Presence of hemispherical power asymmetry within the standard cosmological model

Determine whether hemispherical power asymmetry in cosmic microwave background temperature anisotropies constitutes a genuine feature within the standard ΛCDM cosmological model by rigorously assessing its statistical significance relative to isotropic ΛCDM expectations across angular scales.

Background

Hemispherical power asymmetry (HPA) refers to a large-angle anisotropy in the cosmic microwave background (CMB) where one hemisphere exhibits excess power relative to the opposite hemisphere. It was first identified in WMAP data and has persisted in Planck analyses. A common phenomenological description is a dipole modulation of an otherwise statistically isotropic Gaussian CMB field.

This paper reassesses HPA using the Local Variance Estimator (LVE) on Planck PR4 (2020) SEVEM-cleaned CMB temperature maps, across disc radii from 1° to 90°, supported by 600 Planck FFP12 simulations. The authors revalidate aspects of LVE usage, including variance weighting and nside choices, and examine the reliability range of the method.

Their analysis finds statistically significant dipolar anomalies over several angular scales and broadly consistent preferred directions, while also noting scale dependence of the recovered amplitude. Despite methodological refinements and extensive validation, the authors conclude that the status of HPA within the standard ΛCDM framework remains unresolved.