Although most of what we know about the high-energy universe comes from photons, crucial information is also carried by energetic particles—cosmic rays, neutrinos, and, possibly, exotic particles that could constitute much of the mass of the universe, the so-called dark matter. Cosmic rays are relativistic particles accelerated by supernova shock waves and other energetic phenomena. They play an important role in the ionization, heating, and pressurizing of the interstellar medium and in the production of high-energy photons. The typical cosmic ray has an energy of motion comparable to the energy associated with its rest mass, but the most energetic cosmic rays have energies nearly a trillion times greater. The nature and origin of these ultrahigh-energy cosmic rays are not understood. The Southern Hemisphere Pierre Auger Observatory and the high-resolution Fly’s Eye are two ground-based projects that will soon be under way to detect and characterize these ultrahigh-energy cosmic rays. The composition of lower-energy cosmic rays is being studied by the Advanced Composition Explorer, whereas the Antimatter-Matter Spectrometer will search for the presence of antimatter in the cosmic rays.

The committee notes that a proposed small space mission, the Advanced Cosmic-ray Composition Experiment for the Space Station (ACCESS), shows great promise in being able to characterize the mechanism of cosmic-ray acceleration with far greater precision than heretofore possible. A possible theory challenge for ACCESS is

To study the acceleration and propagation of relativistic particles in astrophysics in order to enable accurate comparison between theory and ACCESS observations.

Neutrinos are produced by nuclear reactions in the interior of stars like the Sun, in supernova explosions, and possibly in gamma-ray bursts and in the regions around supermassive black holes. Most existing neutrino detectors are designed to study the relatively low energy neutrinos from the Sun and are focused primarily on studying the physics of neutrinos; as such, they lie outside the purview of this report. Several projects are under way in Europe and in the United States to search for much-higher-energy neutrinos. The U.S. project, AMANDA, uses a huge volume of subsurface ice at the South Pole to detect neutrinos; a much larger follow-on experiment, Ice Cube, has been proposed.