universe, finding significant anisotropy that suggests the Universe may not be spatially homogeneous on the largest scales.

Contribution

It introduces a novel analysis of ultra-compact radio sources to assess cosmic isotropy, revealing anisotropic features aligned with the CMB dipole.

Findings

Significant anisotropy in Omega_m across the sky.

The anisotropy axis is close to the CMB dipole axis.

Results suggest a non-homogeneous, centrally dense universe model.

Abstract

A 2.29 GHz VLBI all-sky survey of ultra-compact radio sources has formed the basis of a number of cosmological investigations, which examine the relationship between angular-size and redshift. Here I use a sample of 468 such sources with 0.5<z<=3.787, to investigate the isotropy of the Universe. The sample is divided into hemispherical sub-samples, over an all-sky 5 degree x 5 degree array, each of which is allowed to determine a value of Omega_m, assuming that we are living in a spatially flat homogeneous isotropic LambdaCDM model. If we regard the latter as a null hypothesis, then it fails the test -- the results show significant anisotropy, the smallest value of Omega_m being towards (l,b)=(253.9,24.1) degrees, the largest in the opposite direction. This is close to the CMB dipole axis, but in the obverse sense. This is interpreted as meaning that the Universe is not spatially homogeneous on the largest scales, and is better represented at late times by a spherically symmetric model with a density enhancement at its centre.