The LBT Y p Project. V. Cosmological Implications of a New Determination of Primordial 4 He

Abstract

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 p Project’s new measurement of the primordial 4 He mass fraction, Y p = 0.2458 ± 0.0013, is the most precise determination to date. In this paper, we combine our new Y 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 ν = 3, the single free parameter is the cosmic baryon density; the CMB measures this independently, with results consistent with each other. Combining Y p , D/H, BBN, and the CMB gives the cosmic baryon-to-photon ratio η = (6.120 ± 0.038) × 10 −10 , corresponding to a baryon density parameter Ω B h 2 = 0.02236 ± 0.00014. We then allow N ν to vary and thus measure relativistic species present during nucleosynthesis. We find η = (6.101 ± 0.044) × 10 −10 or Ω B h 2 = 0.02229 ± 0.00016, and N ν = 2.925 ± 0.082, and for N ν ≥ 3, Δ N ν = N ν − 3 ≤ 0.125 (95% confidence limit) during BBN and the CMB. Our results demonstrate consistency with the Standard Model of particle physics and the standard cosmology that links BBN at ∼ 1 s and the CMB at ∼400,000 yr.

The authors' abstract, as published at the source. The Astrophysical Journal, 2026 · DOI ↗