Testing the void against cosmological data: fitting CMB, BAO, SN and<i>H</i>₀

Abstract

In this paper, instead of invoking Dark Energy, we try and fit various\ncosmological observations with a large Gpc scale under-dense region (Void)\nwhich is modeled by a Lemaitre-Tolman-Bondi metric that at large distances\nbecomes a homogeneous FLRW metric. We improve on previous analyses by allowing\nfor nonzero overall curvature, accurately computing the distance to the\nlast-scattering surface and the observed scale of the Baryon Acoustic peaks,\nand investigating important effects that could arise from having nontrivial\nVoid density profiles. We mainly focus on the WMAP 7-yr data (TT and TE),\nSupernova data (SDSS SN), Hubble constant measurements (HST) and Baryon\nAcoustic Oscillation data (SDSS and LRG). We find that the inclusion of a\nnonzero overall curvature drastically improves the goodness of fit of the Void\nmodel, bringing it very close to that of a homogeneous universe containing Dark\nEnergy, while by varying the profile one can increase the value of the local\nHubble parameter which has been a challenge for these models. We also try to\ngauge how well our model can fit the large-scale-structure data, but a\ncomprehensive analysis will require the knowledge of perturbations on LTB\nmetrics. The model is consistent with the CMB dipole if the observer is about\n15 Mpc off the centre of the Void. Remarkably, such an off-center position may\nbe able to account for the recent anomalous measurements of a large bulk flow\nfrom kSZ data. Finally we provide several analytical approximations in\ndifferent regimes for the LTB metric, and a numerical module for CosmoMC, thus\nallowing for a MCMC exploration of the full parameter space.\n