Nonlinear preheating with nonminimally coupled scalar fields in the
  Starobinsky model
  We study the preheating after inflation in the Starobinsky model with a
nonminimally coupled scalar field $\chi$. Using the lattice simulation, we
analyze the rescattering between the $\chi$ particles and the inflaton
condensate, and the backreaction effect of the scalar metric perturbations. We
find that the rescattering is an efficient mechanism promoting the growth of
the $\chi$ field variance. Meanwhile, copious inflaton particles can be knocked
out of the inflaton condensate by rescattering. As a result, the inflaton field
can become a nonnegligible gravitational wave source, even comparable with the
$\chi$ field in some parameter regions. For the scalar metric perturbations,
which are on the sub-Hubble scale in our analysis, our results show that they
have negligible effects on the evolution of scalar fields and the production of
gravitational waves in the model considered in present paper.
