Primordial Nucleosynthesis — Big Bang First 3 Minutes

💫 Big Bang Nucleosynthesis

In the first 3 minutes of the universe, the density and temperature were high enough for nuclear reactions. As space expanded and cooled, protons and neutrons fused into deuterium, helium-4, and trace lithium-7.

🎛 Parameters

Baryon density Ω_b h² 0.022 Neutron lifetime τ_n (s) 880

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📐 Final Abundances

Y_p (⁴He)

0.247

D/H (×10⁻⁵)

2.5

³He/H (×10⁻⁵)

1.0

⁷Li/H (×10⁻¹⁰)

4.7

ℹ️ Physics

n/p freeze-out: weak interactions decouple at T ≈ 0.8 MeV (t ≈ 1 s), locking n/p ≈ 1/7.

Deuterium bottleneck: D forms only below T ≈ 0.07 MeV because photons destroy it above; this triggers chain reactions D+D→³He, ³He+D→⁴He.

Y_p ≈ 2(n/p)/(1+n/p): most neutrons end up in ⁴He. Higher Ω_b → more D destruction → slightly higher Y_p.