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 big bang nucleosynthesis and the cosmic microwave background. This leads to new constraints on the parameter space of mass m(V) vs kinetic mixing parameter kappa.