It is well known that there exist extremely high energetic particles in the Universe with energies up to 10^20 eV. How they are produced and how they propagate in the Universe have been a puzzle for a long time. Recent experiments suggest that these particles come from outside the Galaxy. More detailed understanding of their origin will require detecting many more particles. It may also be necessary to observe the production process through other windows. Nucleons and nuclei lose their energies in less than 100 Mpc through interactions with the cosmic microwave background. Furthermore, intergalactic magnetic fields can bend their trajectories, masking their sites of origin. Gamma rays are even more limited in pathlength due to electron pair production when they collide with radio photons.

In this regard, neutrinos have uniquely advantageous characteristics: they can penetrate cosmological distances in the Universe and their trajectories are not deflected because they have no electric charge. They carry information about extremely high energy (EHE) production processes, even in the early Universe. It is therefore important to understand the possible processes for producing EHE neutrinos and to consider the possibilities for detecting the predicted flux resulting from several different models.

The possible mechanisms to produce EHE/UHE neutrinos are:

The Greisen Neutrinos

EHE neutrinos above 10^19 eV are produced by photopion production of the 2.7 K cosmic background radiation photons and the ultra-high energy cosmic ray Nucleons during propagation in intergalactic space.

Neutrinos emitted from Active Galactic Nuclei

Active Galactic Nuclei (AGN) is one of the possible, and likely, acceleration sites to produce EHE cosmic rays, and the accelerated proton energy loss due to proton-proton and/or proton-gamma interactions in the AGN accretion disk or with UV photons in the associated jets are dominant mechanism for neutrino (and photon) production.

Neutrinos emitted from Gamma-Ray Bursters

Gamma-ray Bursters (GRBs) are one of the possible, and likely, acceleration sites to produce EHE cosmic rays, and the accelerated proton energy loss due to proton-proton and/or proton-gamma interactions in the GRB fireball are the source for neutrino (and photon) production.

Annihilation or collapse of topological defects (TDs)