Primordial nuclear freeze-out and the deuterium bottleneck
In hot Big-Bang nucleosynthesis, neutron–proton conversion initially approaches chemical equilibrium.
Key knowledge
In hot Big-Bang nucleosynthesis, neutron–proton conversion initially approaches chemical equilibrium. As its interaction rate Γ becomes too slow relative to expansion H, equilibrium tracking fails. Free neutrons subsequently decay until incorporated into nuclei. Deuterium formation is delayed by photodissociation from the high-energy photon tail; having k_BT below its binding energy alone is insufficient when photons greatly outnumber baryons. Once the bottleneck eases, a reaction network assembles light nuclei; later dilution and cooling limit further processing.
Values and equations
Γ/H compares interaction and expansion rates; nₙ/nₚ≈exp[−(mₙ−mₚ)c²/(k_BT)] in the stated equilibrium limit
- Γ,H
- interaction rate per particle and Hubble expansion rate s⁻¹
- nₙ,nₚ
- neutron and proton number densities m⁻³
- mₙ,mₚ
- neutron and proton masses kg
- c
- vacuum light speed m s⁻¹
- k_B
- Boltzmann constant J K⁻¹
- T
- plasma temperature K
Conditions and scope
- Γ/H is a rate comparison, not a sharp universal switch. Equilibrium nₙ/nₚ≈exp[−(mₙ−mₚ)c²/(k_BT)] additionally assumes nondegenerate species and negligible relevant chemical potentials.
- The abundance calculation requires baryon-to-photon ratio, expansion history, neutron lifetime and actual nuclear rates. No current numerical abundance fit, universal lithium-problem solution or creation of all heavy elements in BBN is asserted.
Sources · 1
References
Sources retain their recorded scope. A source link alone does not establish verification of every statement.
Evidence and review
Read the original review's weak-rate/expansion comparison, neutron decay, photon-to-baryon deuterium bottleneck and bounded light-nucleus network passages. Restored c² and k_B in the equilibrium relation given in source natural units. Excluded dated fits and the source's approximate high-energy-tail expression interpreted as an exact integral or instantaneous freeze-out rule.
2026-10-10 · Independent AI editorial review: All44 exact root-authored compact records, equations/variable units/conditions and140 origin edges accepted after independently reading final6/18 with bounded primary bodies and matching prior38/122 scientific fingerprints unchanged. Exact claim/edge review is distinct from combined inventory scope and separate CLE26 approval; no entire cited books or old-corpus experimental revalidation is asserted. Receipt: cosmos-completion-independent-content-and-inventory-review.1.
Entry details
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- uni-cosmos-primordial-nuclear-freezeout
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- core.1
- Structured reference record
- University core expansion