Baryon acoustic oscillations

Baryon acoustic oscillations are pressure waves in the early universe’s photon–baryon plasma that left a characteristic distance scale in the later distribution of matter and galaxies.

Photon–baryon fluid: The tightly coupled mixture of photons and baryons that filled the early universe before recombination. Its pressure and gravity drove the waves that created the acoustic scale.
Baryon acoustic oscillation survey: An astronomical survey that maps large-scale structure to measure the baryon acoustic scale. These surveys turn galaxy positions into measurements of the standard ruler.
Ralph Alpher: An American physicist whose work helped establish modern physical cosmology and primordial nucleosynthesis. Alpher and Robert Herman discussed sound waves in the early-universe plasma in 1970.
Baryon acoustic oscillation peak: The enhanced galaxy-pair clustering near the characteristic sound-horizon separation. This localized peak is the configuration-space expression of the broader oscillatory signal.
Sound-horizon calibration: The determination of the sound horizon’s physical length from early-universe physics and observations. Distance estimates from oscillations depend on this calibration, which is sensitive to the pre-recombination expansion history.
Recombination (cosmology): The era when electrons and nuclei combined, allowing photons to travel freely through the universe. Photon decoupling froze the plasma’s wave pattern into the matter distribution.
Dark energy: The name for the unknown cause of the universe’s observed accelerated expansion. Comparing the acoustic ruler across cosmic times constrains dark energy’s influence.
Robert Herman: An American physicist who co-developed early predictions of the cosmic microwave background. His work with Alpher connected plasma sound waves to large-scale cosmic structure.
Cosmic microwave background acoustic peaks: A sequence of temperature and polarization peaks in the cosmic microwave background’s angular power spectrum. These peaks are related plasma oscillations observed in radiation, not the late-time galaxy distribution.
Hubble tension: The disagreement between measurements of the present expansion rate from the early and nearby universe. Acoustic-scale inferences contribute to comparisons between early-universe and local expansion measurements.

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