Cosmological Bounces Induced by a Fermion Condensate

We show that it is possible for fermion condensation of the Nambu-Jona-Lasinio type to induce a non-singular bounce that smoothly connects a phase of slow contraction to a phase of expansion. A chiral condensate – a non-zero vacuum expectation value of the spinor bilinear $⟨\bar{\mathrm{\Psi }}\mathrm{\Psi }⟩$ – can form spontaneously after a slow contraction phase smooths and flattens the universe and the Ricci-curvature exceeds a critical value. In this approach, a high density of spin-aligned free fermions is not required, which avoids the problem of generating a large anisotropy and initiating chaotic mixmaster behavior during the bounce phase.