The Pesky Power Asymmetry

TL;DR

This paper reviews models explaining the hemispherical power asymmetry in the CMB, emphasizing the need to satisfy multiple cosmological constraints and proposing tests to differentiate between scenarios.

Contribution

It identifies viable models for the power asymmetry that include modulated isocurvature, reionization, gravitational waves, or spectral index variations.

Findings

Consistent models include modulated isocurvature or reionization effects.

Models must satisfy CMB and quasar constraints.

Proposes tests to distinguish between different scenarios.

Abstract

Physical models for the hemispherical power asymmetry in the cosmic microwave background (CMB) reported by the Planck Collaboration must satisfy CMB constraints to the homogeneity of the Universe and quasar constraints to power asymmetries. We survey a variety of models for the power asymmetry and show that consistent models include a modulated scale-dependent isocurvature contribution to the matter power spectrum or a modulation of the reionization optical depth, gravitational-wave amplitude, or scalar spectral index. We propose further tests to distinguish between the different scenarios.