The Hot Dark Matter

There is a puzzling contradiction: direct observations favor a low-mass-density universe (0.2 ≤ Ωm ≤ 0.6), but the only model which fits universe structure over more than three orders of magnitude in distance scale has a mix of hot (neutrino) and cold dark matter providing a critical density universe. Models of an open universe (low Ωm) or one adding a cosmological constant (Λ) to provide a critical energy density (Ωm + ΩΛ = 1) have probabilities of < 10 . Two-neutrino dark matter works better than having the needed ∼ 5 eV of neutrino mass in one species of neutrino, and this is consistent with the only model which fits all present indications for neutrino mass: νμ → ντ accounting for the atmospheric anomaly (with νμ and ντ being the hot dark matter), ν̄μ → ν̄e being observed by LSND, and νe → νs explaining the solar νe deficit. The LSND/KARMEN results are consistent with the needed mass of hot dark matter. Further support for this mass pattern is provided by the need for the sterile neutrino, νs, to make possible heavyelement nucleosynthesis in supernovae. It is a fascinating question as to whether the hot dark matter paradox will be resolved by better measurements or by the introduction of new physics.