Ultrahigh-energy neutrinos from astrophysical sources and superheavy particle decays

Problems in the fields of neutrino astronomy and ultrahigh-energy astrophysics are reviewed. Neutrino fluxes produced in various astrophysical sources (\it{bottom-up} acceleration scenarios) and resulting from the decay of superheavy particles (\it{top-down} scenarios) are considered. Neutrino oscillation processes and the absorption and regeneration of neutrinos inside the earth are analyzed and some other factors affecting the intensity and flavor composition of astrophysical neutrino fluxes are discussed. Details of ultrahigh-energy neutrino interactions are discussed within the Standard Model, as well as using nonstandard scenarios predicting an anomalous increase in the inelastic neutrino – nucleon cross section. Ultrahigh-energy neutrino detection techniques currently in use in new-generation neutrino telescopes and cosmic ray detectors are also discussed.