Same equations as the 3D version, drawn on a plain 2D canvas instead of a WebGL scene. Temperature falls as T(t) ≈ 1.3·t-1/2 MeV (radiation-dominated era). Above the freeze-out temperature (~0.8 MeV, t≈2s) the neutron/proton ratio tracks thermal equilibrium n/p = exp(-Δmc²/T); below it, it freezes and decays with the 880s neutron lifetime. A logistic "synthesis progress" curve centred near t≈345s·(0.022/Ωbh²) converts free neutrons into deuterium, then helium-4, with density-scaled deuterium/helium-3/lithium-7 yields.