THE world鈥檚 most sensitive cosmic-ray detector has failed to find the ultra-high energy cosmic rays spotted by a smaller Japanese detector 鈥� leaving the standard model鈥檚 limit for cosmic ray energies intact for now.
By the time cosmic rays from outside our galaxy reach Earth, the standard model says they should have a maximum energy of 5 脳 1019 electronvolts, known as the Greisen-Zatsepin-Kuzmin (GZK) limit. This is because cosmic rays lose energy through collisions with photons from the cosmic microwave background as they race through space at close to light speed.
But Japan鈥檚 Akeno Giant Air Shower Array (AGASA) has on 11 occasions sensed cosmic rays above the GZK limit with its 111 detectors. Now the massive Pierre Auger Observatory in Argentina has put 670 (out of its eventual 1400) cosmic-ray detectors on the case.
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But the observatory鈥檚 first results, which were released on 1 July, reveal only low-energy cosmic rays below the GZK limit. Observatory spokesman Alan Watson of Leeds University in the UK says that with less than half its 1400 detectors so far up and running, it is too early to tell whether the rays above the GZK limit will eventually be detected. 鈥淚t鈥檚 sort of tantalising. In two years we will solve the problem,鈥� he promises.
Unlike AGASA鈥檚 100-square-kilometre array, the Auger observatory鈥檚 detectors will be spread across 3000 square kilometres of the Argentinian pampas.
