Unveiling hidden patterns in CMB anisotropy maps

Abstract

Bianchi VII_h models have been recently proposed to explain potential anomalies in the CMB anisotropy as observed by WMAP. We investigate the violation of statistical isotropy due to an embedded Bianchi VII_h template in the CMB anisotropy maps to determine whether the existence of a hidden Bianchi template in the WMAP data is consistent with the previous null detection of the bipolar power spectrum in the WMAP first-year maps. We argue that, although correcting the WMAP maps for the Bianchi template may explain some features in the WMAP data, it may cause other anomalies such as preferred directions leading to detectable levels of violation of statistical isotropy in the Bianchi corrected maps. We compute the bipolar power spectrum for the low density Bianchi VII_h models embedded in the background CMB anisotropy maps with the power spectrum that have been shown in recent literature to best fit the first-year WMAP data. By examining the statistical isotropy of these maps, we put a limit of (σ/H)0≤2.77×10⁻¹⁰(99% C.L.) on the shear parameter in Bianchi VII_h models.