Baryon acoustic oscillations

Baryon acoustic oscillations (BAO) are fluctuations in the density of the visible baryonic matter of the universe caused by acoustic density waves in the primordial plasma of the early universe. These waves created a standard ruler that can be measured by looking at the large scale structure of matter using astronomical surveys. The length of this standard ruler is approximately 490 million light years in today's universe.

The standard ruler provided by BAO helps cosmologists understand more about the nature of dark energy, which causes the accelerating expansion of the universe. By measuring the large scale structure of matter, scientists can constrain cosmological parameters and gain insights into the properties of dark energy.