Bayesian analysis of sparse anisotropic universe models and application to the 5-yr WMAP data

TL;DR

This paper develops a Bayesian framework for analyzing anisotropic universe models using CMB data, applying it to WMAP observations, and finds tentative evidence for a preferred cosmic direction.

Contribution

It introduces a method to perform exact Bayesian analysis of anisotropic models with non-diagonal covariance matrices, applied to real WMAP data for the first time.

Findings

Tentative detection of a preferred direction in the universe.

Estimated anisotropy amplitude g_* ~ 0.15 with directional consistency.

Results are stable across different multipole ranges.

Abstract

We extend the previously described CMB Gibbs sampling framework to allow for exact Bayesian analysis of anisotropic universe models, and apply this method to the 5-year WMAP temperature observations. This involves adding support for non-diagonal signal covariance matrices, and implementing a general spectral parameter MCMC sampler. As a worked example we apply these techniques to the model recently introduced by Ackerman et al., describing for instance violations of rotational invariance during the inflationary epoch. After verifying the code with simulated data, we analyze the foreground-reduced 5-year WMAP temperature sky maps. For l <= 400 and the W-band data, we find tentative evidence for a preferred direction pointing towards (l,b) = (110 deg, 10 deg) with an anisotropy amplitude of g_* = 0.15 +/- 0.039. Similar results are obtained from the V-band data [g_* = 0.10 +/- 0.04; (l,b) = (130 deg, 20 deg)]. Further, the preferred direction is stable with respect to multipole range, seen independently in both l=[2,100] and [100,400], although at lower statistical significance. We have not yet been able to establish a fully satisfactory explanation for the observations in terms of known systematics, such as non-cosmological foregrounds, correlated noise or asymmetric beams, but stress that further study of all these issues is warranted before a cosmological interpretation can be supported.