💫 Primordial Nucleosynthesis (2D)

In the first 3 minutes after the Big Bang, temperature and density were high enough for nuclear fusion. As the universe expanded and cooled, protons and neutrons combined into deuterium, helium-4, and trace lithium-7. This abundance-vs-temperature plot is the standard textbook view of that process.

🎛 Parameters

📊 Live Readout

Cosmic time t
0.001 s
Temperature T
1.50 MeV
¹H fraction
1.000
⁴He fraction
0.000
D/H (×10⁻⁵)
2.5
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 that; this triggers D+D→³He, ³He+D→⁴He.

Y_p ≈ 2(n/p)/(1+n/p): nearly all surviving neutrons end up in ⁴He. A higher η burns more D into He, so D/H falls and Y_p rises slightly with η.