We study gravitational creation of light fermions in the presence of classical scalar metric perturbations about a flat Friedmann-Lemaitre-Robertson-Walker (FLRW) background. These perturbations can be large during preheating, breaking the conformal flatness of the background spacetime. We compute numerically the total number of particles generated by the modes of the metric perturbations which have grown sufficiently to become classical. In the absence of inhomogeneities massless fermions are not gravitationally produced, and then this effect may be relevant for abundance estimates of light gravitational relics.
Bibliographical noteFunding Information:
The authors thank Jürgen Baacke and Fermin Viniegra for contributions at early stages of this project and Robert H. Brandenberger for comments on the draft. S.T. also thanks Alexei A. Starobinsky and Jun'ichi Yokoyama for useful discussions. S.T. is thankful for financial support from the JSPS (No. 04942). The work of M.P. is supported by the European Commission RTN programmes HPRN-CT-2000-00148 and 00152. M.P. thanks Lev Kofman for some useful discussions. He also thanks the Canadian Institute for Theoretical Astrophysics of Toronto and the Astrophysics group of Fermilab for their friendly hospitality during the early stages of this work.