Out of the darkness: probing the inflationary era with dark photon dark matter

A recent hint reported by the TASEH haloscope suggests the possible detection of dark photon dark matter with mass $19.5~{\rm \mu eV}$. Due to their production during inflation, dark photons act as unique messengers from this primordial epoch. We explore the implications that a confirmed detection would have in directly probing the inflationary era for the first time. To resolve the intrinsic degeneracy between the dark photon mixing parameter and its fractional relic abundance introduced by haloscope measurements, we motivate a next-generation light-shining-through-a-wall experiment. Combining these two experiences with cosmological data, particularly measurements of the tensor-to-scalar ratio $r$, we propose an interdisciplinary approach to reconstruct dark photon properties. We delineate a coherent strategy for simultaneously determining the dark photon kinetic coupling, abundance, and properties of the inflationary era.