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.