Precision Cosmology with the Lightest Elements

TL;DR

This work highlights how precision measurements of the primordial deuterium abundance, $(D/H)_{\rm p}$, from metal-poor damped Lyman-Alpha systems enable a robust determination of the baryon density, $\Omega_b h^2$, that agrees with independent CMB inferences. By compiling eight high-quality $(D/H)$ measurements and employing blind analysis to control systematics, the authors obtain $(D/H)_{\rm p} = (2.510 \pm 0.028) \times 10^{-5}$, which translates to $\Omega_b h^2 = 0.02241 \pm 0.00031$ using updated BBN cross-sections and the LINX code. The Planck+ACT CMB result, $\Omega_b h^2 = 0.02250 \pm 0.00011$, shows remarkable concordance, reinforcing the standard cosmological model. Constraints on the effective number of neutrino species, $N_{\rm eff}$, remain consistent with the standard value, with no evidence for additional relativistic components; future improvements in $(D/H)_{\rm p}$ measurements and nuclear physics will sharpen tests for new physics. The work also outlines imminent observational advances (ELT, CUBES) and nuclear-data progress that could push precision below the 1% level, enhancing sensitivity to beyond-Standard-Model scenarios.

Abstract

This is a transcript of the joint talk we gave at the Sixth Gruber Cosmology Conference at Yale University on 3 October 2025. We describe the key role played by Big Bang Nucleosynthesis (BBN) in today's `Precision Cosmology', focusing in particular on the precise determination of the primordial abundance of deuterium. We describe the development of the ideas and methods of BBN research from their beginnings more than 75 years ago to the latest developments, and conclude with a forward look to likely advances expected towards the end of the current decade.

Figures (8)

• Figure 1: The nuclear reactions of Big Bang Nucleosynthesis, leading to the formation of H, He, Li and their isotopes. At the end of BBN, most neutrons are incorporated into $^4$He, with only trace amounts of deuterium, $^3$He, and $^7$Li. (Image reproduced from https://cococubed.com/ courtesy of Frank Timmes).
• Figure 2: The relative abundances of the light elements produced in BBN depend on the baryon to photon ratio, $\eta$, which in turn gives a measure of $\Omega_{\rm b} h^2$. Here $\Omega_{\rm b}$ is the fraction of the critical density contributed by ordinary matter (baryons) and $h$ is Hubble constant in units of 100 km s$^{-1}$ Mpc$^{-1}$. Cyburt16.
• Figure 3: The twin 10-m Keck telescopes on the summit of Manua Kea, on the Big Island of Hawai'i.
• Figure 4: Column density distribution of H i absorbers Zafar13.
• Figure 5: The 'precision sample' of eight measures of the D/H in metal-poor DLAs, plotted against the oxygen abundance. $1\,\sigma$ errors are shown. Seven of the values are from Cooke18, while the eighth is from the recent work by Kislitsyn24. The dotted lines indicate the 1 and 2 $\sigma$ bounds on the weighted mean: $\langle {\rm D/H} \rangle = (2.510 \pm 0.028) \times 10^{-5}$. The black line shows the astration of deuterium estimated from the chemical evolution model of Voort18: for DLAs with [O/H] $\hbox{$\; \buildrel < \over \sim \;$} -1.5$ the correction for astration is much smaller than the errors on the D/H measurements.