Baryon acoustic oscillations

In the young universe, photons, electrons and ordinary matter were coupled in a hot plasma. Small density variations attracted matter through gravity, while pressure from radiation resisted compression. The competition launched sound waves that moved outward from denser regions. As the universe expanded and cooled, electrons combined with nuclei to form neutral atoms. Photons then travelled more freely, pressure support weakened, and the waves stopped propagating. Their influence remained as a subtle excess of matter at a preferred distance from the original density variations.

Over cosmic time, gravity amplified small density differences and helped build galaxies and larger structures. The primordial wave pattern therefore appears statistically in the distribution of galaxies: pairs separated by a particular large distance are slightly more common than pairs at nearby separations. This is not a sharply visible ring around every galaxy; it is a weak statistical feature that must be measured across very large galaxy surveys. The feature is also imprinted in the cosmic microwave background, the relic radiation released when the early universe became transparent.

The characteristic BAO scale is calibrated using physics of the early universe, including measurements of the cosmic microwave background. Because it provides a reference length, astronomers can compare its apparent size and orientation in galaxy maps at different redshifts. Those observations constrain distances and the expansion history of the universe, and can help test models of dark energy and gravity. The inference depends on careful modelling and cross-checking: BAO measurements constrain cosmological models rather than independently proving a particular explanation for cosmic acceleration.

The BAO feature was identified in galaxy-survey data in 2005 by independent teams analysing the Sloan Digital Sky Survey and the 2-degree Field Galaxy Redshift Survey. Subsequent spectroscopic surveys improved measurements by mapping large samples of galaxies and measuring their redshifts. NASA describes the present-day characteristic separation as roughly 500 million light-years; an often-used comoving scale is about 150 megaparsecs. These figures express the same approximate large-scale feature in different units, with cosmological distance conventions important when comparing measurements.