Power asymmetry in CMB polarization maps from PLANCK : a local variance analysis

TL;DR

This paper detects a significant hemispherical power asymmetry in PLANCK 2015 polarization data, suggesting potential deviations from statistical isotropy in the CMB, with implications for understanding cosmological principles.

Contribution

It presents the first analysis of polarization maps revealing hemispherical power asymmetry, extending previous temperature-based findings to polarized CMB data.

Findings

Hemispherical power asymmetry of ~2.6-3.9% in polarization data

Asymmetry direction broadly aligns with CMB kinetic dipole

Future polarization measurements can clarify anisotropy discrepancies

Abstract

A persistent signal of power asymmetry on opposite hemispheres of CMB sky was seen in full-sky temperature measurements made so far. This asymmetry was seen in microwave sky from WMAP as well as PLANCK satellites, and calls for attention the larger question of \emph{statistical isotropy}, one of the foundational principles of modern cosmology. In this work we present an analysis of polarized CMB maps from PLANCK 2015 full mission data. We apply the local variance estimator on low resolution $E-$mode maps from PLANCK 2015 polarization \texttt{Commander} solution. We find a significant hemispherical power asymmetry in polarization data on large angular scales, at the level of $\sim 2.6-3.9\%$ depending on the galactic mask, and the circular disc radius used for computing local variance maps. However the direction is found to be pointing broadly towards CMB kinetic dipole direction. Precise measurements of CMB polarization in future will shed light on this apparent discrepancy in the anisotropy axis seen in temperature and polarized CMB sky, and likely influence of systematics on our findings.