We discuss the gravitational creation of superheavy particles \chi in an inflationary scenario with a quartic potential and a non-minimal coupling between the inflaton \varphi and the Ricci curvature : \xi \varphi ^ { 2 } R / 2 .
We show that for large constants \xi \gg 1 , there can be abundant production of particles \chi with masses largely exceeding the inflationary Hubble rate H _ { infl } , up to ( \text { afew } ) \times \xi H _ { infl } , even if they are conformally coupled to gravity .
We discuss two scenarios involving these gravitationally produced particles \chi .
In the first scenario , the inflaton has only gravitational interactions with the matter sector and the particles \chi reheat the Universe .
In this picture , the inflaton decays only due to the cosmic expansion , and effectively contributes to dark radiation , which can be of the observable size .
The existing limits on dark radiation lead to an upper bound on the reheating temperature .
In the second scenario , the particles \chi constitute Dark Matter , if substantially stable .
In this case , their typical masses should be in the ballpark of the Grand Unification scale .