A Multiresolution View of Thermalization and Entropy Flow after Preheating
Braden, Jonathan (University of Toronto / CITA)

After inflation the coherent energy of the inflaton field must ultimately be transferred into the incoherent energy density of Standard Model particles to begin the hot big bang. In many models, the early stages of this energy transfer are very efficient due to instabilities that arise when the inflaton couples to other fields. This rapid transfer of energy, known as preheating, leads to energy distributions that are sharply peaked in the infrared. In order to connect the end of inflation to the standard hot big bang, we need to understand the evolution from this highly nonequilibrium state to thermal equilibrium. Here, we study thermalization in a preheating model driven by parametric resonance using large parallel lattice simulations. Shortly after nonlinear effects terminate the resonance, the fields enter into a slowly evolving phase during which the one-point distributions of the stress-energy tensor are nearly constant. Concurrently, the power spectrum of energy density fluctuations develops a slowly moving peak whose height increases linearly with time as the overall fluctuations in the system grow and the coherent energy of the inflaton is converted into incoherent excitations. We introduce a nonequilibrium entropy with contributions from all wavenumbers in order to measure the growing complexity of the system. We also use a hierarchical coarse-graining via block smoothing (essentially Wilsonian Renormalization on a lattice) to study the scale dependence of the energy density and find a rapid buildup of fluctuations in an intermediate range of scales. This is followed by a much slower and longer buildup of fluctuations on smaller scales. Our results suggest that despite the complex nature of the system prior to thermalization, there is a simple underlying principle guiding the dynamics of the system whose universality remains to be explored.