Pauli's assertion that the neutrino would be exceptionally elusive proved correct, as it would be another 25 years from the time of its prediction that two experimentalists would finally detect a neutrino. Working with the Savannah River high-flux nuclear reactor in 1956, Fred Reines and Clyde Cowan first detected the signature of a neutrino interaction with matter. Placing a ton-sized detector near the reactor, they were able to decipher the signal of a neutrino scattering off of a proton. Since then, three different neutrino types have been found to exist. Each type, or "flavor", corresponds to a different "lepton" particle: electron, muon, or tau. The neutrino is also known to have its own anti-particle, the anti-neutrino. (In fact, it is an electron anti-neutrino that is produced in beta decay).
With the realization that neutrinos play a key role in a number of astrophysical processes, a new and massive field of research known as "neutrino astronomy" has emerged in the past two decades. Neutrino detectors have sprung up in the USA, Japan, Russia, and Canada.
On the cosmological level, calculations of Big Bang nucleosynthesis indicate the production of neutrinos among the reactions which form the light elements. Therefore, just as these calculations provide some prediction of the observed light element abundances, so too they yield some indication of the neutrino abundance in the Universe. Much as the radiation field which forms today's CMB had "decoupled" from matter a few minutes after the Big Bang, so too did neutrinos decouple from matter--albeit at a much earlier time than the photon background. Hence, there is a predicted cosmic neutrino background as dense as the photons which comprise the CMB.
One of the most revealing criteria as to whether the Universe is dominated by CDM or HDM is the way that matter, in particular galaxies, are distributed throughout the sky. HDM, as represented primarily by neutrinos, does not account for the pattern of galaxies observed in the Universe. Neutrinos, as aforementioned, would have emerged from the Big Bang with such highly relativistic velocities (i.e. close to light speed) that they would tend to smooth out any fluctuations in matter density as they streamed out through the Universe. In the early Universe, the neutrino density was enormous, and so most of the matter density could be accounted for by neutrinos. Given their great speeds, neutrinos would tend to free stream out of any overdense regions--that is, regions with densities greater than the average density in the Universe. This process implies that density fluctuations could appear only after the neutrinos slowed down considerably. (i.e. As the Universe expanded, its temperature decreased, thereby resulting in neutrino "cooling.")
When coupled with the amount of power seen on large scale by the COBE satellite, it turns out that a neutrino-dominated Universe would contain insufficient power on small scales to be compatible with observations. This rules out any purely HDM model of the Universe.
When people discuss HDM nowadays they usually refer to it in the context of "Mixed Dark Matter" of "Cold+Hot Dark Matter" models. In these models the bulk of the dark matter is cold, but a tiny fraction is hot. Current experiments limit the amount of "hot" dark matter in the universe to at most a few percent, with the best fit being unmeasurably small. This is to be compared to the cold dark matter component, which is around 1/3 of the total energy in the universe.
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