Bayesian analysis of an anisotropic universe model: systematics and
  polarization
  We revisit the anisotropic universe model previously developed by Ackerman,
Carroll and Wise (ACW), and generalize both the theoretical and computational
framework to include polarization and various forms of systematic effects. We
apply our new tools to simulated WMAP data in order to understand the potential
impact of asymmetric beams, noise mis-estimation and potential Zodiacal light
emission. We find that neither has any significant impact on the results. We
next show that the previously reported ACW signal is also present in the 1-year
WMAP temperature sky map presented by Liu & Li, where data cuts are more
aggressive. Finally, we reanalyze the 5-year WMAP data taking into account a
previously neglected (-i)^{l-l'}-term in the signal covariance matrix. We still
find a strong detection of a preferred direction in the temperature map.
Including multipoles up to l=400, the anisotropy amplitude for the W-band is
found to be g = 0.29 +- 0.031, nonzero at 9 sigma. However, the corresponding
preferred direction is also shifted very close to the ecliptic poles at (l,b)=
(96,30), in agreement with the analysis of Hanson & Lewis, indicating that the
signal is aligned along the plane of the solar system. This strongly suggests
that the signal is not of cosmological origin, but most likely is a product of
an unknown systematic effect. Determining the nature of the systematic effect
is of vital importance, as it might affect other cosmological conclusions from
the WMAP experiment. Finally, we provide a forecast for the Planck experiment
including polarization.
