baryon acoustic oscillations

History

American astrophysicist Jim Peebles and Russian scientist Yuri Zeldovich predicted this effect in the 1970s, and in 2005 it was first clearly detected in data from the Sloan Digital Sky Survey.

How it works

In the ancient plasma, gravity compresses matter, but light pushes it back — oscillations begin, like in a musical instrument. When the Universe suddenly becomes transparent (the recombination epoch), light escapes, and the 'sound' freezes. The loudest peak of these waves sets a standard scale that we see as a preferred separation of galaxies.

History

American astrophysicist Jim Peebles and Russian scientist Yuri Zeldovich predicted this effect in the 1970s, and in 2005 it was first clearly detected in data from the Sloan Digital Sky Survey.

How it works

In the ancient plasma, gravity compresses matter, but light pushes it back — oscillations begin, like in a musical instrument. When the Universe suddenly becomes transparent (the recombination epoch), light escapes, and the 'sound' freezes. The loudest peak of these waves sets a standard scale that we see as a preferred separation of galaxies.

History

The concept of sound waves in prestellar matter dates back to work by Evgeny Lifshitz (1946) on gravitational instability. In the 1970s, Jim Peebles and Yuri Zeldovich predicted that these waves would leave an observable trace. The first convincing detection of the BAO peak was made in 2005 by the SDSS collaboration using a sample of red galaxies. Research on nucleosynthesis by George Gamow and Ralph Alpher played a key role in determining the composition of the primordial plasma, which allowed calculation of oscillation parameters.

How it works

Before recombination, baryons and photons are tightly coupled by scattering. Gravitational potentials of dark matter attract the baryon-photon fluid; compression increases photon pressure, causing a rebound — this is an acoustic wave. Its speed is determined by the density ratio of baryons to photons: c_s = c / sqrt(3(1+R)), where R = 3ρ_b/4ρ_γ ≈ 0.6 at recombination. By the time of deionization, different oscillation modes freeze in various phases, creating a series of peaks in the power spectrum. The dominant peak corresponds to the wave that traveled a distance r_s ≈ ∫ c_s dt / a(t) from the Big Bang to recombination. Today this scale is seen as a preferred distance between galaxies. Measuring the BAO peak in radial and transverse directions yields the Hubble parameter H(z) and angular diameter distance D_A(z), which is critically important for testing dark energy models.

Open problems & fun facts

Systematic errors in photometric redshifts limit the accuracy of the Hubble diagram; Nonlinear structure evolution and galaxy biases blur and slightly shift the BAO peak; Uncertainty in the sound horizon value requires independent calibrations from the early Universe, e.g., CMB data

💡 In 2014, BAO data detected the accelerated expansion stage of the Universe at a significance level above 5σ independently of type Ia supernovae.