SCALAR FIELD-DYNAMICS IN FRIEDMANN-ROBERTSON-WALKER SPACETIMES

We study the nonlinear dynamics of quantum fields in matter-and radiat ion-dominated universes, using the nonequilibrium field theory approac h combined with the nonperturbative Hartree and the large N approximat ions. We examine the phenomenon of explosive particle production due t o spinodal instabilities and parametric amplification in expanding uni verses with and without symmetry breaking. For a variety of initial co nditions, we compute the evolution of the inflaton, its quantum fluctu ations, and the equation of state. We find explosive growth of quantum fluctuations, although particle production is somewhat sensitive to t he expansion of the universe. In the large N limit for symmetry-breaki ng scenarios, we determine generic late time solutions for any flat Fr iedmann-Robertson-Walker (FRW) cosmology. We also present a complete a nd numerically implementable renormalization scheme for the equation o f motion and the energy momentum tensor in flat FRW cosmologies. In th is scheme the renormalization constants are independent of time and of the initial conditions.