What are Baryon Acoustic Oscillations?
A Baryon is ordinary matter, that is protons, neutrons and other massive but unstable heavier particles called hyperons. At the time of the Big Bang, and for about 400,000 years afterwards, the density of the universe kept photons and Baryons interlocked or entangled together. Only as the universe cooled and expanded did the decoupling take place. So photons now were able to travel largely unimpeded and the speed of sound caused by the decoupling fell as matter began to take shape from separate atoms into clusters of atoms.
