Big Bang Nucleosynthesis: 2015

TL;DR

This work updates Big Bang Nucleosynthesis (BBN) predictions by incorporating NACRE-II nuclear rates and the latest neutron lifetime, then tests those predictions against Planck 2015 CMB data through a Monte Carlo likelihood framework. The authors find that deuterium observations provide the strongest current constraint on the number of relativistic species, yielding $N_\nu < 3.2$ at 2$\sigma$, while the lithium-7 discrepancy with observations remains unresolved, hinting at new physics or astrophysical systematics. The analysis demonstrates that D/H together with CMB data yields a cohesive test of Standard Model cosmology, with D/H becoming the dominant source of theoretical uncertainty and Planck data enabling BBN-era constraints to be probed with CMB-only information. The paper highlights the need for improved nuclear cross sections to further sharpen BBN tests and outlines a path to more precise determinations of primordial abundances and fundamental cosmological parameters through joint BBN-CMB analyses.

Abstract

Big-bang nucleosynthesis (BBN) describes the production of the lightest nuclides via a dynamic interplay among the four fundamental forces during the first seconds of cosmic time. We briefly overview the essentials of this physics, and present new calculations of light element abundances through li6 and li7, with updated nuclear reactions and uncertainties including those in the neutron lifetime. We provide fits to these results as a function of baryon density and of the number of neutrino flavors, N_nu. We review recent developments in BBN, particularly new, precision Planck cosmic microwave background (CMB) measurements that now probe the baryon density, helium content, and the effective number of degrees of freedom, n_eff. These measurements allow for a tight test of BBN and of cosmology using CMB data alone. Our likelihood analysis convolves the 2015 Planck data chains with our BBN output and observational data. Adding astronomical measurements of light elements strengthens the power of BBN. We include a new determination of the primordial helium abundance in our likelihood analysis. New D/H observations are now more precise than the corresponding theoretical predictions, and are consistent with the Standard Model and the Planck baryon density. Moreover, D/H now provides a tight measurement of N_nu when combined with the CMB baryon density, and provides a 2sigma upper limit N_nu < 3.2. The new precision of the CMB and of D/H observations together leave D/H predictions as the largest source of uncertainties. Future improvement in BBN calculations will therefore rely on improved nuclear cross section data. In contrast with D/H and he4, li7 predictions continue to disagree with observations, perhaps pointing to new physics.

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