Litmus Test for Cosmic Hemispherical Asymmetry in the Cosmic Microwave Background B-mode polarization

TL;DR

This paper investigates how cosmic hemispherical asymmetry in the CMB temperature field may also manifest in B-mode polarization due to weak lensing, providing a new method to test statistical isotropy at smaller scales.

Contribution

It introduces a novel test for cosmic hemispherical asymmetry in B-mode polarization caused by weak lensing, extending the analysis beyond temperature measurements.

Findings

Weak lensing induces SI violation in B modes at smaller scales.

Power asymmetry limited to low multipoles does not produce significant effects.

The method can constrain the scale range of hemispherical asymmetry.

Abstract

Recent measurements of the temperature field of Cosmic Microwave Background (CMB) provide tantalising evidence for violation of Statistical Isotropy (SI) that constitutes a fundamental tenet of contemporary cosmology. CMB space based missions, WMAP and Planck have observed a $7\%$ departure in the SI temperature field at large angular scales. However, due to higher cosmic variance at low multipoles, the significance of this measurement is not expected to improve from any future CMB temperature measurements. We demonstrate that weak lensing of the CMB due to scalar perturbations produce a corresponding SI violation in $B$ modes of CMB polarization at smaller angular scales. Measurability of this phenomenon depends upon the scales ($l$ range) over which power asymmetry is present. Power asymmetry which is restricted only to $l<64$ in temperature field cannot lead to any significant observable effect from this new window. However, this effect can put an independent bound on the spatial range of scales of hemispherical asymmetry present in scalar sector.