Nucleosynthesis constraints on the faint vector portal

New Abelian U (1)′ gauge bosons V μ can couple to the Standard Model through mixing of the associated field strength tensor V μν with the one from hypercharge, F μν Y . Here we consider early Universe sensitivity to this vector portal and show that the effective mixing parameter with the photon, κ , is being probed for vector masses in the GeV ballpark down to values 10 −10 ≲ κ ≲ 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.