CONTROVERSY abounds over how to explain the highest-energy cosmic rays, subatomic particles that tear through space at near light speed while packing the punch of rifle bullets. Now a cosmic ray detector in Utah has further deepened the controversy with evidence that the particles may not be so powerful after all.
A new analysis of results from the High Resolution Fly鈥檚 Eye (HiRes) experiment in Salt Lake City has detected a sharp cut-off in the energy spectrum of cosmic rays. This stands in stark contrast to a Japanese experiment that has previously reported particles with bafflingly higher energies.
According to standard physics, cosmic rays with energies larger than about 5 脳 1019 electronvolts will collide with photons left over from the big bang and so lose energy as they cross large distances. This puts a theoretical limit on the energy they can have when they reach Earth, known as the Greisen-Zatsepin-Kuzmin (GZK) cut-off.
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Japan鈥檚 Akeno Giant Air Shower Array (AGASA) caused a sensation in 2004 by revealing 11 cosmic rays with energies greater than the GZK cut-off, detected between 1990 and 2004. In order to reach Earth at such high energy, each of the particles had to have originated somewhere within 100 million light years of Earth. The problem is that no celestial phenomenon known within this radius could account for such energetic rays. So physicists began to dream up new mechanisms beyond standard physics and astronomy, such as the decay of ultra-heavy dark matter particles.
HiRes contradicts the Japanese data by registering just two events above the GZK cut-off in nine years of observations, which in statistical terms is explicable without the need for new physics 鈥淓verything hangs together. We have observed what is actually expected in the standard model,鈥� says HiRes project manager Charles Jui, University of Utah.
鈥淣o celestial phenomenon known within 100 million light years of Earth could account for such energetic rays鈥�
HiRes looked for the faint fluorescent light emitted when a cosmic ray strikes the atmosphere and creates a shower of billions of secondary particles. In contrast, the AGASA method detected the secondary particles as they passed through 111 water tanks spread around the University of Tokyo鈥檚 Akeno Observatory. 鈥淲e must now figure out why ground arrays give different answers than fluorescence detectors,鈥� says Jui.
Scientists from HiRes and AGASA are now collaborating on a new cosmic ray detector called Telescope Array that will meld the AGASA and HiRes methods into a single experiment. Construction has already started, and Jui expects a result within a year.
Meanwhile, Alan Watson of the University of Leeds, UK, says Argentina鈥檚 Pierre Auger Observatory, currently the largest cosmic ray detector in the world, could give the final word on the existence of cosmic rays above the cut-off within six months.
