Baryonic Acoustic Oscillations (BAOs)

Baryonic Acoustic Oscillations (BAOs) are periodic fluctuations in the density of the visible baryonic matter (ordinary matter composed of protons and neutrons) of the universe, caused by pressure (acoustic) waves that propagated in the early universe. These oscillations serve as a “cosmic ruler” in cosmology, providing a standard length scale to measure the expansion history of the universe and to constrain models of dark energy and dark matter.

Background

In the first few hundred thousand years after the Big Bang, the universe was a hot, dense plasma of photons, electrons, and baryons (protons and neutrons). Photons scattered strongly with electrons through Thomson scattering, coupling radiation and matter into a single fluid. Within this plasma, density fluctuations generated regions of compression and rarefaction due to the opposing effects of gravitational attraction and photon pressure.
These oscillations were essentially sound waves travelling through the primordial plasma at about half the speed of light. When the universe cooled sufficiently (~380,000 years after the Big Bang), neutral atoms formed during recombination, photons decoupled from matter (releasing the cosmic microwave background (CMB)), and the acoustic oscillations were “frozen” into the distribution of matter.
The imprints of these oscillations can be observed today both in the anisotropies of the CMB and in the large-scale clustering of galaxies.

Mechanism of BAOs

The key processes that led to BAOs are:

• Initial Perturbations: Tiny quantum fluctuations in the early universe, amplified during inflation, created over-dense regions.
• Photon-Baryon Coupling: The plasma resisted gravitational collapse due to radiation pressure, generating oscillations (similar to sound waves in air).
• Freeze-out at Recombination: When photons decoupled, the baryons were left with a slight preference for a characteristic separation scale, which persists in the matter distribution.

The result was a preferred scale of ~150 megaparsecs (Mpc) (about 490 million light-years), often referred to as the sound horizon at recombination.

Observational Evidence

BAOs manifest themselves as a slight excess probability of finding pairs of galaxies separated by the sound horizon scale. Detecting BAOs requires large galaxy surveys and statistical analysis of clustering patterns:

• Cosmic Microwave Background (CMB): The first evidence of acoustic oscillations came from measurements of CMB anisotropies by missions such as WMAP and Planck, which revealed distinct peaks in the angular power spectrum corresponding to oscillations in the early plasma.
• Galaxy Redshift Surveys: The Sloan Digital Sky Survey (SDSS) and later surveys, such as BOSS (Baryon Oscillation Spectroscopic Survey), directly detected the BAO signal in the large-scale distribution of galaxies.
• Lyman-Alpha Forest: BAOs have also been observed in the distribution of intergalactic gas clouds, offering insights at higher redshifts.

Cosmological Significance

BAOs serve as a cosmological standard ruler, allowing scientists to measure the geometry and expansion history of the universe with high precision:

• Measuring Dark Energy: By comparing the observed scale of BAOs at different redshifts, cosmologists can track how the expansion rate has changed, thereby constraining models of dark energy.
• Determining Cosmological Parameters: BAOs provide independent measurements of the Hubble constant (H₀), matter density (Ωm), and curvature of the universe.
• Cross-Checks with Other Probes: Combined with supernova observations, gravitational lensing, and CMB data, BAOs strengthen the cosmological model known as ΛCDM (Lambda Cold Dark Matter).

Advantages and Limitations

Advantages:

• BAOs are relatively immune to systematic uncertainties compared to other cosmological probes.
• The physics of acoustic oscillations in the early universe is well understood.
• They provide an absolute distance scale tied to fundamental physics rather than astrophysical assumptions.

Limitations:

• The BAO signal is subtle (about a 1% effect in the clustering of galaxies), requiring very large surveys.
• Interpretation depends on accurate modelling of galaxy bias and redshift-space distortions.
• Precision is limited by cosmic variance, as measurements rely on large-scale structures.

Future Prospects

The study of BAOs continues to expand with next-generation surveys:

• DESI (Dark Energy Spectroscopic Instrument) aims to measure BAOs using millions of galaxies and quasars.
• Euclid (ESA) and the Nancy Grace Roman Space Telescope (NASA) will map BAOs across vast cosmic volumes with unprecedented accuracy.
• SKA (Square Kilometre Array) will explore BAOs using hydrogen line (21 cm) observations, probing deeper into cosmic history.