LINEAR VERSUS NONLINEAR RELAXATION - CONSEQUENCES FOR REHEATING AND THERMALIZATION

We consider the case of a scalar field, the inflaton, coupled to both lighter scalars and fermions, and study the relaxation of the inflaton via particle production in both the linear and nonlinear regimes. Thi s has an immediate application to the reheating problem in inflationar y universe models. The linear regime analysis offers a rationale for t he standard approach to the reheating problem, but we make a distincti on between relaxation and thermalization. We find that particle produc tion when the inflaton starts in the nonlinear region is typically a f ar more efficient way of transferring energy out of the inflaton zero mode and into the quanta of the lighter scalar than single particle de cay. For the nonlinear regime we take into account self-consistently t he evolution of the expectation value of the inflaton field coupled to the evolution of the quantum fluctuations. An exhaustive numerical an alysis of the renormalized equations reveals that the distribution of produced particles is far from thermal, and exhibits the effect associ ated with open channels. In the fermionic case, Pauli blocking begins to hinder the transfer of energy into the fermion modes very early on in the evolution of the inflaton. We discuss the issue of thermalizati on and estimate the reheating temperature to be proportional to the in flaton mass. Cosmological implications are discussed in particular for the Polonyi problem.