Big Bang Nucleosynthesis: 2015

Abstract: Big-bang nucleosynthesis (BBN) describes the production of the lightest nuclides via a dynamic interplay among the four fundamental forces during the first seconds of cosmic time. We briefly overview the essentials of this physics, and present new calculations of light element abundances through li6 and li7, with updated nuclear reactions and uncertainties including those in the neutron lifetime. We provide fits to these results as a function of baryon density and of the number of neutrino flavors, N_nu. We review recent developments in BBN, particularly new, precision Planck cosmic microwave background (CMB) measurements that now probe the baryon density, helium content, and the effective number of degrees of freedom, n_eff. These measurements allow for a tight test of BBN and of cosmology using CMB data alone. Our likelihood analysis convolves the 2015 Planck data chains with our BBN output and observational data. Adding astronomical measurements of light elements strengthens the power of BBN. We include a new determination of the primordial helium abundance in our likelihood analysis. New D/H observations are now more precise than the corresponding theoretical predictions, and are consistent with the Standard Model and the Planck baryon density. Moreover, D/H now provides a tight measurement of N_nu when combined with the CMB baryon density, and provides a 2sigma upper limit N_nu < 3.2. The new precision of the CMB and of D/H observations together leave D/H predictions as the largest source of uncertainties. Future improvement in BBN calculations will therefore rely on improved nuclear cross section data. In contrast with D/H and he4, li7 predictions continue to disagree with observations, perhaps pointing to new physics.

• The paper presents updated light element abundance calculations by incorporating revised nuclear reaction rates and neutron lifetime data.
• It leverages Planck CMB data to constrain baryon density and effective neutrino numbers, enhancing precision in early universe modeling.
• The research highlights the persistent lithium-7 discrepancy, suggesting potential deviations from Standard Model predictions and avenues for new physics.

Overview of "Big Bang Nucleosynthesis: 2015"

The paper "Big Bang Nucleosynthesis: 2015" by Cyburt et al. presents a comprehensive analysis of primordial nucleosynthesis, focusing on the synthesis of light elements in the early universe. This research is significant for understanding the physics of the early universe and serves as a test for the Standard Model of cosmology and particle physics.

Key Contributions

1. Updated Light Element Calculations: The authors provide updated calculations for the abundances of light elements up to lithium-7 ($^7\mathrm{Li}$), incorporating recent data on nuclear reaction rates and uncertainties, including a reevaluation of neutron lifetime. These updates are crucial given the sensitivity of Big Bang Nucleosynthesis (BBN) predictions to these parameters.
2. Constraints from Cosmic Microwave Background (CMB): Utilizing the precision measurements from the 2015 Planck mission, the paper leverages CMB data to probe parameters such as baryon density and effective neutrino degrees of freedom. This work highlights the utility of CMB data alone in constraining cosmological models and testing BBN scenarios.
3. Likelihood Analysis: By combining Planck data chains with BBN outputs and astronomical observations of light element abundances, the authors perform a robust likelihood analysis. This analysis assists in refining our understanding of the primordial abundances and their compatibility with theoretical predictions.
4. Deuterium and Helium Measurements: New observations provided a more precise determination of D/H (deuterium to hydrogen ratio), aligning well with the Standard Model predictions. The D/H ratio now constrains the effective number of neutrino species ($N_\nu$), with a two-sigma upper limit set at $N_\nu < 3.2$.
5. Lithium Problem: The research acknowledges the ongoing discrepancy between predicted and observed lithium-7 abundances, known as the "lithium problem." Despite improvements in BBN theory and observational techniques, this disagreement persists, suggesting potential avenues for new physics or revised astrophysical models.

Implications

• Practical Implications: This research enhances the precision of cosmological parameter estimates, contributing to a better understanding of the universe's composition and the conditions of the early universe. These insights are foundational for fields such as astrophysics and cosmology.
• Theoretical Implications: The study's findings, particularly regarding lithium-7, highlight areas where current models of particle physics might be incomplete, indicating potential new physics beyond the Standard Model. Future theoretical models need to address these discrepancies to provide a coherent understanding of early universe physics.
• Future Directions: The paper suggests that future progress in both theoretical and observational aspects of BBN will hinge on improving nuclear reaction data, especially concerning deuterium. As BBN remains a critical probe of fundamental physics, ongoing advancements in both CMB measurements and astronomical techniques will continue to refine our knowledge landscape.

In summary, "Big Bang Nucleosynthesis: 2015" offers pivotal advancements in our understanding of early universe nucleosynthesis, providing both a benchmark for current models and a foundation for future research exploring the intricacies of early cosmological conditions.