Resonant decay of Bose condensates

We present results of fully non-linear calculations of the decay of a coherently oscillating scalar field $\phi$, interacting with another scalar field $X$ in the regime of wide parametric resonance. We numerically study the decay both in flat space-time and in the expanding Universe for cosmologically interesting range of resonance parameter $q$, $q\lsim 10^4$. Combining our numerical results with analytical estimates, we extrapolate them to larger values of $q$. We find that scattering of $X$ fluctuations off the Bose condensate is a very efficient mechanism limiting the resonant growth of $X$ fluctuations. The resulting variance of $X$ is typically several orders of magnitude smaller than that obtained in the Hartree approximation. In the expanding Universe, the dilution of Bose condensate by the expansion makes the scattering less effective, and the fluctuations can grow to larger values than in flat space-time.