2024/01/16 by Tsung‐Han Yeh, Yeh, Tsung-Han, Keith A. Olive +3 · 3 citations
Physics and Astronomy · #Cosmology and Gravitation Theories #Cosmology and Nongalactic Astrophysics (astro-ph.CO) #Dark Matter and Cosmic Phenomena #FOS: Physical sciences #High Energy Physics - Phenomenology (hep-ph) #Particle physics theoretical and experimental studies
paper · pdf · doi:10.48550/arxiv.2401.08795
openalex publication_date 2024/01/16 · openalex created_date 2025/10/10 · openalex updated_date 2026/07/30
Big-bang nucleosynthesis (BBN) probes the cosmic mass-energy density at temperatures ∼ 10 MeV to ∼ 100 keV. 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. We use light element abundances and the cosmic microwave background (CMB) constraints on η and Nν 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 to N\rm eff < 3 for the case of three Standard Model neutrino species. Intriguingly, likelihood results based on \em Planck CMB data alone find Nν = 2.800 ± 0.294, and when combined with standard BBN and the observations of D and \he4 give Nν = 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 results are for a matter species that decays entirely electromagnetically with a lifetime τX = 0.89 \rm sec and pre-decay density that is a fraction ξ= (ρX/ρ\rm rad)|10 \rm MeV = 0.0026 of the radiation energy density at 10 MeV; similarly good fits are found over a range where ξτX1/2 is constant. On the other hand, decaying matter often spoils the BBN+CMB concordance, and we present limits in the (τX,ξ) plane for both electromagnetic and invisible decays. For dark (invisible) decays, standard BBN (i.e. ξ=0) supplies the best fit. We end with a brief discussion of the impact of future measurements including CMB-S4.