Probing the anisotropic local Universe and beyond with SNeIa data

Abstract

The question of the transition to global isotropy from our anisotropic local universe is studied using the Union 2 catalogue of Type Ia supernovae (SNeIa). We construct a 'residual' statistic sensitive to systematic shifts in their brightness in different directions and use this to search in different redshift slices for a preferred direction on the sky in which the SNeIa are brighter or fainter relative to the standard Λcold dark matter (ΛCDM) cosmology. At low redshift (z < 0.05), we find that an isotropic model such as ΛCDM is barely consistent with the SNeIa data at 2σ-3σ. A maximum-likelihood analysis of peculiar velocities confirms this finding - there is a bulk flow of 260 kms-1 extending out to z~ 0.06, which disagrees with ΛCDM at 1σ-2σ. Since the Shapley concentration is believed to be largely responsible for this bulk flow, we make a detailed study of the infall region: the SNeIa falling away from the Local Group towards Shapley are indeed significantly dimmer than those falling towards us on to Shapley. Convergence to the CMB rest frame must occur well beyond Shapley (z > 0.06) so this low-redshift bulk flow will systematically bias any reconstruction of the expansion history of the Universe. At higher redshifts z > 0.15 the agreement between the SNeIa data and the ΛCDM model does improve, however, the sparseness and low quality of the data mean that the latter cannot be singled out as the preferred cosmological model. © 2011 The Authors Monthly Notices of the Royal Astronomical Society © 2011 RAS.