Limits on Non-Relativistic Matter During Big-Bang Nucleosynthesis

Big-bang nucleosynthesis (BBN) probes cosmic mass-energy at temperatures ~1 to ~0.1 MeV. Here we consider the effect of a cosmic matter-like species that is non-relativistic and pressureless during BBN. Such a component must decay; doing so during BBN can alter the baryon-to-photon ratio η and the effective number of neutrino species N_ν. We use light element abundances and the cosmic microwave background (CMB) constraints on η and N_ν to place constraints on such a matter component. We find that electromagnetic decays heat the photons relative to neutrinos, and thus dilute the effective number of relativistic species for the case of three Standard Model neutrinos. Intriguingly, likelihood results based on Planck CMB data alone find N_ν = 2.800 ± 0.294, and when combined with standard BBN and the observations of D and ⁴He give N_ν = 2.898 ± 0.141. While both results are consistent with the Standard Model, we find that a nonzero abundance of electromagnetically decaying matter gives a better fit to these results. Our best-fit is for a matter species that decays entirely electromagnetically with a lifetime τ_X = 0.89 s and pre-decay density that is a fraction ξ = (ρ_X/ρ_rad)_10MeV = 0.0026 of the radiation energy density at 10 MeV; similarly good fits are found over a range of constant ξτ^1/2. On the other hand, decaying matter often spoils the BBN+CMB concordance, and we present limits in the (τ_X, ξ) plane for both electromagnetic and invisible decays. We end with a brief discussion of the impact of future measurements including CMB-S4.