Bayesian Analysis of Sparse Anisotropic Universe Models and Application to the Five-Yearwmapdata

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.