Establishing homogeneity of the universe in the shadow of dark energy

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?

40 pages, 15 figures. Accepted review article to appear in a special volume of the "Comptes Rendus de l'Academie des Sciences" about Dark Energy and Dark Matter