A reassessment of LVE method and hemispherical power asymmetry in CMB temperature data from Planck PR4

TL;DR

This paper reexamines the hemispherical power asymmetry in Planck CMB data using the local variance estimator, confirming its scale dependence and reaffirming it as a significant anomaly challenging the standard cosmological model.

Contribution

The study reassesses the LVE method's reliability and details its application to Planck PR4 data, emphasizing the scale dependence of the hemispherical power asymmetry.

Findings

HPA is confined to large angular scales in the CMB.

Dipole anisotropy detected with >2σ significance at certain scales.

HPA exhibits a scale dependence modeled as a power-law.

Abstract

We undertake a reassessment of one of the large angular scale anomalies observed in cosmic microwave background (CMB) temperature signal referred to as Hemispherical Power Asymmetry (HPA). For the present analysis we use \texttt{SEVEM} cleaned CMB maps from \emph{Planck}'s 2020 final data release (public release 4/PR4). To probe HPA, we employed the local variance estimator (LVE) method with different disc radii ranging from $0.5^\circ$ to $90^\circ$. Our emphasis here is to revalidate the LVE method in various ways for its optimal usage and probe the hemispherical power asymmetry in the form of a dipole modulation field underlying CMB sky. By and large, our results are in agreement with earlier reported ones with more detailed presentation of explicit and not-so-explicit assumptions involved in the estimation process. It is reaffirmed that HPA is confined to low multipoles or large angular scales of the CMB sky. A dipole like anisotropy was found in the LVE maps with anomalous power for disc radii of $2^\circ$ and upward up to $36^\circ$ at $\gtrsim2\sigma$ level. In the range $4^\circ$ to $10^\circ$ none of the 600 \sevem\ CMB simulations were found to have a dipole amplitude higher than the data when using LVE method as originally proposed. The above reported values fall in the reliability range of LVE method after this extensive re-evaluation. We also observe a scale dependence of the HPA dipole amplitude and model it as a power-law. We conclude that the hemispherical power asymmetry still remains as a challenge to the standard model.