The Primordial Lithium Problem (1203.3551v1)

Abstract: Big-bang nucleosynthesis (BBN) theory, together with the precise WMAP cosmic baryon density, makes tight predictions for the abundances of the lightest elements. Deuterium and 4He measurements agree well with expectations, but 7Li observations lie a factor 3-4 below the BBN+WMAP prediction. This 4-5\sigma\ mismatch constitutes the cosmic "lithium problem," with disparate solutions possible. (1) Astrophysical systematics in the observations could exist but are increasingly constrained. (2) Nuclear physics experiments provide a wealth of well-measured cross-section data, but 7Be destruction could be enhanced by unknown or poorly-measured resonances, such as 7Be + 3He -> 10C^* -> p + 9B. (3) Physics beyond the Standard Model can alter the 7Li abundance, though D and 4He must remain unperturbed; we discuss such scenarios, highlighting decaying Supersymmetric particles and time-varying fundamental constants. Present and planned experiments could reveal which (if any) of these is the solution to the problem.

• The paper highlights a 3-4 times lower observed lithium-7 abundance compared to Big Bang Nucleosynthesis predictions.
• It examines potential causes including measurement uncertainties in low-metallicity stars and adjustments to nuclear reaction rates.
• It explores beyond Standard Model physics, such as decaying dark matter and variable fundamental constants, as promising avenues for resolution.

An Expert Review of "The Primordial Lithium Problem"

The paper "The Primordial Lithium Problem" by Brian D. Fields discusses one of the significant discrepancies in cosmological observations and theoretical predictions concerning the nucleosynthesis of light elements immediately following the Big Bang. This issue, commonly referred to as the Lithium Problem, arises due to a mismatch between the observed abundance of lithium-7 ($^7\text{Li}$) in the universe and the theoretical predictions made by Big Bang Nucleosynthesis (BBN) models and the cosmic baryon density measured by WMAP.

Big Bang Nucleosynthesis and Observable Discrepancies

BBN remains a vital theoretical framework for understanding the production of light elements such as deuterium, helium-3, helium-4, and lithium-7 in the early universe, mere seconds to minutes after the Big Bang. While observations of deuterium and helium-4 largely align with the predicted abundances, lithium-7 remains a notable exception. The observed abundance of $^7\text{Li}$ is consistently a factor of 3-4 below the values derived from BBN theory, corresponding to a statistically significant 4-5σ discrepancy.

Addressing Potential Solutions

1. Astrophysical Explanations: The potential errors in the observational data regarding lithium-7 might arise due to astrophysical factors like uncertainties in determining the stellar temperature scale or systematic issues in measuring the lithium abundance in low-metallicity stars. However, this explanation remains tentative, largely due to the lack of observational verification of increased lithium abundances near the predicted BBN values.
2. Nuclear Physics Considerations: The paper thoroughly evaluates whether adjustments in nuclear reaction rates within the BBN could bridge this discrepancy. Proposed reactions like $\text{Be}^7+\text{d} \rightarrow \text{B}^9$ or other resonances have been explored as potential solutions, yet empirical data often fail to support them significantly.
3. Beyond Standard Model Physics: Exploring physics beyond the Standard Model, including decaying dark matter or variations in fundamental constants, remains a fertile ground for resolving the lithium anomaly. Supersymmetric particle decay scenarios and time-variant interaction constants have been posited to potentially alleviate the discrepancy, although these require more substantial empirical evidence and theoretical development.

Implications and the Future Outlook

The resolution of the lithium problem bears significant implications for both cosmology and nuclear physics. Practically, it influences the understanding of stellar processes and nucleosynthetic pathways. Theoretically, discrepancies might indicate new physics beyond the well-established models, potentially necessitating revisions in the standard understanding of the early universe's conditions.

Future work should focus on enhanced observations of primordial lithium in extragalactic environments or refined constraints from cosmic microwave background experiments, which may substantiate or refute existing nucleosynthetic models. Large Hadron Collider (LHC) findings regarding potential supersymmetric partners could provide tangible pathways to reconciling these differences.

Overall, "The Primordial Lithium Problem" remains an open question at the intersection of astrophysics, particle physics, and cosmology, with significant consequences for our understanding of the universe's origin and composition. Continued interdisciplinary efforts are crucial in addressing this complex issue.