  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.
