Fermion Production in Bouncing Cosmologies

Abstract: We address the issue of fermionic particle creation in cosmological bouncing models governed by General Relativity, but where the bounce itself takes place due to quantum effects. If the energy scale of the bounce is not very close to the Planck energy, the Wheeler-DeWitt approach can be used to furnish sensible singularity-free background models with a contracting phase preceding an expanding phase in which the fermions evolve. The fermionic fields considered are massive, neutral and minimally coupled to gravity. We are particularly interested in neutrinos, neutrons and heavy neutrinos motivated by theories beyond the Standard Model of Particle Physics. We perform a numerical analysis for a bouncing model containing radiation and a pressureless fluid. The results reveal that the fermionic production is very small, with no back-reaction, unless the fermions are very heavy with masses up to $10^9$GeV. Hence, investigations concerning baryogenesis in such bouncing models should either go beyond the minimal coupling between gravity and the fermionic fields considered here, or assume the existence of such heavy fermions as a starting point.