Cosmology
Baryon Acoustic Oscillations
Sound waves in early universe — frozen at recombination, leaving cosmic ruler
Baryon acoustic oscillations (BAO) are sound waves that propagated through the hot plasma of the early universe before recombination. Sound waves: pressure waves in baryon-photon plasma. At recombination (~380,000 yr after BB), waves froze — leaving distinctive imprint at characteristic scale ~150 Mpc (490 million ly). This "BAO scale" appears in galaxy distribution today as a "standard ruler." Used to measure universe geometry and dark energy. Detected in galaxy surveys (SDSS, BOSS, eBOSS, DESI). Independent confirmation of cosmological parameters.
- BAO scale~150 Mpc (~490 million ly)
- SourceSound waves in early universe plasma
- Frozen atRecombination (~380,000 yr after BB)
- Detected inSDSS, BOSS, eBOSS, DESI galaxy surveys
- Used asStandard ruler for cosmology
- MeasuresUniverse geometry, expansion history, dark energy
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Why BAO matters
- Cosmology. Independent measurements of expansion.
- Standard ruler. Distinct from Type Ia SN.
- Dark energy. Constrains equation of state.
- Hubble constant. Cross-check.
- Early universe. Probe of recombination physics.
- Survey science. SDSS, BOSS, DESI driving frontier.
- Cosmological tests. Tests of ΛCDM model.
Common misconceptions
- BAO is recent invention. Predicted decades ago; detected 2005.
- BAO is just CMB feature. Distinct large-scale structure feature.
- Sound waves still propagate. Frozen at recombination.
- BAO supports specific cosmology. Tests; results depend on model.
- Need CMB to use BAO. Independent measurement; cross-validation.
- BAO scale changes. Constant comoving scale.
Frequently asked questions
How do BAO form?
Early universe (before recombination): hot plasma. Density perturbations create pressure differences. Pressure waves (sound) propagate at c/√3 (sound speed in plasma). Waves spread outward from each density peak. At recombination: photons free-stream away; baryon-photon coupling ends. Sound waves freeze in place. Ripple pattern in mass density.
What's the BAO scale?
Distance sound waves traveled before recombination. Sound horizon at last scattering: r_d ≈ 150 Mpc (in physical coordinates today). This sets characteristic scale: nearby galaxies cluster at this scale slightly more than expected. Statistical signal — small effect (~1% over and above smooth distribution) but precise.
How is it detected?
Galaxy surveys with millions of galaxies. Compute correlation function — galaxy-galaxy clustering as function of separation. Look for peak at BAO scale. SDSS (2005) first detected. BOSS, eBOSS, DESI continue with higher precision. Current: BAO scale measured to ~1% precision.
Why is it a "standard ruler"?
Same physical scale set by early-universe physics. We can calculate its physical size precisely from CMB (which was emitted at recombination). Measure angular/redshift size in galaxy survey → cosmological distance and expansion history. Independent of stellar physics — different systematic from Type Ia SN.
How does this help cosmology?
BAO measures: (1) Hubble parameter H(z) — expansion rate at different redshifts. (2) Angular diameter distance — universe geometry. (3) Equation of state of dark energy w(z). (4) Hubble constant H₀ when combined with CMB. Provides independent constraints — checks dark energy theories.
Is BAO consistent with other measurements?
Largely yes. Combined with CMB and Type Ia SN, BAO supports flat universe with dark energy ~70%, matter ~30%. BAO measurements consistent across surveys. Some tension with Hubble (H₀) but overall agreement strong.
What's DESI?
Dark Energy Spectroscopic Instrument (started 2021). Aims to measure 30+ million galaxy spectra. Will produce most precise BAO measurements. Targets: dark energy properties, modified gravity tests. First year data released 2024 — interesting hints about evolving dark energy. Active frontier.