CMB low multipole alignments across WMAP and Planck data releases

Abstract

The first observations of the cosmic microwave background (CMB) from NASA’s Wilkinson Microwave Anisotropy Probe (WMAP) led to finding ‘alignment’ anomalies not expected from fluctuations in the isotropic cosmological model. We study the data of all 8 full-sky public releases since then to test for anomalous alignments and shapes of the first 60 multipoles, i.e. over the range 2 ≤ l ≤ 61. We use rotationally invariant and covariant statistics to test isotropy of all subsequent WMAP data releases, along with those from the ESA’s Planck mission. Anomalous alignments among the multipoles l = 1, 2, 3 are very consistent and robust. More alignments are detected, some of them new, while significance is diluted by the large range of the search. Power entropy, a measure of the randomness of the multipoles, is consistently anomalous at about 2σ level or better across all data releases. It appears that the CMB is not as random as the cosmological principle predicts on large angular scales. © 2025 The Author(s).