A nonperturbative correction to the thermal nucleation rate of critical bubbles in a first order phase transition is estimated. The correction originates from large-amplitude fluctuations which may be present before the transition occurs. Using a simple model of a scalar field in a double-well potential, we present a method to obtain a corrected potential which incorporates the free-energy density available from large-amplitude fluctuations, which is not included in the usual perturbative calculation. For weaker phase transitions, the nucleation rate can be much larger than the rate calculated via perturbation theory. As an application of our method, we show how nonperturbative corrections can both qualitatively and quantitatively explain anomalously high nucleation rates observed in 2-d numerical simulations.

Latex, 8 pages, 2 postscript figures, submitted to PRL. Also available at http://fnas08.fnal.gov/