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.
