High-energy neutrino astrophysics

High-energy (> 100 MeV) neutrino astrophysics enters an era of opportunity and discovery as the sensitivity of detectors approaches astrophysically re levant flux levels. We review the major challenges for this emerging field, among which the nature of dark matter, the origin of cosmic rays, and the physics of extreme objects such as active galactic nuclei, gamma-ray bursts , pulsars, and supernova remnants are of prime importance. Variable sources at cosmological distances allow the probing of neutrino propagation proper ties over baselines up to about 20 orders of magnitude larger than those pr obed by terrestrial long-baseline experiments. We review the possible astro physical sources of high-energy neutrinos, which also act as an irreducible background to searches for phenomena at the electroweak and grand-unified- theory symmetry-breaking scales related to possible supersymmetric dark mat ter and topological defects. Neutrino astronomy also has the potential to d iscover previously unimagined high-energy sources invisible in other channe ls and provides the only means for direct observations of the early univers e prior to the era of decoupling of photons and matter. We conclude with a discussion of experimental approaches and a short report on present project s and prospects. We look forward to the day when it will be possible to see the universe through a new window in the light of what may be its most num erous particle, the elusive neutrino.