Gravitational Fermion Production in Inflationary Cosmology
  We revisit the gravitational production of massive Dirac fermions in inflationary cosmology with a focus on clarifying the analytic computation of the particle number density in both the large and the small mass regimes. For the case in which the masses of the gravitationally produced fermions are small compared to the Hubble expansion rate at the end of inflation, we obtain a universal result for the number density that is nearly independent of the details of the inflationary model. The result is identical to the case of conformally coupled scalars up to an overall multiplicative factor of order unity for reasons other than just counting the fermionic degrees of freedom. 
Daniel J.H. Chung, Lisa L. Everett, Hojin Yoo and Peng Zhou
astro-ph.CO hep-ph hep-th