Primordial Nucleosynthesis for the New Cosmology: Determining Uncertainties and Examining Concordance
Richard H. Cyburt
TL;DR
This work develops a rigorous, data-driven framework to quantify uncertainties in Big Bang Nucleosynthesis predictions by explicitly incorporating correlations and normalization systematics in nuclear cross sections. It maps energy-dependent cross sections onto temperature-dependent thermonuclear rates using well-mounded kernels $W(E,T)$ and carefully propagates uncertainties into $\lambda(T)$, revealing how correlated data reduce rate uncertainties. Applying this to 12 key BBN reactions and updating inputs (neutron lifetime, $G_N$, and the $np\to d$ and $d(p,\gamma)^{3}$He channels) yields refined light-element forecasts $Y_p$, D/H, $^{3}$He/H, and $^{7}$Li/H$ across the baryon density parameter $\eta$ or equivalently $\Omega_B h^2$, which are then confronted with deuterium, helium, lithium observations and the CMB (WMAP) baryon density. The analysis finds good concordance for deuterium with CMB-derived baryon density, while helium and lithium confront larger systematic uncertainties and potential new-physics interpretations; importantly, this framework shows how improved cross-section data and larger high-quality D observations can sharpen tests of standard cosmology and constrain non-standard BBN scenarios via $N_{\nu,\mathrm{eff}}$.
Abstract
Big bang nucleosynthesis (BBN) and the cosmic microwave background (CMB) have a long history together in the standard cosmology. The general concordance between the predicted and observed light element abundances provides a direct probe of the universal baryon density. Recent CMB anisotropy measurements, particularly the observations performed by the WMAP satellite, examine this concordance by independently measuring the cosmic baryon density. Key to this test of concordance is a quantitative understanding of the uncertainties in the BBN light element abundance predictions. These uncertainties are dominated by systematic errors in nuclear cross sections. We critically analyze the cross section data, producing representations that describe this data and its uncertainties, taking into account the correlations among data, and explicitly treating the systematic errors between data sets. Using these updated nuclear inputs, we compute the new BBN abundance predictions, and quantitatively examine their concordance with observations. Depending on what deuterium observations are adopted, one gets the following constraints on the baryon density: OmegaBh^2=0.0229\pm0.0013 or OmegaBh^2 = 0.0216^{+0.0020}_{-0.0021} at 68% confidence, fixing N_{ν,eff}=3.0. Concerns over systematics in helium and lithium observations limit the confidence constraints based on this data provide. With new nuclear cross section data, light element abundance observations and the ever increasing resolution of the CMB anisotropy, tighter constraints can be placed on nuclear and particle astrophysics. ABRIDGED