Astrophysics & Particle Physics-APPC 2026

Baryon Acoustic Oscillations (BAO) are faint, periodic matter distribution variations in the universe, left behind by sound waves in the early universe's hot plasma. These fluctuations began approximately 380,000 years following the Big Bang, at the stage of recombination, when photons detached from matter to give rise to the Cosmic Microwave Background (CMB).Competition between radiation pressure and gravitational attraction in the early universe generated pressure waves—"cosmic sound waves"—that traveled in the baryon-photon plasma. When the universe cooled and photons decoupled, these oscillations were imprinted into the matter distribution, resulting in a characteristic scale of approximately 150 million parsecs (≈500 million light-years). This scale was inserted in the large-scale structure of galaxies and is a cosmic standard ruler. Cosmologists can trace the expansion of the universe by measuring the BAO scale at various redshifts. This is one of the strongest and most self-consistent techniques for limiting the properties of dark energy, the enigmatic entity that governs the accelerated expansion of the universe.BAO had its beginnings with observations from the Sloan Digital Sky Survey (SDSS) and since then has been further improved with surveys such as BOSS (Baryon Oscillation Spectroscopic Survey), eBOSS, and WiggleZ, and future missions such as DESI (Dark Energy Spectroscopic Instrument), Euclid, and the Nancy Grace Roman Space Telescope hold the promise for even greater precision.

These observations supplement other cosmological probes such as the CMB anisotropies, supernovae, and weak lensing to check the validity of the standard ΛCDM cosmological model.Together with dark energy, the BAO also evidence cosmic inflation because their origin can be traced to primordial fluctuations smoothened out over large scales. They also assist in sharpening measurements of the Hubble constant (H₀), fueling the controversy among varying determinations of the rate of expansion of the universe.Summing up, baryon acoustic oscillations are fossil sound waves serving as a cosmic ruler, allowing astronomers to trace the history of the expansion of the universe, explore the nature of dark energy, and uncover more secrets about the early universe.