Baryon acoustic oscillations
In cosmology, baryon acoustic oscillations (BAO) are fluctuations in the density of the visible baryonic matter (normal matter) of the universe, caused by acoustic density waves in the primordial plasma of the early universe.
The length of this standard ruler (≈490 million light years in today's universe[3]) can be measured by looking at the large scale structure of matter using astronomical surveys.
[3] BAO measurements help cosmologists understand more about the nature of dark energy (which causes the accelerating expansion of the universe) by constraining cosmological parameters.
[2] The early universe consisted of a hot, dense plasma of electrons and baryons (which include protons and neutrons).
[4] The cosmic microwave background (CMB) radiation is light that was scattered just before, and emitted by, recombination, now seen with our telescopes as radio waves all over the sky since it is red-shifted.
[4] WMAP indicates (Figure 1) a smooth, homogeneous universe with density anisotropies of 10 parts per million.
While this region of overdensity gravitationally attracts matter towards it, the heat of photon-matter interactions creates a large amount of outward pressure.
The dark matter interacts only gravitationally, and so it stays at the center of the sound wave, the origin of the overdensity.
[3] Without the photo-baryon pressure driving the system outwards, the only remaining force on the baryons was gravitational.
[11] As an analogy, imagine dropping many pebbles into a pond and watching the resulting wave patterns in the water.
The physics of the propagation of the baryon waves in the early universe is fairly simple; as a result cosmologists can predict the size of the sound horizon at the time of recombination.
A better understanding of the acceleration of the universe, or dark energy, has become one of the most important questions in cosmology today.
In order to understand the nature of the dark energy, it is important to have a variety of ways of measuring the acceleration.
[3] Thus BAO provides a measuring stick with which to better understand the nature of the acceleration, completely independent from the supernova technique.
The Sloan Digital Sky Survey (SDSS) is a major multi-spectral imaging and spectroscopic redshift survey using the dedicated 2.5-metre wide-angle SDSS optical telescope at Apache Point Observatory in New Mexico.
The goal of this five-year survey was to take images and spectra of millions of celestial objects.
The SDSS catalog provides a picture of the distribution of matter in a large enough portion of the universe that one can search for a BAO signal by noting whether there is a statistically significant overabundance of galaxies separated by the predicted sound horizon distance.
The SDSS team looked at a sample of 46,748 luminous red galaxies (LRGs), over 3,816 square-degrees of sky (approximately five billion light years in diameter) and out to a redshift of z = 0.47.
The BAO signal would show up as a bump in the correlation function at a comoving separation equal to the sound horizon.
[3][14] SDSS confirmed the WMAP results that the sound horizon is ~150 Mpc in today's universe.
[18] Both teams are credited and recognized for the discovery by the community as evidenced by the 2014 Shaw Prize in Astronomy[19] which was awarded to both groups.
[22] The BAO in the radial and transverse directions provide measurements of the Hubble parameter and angular diameter distance, respectively.
The angular diameter distance and Hubble parameter can include different functions that explain dark energy behavior.
The Friedmann equations express the expansion of the universe in terms of Newton's gravitational constant,
Therefore, the following are possible explanations:[26] In order to differentiate between these scenarios, precise measurements of the Hubble parameter as a function of redshift are needed.
Measurements of the CMB from WMAP put tight constraints on many of these parameters; however it is important to confirm and further constrain them using an independent method with different systematics.
The BAO signal is a standard ruler such that the length of the sound horizon can be measured as a function of cosmic time.
