Rotation of Linear Polarization Plane and Circular Polarization from   Cosmological Pseudoscalar Fields
  We discuss the rotation of the linear polarization plane and the production of circular polarization generated by a pseudoscalar field acting as dark matter. We compute analytically and numerically the propagation of the Stokes parameters from the last scattering surface for an oscillating and a monotonic decreasing pseudoscalar field. For the models studied in this paper, we show the comparison between the widely used approximation in which the rotation angle is constant in time and the exact result. 
Fabio Finelli, Matteo Galaverni