Review of Big Bang Nucleosynthesis and Primordial Abundances

TL;DR

BBN provides a testable link between early Universe physics and light-element abundances, with the primordial yields of D, $^3$He, $^4$He, and $^7$Li primarily governed by the baryon-to-photon ratio $\eta$. The most reliable constraint arises from deuterium in quasar absorbers, yielding $\eta \approx 5.1\times10^{-10}$ and $\Omega_b h^2 \approx 0.019$, which harmonizes with CMB and cluster measurements. Non-standard BBN scenarios exist but are increasingly constrained by D/H and $Y_p$ data; overall, standard BBN remains consistent with observations, though systematics in $^4$He and $^7$Li continue to fuel discussion. The convergence of BBN with independent probes supports a coherent cosmological picture, while future precise measurements (notably CMB and Ly$\alpha$ studies) will tighten these fundamental constraints.

Abstract

Big Bang Nucleosynthesis (BBN) is the synthesis of the light nuclei, Deuterium, He3, He4 and Li7, during the first few minutes of the universe. This review concentrates on recent improvements in the measurement of the primordial (after BBN, and prior to modification) abundances of these nuclei. We mention improvement in the standard theory, and the non-standard extensions which are limited by the data. (abridged)

Figures (3)

• Figure 1: Mass fraction of nuclei as a function of temperature for $\eta = 5.1 \times 10^{-10}$, from Nollet & Burles (1999) and Burles et al. (1999).
• Figure 2: Abundances expected for the light nuclei $^4$He, D, $^3$He and $^7$Li (top to bottom) calculated in standard BBN. New estimates of the nuclear cross-section errors from Burles et al. (1999a) and Nollet & Burles (1999) were used to estimate the 95% confidence intervals which are shown by the vertical widths of the abundance predictions. The horizontal scale, $\eta$, is the one free parameter in the calculations. It is expressed in units of the baryon density or critical density for a Hubble constant of 65 kms$^{-1}$Mpc$^{-1}$. The 95% confidence intervals for data, shown by the rectangles, are from Izotov and Thuan 1998a ($^4$He); Burles & Tytler 1998a (D); Gloeckler & Geiss 1996 ($^3$He); Bonifacio and Molaro 1997 ($^7$Li extended upwards by a factor of two to allow for possible depletion).
• Figure 3: Optical spectrum of quasar 1937--1009, which shows the best example of primordial D/H. The top spectrum, from the Kast spectrograph on the 3-m telescope at Lick observatory, is of low spectral resolution, and high signal to noise. The continuum emission, from the accretion disk surrounding the black hole at the center of the quasar, is at about 6 flux units. The emission lines showing more flux (near 4950, 5820, 5940, 6230, 6700 & 7420 Å ) arise in gas near the quasar. The absorptoin lines, showing less flux, nearly all arise in gas which is well separated from, and unrelated to the quasar. The numerous absorption lines at 4200 -- 5800 Å are H I Ly$\alpha$ from the gas in the intergalactic medium. This region of the spectrun is called the Ly$\alpha$ forest. This gas fills the volume of the intergalactic medium, and the absorption lines arise from small, factor of a few, fluctuations in the density of the gas on scales of a few hundred kpc. The Ly$\alpha$ lines were all created by absorption of photons with wavelengths of 1216Å . They appear at a range of observed wavelengths because they have different redshifts. Hence Ly$\alpha$ absorption at 5800Å is near the QSO, while that at 5000Å is nearer to us. The abrupt drop in flux at 4180 Å is caused by H I Lyman continuum absorption in the absorber at $z=3.572$. Photons now at $<4180$ Å had more than 13.6 eV when they passed though the absorber, and they ionized its H I. The 1% residual flux in this Lyman continuum region has been measured in spectra of higher signal to noise (Burles & Tytler 1997) and gives the H I column density, expressed as H I atoms per cm$^{-2}$ through the absorbing gas. The lower plot shows a portion of a spectrum with much higher resolution taken with the HIRES spectrograph on the Keck-1 telescope. We mark the Ly$\alpha$ absorption lines of H I and D from the same gas. The column density of D is measured from this spectrum. Dividing these two column densities we find D/H $= 3.3 \pm 0.3 \times 10^{-5}$ (95% confidence), which is believed to be the primoridal value, and using SBBN predictions, this gives the most accurate measurements of $\eta$ and $\Omega_b$.