Exploring the Universe beyond the Photon Window

Conventional astronomy spans about 18 decades in photon wavelengths, from 104 cm radio-waves to 10−14 cm γ-rays of GeV energy. Because the universe is opaque to photons of TeV energy and above (see Fig. 1) [1], present studies focus on hadrons, neutrinos, and gravitational waves as messengers probing the high energy universe. The best candidates to serve as messengers in a new astronomy of the high energy behavior of distant sources are neutral particles. This is because the orbit of a charged cosmic ray can be substantially bent, both by extragalactic magnetic fields [2] and by the ambient magnetic field of our own Galaxy [3]; destroying the possibility of locating the source. The most promising messenger is the neutrino: it can be copiously produced in cosmic beam dumps and can traverse unscathed dense astrophysical environments. In this talk I delineate the prospects to identify high energy neutrino emitters.