Arman Shafieloo
IEU Korea
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. 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 we cannot single out any particular
model as the preferred cosmological model.