Cross-correlation of the Polarizations of the 21-cm and Cosmic Microwave Backgrounds

TL;DR

This paper explores the potential of cross-correlating 21-cm polarization with CMB polarization to gain insights into reionization, analyzing the detectability and null hypothesis testing with future observational missions.

Contribution

It introduces an analytical model for reionization, derives the 21-cm and CMB polarization cross-correlation spectrum, and assesses the observational prospects for detecting this signal.

Findings

Cross-correlation could help detect reionization signals with next-generation missions.

Null hypothesis testing shows some questions are within reach, others remain challenging.

Synergy of 21-cm and CMB data enhances understanding of reionization.

Abstract

The polarization of the 21-cm radiation from the epoch of reionization arises from Thomson scattering of 21-cm photons from free electrons and provides information that complements that from the intensity fluctuation. Previous work showed that a direct detection of this signal will be difficult, and hinted that the signal might be enhanced via correlation with other tracers. Here, we discuss the cross-correlation between the cosmic microwave background (CMB) polarization and the 21-cm polarization. We treat reionization using an analytical model with parameters calibrated by semi-numerical simulations. We then derive the cross-correlation angular power spectrum using the total-angular-momentum formalism. We also provide a noise analysis to test against two closely related, but subtly different, null hypotheses. First, we assume no reionization as a null hypothesis, and determine how well this null hypothesis could be ruled out by an observed 21cm-CMB polarization correlation. Second, we determine how well the null hypothesis of no 21-cm polarization can be ruled out by seeking the cross-correlation, assuming reionization is established from the CMB. We find that the first question could be answered by a synergy of ambitious next-generation 21-cm and CMB missions, whereas the second question will still remain out of reach.