  Astronomy at the highest energies observed must be performed by studying
neutrinos rather than photons because the universe is opaque to photons of
these energies. By making observations of neutrinos with energies above 10 EeV
one can deduce information about the distribution and cosmological history of
cosmic rays which may have been accelerated to energies above 100 EeV. Very
large fluxes of neutrinos at these energies which exhibit a hard spectrum would
be evidence of a GUT phase transition or superheavy dark matter particles.
Neutrinos at energies around a PeV may be produced in observable quantities in
active galaxies from the acceleration and interactions of cosmic rays in the
vicinity of massive black holes or in relativistic jets produced by these black
holes. Cosmological gamma-ray bursts may also produce PeV neutrinos. Neutrinos
at these energies may oscillate into tau neutrinos which can travel through the
Earth by the process of regeneration and produce upward moving air-showers.
Neutrinos of energies above of 1000 EeV can produce ultrahigh energy cosmic
rays thorugh interactions with big-bang neutrinos, producing Z bosons. This
"Z-burst" phenomenon can be used to study the mass and distribution of big-bang
neutrinos.
