Nucleosynthesis constraints on the faint vector portal
  New Abelian U(1)' gauge bosons $V_{\mu}$ can couple to the Standard Model
through mixing of the associated field strength tensor $V_{\mu\nu}$ with the
one from hypercharge, $F_{\mu\nu}^Y$. Here we consider early Universe
sensitivity to this vector portal and show that the effective mixing parameter
with the photon, $\kappa$, is being probed for vector masses in the GeV
ballpark down to values $10^{-10} \lesssim \kappa \lesssim 10^{-14}$ where no
terrestrial probes exist. The ensuing constraints are based on a detailed
calculation of the vector relic abundance and an in-depth analysis of relevant
nucleosynthesis processes.
