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The 2024 BBN baryon abundance update

Nils Schöneberg

Abstract

We revisit the state of the light element abundances from big bang nucleosynthesis in early 2024 with particular focus on the derived baryon abundance. We find that the largest differences between the final baryon abundances are typically driven by the assumed Deuterium burning rates, characterized in this work by the underlying code. The rates from theoretical ab-initio calculations favor smaller baryon abundances, while experimentally-determined rates prefer higher abundances. Through robust marginalization over a wide range of nuclear rates, the recently released $\mathtt{PRyMordial}$ code allows for a conservative estimate of the baryon abundance at $Ω_b h^2 = 0.02218 \pm 0.00055$ (using PDG-recommended light element abundances) in $Λ$CDM and $Ω_b h^2 = 0.02196 \pm 0.00063$ when additional ultra-relativistic relics are considered ($Λ$CDM + $N_\mathrm{eff}$). These additional relics themselves are constrained to $ΔN_\mathrm{eff} = -0.14 \pm 0.21$ by light element abundances alone.

The 2024 BBN baryon abundance update

Abstract

We revisit the state of the light element abundances from big bang nucleosynthesis in early 2024 with particular focus on the derived baryon abundance. We find that the largest differences between the final baryon abundances are typically driven by the assumed Deuterium burning rates, characterized in this work by the underlying code. The rates from theoretical ab-initio calculations favor smaller baryon abundances, while experimentally-determined rates prefer higher abundances. Through robust marginalization over a wide range of nuclear rates, the recently released code allows for a conservative estimate of the baryon abundance at (using PDG-recommended light element abundances) in CDM and when additional ultra-relativistic relics are considered (CDM + ). These additional relics themselves are constrained to by light element abundances alone.
Paper Structure (10 sections, 6 equations, 6 figures, 2 tables)

This paper contains 10 sections, 6 equations, 6 figures, 2 tables.

Table of Contents

  1. Introduction
  2. A short theory summary
  3. Method and Data
  4. Results
  5. Sensitivity to employed code
  6. Sensitivity to employed datasets
  7. The Helium anomaly
  8. Additional relativistic degrees of freedom
  9. Conclusions
  10. Notes on the employed data sets

Figures (6)

  • Figure 1: Light element abundances as a function of baryon abundance $\Omega_b h^2$ and number of effective additional neutrino species $\Delta N_\mathrm{eff}$. The green and blue line and contours represent the mean, and the $1\sigma$ and $2\sigma$ contours of the Deuterium and Helium recommendation from PDG ParticleDataGroup:2022pth, respectively. Computed from the PArthENoPE v3.0 code.
  • Figure 2: Impact of marginalization over nuclear reaction rates for the PRyMordial code, compared to the PArthENoPE v3.0 adopted uncertainties (as described in the text). The remaining width of the PRyMordial code without marginalization directly represents the uncertainty of the deuterium determination.
  • Figure 3: Comparison of the $\Omega_b h^2$ constraints in the $\Lambda$CDM model between different codes (or settings) for the same underlying data titled "BAO+BBN papers" in \ref{['tab:bbn_data']}.
  • Figure 4: Comparison of the baryon abundance constraints from different data sets when using the PArthENoPE v3.0 code (top left), using the PRyMordial code in NACRE II mode (top right), and using the PRyMordial code in its PRIMAT mode (bottom). For the datasets, see \ref{['tab:bbn_data']}.
  • Figure 5: A comparison of various constraints in the $\Delta N_\mathrm{eff}-\Omega_b h^2$ plane. In black and blue we show the parameters compatible with the PDG-recommended measurements for the Deuterium and Helium abundances (mean+$1\sigma$), respectively (see \ref{['tab:bbn_data']}). In green we show the parameters compatible with the EMPRESS Helium abundance measurement (mean+$1\sigma$). The BBN computations were performed here with PArthENoPE v3.0. Finally, the red contours show the baryon abundance ($1\sigma$ and $2\sigma$) from Planck Planck:2018vyg as a comparison.
  • ...and 1 more figures