Examining statistical isotropy of CMB low multipoles from Planck PR4 data

TL;DR

This study analyzes Planck PR4 data to assess statistical isotropy in low multipoles of the CMB, finding significant anisotropy and alignments, but no intrinsic planarity, with implications for understanding cosmic anisotropies.

Contribution

It applies the Power tensor method to the latest Planck data to evaluate anisotropy and alignments in low multipoles, providing updated insights into CMB anomalies.

Findings

Significant anisotropic modes with 0.3% probability

Quadrupole and octopole remain aligned

Collective multipole alignment axis close to dipole axis

Abstract

Low multipoles ($l$) in cosmic microwave background (CMB) temperature anisotropies have shown some `peculiarities' when examined since the release of the full sky CMB maps, using a variety of tests. In this paper, we concern ourselves with the very first peculiarities seen in CMB data viz., a breakdown of statistical isotropy in the form of axiality and planarity of these low-$l$ modes, and preferred alignments among them. We scrutinize the latest CMB data from ESA's Planck mission, PR4, to evaluate the current status of these deviations. We employ the Power tensor method which allows an invariant characterization of the distribution of power in a given multipole, and apply it to probe the first sixty multipoles i.e., $l=2$ to 61. We find that there are significant number of modes that are intrinsically anisotropic with a cumulative probability of $0.3\%$. However since the planarity study reveals that those modes that are unusually planar are subset of these anisotropic modes, we conclude that they may not be intrinsically planar. The quadrupole is still well aligned with the octopole. Besides, $l=3$, higher multipoles aligned with quadrupole are found to be insignificant. Interestingly, the collective alignment axis of the first sixty multipoles is found to be broadly closer to the axis of dipole, quadrupole, octopole and other modes aligned with $l=2$.