What are baryon acoustic oscillations?

Baryon acoustic oscillations (BAOs) are a pattern of sound waves that originated in the early universe, shortly after the Big Bang. These waves left an imprint on the distribution of matter in the universe, which can still be observed today. Baryons are particles that make up ordinary matter, such as protons and neutrons, while dark matter is an invisible substance that makes up most of the mass in the universe. The pattern of baryon acoustic oscillations provides a standard ruler that astronomers can use to measure the expansion history of the universe.

How do baryon acoustic oscillations form?

Baryon acoustic oscillations formed during the first few hundred thousand years after the Big Bang, when the universe was hot and dense. At this time, radiation and matter were tightly coupled, and sound waves could propagate through the plasma. However, as the universe expanded and cooled, the radiation and matter decoupled, freezing the sound waves in place. These frozen waves correspond to fluctuations in the density of matter over large scales, about 500 million light-years across.

Measuring baryon acoustic oscillations

Measuring the pattern of baryon acoustic oscillations requires observing the large-scale structure of the universe. Astronomers use telescopes to observe the positions and distances of galaxies in the sky. By studying how the density of galaxies varies over large distances, they can infer the size of the baryon acoustic oscillations. The most precise measurements of BAOs come from surveys of hundreds of thousands of galaxies, such as the Sloan Digital Sky Survey.

Example: Importance of baryon acoustic oscillations

The measurement of baryon acoustic oscillations provides a powerful tool for studying the expansion history of the universe. By comparing the size of the BAOs at different distances, astronomers can measure how the universe has expanded over time. This has led to the discovery that the expansion of the universe is accelerating, which is attributed to the mysterious dark energy. BAO measurements also help to determine other cosmological parameters, such as the curvature of the universe and the density of dark matter. As such, baryon acoustic oscillations are a valuable tool for understanding the fundamental properties of the universe.