[Paper Review] Big Bang Nucleosynthesis (in "The Review of Particle Properties" 2004)

Brian D. Fields, S. Sarkar|arXiv (Cornell University)|Jun 29, 2004

Cosmology and Gravitation TheoriesPhysics and Astronomy2,217 citations

TL;DR

The paper reviews Big-Bang Nucleosynthesis (BBN) predictions for D, 3He, 4He, and 7Li, compares them with observations to constrain the baryon-to-photon ratio and baryon density, and discusses implications for physics beyond the Standard Model and the CMB. It emphasizes concordance of standard BBN within a specific η range and uses it to test cosmology and new physics.

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 helium-4 and lithium-7 is presently ~2 σbelow the value implied by the abundance of deuterium. The latter value is also coincident with the independent determination of ηfrom WMAP observations of CMB anisotropy. However taking systematic uncertainties in the abundance estimates into account, there is overall concordance in the range η= (3.4 - 6.9) x 10^{-10} @ 95% c.l. corresponding to a cosmological baryon density Ω_B h^2 = 0.012 - 0.025. If the above discrepancy is due to a neutrino chemical potential, then upto 7.1 effective neutrino species are allowed by nucleosynthesis. Other constraints on new physics are briefly discussed.

Motivation & Objective

• Assess how BBN predictions of light-element abundances constrain the baryon-to-photon ratio η.
• Evaluate concordance between BBN predictions and observational primordial abundances (D, 3He, 4He, 7Li).
• Explore implications for the baryon density ΩB and the presence of new physics (e.g., additional neutrino species) during BBN.
• Compare BBN-derived η with CMB measurements to test the standard cosmology across epochs.

Proposed method

• Describe the freeze-out of n/p ratio at Tfr ≃ 1 MeV and its dependence on weak, strong, electromagnetic, and gravitational interactions.
• Use η = nB/nγ to normalize reaction rates and predict light-element abundances (e.g., D/H, 4He mass fraction Yp, 7Li/H) via the Wagoner code.
• Incorporate radiative corrections, non-equilibrium neutrino heating, and finite nucleon mass effects into 4He predictions.
• Map predicted abundances as functions of η10 in light of updated nuclear cross sections and neutron lifetime τn = 885.7 ± 0.8 s.

Experimental results

Research questions

• RQ1 What η range simultaneously fits the observed primordial abundances of D, 3He, 4He, and 7Li?
• RQ2 How does the inferred baryon density from BBN compare with CMB measurements and with other cosmological probes?
• RQ3 What do light-element abundances imply about the number of relativistic species (Nν) during BBN and constraints on beyond-Standard-Model physics?
• RQ4 To what extent do systematic uncertainties in observations (especially 4He and D) affect the concordance of BBN with the standard cosmology?

Key findings

• BBN predictions for D, 3He, 4He, and 7Li are broadly concordant with observations within η10 ≃ 3.4–6.9 (95% CL).
• The inferred baryon density is 0.012 ≤ ΩBh2 ≤ 0.025 (95% CL) given η10 in the concordant range.
• The primordial 4He abundance is Yp ≈ 0.238 ± 0.002(stat) ± 0.005(sys), consistent with multiple analyses within systematic uncertainties.
• CMB-derived ηCMB (from WMAP first-year data) is consistent with BBN in the standard framework, supporting a zero-parameter BBN when η is fixed by CMB.
• BBN places strong constraints on extra relativistic species (Nν), with fits often close to Nν ≈ 3 but allowing ranges (e.g., 1.7 ≤ η10 ≤ 4.3 and 1.4 ≤ Nν ≤ 4.9) under certain assumptions; tighter bounds arise with CMB priors.
• BBN constraints extend to new physics scenarios (e.g., sterile neutrinos, non-standard decays, extra dimensions) by limiting how the expansion rate and entropy are altered during nucleosynthesis.