What is the origin and nature of ultra-high-energy cosmic rays?
Cosmic rays are high-energy particles, mainly protons and alpha particles, which come from outer space and hit the Earth's atmosphere producing a shower of other particles. Most of these are believed to have picked up their energy by interacting with shock waves in the interstellar medium. But, the highest-energy ones remain mysterious - nobody knows how they could have acquired such high energies.
The record is a 1994 event detected by the Fly's Eye in Utah, which recorded a shower of particles produced by a cosmic ray of about 300 EeV. A similar event has been detected by the Japanese scintillation array AGASA. An EeV is an "exa-electron-volt", which is the energy an electron picks up going through a potential of 10^18 volts. 300 Eev is about 50 joules - the energy of a one-kilogram mass moving at 10 meters/second, presumably all packed into one particle! Nobody knows how such high energies are attained - perhaps as a side-effect of the shock made by a supernova or gamma-ray burster? The puzzle is especially acute because because particles with energies over 50 EeV are expected to interact with the cosmic microwave background radiation and lose energy after travelling only moderate extragalactic distances, say 30 megaparsecs. This effect is called the Greisen-Zatsepin-Kuz'min (or GZK) cutoff. So, either our understanding of the GZK cutoff is mistaken, or ultra-high-energy cosmic rays come from fairly nearby sources - in cosmological terms, that is.
Right now the data is confusing, because two major experiments on ultra-high-energy cosmic rays have yielded conflicting results. The Fly's Eye seems to see a sharp dropoff in the number of cosmic rays above 100 Eev, while the AGASA detector in Japan does not. People hope that the Pierre Auger cosmic ray observatory, being built in western Argentina, will settle the question.