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. In the same way that supernovae provide a “standard candle” for astronomical observations,[1] BAO matter clustering provides a “standard ruler” for length scale in cosmology.[2] The length of this standard ruler is given by the maximum distance the acoustic waves could travel in the primordial plasma before the plasma cooled to the point where it became neutral atoms (the epoch of recombination), which stopped the expansion of the plasma density waves, “freezing” them into place. 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]
Baryon
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“Baryonic” redirects here. For the dinosaur, see Baryonyx.
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Elementary particles of the Standard Model
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In particle physics, a baryon is a type of composite subatomic particle which contains an odd number of valence quarks (at least 3).[1] Baryons belong to the hadron family of particles; hadrons are composed of quarks. Baryons are also classified as fermions because they have half-integer spin.
The name “baryon”, introduced by Abraham Pais,[2] comes from the Greek word for “heavy” (βαρύς, barýs ), because, at the time of their naming, most known elementary particles had lower masses than the baryons. Each baryon has a corresponding antiparticle (antibaryon) where their corresponding antiquarks replace quarks. For example, a proton is made of two up quarks and one down quark; and its corresponding antiparticle, the antiproton, is made of two up antiquarks and one down antiquark.
Because they are composed of quarks, baryons participate in the strong interaction, which is mediated by particles known as gluons. The most familiar baryons are protons and neutrons, both of which contain three quarks, and for this reason they are sometimes called triquarks . These particles make up most of the mass of the visible matter in the universe and compose the nucleus of every atom. (Electrons, the other major component of the atom, are members of a different family of particles called leptons; leptons do not interact via the strong force.) Exotic baryons containing five quarks, called pentaquarks, have also been discovered and studied.
A census of the Universe’s baryons indicates that 10% of them could be found inside galaxies, 50 to 60% in the circumgalactic medium,[3] and the remaining 30 to 40% could be located in the warm–hot intergalactic medium (WHIM).[4]
Baryogenesis[edit]
Main article: Baryogenesis
Experiments are consistent with the number of quarks in the universe being a constant and, to be more specific, the number of baryons being a constant (if antimatter is counted as negative);[ citation needed ] in technical language, the total baryon number appears to be conserved. Within the prevailing Standard Model of particle physics, the number of baryons may change in multiples of three due to the action of sphalerons, although this is rare and has not been observed under experiment. Some grand unified theories of particle physics also predict that a single proton can decay, changing the baryon number by one; however, this has not yet been observed under experiment. The excess of baryons over antibaryons in the present universe is thought to be due to non-conservation of baryon number in the very early universe, though this is not well understood.
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