Is the Cosmic Microwave Background Circularly Polarized?
  The primordial anisotropies of the cosmic microwave background (CMB) are linearly polarized via Compton-scattering. The Faraday conversion process during the propagation of polarized CMB photons through regions of the large-scale structure containing magnetized relativistic plasma, such as galaxy clusters, will lead to a circularly polarized contribution. Though the resulting Stokes-V parameter is of order 10^-9 at frequencies of 10 GHz, the contribution can potentially reach the total Stokes-U at low frequencies due to the cubic dependence on the wavelength. In future, the detection of circular polarization of CMB can be used as a potential probe of the physical properties associated with relativistic particle populations in large-scale structures. 
Asantha Cooray (Caltech), Alessandro Melchiorri (Oxford), Joseph Silk   (Oxford)