Supersonic baryon-CDM velocities and CMB B-mode polarization

TL;DR

Supersonic baryon-CDM velocities during reionization can generate a distinctive CMB B-mode polarization pattern, but the predicted signal is too weak to detect with current models, though future experiments might constrain early reionization scenarios.

Contribution

This work calculates the CMB B-mode polarization power spectrum induced by supersonic baryon-CDM velocities during reionization, revealing its characteristic shape and amplitude.

Findings

B-mode power spectrum has acoustic peaks at l ~ 200, 400.

Signal amplitude is too small for current reionization models.

Future experiments could constrain early reionization scenarios.

Abstract

It has recently been shown that supersonic relative velocities between dark matter and baryonic matter can have a significant effect on formation of the first structures in the universe. If this effect is still non-negligible during the epoch of hydrogen reionization, it generates large-scale anisotropy in the free electron density, which gives rise to a CMB B-mode. We compute the B-mode power spectrum and find a characteristic shape with acoustic peaks at l ~ 200, 400, ... The amplitude of this signal is a free parameter which is related to the dependence of the ionization fraction on the relative baryon-CDM velocity during the epoch of reionization. However, we find that the B-mode signal is undetectably small for currently favored reionization models in which hydrogen is reionized promptly at z ~ 10, although constraints on this signal by future experiments may help constrain models in which partial reionization occurs at higher redshift, e.g. by accretion onto primordial black holes.