FIRST-ORDER NONTHERMAL PHASE-TRANSITION AFTER PREHEATING

During preheating after inflation, parametric resonance rapidly genera tes very large fluctuations of scalar fields. In models where the infl aton field phi oscillates in a double-well potential and interacts wit h another scalar field X, fluctuations of X can keep the phi --> -phi symmetry temporarily restored. If the coupling of phi to X is much str onger than the inflaton self-coupling, the subsequent symmetry breakin g is a first-order phase transition. we demonstrate the existence of t his nonthermal phase transition with lattice simulations of the full n onlinear dynamics of the interacting fields. In particular, we observe nucleation of an expanding bubble.