  The recent discovery of apparent cosmic acceleration has highlighted the
depth of our ignorance of the fundamental properties of nature. It is commonly
assumed that the explanation for acceleration must come from a new form of
energy dominating the cosmos - dark energy - or a modification of Einstein's
theory of Relativity. It is often overlooked, however, that a currently viable
alternative explanation of the data is radial inhomogeneity which alters the
Hubble diagram without any acceleration. This explanation is often ignored for
two reasons: radial inhomogeneity significantly complicates analysis and
predictions, and so the full details have not been investigated; and it is a
philosophically highly controversial idea, revoking as it does the long-held
Copernican Principle. To date, there has not been a general way of determining
the validity if the Copernican Principle -- that we live at a typical position
in the universe -- significantly weakening the foundations of cosmology as a
scientific endeavour. Here we present an observational test for the Copernican
assumption which can be automatically implemented while we search for dark
energy in the coming decade. Our test is entirely independent of any model for
dark energy or theory of gravity and thereby represents a model-independent
test of the Copernican Principle.
