Big Bang Nucleosynthesis
Light Element Formation
Why nuclei form early
Big Bang nucleosynthesis (BBN) occurred when the Universe was about minutes old and hot enough for nuclear reactions but cool enough for deuterium to survive against rapid photodissociation. BBN is one of the strongest tests of the hot Big Bang because it predicts primordial light-element abundances from only a few parameters, especially the baryon density.
Main BBN products
| Species | Why it matters |
|---|---|
| Deuterium | Sensitive baryometer |
| Helium-4 | Tracks neutron-to-proton ratio |
| Helium-3 | Secondary product of nuclear pathways |
| Lithium-7 | Important but historically difficult to match exactly |
Deuterium is special
Deuterium is fragile, so its final abundance is highly sensitive to the baryon density and reaction rates.
What does the primordial deuterium abundance most directly constrain?
Deuterium is an excellent probe of the baryon-to-photon ratio and thus the baryon density.
Correct answer: The baryon density
Why can deuterium not survive until temperatures are very high?
At early times, photon energies are sufficient to break apart weakly bound deuterium nuclei.
Correct answer: High-energy photons photodissociate it
Reasoning through BBN
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1
Estimate the temperature and time window for nuclear reactions.
-
2
Use the neutron-to-proton ratio to infer helium production.
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3
Track how deuterium bottleneck release enables heavier nuclei.
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4
Compare predicted abundances with observations.