Photon-photon interactions as a source of CMB circular polarization

TL;DR

This paper predicts that photon-photon interactions mediated by neutral hydrogen can generate detectable levels of circular polarization in the cosmic microwave background, especially in regions with high plane polarization.

Contribution

It introduces a new mechanism for CMB circular polarization via photon-photon interactions involving neutral hydrogen, with quantitative predictions.

Findings

Circular polarization can reach levels of several times 10^{-5}.

Significant polarization effects occur in small angular regions less than 1/300 degrees.

The process involves forward scattering that changes photon polarization states.

Abstract

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.