Bayesian analysis of anisotropic cosmologies: Bianchi VII_h and WMAP

TL;DR

This paper performs a Bayesian analysis of Bianchi VII_h anisotropic cosmologies using WMAP data, finding no significant evidence for such models over the standard LCDM cosmology but constraining their vorticity.

Contribution

The study develops Bayesian techniques for analyzing anisotropic cosmologies with full-sky and partial-sky CMB data, applying them to WMAP observations to test Bianchi VII_h models.

Findings

No evidence for Bianchi VII_h in WMAP data.

Bayes factors slightly favor phenomenological models with Bianchi components.

Constraints on Bianchi vorticity are established at $(7;H)_0 < 8.6 imes 10^{-10}$ with 95% confidence.

Abstract

We perform a definitive analysis of Bianchi VII_h cosmologies with WMAP observations of the cosmic microwave background (CMB) temperature anisotropies. Bayesian analysis techniques are developed to study anisotropic cosmologies using full-sky and partial-sky, masked CMB temperature data. We apply these techniques to analyse the full-sky internal linear combination (ILC) map and a partial-sky, masked W-band map of WMAP 9-year observations. In addition to the physically motivated Bianchi VII_h model, we examine phenomenological models considered in previous studies, in which the Bianchi VII_h parameters are decoupled from the standard cosmological parameters. In the two phenomenological models considered, Bayes factors of 1.7 and 1.1 units of log-evidence favouring a Bianchi component are found in full-sky ILC data. The corresponding best-fit Bianchi maps recovered are similar for both phenomenological models and are very close to those found in previous studies using earlier WMAP data releases. However, no evidence for a phenomenological Bianchi component is found in the partial-sky W-band data. In the physical Bianchi VII_h model we find no evidence for a Bianchi component: WMAP data thus do not favour Bianchi VII_h cosmologies over the standard Lambda Cold Dark Matter (LCDM) cosmology. It is not possible to discount Bianchi VII_h cosmologies in favour of LCDM completely, but we are able to constrain the vorticity of physical Bianchi VII_h cosmologies at $(\omega/H)_0 < 8.6 \times 10^{-10}$ with 95% confidence.