Cross-correlation of the polarizations of the 21-cm and cosmic microwave backgrounds

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 polarization and the 21-cm polarization. We treat reionization using an analytical model with parameters calibrated by seminumerical 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 the null hypothesis of no 21-cm polarization. We find that, due to the smallness of the signal, it will be difficult to reject this null hypothesis even with a synergy of ambitious next-generation 21-cm and cosmic microwave background missions.

Automated summary

This study discusses the cross-correlation between the cosmic microwave background polarization and the 21-cm radiation polarization, using an analytical model and angular power spectrum analysis. The results indicate that it will be difficult to reject the null hypothesis of no 21-cm polarization, even with the synergy of next-generation 21-cm and cosmic microwave background missions.