Primordial Light Element Abundances
The proportions of hydrogen, helium, and trace light elements left over from the Big Bang, measured in pristine cosmic environments, provide a sensitive test of early-universe physics.
Definition
Primordial light element abundances are the relative amounts of the lightest nuclei produced by Big Bang nucleosynthesis, expressed as ratios to hydrogen or as mass fractions, as they existed before being modified by later stellar nucleosynthesis.
Scope
This topic covers the measured primordial abundances of deuterium, helium-3, helium-4, and lithium-7, the astronomical environments used to infer them before stellar processing alters them, the comparison of these measurements with nucleosynthesis predictions, and the outstanding lithium discrepancy.
Core questions
- What are the primordial abundances of the light elements?
- How are these abundances measured without contamination from stars?
- Why does the lithium abundance disagree with theory?
Key concepts
- Deuterium abundance
- Helium-4 mass fraction
- Helium-3
- Lithium-7
- Metal-poor environments
- Baryometer
- Lithium problem
Key theories
- Deuterium as a baryometer
- Deuterium is only destroyed, never produced, after the Big Bang, and its primordial abundance is highly sensitive to the baryon density, making measurements in pristine gas a precise probe of cosmic baryon content.
- Helium-4 mass fraction
- About a quarter of the baryonic mass emerged as helium-4, a robust prediction weakly dependent on baryon density but sensitive to the expansion rate and neutron lifetime, measured in metal-poor extragalactic gas.
Mechanisms
Astronomers measure deuterium in absorption lines of pristine gas clouds along quasar sightlines, helium-4 in emission from metal-poor dwarf galaxies, and lithium in the atmospheres of old halo stars, extrapolating to zero metallicity to recover the values set by Big Bang nucleosynthesis.
Clinical relevance
The agreement of measured deuterium and helium abundances with theory across many orders of magnitude validates the hot Big Bang and fixes the cosmic baryon density independently of the cosmic microwave background, while the persistent lithium discrepancy serves as a possible window onto new physics.
History
Early helium measurements in the 1970s supported the Big Bang, and high-resolution quasar spectroscopy from the 1990s onward delivered precise deuterium abundances; as measurements sharpened, the lithium-7 abundance emerged as a stubborn discrepancy with otherwise excellent agreement.
Debates
- The lithium problem
- Measured lithium-7 in old stars is several times lower than predicted from the cosmic-microwave-background baryon density, with proposed explanations ranging from stellar depletion to uncertain nuclear rates to physics beyond the standard model, none yet conclusive.
Key figures
- Gary Steigman
- Keith Olive
- Brian Fields
- David Tytler
Related topics
Seminal works
- cyburt2016
Frequently asked questions
- Why measure abundances in metal-poor environments?
- Stars produce and destroy light elements over cosmic time, so to recover the primordial values astronomers seek the most chemically pristine, metal-poor gas and stars, where contamination from later stellar processing is smallest.
- What is the lithium problem?
- It is the mismatch between the lithium-7 abundance predicted by Big Bang nucleosynthesis using the baryon density from the cosmic microwave background and the lower amount actually observed in ancient stars, a discrepancy that remains unexplained.