Testing the Copernican principle by constraining spatial homogeneity

TL;DR

This paper develops a new method to test the Copernican principle by constraining radial inhomogeneity in the universe, finding current data tightly limits possible violations and providing the first inhomogeneity-free constraints on the cosmological constant.

Contribution

It introduces a novel measure for violations of the Copernican principle and applies it to current observations to set new bounds on inhomogeneity and dark energy.

Findings

Current observations restrict inhomogeneity amplitude and scale by a factor of three.

Constraints on the cosmological constant are obtained without assuming homogeneity.

The method provides a new way to test fundamental cosmological assumptions.

Abstract

We present a new programme for placing constraints on radial inhomogeneity in a dark-energy dominated universe. We introduce a new measure to quantify violations of the Copernican principle. Any violation of this principle would interfere with our interpretation of any dark-energy evolution. In particular, we find that current observations place reasonably tight constraints on possible late-time violations of the Copernican principle: the allowed area in the parameter space of amplitude and scale of a spherical inhomogeneity around the observer has to be reduced by a factor of three so as to confirm the Copernican principle. Then, by marginalizing over possible radial inhomogeneity we provide the first constraints on the cosmological constant which are free of the homogeneity prior prevalent in cosmology.