Cosmic Microwave Background Constraints on Very Dark Photons

We analyze the effects of a massive dark photon with mV > 1 MeV on the Cosmic Microwave Background (CMB). We calculate the freeze-in abundance and derive limits on the parameter space from energy injections in the CMB. We find that for dark vectors in the mass range 1 MeV < mV 300 MeV, the model is already ruled out for effective electromagnetic coupling as low as 10−34 – 10−37.