Despicable dark relics : generated by gravity with unconstrained masses

Abstract

We demonstrate the existence of a generic, efficient and purely gravitational channel producing a significant abundance of dark relics during reheating after the end of inflation. The mechanism is present for any inert scalar with the non-minimal curvature coupling ξ Rχ2 and the relic production is efficient for modest values ξ = Script O(1). The observed dark matter abundance can be reached for a broad range of relic masses extending from m ~ 1 k eV to m ~ 108 GeV, depending on the scale of inflation and the dark sector couplings. Frustratingly, such relics escape direct, indirect and collider searches since no non-gravitational couplings to visible matter are needed.