Cosmological Constraints on Very Dark Photons
  We explore the cosmological consequences of kinetically mixed dark photons
with a mass between 1 MeV and 10 GeV, and an effective electromagnetic fine
structure constant as small as $10^{-38}$. We calculate the freeze-in abundance
of these dark photons in the early Universe and explore the impact of late
decays on BBN and the CMB. This leads to new constraints on the parameter space
of mass $m_V$ vs kinetic mixing parameter $\kappa$.
