Baryon Acoustic Oscillations (BAO)

Baryon Acoustic Oscillations (BAO) refer to the pattern of regular ripples in the distribution of matter across the universe, originating from sound waves in the hot, dense early universe. When the universe was very young, pressure waves traveled through the hot plasma of particles, creating slight over-densities of matter at certain scales. After the universe cooled and particles combined into atoms, these imprints remained as a preferred distance scale (~500 million light-years), which we observe today as a subtle excess in galaxy clustering. Studying BAO helps astronomers measure the universe’s expansion rate and understand the nature of dark energy.