Dark matter from gravitational particle production at reheating
  We show that curvature induced particle production at reheating generates
adiabatic dark matter if there are non-minimally coupled spectator scalars
weakly coupled to visible matter. The observed dark matter abundance implies an
upper bound on spectator masses $m$ and non-minimal coupling values $\xi$. For
example, assuming quadratic inflation, instant reheating and a single spectator
scalar with only gravitational couplings, the observed dark matter abundance is
obtained for $m\sim 0.1$ GeV and $\xi \sim 1$. Larger mass and coupling values
of the spectator are excluded as they would lead to overproduction of dark
matter.
