Cyclotron Emission and Thermalization of the CMB Spectrum
  If there was a weak magnetic field in the early universe, cyclotron emission could play an important role in the thermalization of the CMB. We study this process in the tightly coupled primordial electron-photon plasma and find that if the magnetic field is large enough so that the plasma effects allow emission of cyclotron photons, this process will wipe out deviations from the black body spectrum. 
Niayesh Afshordi