Robust Likelihoods for Inflationary Gravitational Waves from Maps of Cosmic Microwave Background Polarization

TL;DR

This paper develops a joint pixel-space likelihood method for analyzing CMB B-mode polarization data to better reject contamination and improve the detection of inflationary gravitational waves.

Contribution

It introduces a new joint pixel-space likelihood approach that accounts for contamination and provides null tests to identify excess variance in CMB polarization data.

Findings

The likelihood helps reject contamination effectively.

Null tests can identify unmodeled excess variance.

Method improves robustness of inflationary gravitational wave detection.

Abstract

The B-mode polarization of the cosmic microwave background (CMB) provides a unique window into tensor perturbations from inflationary gravitational waves. Survey effects complicate the estimation and description of the power spectrum on the largest angular scales. The pixel-space likelihood yields parameter distributions without the power spectrum as an intermediate step, but it does not have the large suite of tests available to power spectral methods. Searches for primordial B-modes must rigorously reject and rule out contamination. Many forms of contamination vary or are uncorrelated across epochs, frequencies, surveys, or other data treatment subsets. The cross power and the power spectrum of the difference of subset maps provide approaches to reject and isolate excess variance. We develop an analogous joint pixel-space likelihood. Contamination not modeled in the likelihood produces parameter-dependent bias and complicates the interpretation of the difference map. We describe a null test that consistently weights the difference map. Excess variance should either be explicitly modeled in the covariance or be removed through reprocessing the data.