Baryon acoustic oscillations (BAO) are a key signature of the early universe that provide important constraints on cosmological models and the expansion history of the universe. BAO are sound waves that propagated through the early universe, leaving a distinctive imprint on the distribution of matter. By studying the clustering of galaxies and measuring the characteristic scale of these oscillations, astronomers can infer the expansion rate of the universe at different points in cosmic history. This allows for precise measurements of key cosmological parameters such as the density of dark energy and dark matter. BAO studies are an important tool for understanding the large-scale structure of the universe and testing theories of cosmic evolution.