Cosmic Recombination 2D — 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 2D simulator solves the real Saha ionization equation frame-by-frame from fundamental constants to drive the ionization fraction of a field 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. A synced curve panel plots the full X_e(z) transition for the chosen baryon density so you can see exactly where the current redshift sits on the recombination curve. Drag to pan the cloud and scroll to zoom, and watch the plasma of free protons and electrons settle into neutral gas — catching the exact moment the photon stops scattering and free-streams away at the last scattering surface that produced the cosmic microwave background.