Bayesian analysis of an anisotropic universe model: systematics and polarization

TL;DR

This paper revisits the anisotropic universe model, incorporating polarization and systematics, and finds that the detected anisotropy in WMAP data is likely due to systematic effects rather than cosmological origin.

Contribution

The authors extend the ACW anisotropic model to include polarization and systematics, reanalyze WMAP data, and identify potential non-cosmological origins of the anisotropy signal.

Findings

No significant impact of asymmetric beams or noise mis-estimation.

Detection of a preferred direction aligned with the solar system plane.

Signal likely caused by systematic effects, not cosmological.

Abstract

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.