Testing the void against cosmological data: fitting CMB, BAO, SN and H0

Abstract

In this paper, instead of invoking Dark Energy, we try and fit various cosmological observations with a large Gpc scale under-dense region (Void) which is modeled by a Lemaître-Tolman-Bondi metric that at large distances becomes a homogeneous FLRW metric.We improve on previous analyses by allowing for nonzero overall curvature, accurately computing the distance to the last-scattering surface and the observed scale of the Baryon Acoustic peaks, and investigating important effects that could arise from having nontrivial Void density profiles. We mainly focus on the WMAP 7-yr data (TT and TE), Supernova data (SDSS SN), Hubble constant measurements (HST) and Baryon Acoustic Oscillation data (SDSS and LRG). We find that the inclusion of a nonzero overall curvature drastically improves the goodness of fit of the Void model, bringing it very close to that of a homogeneous universe containing Dark Energy,while by varying the profile one can increase the value of the local Hubble parameter which has been a challenge for these models.We also try to gauge how well our model can fit thelarge-scale-structure data, but a comprehensive analysis will requirethe knowledge of perturbations on LTB metrics. The model is consistentwith the CMB dipole if the observer is about 15 Mpc off the centre of the Void. Remarkably, such an off-center position may be able to account for the recent anomalous measurements of a large bulk flow from kSZ data. Finally we provide several analytical approximations in different regimes for the LTB metric, and a numerical module for cosmomc, thus allowing for a MCMC exploration of the full parameter space.