Neutrinos And Big Bang Nucleosynthesis

TL;DR

This paper reviews how Big Bang Nucleosynthesis constrains three key early-universe parameters: the baryon density via $eta10$, the expansion rate via $DeltaNnu$ and the associated $S$, and a possible neutrino degeneracy via $xi$. Using primordial deuterium and helium-4 abundances, it finds $eta10 = 6.27 \pm 0.34$ and $DeltaNnu = 0.66^{+0.47}_{-0.45}$ (giving $N_eff = 3.71^{+0.47}_{-0.45}$) for $xi = 0$, with a small negative $xi$ of $-0.038 \pm 0.026$ when $DeltaNnu = 0$; lithium remains a problem with $A(Li) \approx 2.70 \pm 0.06$. The results are broadly consistent with CMB constraints on the baryon density and dark radiation, and Planck-era forecasts suggest tighter limits (e.g., $\sigma(DeltaNnu) \sim 0.2$ and $\sigma(eta10) \sim 0.03$). When combined with CMB data, these analyses can constrain or reveal a nonzero neutrino degeneracy and the presence of dark radiation, while the persistent lithium discrepancy indicates physics beyond standard BBN is still required to reconcile observations.

Abstract

According to the standard models of particle physics and cosmology, there should be a background of cosmic neutrinos in the present Universe, similar to the cosmic microwave photon background. The weakness of the weak interactions renders this neutrino background undetectable with current technology. The cosmic neutrino background can, however, be probed indirectly through its cosmological effects on big bang nucleosynthesis (BBN) and the cosmic microwave background (CMB) radiation. In this BBN review, focused on neutrinos and, more generally on dark radiation, the BBN constraints on the number of "equivalent neutrinos" (dark radiation), on the baryon asymmetry (baryon density), and on a possible lepton asymmetry (neutrino degeneracy) are reviewed and updated. The BBN constraints on dark radiation and on the baryon density following from considerations of the primordial abundances of deuterium and helium-4 are in excellent agreement with the complementary results from the CMB, providing a suggestive, but currently inconclusive, hint of the presence of dark radiation and, they constrain any lepton asymmetry. For all the cases considered here there is a "lithium problem": the BBN-predicted lithium abundance exceeds the observationally inferred primordial value by a factor of ~3.