Dark matter decay through gravity portals
  Motivated by the fact that, so far, the whole body of evidence for dark
matter is of gravitational origin, we study the decays of dark matter into
Standard Model particles mediated by gravity portals, i.e., through nonminimal
gravitational interactions of dark matter. We investigate the decays in several
widely studied frameworks of scalar and fermionic dark matter where the dark
matter is stabilized in flat spacetime via global symmetries. We find that the
constraints on the scalar singlet dark matter candidate are remarkably strong
and exclude large regions of the parameter space, suggesting that an additional
stabilizing symmetry should be in place. In contrast, the scalar doublet and
the fermionic singlet candidates are naturally protected against too fast
decays by gauge and Lorentz symmetry, respectively. For a nonminimal coupling
parameter $\xi\sim {\cal{O}}(1)$, decays through the gravity portal are
consistent with observations if the dark matter mass is smaller than $\sim
10^5$ GeV, for the scalar doublet, and $\sim 10^6$ GeV, for the fermionic
singlet.
