Cosmic Recombination — The Saha Equation & Last Scattering

Roughly 380,000 years after the Big Bang, the expanding universe cooled past ≈3000 K and hydrogen nuclei captured free electrons for the first time — cosmological recombination. This simulator solves the real Saha ionization equation frame-by-frame from fundamental constants to drive the ionization fraction of a cloud of hydrogen atoms as you sweep redshift, while a single CMB photon random-walks between scattering events whose frequency is set by the physical mean free path. Watch the plasma of free protons and electrons settle into neutral gas, and catch the exact moment the photon stops scattering and free-streams away — the last scattering surface that produced the cosmic microwave background.