118 separately heavy sterile neutrinos exist in non

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[118]Separately, heavy sterile neutrinos exist in non-supersymmetric extensions to the standard model which explain thesmall neutrino mass through the seesaw mechanism.Warm dark matter comprises particles with an FSL comparable to the size of a protogalaxy. Predictions based on warm darkmatter are similar to those for cold dark matter on large scales, but with less small-scale density perturbations. This reduces thepredicted abundance of dwarf galaxies and may lead to lower density of dark matter in the central parts of large galaxies. Someresearchers consider this a better fit to observations. A challenge for this model is the lack of particle candidates with the requiredmass ≈ 300 eV to 3000 eV.No known particles can be categorized as warm dark matter. A postulated candidate is the sterile neutrino: A heavier, slower formof neutrino that does not interact through the weak force, unlike other neutrinos. Some modified gravity theories, such as scalar–tensor–vector gravity, require "warm" dark matter to make their equations work.Cold dark matterWarm dark matterHot dark matter
Hot dark matter consists of particles whose FSL is much larger than the size of a protogalaxy. The neutrino qualifies as suchparticle. They were discovered independently, long before the hunt for dark matter: they were postulated in 1930, and detected in1956. Neutrinos' mass is less than 10−6that of an electron. Neutrinos interact with normal matter only via gravity and the weakforce, making them difficult to detect (the weak force only works over a small distance, thus a neutrino triggers a weak forceevent only if it hits a nucleus head-on). This makes them 'weakly interacting light particles' (WILPs), as opposed to WIMPs.The three known flavours of neutrinos are the electron, muon, and tau. Their masses are slightly different. Neutrinos oscillateamong the flavours as they move. It is hard to determine an exact upper bound on the collective average mass of the threeneutrinos (or for any of the three individually). For example, if the average neutrino mass were over 50 eV/c2(less than 10−5ofthe mass of an electron), the universe would collapse. CMB data and other methods indicate that their average mass probablydoes not exceed 0.3 eV/c2. Thus, observed neutrinos cannot explain dark matter.[119]Because galaxy-size density fluctuations get washed out by free-streaming, hot dark matter implies the first objects that can formare huge supercluster-size pancakes, which then fragment into galaxies. Deep-field observations show instead that galaxiesformed first, followed by clusters and superclusters as galaxies clump together.If dark matter is made up of sub-atomic particles, then millions, possibly billions, of such particles must pass through everysquare centimeter of the Earth each second.

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Term
Fall
Professor
Miguel Mosquera

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