The frozen imprint of sound waves in the plasma of the early Universe which became stuck in place when matter and radiation decoupled (see decoupling). Baryon acoustic oscillations result in a preferred scale of the clustering of baryons in the Universe. This has been detected as a slight tendency for galaxies to be found separated by the predicted scale (of about 490 million light years) more than at slightly larger or smaller scales. BAO provides a cosmic standard ruler, allowing large-redshift surveys to measure the expansion history of the Universe and hence provide constraints on the properties of dark energy.