Cosmological three-level neutrino laser

We present a calculation of a neutrino decay scenario in the early Universe. The specific decay is nu(2)-->nu(1)+phi, where phi is a boson. If there is a neutrino mass hierarchy, m(ve)<m(v mu)<m(vT), we show that it is possible to generate stimulated decay and effects similar to atomic lasing without invoking new neutrinos, even starting from identical neutrino distributions. Under the right circumstances the decay can be to very low-momentum boson states thereby producing something similar to a Bose condensate, with possible consequences for structure formation. Finally, we argue that this type of decay may also be important elsewhere in early Universe physics.