The Frozen Ripples

In the very early universe, when electrons, protons, and photons were all mixed up in a soup and interacting with each other, sound waves propagating in the soup produced rarefaction and compression of the baryonic matter. When the photons stopped interacting with the matter particles, the ripples froze—and the frozen wave patterns expanded with the expansion of the universe. These patterns ultimately gave rise to the large-scale structure of the universe, consisting of superclusters of galaxies. This chapter is on that frozen ripples, called Baryon Acoustic Oscillation (BAO).