A Pixel Space Method for Testing Dipole Modulation in the CMB Polarization

TL;DR

This paper presents a new pixel space method for detecting dipole modulation in CMB polarization, capable of analyzing partial sky data and potentially identifying hemispherical power asymmetry with high confidence in future observations.

Contribution

The paper introduces a novel pixel space technique using squared polarized flux to detect dipole modulation in CMB polarization, applicable to partial sky data and tested with simulations and real data.

Findings

Method can detect signals at 2.7σ level in future missions

Application to Planck data shows no significant effect

Strong bias found in simulations may mask real signals

Abstract

We introduce a pixel space method to detect dipole modulation or hemispherical power asymmetry in the cosmic microwave background (CMB) polarization. The method relies on the use of squared total polarized flux whose ensemble average picks up a dipole due to the dipole modulation in the CMB polarization. The method is useful since it can be applied easily to partial sky. We define several statistics to characterize the amplitude of the detected signal. By simulations we show that the method can be used to reliably extract the signal at 2.7$\sigma$ level or higher in future CORE-like missions, assuming that the signal is present in the CMB polarization at the level detected by the Planck mission in the CMB temperature. An application of the method to the 2018 Planck data does not detect a significant effect, when taking into account the presence of correlated detector noise and residual systematics in data. Using the FFP10 we find the presence of a very strong bias which might be masking any real effect.