Ultra-High Energy Cosmic Rays: A Probe of Physics and Astrophysics at Extreme Energies

The origin of cosmic rays is one of the major unresolved questions in astrophysics. In particular, the highest energy cosmic rays observed possess macroscopic energies and their origin is likely associated with the most energetic processes in the Universe. They thus provide a probe of physics and astrophysics at energies that are unreached in laboratory experiments. Theoretical explanations range from acceleration of charged particles in astrophysical environments to particle physics beyond the well established Standard Model, and processes taking place at the earliest moments of our Universe. Distinguishing between these scenarios requires detectors with effective areas in the 1000 km^2 range which are now under construction or in the planning stage. Close connections with gamma-ray and neutrino astrophysics add to the interdisciplinary character of this field.