Asymmetric Beams and CMB Statistical Anisotropy

TL;DR

This paper investigates how beam asymmetries in CMB observations induce statistical anisotropy, demonstrating that they can fully explain certain observed anomalies like the quadrupolar modulation in WMAP data.

Contribution

It develops analytical tools to study the impact of beam asymmetries on quadratic estimators of anisotropy, applicable to full-sky and realistic data analysis.

Findings

Beam asymmetries can mimic anisotropic signals in CMB maps.

The quadrupolar anomaly in WMAP data is explained by beam effects.

Methodology applies to analyzing anisotropy in CMB observations.

Abstract

Beam asymmetries result in statistically-anisotropic cosmic microwave background (CMB) maps. Typically, they are studied for their effects on the CMB power spectrum, however they more closely mimic anisotropic effects such as gravitational lensing and primordial power asymmetry. We discuss tools for studying the effects of beam asymmetry on general quadratic estimators of anisotropy, analytically for full-sky observations as well as in the analysis of realistic data. We demonstrate this methodology in application to a recently-detected 9 sigma quadrupolar modulation effect in the WMAP data, showing that beams provide a complete and sufficient explanation for the anomaly.