Baryon Acoustic Oscillations — The Frozen Sound Wave

In the first few hundred thousand years after the Big Bang, the universe was a hot, opaque plasma of photons and baryons locked together by Thomson scattering. Density ripples in that plasma propagated outward as genuine sound waves, at speeds set by radiation pressure. This simulation numerically integrates the real cosmological sound-speed and sound-horizon equations to grow a spherical wavefront from a point-like seed, then freezes it the instant recombination decouples photons from baryons at z ≈ 1090 — reproducing the ≈144 Mpc comoving Baryon Acoustic Oscillation scale that cosmologists still measure today in the clustering of galaxies and the cosmic microwave background. A separate, faster shell tracks the photons that keep streaming outward at light speed after decoupling, visualising the difference between the frozen baryon ruler and the ever-growing photon horizon.