Extraterrestrial neutrinos provide a unique window to probe the deepest reaches of stars, quasars, and exotic structures in the cosmos. In contrast to all other SM particles, they can escape from dense astrophysical environments and propagate to the Earth unscathed, arriving aligned with their source. Even at the highest energies the annihilation mean free path on the cosmic neutrino background,
is somewhat above the present size of the horizon (recall that H0-1 ~ 1028 cm) .
In addition, the fluxes of ultra-high energy cosmic neutrinos offer clues to the properties of neutrinos themselves and provide an important probe of new ideas in particle physics. Namely, their known interactions are so weak that new physics may easily alter neutrino properties. This is especially relevant for extraterrestrial neutrinos with energies 106 GeV that interact with nucleons in the Earth atmosphere with center-of-mass energies above the electroweak scale, where the SM is expected to be modified by new physics.
In summary, ultra high energy extraterrestrial neutrinos are inextricably linked with the physics processes discussed throughout this review. This chapter will therefore focus on the main features of high energy neutrino astronomy. For comprehensive reviews on cosmic neutrinos the reader is referred to [466, 467, 468, 469].