Cosmological Bounds to the Magnetic Moment of Heavy Tau Neutrinos
  The magnetic moment of tau neutrinos in the MeV mass range may be large enough to modify the cosmological freeze-out calculation and determine the tau-neutrino relic density. In this paper we revisit such a possibility. We calculate the evolution and freeze-out of the tau neutrino number density as a function of its mass and magnetic moment. We then determine its relic density, then calculate its effect upon primordial nucleosynthesis including previously neglected effects. 
Dario Grasso and Edward W. Kolb