Photon-photon interactions as a source of CMB circular polarization
  Photon-photon interactions mediated by the neutral hydrogen background can
transform plane polarization into circular polarization, through completely
forward processes, [gamma+gamma+atom-> gamma+ gamma+atom], in which only the
photon polarizations are changed. The ratio of circular to plane polarization
intensities is predicted to be at the level of several times 10^{-5} for some
regions of angular size less than 1/300 and with large plane polarizations.
