| Abstract
| - We have extended the previous analyses of Jaffe et al. to a complete Markov chain Monte Carlo (MCMC) parameter space study of the Bianchi-type VIIh models including a dark energy density, using Wilkinson Microwave Anisotropy Probe (WMAP) cosmic microwave background (CMB) data from the 1- and 3-yr releases. Since we perform the analysis in a Bayesian framework our entire inference is contained in the multidimensional posterior distribution from which we can extract marginalized parameter constraints and the comparative Bayesian evidence. Treating the left-handed Bianchi CMB anisotropy as a template centred upon the ‘coldspot’ in the Southern hemisphere, the parameter estimates derived for the total energy density, ‘tightness’ and vorticity from 3-yr data are found to be: Ωtot= 0.43 ± 0.04, h= 0.32+0.02−0.13, ω= 9.7+1.6−1.5× 10−10 with orientation ). This template is preferred by a factor of roughly unity in log-evidence over a concordance cosmology alone. A Bianchi-type template is supported by the data only if its position on the sky is heavily restricted. All other Bianchi VIIh templates including all right-handed models, are disfavoured. The low total energy density of the preferred template, implies a geometry that is incompatible with cosmologies inferred from recent CMB observations. Jaffe et al. found that extending the Bianchi model to include a term in ΩΛ creates a degeneracy in the plane. We explore this region fully by MCMC and find that the degenerate likelihood contours do not intersect areas of parameter space that 1- or 3-yr WMAP data would prefer at any significance above 2σ. Thus we can confirm the conclusion that a physical Bianchi VIIh model is not responsible for this signature, which we have treated in our analysis as merely a template.
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