Establishing homogeneity of the universe in the shadow of dark energy

TL;DR

This paper discusses the importance of empirically testing the assumption of universe homogeneity and the Copernican principle, especially in light of alternative models that challenge dark energy explanations.

Contribution

It highlights the need for independent tests of the universe's homogeneity, questioning the foundational assumption in cosmology beyond standard models.

Findings

Radial inhomogeneity constraints are weak.

Alternative models challenge the Copernican principle.

Testing the Copernican principle is crucial for cosmology.

Abstract

Assuming the universe is spatially homogeneous on the largest scales lays the foundation for almost all cosmology. This idea is based on the Copernican principle, that we are not at a particularly special place in the universe. Surprisingly, this philosophical assumption has yet to be rigorously demonstrated independently of the standard paradigm. This issue has been brought to light by cosmological models which can potentially explain apparent acceleration by spatial inhomogeneity rather than dark energy. These models replace the temporal fine tuning associated with Lambda with a spatial fine tuning, and so violate the Copernican assumption. While is seems unlikely that such models can really give a realistic solution to the dark energy problem, they do reveal how poorly constrained radial inhomogeneity actually is. So the bigger issue remains: How do we robustly test the Copernican principle independently of dark energy or theory of gravity?