On gravitational preheating
  We consider dark matter production during the inflaton oscillation epoch. It
is conceivable that renormalizable interactions between dark matter and
inflaton may be negligible. In this case, the leading role is played by higher
dimensional operators generated by gravity and thus suppressed by the Planck
scale. We focus on dim-6 operators and study the corresponding particle
production in perturbative and non-perturbative regimes. We find that the dark
matter production rate is dominated by non-derivative operators involving
higher powers of the inflaton field. Even if they appear with small Wilson
coefficients, such operators can readily account for the correct dark matter
abundance.
