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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 $1,text{MeV} lesssim m_chi lesssim 1,text{GeV}$. For $p-wave suppressed annihilations, constraints from BBN turn out to be significantly stronger than the ones from CMB observations, and are competitive with the strongest bounds from other indirect searches. We furthermore update the lower bound from BBN on the mass of thermal dark matter using improved determinations of primordial abundances. While being of similar strength as the corresponding bound from CMB, it is significantly more robust to changes in the particle physics model.
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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 universe. We study generic constraints from Big Bang Nucleosynthesis on such a setup, with a particular emphasis on MeV-scale particles which are neither fully relativistic nor non-relativistic during all times relevant for Big Bang Nucleosynthesis. We apply our results to a simple model of self-interacting dark matter with a light scalar mediator. This setup turns out to be severely constrained by these considerations in combination with direct dark matter searches and will be fully tested with the next generation of low-threshold direct detection experiments.
In this work, we revise and update model-independent constraints from Big Bang Nucleosynthesis on MeV-scale particles $phi$ which decay into photons and/or electron-positron pairs. We use the latest determinations of primordial abundances and extend the analysis in arXiv:1808.09324 by including all spin-statistical factors as well as inverse decays, significantly strengthening the resulting bounds in particular for small masses. For a very suppressed initial abundance of $phi$, these effects become ever more important and we find that even a pure freeze-in abundance can be significantly constrained. In parallel to this article, we release the public code ACROPOLIS which numerically solves the reaction network necessary to evaluate the effect of photodisintegration on the final light element abundances. As an interesting application, we re-evaluate a possible solution of the lithium problem due to the photodisintegration of beryllium and find that e.g. an ALP produced via freeze-in can lead to a viable solution.
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 triggered a new set of global BBN analyses within the context of the Standard Model. In this addendum to JCAP 01 (2020) 004 (arXiv:1910.01649), we consider the implications of these new results for our constraints on MeV-scale dark sectors. Importantly, we find that our bounds in the BBN-only and Planck-only analyses are insensitive to these updates. Similarly, we find that our constraints derived using BBN and CMB data simultaneously are not significantly modified for neutrinophilic particles. The bounds on electrophilic dark sector states, however, can vary moderately when combining BBN and CMB observations. We present updated results for all the relevant light dark sector states, calculated using the rates obtained by the leading groups performing standard BBN analyses.
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 cosmological evolution of the dark matter, taking into account the modifications of the scalar mass by the environment as well as the full dynamics of Big Bang Nucleosynthesis. We find that precision measurements of the helium-4 abundance set stringent constraints on the available parameter space, and that these constraints are strongly affected by both the dark matter environmental mass and the dynamics of the neutron freeze-out. Furthermore, we perform the analysis in both the Einstein and Jordan frames, the latter of which allows us to implement the model into numerical Big Bang Nucleosynthesis codes and analyze additional light elements. The numerical analysis shows that the constraint from helium-4 dominates over deuterium, and that the effect on lithium is insufficient to solve the lithium problem. Comparing to several other probes, we find that Big Bang Nucleosynthesis sets the strongest constraints for the majority of the parameter space.
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.
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