Closing in on the large-scale CMB power asymmetry

TL;DR

This paper evaluates models explaining the large-scale CMB power asymmetry, finding some are compatible with observations while others are ruled out due to overproduction of isotropic power.

Contribution

It systematically tests physical position-space modulation models against CMB data, constraining their viability based on isotropic power overproduction.

Findings

Some models with standard isotropic power are allowed.

Models with modulated tensor or uncorrelated isocurvature modes are ruled out.

Adding anti-correlated isocurvature modes does not resolve overproduction issues.

Abstract

Measurements of the cosmic microwave background (CMB) temperature anisotropies have revealed a dipolar asymmetry in power at the largest scales, in apparent contradiction with the statistical isotropy of standard cosmological models. The significance of the effect is not very high, and is dependent on a posteriori choices. Nevertheless, a number of models have been proposed that produce a scale-dependent asymmetry. We confront several such models for a physical, position-space modulation with CMB temperature observations. We find that, while some models that maintain the standard isotropic power spectrum are allowed, others, such as those with modulated tensor or uncorrelated isocurvature modes, can be ruled out on the basis of the overproduction of isotropic power. This remains the case even when an extra isocurvature mode fully anti-correlated with the adiabatic perturbations is added to suppress power on large scales.