Power Asymmetries in the Cosmic Microwave Background Temperature and Polarization patterns

TL;DR

This study investigates hemispherical asymmetries in the Cosmic Microwave Background's temperature and polarization, confirming temperature asymmetry in the Southern hemisphere and exploring polarization data for additional insights, with implications for future observations.

Contribution

It provides the first joint analysis of temperature and polarization asymmetries in the CMB, confirming temperature asymmetry and assessing polarization's role using WMAP data and forecasting Planck's capabilities.

Findings

Significant temperature asymmetry in the Southern hemisphere (3-4 sigma).

No significant asymmetry found in polarization spectra (EE, BB, TE, TB, EB).

Cold Spot does not affect the asymmetry significance.

Abstract

We test the asymmetry of the Cosmic Microwave Background anisotropy jointly in temperature and polarization. We study the hemispherical asymmetry, previously found only in the temperature field, with respect to the axis identified by Hansen et al. (2009). To this extent, we make use of the low resolution WMAP 5 year temperature and polarization Nside=16 maps and the optimal angular power spectrum estimator BolPol (Gruppuso et al. 2009). We consider two simple estimators for the power asymmetry and we compare our findings with Monte Carlo simulations which take into account the full noise covariance matrix. We confirm an excess of power in temperature angular power spectrum in the Southern hemisphere at a significant level, between 3 sigma and 4 sigma depending on the exact range of multipoles considered. We do not find significant power asymmetry in the gradient (curl) component EE (BB) of polarized angular spectra. Also cross-correlation power spectra, i.e. TE, TB, EB, show no significant hemispherical asymmetry. We also show that the Cold Spot found by Vielva et al. (2004) in the Southern Galactic hemisphere does not alter the significance of the hemispherical asymmetries on multipoles which can be probed by maps at resolution Nside=16. Although the origin of the hemispherical asymmetry in temperature remains unclear, the study of the polarization patter could add useful information on its explanation. We therefore forecast by Monte Carlo the Planck capabilities in probing polarization asymmetries.