The Cosmological Evolution of Light Dark Photon Dark Matter

Light dark photons are subject to various plasma effects, such as Debye screening and resonant oscillations, which can lead to a more complex cosmological evolution than is experienced by conventional cold dark matter candidates. Maintaining a consistent history of dark photon dark matter requires ensuring that the super-thermal abundance present in the early Universe $\textit{(i)}$ does not deviate significantly after the formation of the CMB, and $\textit{(ii)}$ does not excessively leak into the Standard Model plasma after BBN. We point out that the role of non-resonant absorption, which has previously been neglected in cosmological studies of this dark matter candidate, produces strong constraints on dark photon dark matter with mass as low as $10^{-22}$ eV. Furthermore, we show that resonant conversion of dark photons after recombination can produce excessive heating of the IGM which is capable of prematurely reionizing hydrogen and helium, leaving a distinct imprint on both the Ly$-\alpha$ forest and the integrated optical depth of the CMB. Our constraints surpass existing cosmological bounds by more than five orders of magnitude across a wide range of dark photon masses.