Hemispherical Power Asymmetry in intensity and polarization for Planck PR4 data

TL;DR

This paper investigates hemispherical power asymmetry in the CMB intensity and polarization data from Planck PR4, finding similar results to previous releases with slight reductions in significance and hints of alignment, suggesting potential deviations from isotropy.

Contribution

It provides a new analysis of Planck PR4 data including polarization, using an improved inpainting method and exploring asymmetry and axis alignment with slightly updated significance levels.

Findings

Hemispherical asymmetry detected at 2.8% significance in E-mode variance

Possible T-E axis alignment at ~5% significance

Results consistent with previous PR3 analysis, with lower significance

Abstract

One of the foundations of the Standard Model of Cosmology is statistical isotropy, which can be tested, among other probes, through the study of the Cosmic Microwave Background (CMB). However, a hemispherical power asymmetry on large scales has been reported for WMAP and Planck data by different works. The statistical significance is above 3${\sigma}$ for temperature, suggesting a directional dependence of the local power spectrum, and thus a feature beyond the ${\Lambda}$CDM model. With the third release of the Planck data (PR3), a new analysis was performed including the E-mode polarization maps, finding an asymmetry at a modest level of significance. In this work, we perform an asymmetry analysis in intensity and polarization maps for the latest Planck processing pipeline (PR4). We obtain similar results to those obtained with PR3, with a slightly lower significance (2.8% for the Sevem method) for the amplitude of the E-mode local variance dipole as well as a significant variability with the considered mask. In addition, a hint of a possible T-E alignment between the asymmetry axes is found at the level of $\sim$ 5%. For the analysis, we have implemented an alternative inpainting approach in order to get an accurate reconstruction of the E-modes. More sensitive all-sky CMB polarization data, such as those expected from the future LiteBIRD experiment, are needed to reach a more robust conclusion on the possible existence of deviations from statistical isotropy in the form of a hemispherical power asymmetry.