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Dark sectors provide a compelling theoretical framework for thermally producing sub-GeV dark matter, and motivate an expansive new accelerator and direct-detection experimental program. We demonstrate the power of constraining such dark sectors using the measured effective number of neutrino species, $N_text{eff}$, from the Cosmic Microwave Background (CMB) and primordial elemental abundances from Big Bang Nucleosynthesis (BBN). As a concrete example, we consider a dark matter particle of arbitrary spin that interacts with the Standard Model via a massive dark photon, accounting for an arbitrary number of light degrees of freedom in the dark sector. We exclude dark matter masses below $sim$ 4 MeV at 95% confidence for all dark matter spins and dark photon masses. These bounds hold regardless of additional new light, inert degrees of freedom in the dark sector, and for dark matter-electron scattering cross sections many orders of magnitude below current experimental constraints. The strength of these constraints will only continue to improve with future CMB experiments.
Very recently, the LUNA collaboration has reported a new measurement of the $d+pto {}^{3}text{He}+gamma$ reaction rate, which plays an important role in the prediction of the primordial deuterium abundance at the time of BBN. This new measurement has
Meta-stable dark sector particles decaying into electrons or photons may non-trivially change the Hubble rate, lead to entropy injection into the thermal bath of Standard Model particles and may also photodisintegrate light nuclei formed in the early
The existence of dark radiation that is completely decoupled from the standard model in the early Universe leaves open the possibility of an associated dark radiation isocurvature mode. We show that the presence of dark radiation isocurvature leads t
Ultralight scalar dark matter can interact with all massive Standard Model particles through a universal coupling. Such a coupling modifies the Standard Model particle masses and affects the dynamics of Big Bang Nucleosynthesis. We model the cosmolog
Thermal dark matter at the MeV scale faces stringent bounds from a variety of cosmological probes. Here we perform a detailed evaluation of BBN bounds on the annihilation cross section of dark matter with a mass $1,text{MeV} lesssim m_chi lesssim 1,t