Physics and Astronomy Colloquium

Ultra-high energy cosmic rays (UHECRs) are microscopic particles that
carry macroscopic energies. Discovered by Rossi and Auger in the 1930s,
their origin remains an unsolved problem. Because they cannot be
contained by the Galactic magnetic field, they must be accelerated by
sources outside our Galaxy. The Pierre Auger Observatory in Argentina
has opened the field of charged-particle astronomy by associating the
arrival directions of UHECRs with active galaxies in the supergalactic
plane within 100 Megaparsecs. The recently launched Fermi Gamma-ray
Space Telescope identifies sites of energetic particle acceleration
through gamma-ray emission. The Fermi Telescope and its first results
are described in view of prospects that gamma-ray bursts and active
galactic nuclei are the sources of UHECRs. Definitive resolution of this
problem may await the completion of IceCube, the high-energy neutrino
telescope at the South Pole.