  We perform calculations of dark photon production and decay in the early
universe for ranges of dark photon masses and vacuum coupling with standard
model photons. Simultaneously and self-consistently with dark photon production
and decay, our calculations include a complete treatment of weak decoupling and
big bang nucleosynthesis (BBN) physics. These calculations incorporate all
relevant weak, electromagnetic, and strong nuclear reactions, including
charge-changing (isospin-changing) lepton capture and decay processes. They
reveal a rich interplay of dark photon production, decay, and associated
out-of-equilibrium transport of entropy into the decoupling neutrino seas. Most
importantly, the self-consistent nature of our simulations allows us to capture
the magnitude and phasing of entropy injection and dilution. Entropy
injection-induced alteration of the time-temperature-scale factor relation
during weak decoupling and BBN leads to changes in the light element abundance
yields and the total radiation content (as parametrized by $N_{\rm eff}$).
These changes suggest ways to extend previous dark photon BBN constraints.
However, our calculations also identify ranges of dark photon mass and
couplings not yet constrained, but perhaps accessible and probable, in future
Stage-4 cosmic microwave background experiments and future high precision
primordial deuterium abundance measurements.
