  The standard Friedmann model of cosmology is based on the Copernican
Principle, i.e. the assumption of a homogeneous background on which structure
forms via perturbations. Homogeneity underpins both general relativistic and
modified gravity models and is central to the way in which we interpret
observations of the CMB and the galaxy distribution. It is therefore important
to probe homogeneity via observations. We describe a test based on the fossil
record of distant galaxies: if we can reconstruct key intrinsic properties of
galaxies as functions of proper time along their worldlines, we can compare
such properties at the same proper time for our galaxy and others. We achieve
this by computing the lookback time using radial Baryon Acoustic Oscillations,
and the time along galaxy world line using stellar physics, allowing us to
probe homogeneity, in principle anywhere inside the past light cone. Agreement
in the results would be an important consistency test -- although it would not
in itself prove homogeneity. Any significant deviation in the results however
would signal a breakdown of homogeneity.
