Sterile neutrinos in the universe

In this chapter, we investigate observable consequences of sterile neutrinos, in galactic, cluster, and cosmological scales. We first review some basic physics of the formation of sterile neutrinos from the oscillation of active neutrinos in the early universe. Then we discuss the possibility of having sterile neutrino halos in the Milky Way and galaxy clusters. The radiative decay of the sterile neutrinos gives energy to heat up the interstellar medium which can simultaneously explain the cooling flow problem in galaxy clusters and the hot gas problem in the Milky Way. Also, this model enables us to obtain the possible ranges of rest mass, decay rate and mixing angle of sterile neutrinos. The mixing angle obtained is not as small as we expected before so that the oscillation of active-sterile neutrino may be visible in near future experiments. However, the existence of the sterile neutrinos cannot be treated as the major component of the dark matter in the universe. Copyright © 2011 by Nova Science Publishers, Inc. All rights reserved.