Dark Sector Equilibration During Nucleosynthesis

Understand

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.