Non-Gaussian Structure of B-mode Polarization after Delensing

TL;DR

This paper investigates the non-Gaussian statistical properties of delensed B-mode polarization in the cosmic microwave background, revealing that delensing reduces correlations but introduces non-Gaussian features affecting gravitational wave detection sensitivity.

Contribution

It provides a detailed analysis of the non-Gaussian structure of delensed B-modes, including power spectrum correlations and PDFs, with an analytic model based on Edgeworth expansion.

Findings

Delensing reduces correlations between multipoles.

The PDF of the power spectrum amplitude is approximately Gaussian with increased variance.

Non-Gaussianity slightly degrades constraints on the tensor-to-scalar ratio.

Abstract

The B-mode polarization of the cosmic microwave background on large scales has been considered as a probe of gravitational waves from the cosmic inflation. Ongoing and future experiments will, however, suffer from contamination due to the B-modes of non-primordial origins, one of which is the lensing induced B-mode polarization. Subtraction of the lensing B-modes, usually referred to as delensing, will be required for further improvement of detection sensitivity of the gravitational waves. In such experiments, knowledge of statistical properties of the B-modes after delensing is indispensable to likelihood analysis particularly because the lensing B-modes are known to be non-Gaussian. In this paper, we study non-Gaussian structure of the delensed B-modes on large scales, comparing them with those of the lensing B-modes. In particular, we investigate the power spectrum correlation matrix and the probability distribution function (PDF) of the power spectrum amplitude. Assuming an experiment in which the quadratic delensing is an almost optimal method, we find that delensing reduces correlations of the lensing B-mode power spectra between different multipoles, and that the PDF of the power spectrum amplitude is well described as a normal distribution function with a variance larger than that in the case of a Gaussian field. These features are well captured by an analytic model based on the 4th order Edgeworth expansion. As a consequence of the non-Gaussianity, the constraint on the tensor-to-scalar ratio after delensing is degraded within approximately a few percent, which depends on the multipole range included in the analysis.