Really energetic particles should hit Earth only very rarely. After all, the number of cosmic rays pelting Earth decreases steadily as the energy of the rays increases. Above a specific energy, the rate ought to drop even faster. For example, if the rays consist mainly of protons, then at such tremendous energies they ought to break into other subatomic particles when they collide with photons in the afterglow of the big bang, the cosmic microwave background. That effect is known as the GZK cutoff and it should limit the energy of cosmic rays to about 1018 electron volts, a million times the energy achieved with particle accelerators. But from 1990 to 2004, physicists working with the Akeno Giant Air Shower Array (AGASA) west of Tokyo spotted roughly a dozen particles with energies 100 times higher (Science, 14 August 1998, p. 891). That excess puzzled physicists, both because they could not explain how the rays got past the GZK cutoff or how they could gain so much energy in the first place.
The latest results came from the Auger Observatory project, which I'm involved with indirectly via the AirFly collaboration.
Now, a new gigantic cosmic ray detector that employs both techniques has settled the issue. The almost-completed Pierre Auger Observatory on the plains of the Pampa Amarilla in western Argentina comprises more than 1200 ground detectors and 24 telescopes and covers an area of 300 square kilometers (Science, 21 June 2002, p. 2134). And the array has already collected enough data to rule out an excess in cosmic rays above 10^20 eV. "If the AGASA had been correct, then we should have seen 30 events [at or above 10^20 eV], and we see two," says Alan Watson, a physicist from the University of Leeds, U.K., and spokesperson for the Auger collaboration. The team will present its results here this week at the 30th International Cosmic Ray Conference.