Cosmic rays are high energy particles that bombard the Earth from outer space. The origin of the very highest energy rays is a mystery, but astronomers have long thought that interactions with the cosmic microwave background - photons left over from the big bang - place an upper limit on their energy. This so-called Greisen-Zatsepin-Kuz'min limit is about 5 x 1019 eV, which is millions of times higher than the most energetic particle accelerators. However, astronomers at the Akeno Giant Air Shower Array (AGASA) in Japan now have evidence for six cosmic rays with energies above this limit. (Phys. Rev. Lett. 81 1163).
Speculation on the origin of high energy cosmic rays has existed for some time. The best candidates are massive supernova explosions or the decay of superheavy particles near the beginning of the universe. The AGASA results show that the energy spectrum extends beyond 1020 eV, but they have been unable to identify sources for any of the six highest-energy rays they have detected – although they do know that the sources must be within 50 megaparsecs of the solar system, which is “nearby” in astrophysical terms.
The team says that the results lead “highest energy cosmic-ray physics into a much more exciting stage” and that these mysteries might be solved by a new generation of cosmic-ray experiments such as the Telescope Array, the High Resolution Fly’s Eye and the Auger project.
