Magnetized birefringence and CMB polarization
  The polarization plane of the cosmic microwave background radiation can be rotated either in a magnetized plasma or in the presence of a quintessential background with pseudoscalar coupling to electromagnetism. A unified treatment of these two phenomena is presented for cold and warm electron-ion plasmas at the pre-recombination epoch. The electron temperature is only relevant to the relativistic correction of the cold plasma results. The spectrum of plasma excitations is obtained from a generalized Appleton--Hartree equation, describing simultaneously the high-frequency propagation of electromagnetic waves in a magnetized plasma with a dynamical quintessence field. It is shown that these two effects are comparable for the plausible range of parameters allowed by present constraints. It is then argued that the generalized expressions derived in the present study may be relevant for direct searches of a possible rotation of the cosmic microwave background polarization. 
Massimo Giovannini