Neutrinos in cosmology

Abstract

About neutrinos, we only know that the three left handed \(\nu_{\rm e_{\rm{L}}} , \nu_{\mu_{\rm{L}}} , \nu_\tau\) exist, that they have weak interactions and that they are much lighter than their corresponding charged leptons. They may be massive or massless, stable or not. If they are massive, they may be the left-handed part of the Majorana or Dirac fermions. If neutrinos are Majorana fermions, the right-handed neutrinos νR may or may not exist and may be heavy or light. If neutrinos are Dirac fermions, the νR must exist. Neutrinos may have new interactions, other than weak, which may allow them to annihilate or decay faster. Not just laboratory experiments can investigate these possibilities: because neutrinos were present in large numbers in the early universe, their properties affect the evolution and the present structure of the universe. Our knowledge of the history of the universe is thus used as a laboratory for placing limits on the mass, lifetime and number of flavours of neutrinos. Also astrophysics leads to limits on neutrinos, the recent observations of neutrinos from the supernova SN 1987A being an outstanding example. On the other hand, the knowledge of some properties of neutrinos may be crucial for cosmology and astrophysics. For example, the solution to a major cosmological problem would be given by a neutrino mass of around 30eV. This would mean that the “dark matter” consists of light neutrinos.