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Following our current knowledge, the neutrino was born about 15 billions years ago, just after the birth of the universe. Since this time, the universe has continuously expanded and cooled, and neutrinos have travelled a lot. Theoretically, they are now many and constitute a a cosmic background radiation whose temperature is 1.9 degree Kelvin (-271.2 degree Celsius).

This neutrinos cosmic background radiation comes from the decoupling of the neutrinos, which, until universe became sufficiently cold, were appearing and disappearing, absorbed by protons or emitted by neutrons. At a given temperature of the universe, about 1 second after the Big-Bang, protons have only a weak probability to absorb a neutrino, which thus decouple.
Some hundreds of thousand of years later, photons will decouple the same way from charged particles like the electrons and will become the light cosmic background radiation at 2.73 degree Kelvin.

The theory estimated also the neutrino density in the universe: about 110 per cm3, for each neutrino family, that means 330 neutrinos per cm3 as a whole. 330 millions per m3. This is quite lot!... By comparison, you find a mean of 0.5 proton and one billion photons per cubic meter of universe. Thus, you find about one billion more neutrinos than protons. And those Big-Bang neutrinos are approximately as numerous as the Big-Bang photons. The matter, which composes us (protons, neutrons, electrons) is not very important... quantitatively.

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Last update: 26/06/1999 :
Didier Verkindt