Sound waves in the early Universe, frozen at recombination — a standard ruler carved into the large-scale structure
BAO: Before recombination (z≈1100), photons and baryons formed a tightly coupled plasma. Dark matter perturbations seeded acoustic waves that propagated outward at the sound speed cs ≈ c/√3. At recombination (~380,000 years), these waves froze, imprinting a characteristic scale of ~150 Mpc in the galaxy distribution. This BAO peak serves as a standard ruler for measuring cosmic expansion history.