The LBT $Y_{\rm p}$ Project V: Cosmological Implications of a New Determination of Primordial $^4$He

TL;DR

This study presents the most precise measurement of primordial helium abundance, combining it with deuterium data to refine cosmological parameters and test the consistency of the Standard Model and standard cosmology.

Contribution

The paper provides the most precise primordial helium measurement and explores its implications for cosmological parameters, including baryon density and relativistic species during nucleosynthesis.

Findings

Primordial helium abundance measured as Yp=0.2458 ± 0.0013.

Cosmic baryon-to-photon ratio determined as η=(6.120 ± 0.038)×10^(-10).

Number of relativistic species during nucleosynthesis consistent with three.

Abstract

The primordial abundance of $^4$He plays a central role in big-bang nucleosynthesis (BBN) and in the cosmic microwave background (CMB). The LBT $Y_{\rm p}$ Project's new measurement of the primordial $^4$He mass fraction $Y_{\rm p} =0.2458 \pm 0.0013$ is the most precise determination to date. In this paper, we combine our new $Y_{\rm p}$ value with the latest primordial deuterium measurement, and assess the consequences for cosmology. For Standard BBN, where the number of light neutrino species is fixed at $N_\nu=3$, the single free parameter is the cosmic baryon density; the CMB measures this independently, with results consistent with each other. Combining $Y_{\rm p}$ , D/H, BBN, and the CMB, gives the cosmic baryon-to-photon ratio $\eta = (6.120 \pm 0.038) \times 10^{-10}$, corresponding to a baryon density parameter $\Omega_{\rm B} h^2 = 0.02236 \pm 0.00014$. We then allow $N_\nu$ to vary and thus measure relativistic species present during nucleosynthesis. We find $\eta = (6.101 \pm 0.044) \times 10^{-10}$ or $\Omega_{\rm B} h^2= 0.02229 \pm 0. 00016$, and $N_\nu = 2.925 \pm 0.082$, and for $N_\nu \ge 3$, $\Delta N_\nu= N_\nu-3 \le 0.125$ (95\% CL) during BBN and the CMB. Our results demonstrate consistency with the Standard Model of particle physics, and with the standard cosmology that links BBN at $\sim 1 \ \rm sec$ and the CMB at $\sim 400,000$ yr.