  New dark vector bosons that couple very feebly to regular matter can be
created in the early universe and decay after the onset of big bang
nucleosynthesis (BBN) or the formation of the cosmic microwave background (CMB)
at recombination. The energy injected by such decays can alter the light
element abundances or modify the power and frequency spectra of the CMB. In
this work we study the constraints implied by these effects on a range of
sub-GeV dark vectors including the kinetically mixed dark photon, and the
$B-L$, $L_e-L_\mu$, $L_e-L_\tau$ , and $L_\mu-L_\tau$ dark U(1) bosons. We
focus on the effects of electromagnetic energy injection, and we update
previous investigations of dark photon and other dark vector decays by taking
into account non-universality in the photon cascade spectrum relevant for BBN
and the energy dependence of the ionization efficiency after recombination in
our treatment of modifications to the CMB.
