A general test of the Copernican Principle

Abstract

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.