[Paper Review] Big-Bang Nucleosynthesis

Brian D. Fields, P. Molaro+1 more|arXiv (Cornell University)|Dec 3, 2014

Particle physics theoretical and experimental studiesPhysics and Astronomy5 references3,416 citations

TL;DR

This paper reviews standard Big-Bang Nucleosynthesis (BBN), detailing how light-element abundances (D, 3He, 4He, 7Li) depend on the baryon density eta, and discusses concordance with CMB results and implications for beyond-Standard-Model physics, including the lithium problem.

ABSTRACT

A critical review is given of the current status of cosmological nucleosynthesis. In the framework of the Standard Model with 3 types of relativistic neutrinos, the baryon-to-photon ratio, $η$, corresponding to the inferred primordial abundances of deuterium and helium-4 is consistent with the independent determination of $η$ from observations of anisotropies in the cosmic microwave background. However the primordial abundance of lithium-7 inferred from observations is significantly below its expected value. Taking systematic uncertainties in the abundance estimates into account, there is overall concordance in the range $η= (5.7-6.7) imes 10^{-10}$ at 95% CL (corresponding to a cosmological baryon density $Ω_B h^2 = 0.021 - 0.025$). The D and He-4 abundances, when combined with the CMB determination of $η$, provide the bound $N_ν=3.28 \pm 0.28$ on the effective number of neutrino species. Other constraints on new physics are discussed briefly.

Motivation & Objective

• Assess how early-Universe conditions determine primordial light-element abundances (D, 3He, 4He, 7Li).
• Quantify the baryon-to-photon ratio eta and its impact on element yields.
• Compare BBN predictions with observations and CMB-derived baryon density to test standard cosmology and constrain new physics.
• Discuss the lithium problem and potential beyond-Standard-Model solutions.
• Outline how BBN constrains relativistic species and decays during nucleosynthesis.

Proposed method

• Describe the freeze-out of n/p at T_fr ~ 1 MeV and its dependence on weak and gravitational interactions.
• Explain the nucleosynthesis chain starting from deuterium formation and the bottleneck due to photo-dissociation.
• Use updated nuclear reaction rates and a Wagoner-code-based framework to compute abundances as functions of eta_10.
• Incorporate corrections to 4He from radiative processes, non-equilibrium neutrino heating, and finite nucleon mass effects.
• Employ Monte Carlo methods to estimate uncertainties in D, 3He, and 7Li from nuclear cross sections; fit predicted abundances with polynomial forms and provide error correlations.
• Compare predictions to observational priors (D/H in DLAs, 4He in H II regions, Li in Pop II stars) and with CMB-derived eta.

Experimental results

Research questions

• RQ1 What baryon-to-photon ratio eta best reconciles BBN predictions with observed abundances of D, 3He, 4He, and 7Li?
• RQ2 How does the inclusion of standard-model physics (e.g., N_nu, neutrino heating) affect predicted abundances and concordance with observations?
• RQ3 To what extent do observational systematics and stellar processes influence the inferred primordial abundances, especially for Li and He?
• RQ4 Can BBN constraints limit beyond-Standard-Model scenarios (additional relativistic species, decays, or varying constants) during the nucleosynthesis era?
• RQ5 Is there evidence for new physics that can resolve the lithium problem without spoiling agreement for D/H and 4He?

Key findings

• Predicted light-element abundances span nine orders of magnitude and are in good overall agreement with primordial observations within uncertainties.
• Concordance yields for eta_10 lie in the range 5.7–6.7 (95% CL) based on D/H and 4He data, corresponding to a baryon density today of rho_b ≈ (3.9–4.6)×10^-31 g cm^-3.
• The CMB-derived baryon density eta_10 ≈ 6.047±0.074 (Planck) is consistent with BBN concordance and high-redshift D/H measurements.
• The primordial 4He abundance is currently estimated as Y_p ≈ 0.2465±0.0097, with CMB measurements also consistent with this value.
• The lithium problem persists: Li/H inferred from Pop II stars is in tension with D/H and 4He constraints, suggesting either systematic uncertainties or new physics during BBN.
• BBN provides strong constraints on beyond-Standard-Model physics, including limits on extra relativistic species (N_nu), decays of massive particles during BBN, and scenarios with modified gravity or extra dimensions.