Completely Dark Photons from Gravitational Particle Production During Inflation.

Starting with the de Broglie--Proca Lagrangian for a massive vector field, we calculate the number density of particles resulting from gravitational particle production (GPP) during inflation, with detailed consideration to the evolution of the number density through the reheating. We find plausible scenarios for the production of dark-photon dark matter of mass in a wide range, as low as a micro-electron volt to $10^{14}$ GeV. Gravitational particle production does not depend on any coupling of the dark photon to standard-model particles.