Limits on Non-Relativistic Matter During Big-Bang Nucleosynthesis

TL;DR

This paper investigates how non-relativistic, pressureless matter that decays during Big-Bang Nucleosynthesis affects cosmic parameters, using light element abundances and CMB data to constrain its properties and decay modes.

Contribution

It provides new constraints on decaying matter species during BBN, including their decay lifetimes and densities, and explores their impact on the effective number of neutrino species and cosmological observations.

Findings

Electromagnetic decays dilute the effective number of neutrino species to N_eff < 3.

Best-fit decay lifetime is approximately 0.89 seconds with a small pre-decay density fraction.

Nonzero electromagnetic decaying matter improves fit to Planck CMB data and BBN observations.

Abstract

Big-bang nucleosynthesis (BBN) probes the cosmic mass-energy density at temperatures $\sim 10$ MeV to $\sim 100$ 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, $\eta$, and the effective number of neutrino species. We use light element abundances and the cosmic microwave background (CMB) constraints on $\eta$ and $N_\nu$ 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 $N_{\nu} = 2.800 \pm 0.294$, and when combined with standard BBN and the observations of D and \he4 give $N_{\nu} = 2.898 \pm 0.141$. 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 $\tau_X = 0.89 \ \rm sec$ and pre-decay density that is a fraction $\xi = (\rho_X/\rho_{\rm rad})|_{10 \ \rm MeV} = 0.0026$ of the radiation energy density at 10 MeV; similarly good fits are found over a range where $\xi \tau_X^{1/2}$ is constant. On the other hand, decaying matter often spoils the BBN+CMB concordance, and we present limits in the $(\tau_X,\xi)$ plane for both electromagnetic and invisible decays. For dark (invisible) decays, standard BBN (i.e. $\xi=0$) supplies the best fit. We end with a brief discussion of the impact of future measurements including CMB-S4.