Dark Sector Equilibration During Nucleosynthesis
  Light, weakly-coupled dark sectors may be naturally decoupled in the early
universe and enter equilibrium with the Standard Model bath during the epoch of
primordial nucleosynthesis. The equilibration and eventual decoupling of dark
sector states modifies the expansion rate of the universe, which alters the
predicted abundances of the light elements. This effect can be encompassed in a
time-varying contribution to $N_{\mathrm{eff}}$, the effective number of
neutrino species, such that $N_{\mathrm{eff}}$ during nucleosynthesis differs
from its measured value at the time of recombination. We investigate the impact
of such variations on the light element abundances with model-independent
templates for the time-dependence of $N_{\mathrm{eff}}$ as well as in specific
models where a dark sector equilibrates with neutrinos or photons. We find that
significant modifications of the expansion rate are consistent with the
measured abundances of light nuclei, provided that they occur during specific
periods of nucleosynthesis. In constraining concrete models, the relative
importance of the cosmic microwave background and primordial nucleosynthesis is
highly model-dependent.
