Examination of frequency and scale dependence of CMB hemispherical power asymmetry

TL;DR

This paper investigates the hemispherical power asymmetry in the CMB using Planck and WMAP data, modeling it as a scale-dependent dipole modulation, and finds consistent asymmetry across frequencies and scales, suggesting a potential cosmological origin.

Contribution

The study introduces a scale-dependent dipole modulation model for the CMB hemispherical power asymmetry and tests its robustness across multiple datasets and frequencies.

Findings

HPA is present across all examined frequency maps.

The asymmetry shows consistent preferred direction and scale dependence.

Results suggest a cosmological origin rather than instrumental effects.

Abstract

In this study, we revisit the well-known cosmic microwave background (CMB) anomaly referred to as Hemispherical Power Asymmetry (HPA), using CMB temperature maps from the Planck mission public release 4 (PR4) and the WMAP nine-year data release. Employing the Local Variance Estimator (LVE) method, we systematically reexamine the properties of HPA to investigate possible frequency dependence as well as scale dependence in its amplitude and direction. We model the HPA as a scale-dependent dipole modulation following a power-law form, rather than assuming a scale-invariant case. Our analysis incorporates seven cleaned frequency-specific CMB temperature maps from both the Planck and WMAP missions to test the robustness of the observed asymmetry across instruments and frequency channels. We find that the dipolar modulation characteristic of HPA is present in all cases examined, with consistent estimates of the preferred direction and scale-dependent variation in dipole amplitudes. These results support the conclusion that the observed asymmetry is unlikely to arise from instrumental artifacts or data-processing effects, and instead points toward a persistent large-scale feature of the CMB sky with a possible cosmological origin.