Baryon Acoustic Oscillations
A standard ruler imprinted by sound waves in the early photon-baryon plasma.
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Baryon Acoustic Oscillations
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Origin
Before recombination, photons and baryons behaved as a coupled plasma. Pressure-supported sound waves propagated through this medium.
Standard Ruler
The maximum distance such waves could travel before decoupling defines the sound horizon. This leaves a preferred scale in the distribution of galaxies.
Cosmological Use
BAO measurements constrain the expansion history by comparing the observed scale to the predicted sound horizon.
Complete Cosmology
A standard introductory path through expansion, Friedmann dynamics, distance scales, and the thermal history of the universe.
Cosmic Microwave Background
Relic blackbody radiation from the epoch when photons decoupled from matter.
Matter-Radiation Equality
The epoch when matter and radiation energy densities were equal.