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Primordial Nucleosynthesis in Light of WMAP

Richard H. Cyburt, Brian D. Fields, Keith A. Olive

TL;DR

This paper assesses primordial nucleosynthesis in light of WMAP by comparing BBN-derived baryon densities with CMB-derived constraints, confirming broad concordance of $\eta$ between the two independent probes and thereby reinforcing hot Big Bang cosmology. It demonstrates how the precise CMB baryometer can be fed into BBN to predict primordial light-element abundances, providing new astrophysical and particle-physics tests, especially for nonstandard physics via $N_{\nu,\mathrm{eff}}$. Deuterium remains a particularly powerful baryometer, with D/H observations largely aligning with BBN predictions, while ${}^4{\rm He}$ and ${}^7{\rm Li}$ show persistent tensions that may reflect systematics or new physics. The work highlights the need for improved nuclear reaction cross sections and more precise D/H measurements to fully exploit the WMAP-era data and to sharpen constraints on early-universe physics.

Abstract

Big bang nucleosynthesis has long provided the primary determination of the cosmic baryon density $\omb h^2$, or equivalently the baryon-to-photon ratio, η. Recently, data on CMB anisotropies have become increasingly sensitive to η. The comparison of these two independent measures provides a key test for big bang cosmology. The first release of results from the Wilkinson Microwave Anisotropy Probe (WMAP) marks a milestone in this test. With the precision of WMAP, the CMB now offers a significantly stronger constraint on η. We discuss the current state of BBN theory and light element observations (including their possible lingering systematic errors). The resulting BBN baryon density prediction is in overall agreement with the WMAP prediction, an important and non-trivial confirmation of hot big bang cosmology. Going beyond this, the powerful CMB baryometer can be used as an input to BBN and one can accurately predict the primordial light element abundances. By comparing these with observations one can obtain new insight into post-BBN nucleosynthesis processes and associated astrophysics. Finally, one can test the possibility of nonstandard physics at the time of BBN, now with all light elements available as probes. Indeed, with the WMAP precision η, deuterium is already beginning to rival \he4's sensitivity to nonstandard physics, and additional D/H measurements can improve this further.

Primordial Nucleosynthesis in Light of WMAP

TL;DR

This paper assesses primordial nucleosynthesis in light of WMAP by comparing BBN-derived baryon densities with CMB-derived constraints, confirming broad concordance of between the two independent probes and thereby reinforcing hot Big Bang cosmology. It demonstrates how the precise CMB baryometer can be fed into BBN to predict primordial light-element abundances, providing new astrophysical and particle-physics tests, especially for nonstandard physics via . Deuterium remains a particularly powerful baryometer, with D/H observations largely aligning with BBN predictions, while and show persistent tensions that may reflect systematics or new physics. The work highlights the need for improved nuclear reaction cross sections and more precise D/H measurements to fully exploit the WMAP-era data and to sharpen constraints on early-universe physics.

Abstract

Big bang nucleosynthesis has long provided the primary determination of the cosmic baryon density , or equivalently the baryon-to-photon ratio, η. Recently, data on CMB anisotropies have become increasingly sensitive to η. The comparison of these two independent measures provides a key test for big bang cosmology. The first release of results from the Wilkinson Microwave Anisotropy Probe (WMAP) marks a milestone in this test. With the precision of WMAP, the CMB now offers a significantly stronger constraint on η. We discuss the current state of BBN theory and light element observations (including their possible lingering systematic errors). The resulting BBN baryon density prediction is in overall agreement with the WMAP prediction, an important and non-trivial confirmation of hot big bang cosmology. Going beyond this, the powerful CMB baryometer can be used as an input to BBN and one can accurately predict the primordial light element abundances. By comparing these with observations one can obtain new insight into post-BBN nucleosynthesis processes and associated astrophysics. Finally, one can test the possibility of nonstandard physics at the time of BBN, now with all light elements available as probes. Indeed, with the WMAP precision η, deuterium is already beginning to rival \he4's sensitivity to nonstandard physics, and additional D/H measurements can improve this further.

Paper Structure

This paper contains 6 sections, 6 equations, 3 figures.

Table of Contents

  1. Introduction
  2. The Baryon Density from BBN (Pre-WMAP)
  3. The Baryon Density from the CMB and Beyond
  4. Using BBN and the CMB to Probe Astrophysics
  5. Using BBN and the CMB to Probe Particle Physics
  6. Discussion and Conclusions

Figures (3)

  • Figure 1: Abundance predictions for standard BBN cfo1; the width of the curves give the $1-\sigma$ error range. The WMAP $\eta$ range (eq. \ref{['eq:eta-cmb']}) is shown in the vertical (yellow) band.
  • Figure 2: Primordial light element abundances as predicted by BBN and WMAP (dark shaded regions). Different observational assessments of primordial abundances are plotted as follows: (a) the light shaded region shows ${\rm D/H} = (2.78 \pm 0.29) \times 10^{-5}$bt-pet, while the dashed curve shows ${\rm D/H} = (2.49 \pm 0.18) \times 10^{-5}$omearakirkman; (b) no observations plotted (c) the light shaded region shows $Y_p = 0.238 \pm 0.002 \pm 0.005$fo98, while the dashed curve shows $Y_p = 0.244 \pm 0.002 \pm 0.005$iz; (d) the light shaded region shows ${}^{7}{\rm Li}$/H = $1.23^{+0.34}_{-0.16} \times 10^{-10}$ryan, while the dashed curve shows ${}^{7}{\rm Li}$/H = $(2.19\pm 0.28) \times 10^{-10}$bon.
  • Figure 3: Likelihoods for $N_{\rm \nu,eff}$ as predicted by the WMAP $\eta$ (eq. \ref{['eq:eta-cmb']}) and light element observations as in Fig. \ref{['fig:abs-MAP']}.