Big Bang Nucleosynthesis (BBN)

This interactive simulator explores Big Bang Nucleosynthesis (BBN) in Astronomy & The Sky. Light-element abundance curves H, ⁴He, D, ³He, ⁷Li vs cosmic time / temperature. Weak freeze-out, neutron decay gap, deuterium bottleneck → Y_p ≈ 0.245. Slide η₁₀ and N_eff over the classic BBN curves; observed values overlaid. Use the controls to change the scenario; watch the visualization and any graphs or readouts to connect the model with lectures, labs, and homework.

Who it's for: For learners comfortable with heavier math or second-level detail. Typical context: Astronomy & The Sky.

Key terms

big
bang
nucleosynthesis
bbn
big bang nucleosynthesis
astronomy

How it works

**Big-Bang nucleosynthesis (BBN)** — within the first ~3 minutes the universe forged the bulk of its primordial light elements. Watch the **n/p ratio** drop from equilibrium toward freeze-out at **T_F ≈ 0.8 MeV**, decay slowly through neutron β-decay, and then suddenly lock into **He-4** when the deuterium bottleneck breaks at **T ≈ 0.07 MeV**. Trace amounts of **D, ³He and ⁷Li** survive. The asymptotic abundances depend almost only on the **baryon-to-photon ratio η₁₀** (and a little on the **effective neutrino number N_eff**) — moving the slider walks you across the famous BBN curves and explains why measuring just **D/H** in distant quasars or **Y_p** in HII regions pins down Ω_b h² independently of the CMB. Right-edge ticks mark the observationally inferred values; note the long-standing ⁷Li discrepancy.

Key equations

T(t) = (1.32 s·MeV²)^{1/2} / √t (radiation era)

(n/p)_eq = exp(−Q/T), Q = m_n − m_p = 1.293 MeV

Y_p ≈ 2(n/p) / (1 + n/p) at BBN onset

D/H ∝ η₁₀^{−1.6}, ³He/H ∝ η₁₀^{−0.6}, ⁷Li/H : U-shape vs η

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