  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?
