Last scattering, relic gravitons, and the circular polarization of the CMB

The tensor contribution to the V-mode polarization induced by a magnetized plasma at last scattering vanishes exactly when the magnetic field is parallel to the direction of propagation of the gravitational wave. For propagation perpendicular to the magnetic field orientation the effect depends on the ratio between the Larmor frequency of the electrons and the observational frequency. If the source term of the V-mode equation is averaged over the directions of the magnetic field, the circular polarization evolves independently from all the other Stokes parameters.