A Warm-Plus-Hot Dark Matter Universe

We investigate a new hybrid-model universe containing two types of dark matter, one ``warm'' and the other ``hot''. The hot component is an ordinary light neutrino with mass $\sim 25h^2$~eV while the warm component is a sterile neutrino with mass $\sim 700h^2$~eV. The two types of dark matter arise entirely within the neutrino sector and do not require separate physical origins. We calculate the linear transfer functions for a representative sample of warm-plus-hot models. The transfer functions, and results from several observational tests of structure formation, are compared with those for the cold-plus-hot models that have been studied extensively in the literature. On the basis of these tests, we conclude that warm-plus-hot dark matter is essentially indistinguishable from cold-plus-hot dark matter, and therefore provides a viable scenario for large scale structure. We demonstrate that a neutrino mass matrix can be constructed which provides the requisite dark matter constituents, while remaining consistent with all cosmological bounds.