Runaway Gravitational Production of Dark Photons.

We demonstrate that gravitational particle production of a massive, Abelian, vector (Proca) field during inflation in the presence of nonminimal coupling to gravity may suffer from an instability which leads to runaway production of high-momentum modes. This is untenable unless there is some mechanism to regulate the runaway. We discuss the parameter space of the particle mass and nonminimal couplings where such a runaway occurs and possible ways to tame the runaway. We find that there is no obvious way to resolve the runaway in a UV completion or with kinetic mixing to the standard model.

Gravitino Swampland Conjecture.

We extend the swampland from effective field theories (EFTs) inconsistent with quantum gravity to EFTs inconsistent with quantum supergravity. This enlarges the swampland to include EFTs that become inconsistent when the gravitino is quantized. We propose the "gravitino swampland conjecture": the gravitino sound speed must be nonvanishing in all EFTs that are low-energy limits of quantum supergravity. This seemingly simple statement has important consequences for both theories and observations. The conjecture is consistent with and supported by the Kachru-Kallosh-Linde-Trivedi and large volume scenarios for moduli stabilization in string theory.