Astrophysics and cosmology closing in on neutrino masses.

Massive neutrinos are expected in most grand unified theories that attempt to unify the strong and electroweak interactions. So far, heroic laboratory experiments have yielded only upper bounds on the masses of the elusive neutrinos. These bounds, however, are not very restrictive and cannot even exclude the possibility that the dark matter in the universe consists of neutrinos. The astrophysical and cosmological bounds on the masses of the muon and tau neutrinos, mv(vmicro) and mv(vtau), which already are much more restrictive than the laboratory bounds, and the laboratory bound on the mass of the electron neutrino, mv(vc), can be improved significantly by future astrophysical and cosmological observations that perhaps will pin down the neutrino masses. Indeed, the recent results from the solar neutrino experiments combined with the seesaw mechanism for generating neutrino masses suggest that mv(vc) approximately 10(-8) electron volts, mv(vmicro) approximately 10(-3) electron volts, and mv(vtau) approximately 10 electron volts, which can be tested in the near future by solar neutrino and accelerator experiments.