Interacting Dirac fields in an expanding universe: Dynamical condensates and particle production

The phenomenon of particle production for quantum field theories in curved spacetimes is crucial to understand the large-scale structure of the universe from an inflationary epoch. In contrast to the free and fixed-background case, the production of particles with strong interactions and backreaction is not completely understood, especially in situations that require going beyond perturbation theory. In this work, we present advances in this direction by focusing on a self-interacting field theory of Dirac fermions in an expanding Friedmann-Robertson-Walker universe. By using a Hamiltonian lattice regularization with continuous conformal time and rescaled fields, this model becomes amenable to either a cold-atom analog-gravity quantum simulation, or a dynamical variational approach. Leveraging a family of variational fermionic Gaussian states, we investigate how dynamical mass generation and the formation of fermion condensates associated to certain broken symmetries modify some well-known results of the free field theory. In particular, we study how the nonperturbative condensates arise and, more importantly, how their real-time evolution has an impact on particle production. Depending on the Hubble expansion rate, we find an interesting interplay of interactions and particle production, including a nontrivial backreaction on the condensates and a parity-breaking spectrum of produced particles.