Baryon acoustic oscillations
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In cosmology, baryon acoustic oscillations (BAO) refers to regular, periodic fluctuations in the density of the visible baryonic matter of the universe, caused by acoustic waves which existed in the early universe. In the same way that supernova experiments provide a "standard candle" for astronomical observations, BAO matter clustering provides a "standard ruler" for length scale in cosmology. The length of this standard ruler (~490 million light years in today's universe) can be measured by looking at the large scale structure of matter using astronomical surveys. BAO measurements help cosmologists understand more about the nature of dark energy (which causes the apparent slight acceleration of the expansion of the universe) by constraining cosmological parameters.
The Early Universe 
The early universe consisted of a hot, dense plasma of electrons and baryons (protons and neutrons). Photons (light particles) traveling in this universe were essentially trapped, unable to travel for any considerable distance before interacting with the plasma via Thomson scattering. As the universe expanded, the plasma cooled to below 3000 K—a low enough energy such that the electrons and protons in the plasma could combine to form neutral hydrogen atoms. This recombination happened when the universe was around 379,000 years old, or at a redshift of z = 1089. Photons interact to a much lesser degree with neutral matter, therefore at recombination the universe suddenly became transparent to photons, allowing them to decouple from the matter and free-stream through the universe. Technically speaking, the mean free path of the photons became on the order of the size of the universe. The cosmic microwave background (CMB) radiation is light emitted after recombination which is only now reaching our telescopes. Therefore when we look at Wilkinson Microwave Anisotropy Probe (WMAP) data, we are looking back in time to see an image of the universe when it was only 379,000 years old.
WMAP indicates (Figure 1) a smooth, homogeneous universe with density anisotropies of 10 parts per million. However, when we observe the universe today we find large structure and density fluctuations. Galaxies, for instance, are a million times more dense than the universe's mean density. The current belief is that the universe was built in a bottom-up fashion, meaning that the small anisotropies of the early universe acted as gravitational seeds for the structure we see today. Overdense regions attract more matter, while underdense regions attract less, and thus these small anisotropies we see in the CMB become the large scale structures we observe in the universe today.
Cosmic Sound 
Imagine an overdense region of the primordial plasma. While this overdensity gravitationally attracts matter towards it, the heat of photon-matter interactions creates a large amount of outward pressure. These counteracting forces of gravity and pressure create oscillations, analogous to sound waves created in air by pressure differences.
Consider a single wave originating from this overdense region in the center of the plasma. This region contains dark matter, baryons and photons. The pressure results in a spherical sound wave of both baryons and photons moving with a speed slightly over half the speed of light outwards from the overdensity. The dark matter only interacts gravitationally and so it stays at the center of the sound wave, the origin of the overdensity. Before decoupling, the photons and baryons move outwards together. After decoupling the photons are no longer interacting with the baryonic matter so they diffuse away. This relieves the pressure on the system, leaving a shell of baryonic matter at a fixed radius. This radius is often referred to as the sound horizon. Without the photo-baryon pressure driving the system outwards, the only remaining force on the baryons is gravitational. Therefore, the baryons and dark matter (still at the center of the perturbation) form a configuration which includes overdensities of matter both at the original site of the anisotropy and in a shell at the sound horizon.
The ripples in the density of space continue to attract matter and eventually galaxies formed in a similar pattern, therefore one would expect to see a greater number of galaxies separated by the sound horizon than by nearby length scales. This particular configuration of matter occurred at each anisotropy in the early universe, and therefore the universe is not composed of one sound ripple, but many overlapping ripples. As an analogy, imagine dropping many pebbles into a pond and watching the resulting wave patterns in the water. It is not possible to observe this preferred separation of galaxies on the sound horizon scale by eye, but one can measure this signal statistically by looking at the separations of large numbers of galaxies.
Standard Ruler 
The physics of the propagation of the baryon waves in the early universe is fairly simple, so cosmologists can predict the size of the sound horizon at recombination. In addition the CMB provides a measurement of this scale to high accuracy. However in the time between recombination and present day, the universe has been expanding. This expansion is well supported by observations and is one of the foundations of the Big Bang Model. In the late 90's, observations of supernova determined that not only is the universe expanding, it is expanding at an increasing rate. Better understanding 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 this acceleration. BAO can add to the body of knowledge about this acceleration by comparing observations of the sound horizon today (using clustering of galaxies) to the sound horizon at the time of recombination (using the CMB). Thus BAO provides a measuring stick with which to better understand the nature of the acceleration, completely independent from the supernova technique.
BAO Signal in the Sloan Digital Sky Survey 
The Sloan Digital Sky Survey (SDSS) is a 2.5-m wide-angle 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 result of compiling the Sloan data is a three-dimensional map of the objects in the nearby universe. The SDSS catalog provides a picture of the distribution of matter such that one can search for a BAO signal by seeing if there is a larger number of galaxies separated at the sound horizon.
The Sloan Team looked at a sample of 46,748 luminous red galaxies (LRGs), over 3816 square-degrees of sky (approximately five billion light years in diameter) and out to a redshift of z = 0.47. They analyzed the clustering of these galaxies by calculating a two-point correlation function on the data. The correlation function () is a function of comoving galaxy separation distance (s) and describes the probability that one galaxy will be found within a given distance bin of another. One would expect a high correlation of galaxies at small separation distances (due to the clumpy nature of galaxy formation) and a low correlation at large differences. The BAO signal would show up as a bump in the correlation function at a comoving separation equal to the sound horizon. This signal was detected by the SDSS team in 2005. SDSS confirmed the WMAP results that the sound horizon is ~150 Mpc in today's universe.
Detection in other galaxy surveys 
The 2dFGRS collaboration reported a detection of the BAO signal in the power spectrum at the same time as the SDSS collaboration. Since then, further detections have been reported in the 6dF Galaxy Survey (6dFGS), WiggleZ and BOSS.
BAO and Dark Energy Formalism 
General Relativity and Dark Energy 
The Hubble parameter, , in terms of the scale factor is:
where is the time-derivative of the scale factor. The Friedmann equations express the expansion of the universe in terms of Newton's gravitational constant, , the mean pressure, , the Universe's density , the curvature, , and the cosmological constant, :
Observational evidence of the acceleration of the universe implies that (at present time) . Therefore the following are possible explanations:
- The universe is dominated by some field or particle that has negative pressure such that the equation of state:
- There is a non-zero cosmological constant, .
- The Friedmann equations are incorrect since they contain over simplifications in order to make the general relativistic field equations easier to compute.
Measured Observables of Dark Energy 
where m is matter, r is radiation, k is curvature, Λ is dark energy, and w is the equation of state. 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. This measures two cosmological distances: the Hubble parameter, , and the angular diameter distance, , as a function of redshift . By measuring the subtended angle, , of the ruler of length, , these parameters are determined as follows:
the redshift interval, , can be measured from the data and thus determining the Hubble parameter as a function of redshift:
Therefore the BAO technique helps constrain cosmological parameters and provide further insight into the nature of dark energy.
See also 
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