Parametric Resonance in an Expanding Universe

Parametric resonance has been discussed as a mechanism for copious particle production following inflation. Here we present a simple and intuitive calculational method for estimating the efficiency of parametric amplification as a function of parameters. This is important for determining whether resonant amplification plays an important role in the reheating process. We find that significant amplification occurs only for a limited range of couplings and interactions.Comment: 18 pages, Latex, 4 figure

Topologically nontrivial time-dependent chiral condensates

Topologically nontrivial time-dependent solutions of the classical nonlinear sigma model are studied as candidates of the disoriented chiral condensate (DCC) in 3+1 dimensions. Unlike the analytic solutions so far discussed, these solutions cannot be transformed into isospin-uniform ones by chiral rotations. If they are produced as DCC's, they can be detected by a distinct pattern in the angle-rapidity distribution of the neutral-to-chrged-pion ratio.Comment: 11 pages, 3 figures available upon request to autho