Cosmological Constraints on Very Dark Photons

Understand

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 $\kappa$.