Standard Model Prediction for Cosmological 21cm Circular Polarization
  Before cosmic reionization, hydrogen atoms acquire a spin polarization
quadrupole through interaction with the anisotropic 21-cm radiation field. The
interaction of this quadrupole with anisotropies in the cosmic microwave
background (CMB) radiation field gives a net spin orientation to the hydrogen
atoms. The 21-cm radiation emitted by these spin-oriented hydrogen atoms is
circularly polarized. Here, we reformulate succinctly the derivation of the
expression for this circular polarization in terms of Cartesian (rather than
spherical) tensors. We then compute the angular power spectrum of the observed
Stokes-$V$ parameter in the standard $\Lambda$CDM cosmological model and show
how it depends on redshift, or equivalently, the observed frequency.
