The Hot Dark Matter

Abstract

There is a puzzling contradiction: direct observations favor a low-mass-density universe ($0.2\leΩ_m\le0.6$), 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 $Ω_m$) or one adding a cosmological constant ($Λ$) to provide a critical energy density ($Ω_m+ Ω_Λ=1$) have probabilities of $<10^{-3}$. Two-neutrino dark matter works better than having the needed $\sim5$ 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: $ν_μ\toν_τ$ accounting for the atmospheric anomaly (with $ν_μ$ and $ν_τ$ being the hot dark matter), $\barν_μ\to\barν_e$ being observed by LSND, and $ν_e\toν_s$ explaining the solar $ν_e$ 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, $ν_s$, to make possible heavy-element 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.