In an analogous fashion, approximately 370,000 years after the Big Bang, electrons and protons combined to form neutral hydrogen, "freezing" in place acoustic pressure waves that had been created when the universe first began to form structure. These pressure waves are called baryon acoustic oscillations (BAO) and the distance they have traveled is known as the sound horizon. This distance is just the speed of sound times the age of the universe when they froze. Just as there is an increased air density in a normal sound wave, there is a slight increase in the chance of finding lumps of matter, and therefore galaxies, separated by the sound horizon distance. Today, this sound horizon distance is about 450 million light years, and it provides a standard ruler for cosmological distance measurements.
DES will measure the clustering on the sky of hundreds of millions of galaxies at different distances from us. These measurements will determine the angular scale of the sound horizon for galaxies at different redshifts. Combining those measurements together will provide information on the history of the cosmic expansion rate that complements the Type Ia supernova measurements.