Probing the anisotropic local universe and beyond with SNe Ia data

TL;DR

This study investigates local anisotropies in the universe using Type Ia supernovae data, revealing significant deviations from isotropy at low redshifts and implications for cosmic flow and expansion history reconstructions.

Contribution

It introduces a residual statistic sensitive to directional brightness shifts in SNe Ia and analyzes their anisotropy, providing new insights into local universe dynamics and their impact on cosmological models.

Findings

Low redshift isotropy is barely consistent with LCDM at 2-3 sigma.

Bulk flow of ~260 km/sec at z~0.06 conflicts with LCDM predictions.

Anisotropic effects diminish at higher redshifts, but data quality limits conclusions.

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 (SNe Ia). We construct a "residual" statistic sensitive to systematic shifts in their brightness in different directions and use this to search in different redshift bins for a preferred direction on the sky in which the SNe Ia are brighter or fainter relative to the 'standard' LCDM cosmology. At low redshift (z<0.05) we find that an isotropic model such as LCDM is barely consistent with the SNe Ia data at 2-3 sigma. A complementary maximum likelihood analysis of peculiar velocities confirms this finding -- there is a bulk flow of around 260 km/sec at z \sim 0.06, which disagrees with LCDM at 1-2 sigma. Since the Shapley concentration is believed to be largely responsible for this bulk flow, we make a detailed study of the infall region: the SNe Ia falling away from the Local Group towards Shapley are indeed significantly dimmer than those falling towards us and on to Shapley. Convergence to the CMB rest frame must occur well beyond Shapley (z>0.06) so the low redshift bulk flow can systematically bias any reconstruction of the expansion history of the Universe. At high redshifts z>0.15 the agreement between the SNe Ia data and the isotropic LCDM model does improve, however, the sparseness and low quality of the data means that LCDM cannot be singled out as the preferred cosmological model.