Baryon Acoustic Oscillations

Baryon Acoustic Oscillations (BAO) are regular, large-scale ripples in the distribution of matter in the universe, caused by pressure waves in the hot, early universe's plasma. When the universe was young, these sound waves traveled through the dense plasma of particles, creating regions of slightly higher density that eventually formed galaxies and galaxy clusters. Today, BAO appear as a consistent preferred separation scale between galaxies, acting like a cosmic "standard ruler." By measuring these patterns, scientists can better understand the universe’s expansion history, its composition, and dark energy.