The scale of cosmic isotropy

The most fundamental premise to the standard model of the universe states that the large-scale properties of the universe are the same in all directions and at all comoving positions. Demonstrating this hypothesis has proven to be a formidable challenge. The cross-over scale R-iso above which the galaxy distribution becomes statistically isotropic is vaguely defined and poorly (if not at all) quantified. Here we report on a formalism that allows us to provide an unambiguous operational definition and an estimate of R-iso. We apply the method to galaxies in the Sloan Digital Sky Survey (SDSS) Data Release 7, finding that R-iso similar to 150h(-1)Mpc. Besides providing a consistency test of the Copernican principle, this result is in agreement with predictions based on numerical simulations of the spatial distribution of galaxies in cold dark matter dominated cosmological models.