Supersonic baryon-CDM velocities and CMB B-mode polarization
  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. 
Simone Ferraro (1), Kendrick M. Smith (1), Cora Dvorkin (2) ((1)   Princeton University, (2) Institute for Advanced Study, P...
astro-ph.CO