Number 137 (Story #1), July 19, 1993 by Phillip F. Schewe and Ben Stein NEUTRINO LASING IN THE EARLY UNIVERSE . Some scientists believe that a mixed dark matter (MDM) cosmology, one featuring both cold and hot dark matter, would better explain the observed large-scale structure of the universe than models using either cold or hot dark matter alone. Astronomers at the Canadian Institute of Theoretical Astrophysics in Toronto suggest that such a MDM model could be based, at least in part, on the decay of primordial heavy neutrinos into light fermions (F) and bosons (B). In this theory many low-energy B's are created when an initial neutrino decay triggers subsequent decays in an exponential cascade process analogous to the stimulated emission of light that occurs in a laser. In this case the B's are analogous to laser photons. The colder B's can condense into a state which could be considered cold dark matter, while the warmer B's and some of the fermions would constitute a form of hot dark matter. (N. Kaiser, R.A. Malaney, and G.D. Starkman, Physical Review Letters, 2 August 1993; contact Robert A. Malaney, 416-978-8194.)