Superenergetic cosmic rays could reveal
Meteorites have been called the poor man's space probe--cheap samples of the beyond. In that case, cosmic rays must be the poor man's particle accelerator. A cosmic-ray particle coming from the direction of the constellation Auriga, detected by an instrument in Utah in 1991, had an energy of 3 x 1020 electron volts--more than 100 million times beyond the range of present accelerators. Such natural largesse achieves what purpose-built machines have long sought: a probe of physics underlying the current Standard Model.
For years, people thought the 1991 ray and a few similar ones--registered, for example, by the Akeno Giant Air Shower Array (AGASA) west of Tokyo--might have been œukes. But last summer Masahiro Takeda of the University of Tokyo and the rest of the AGASA team reported five more such events. Roughly one is seen by the array each year, and there is no indication of any limit to their energy.
Current theories say that is impossible. If these cosmic rays are protons or atomic nuclei, as the experiments hint, they must be moving almost at the speed of light. At that clip, the cosmic microwave background, a tenuous gas of primordial radiation that fills space, looks like a thick sea. Particles wading through it lose energy until they fall below 5 x 1019 eV, known as the Greisen-Zatsepin-Kuzmin cutoff. After traveling 150 million light-years, no ordinary particle could still have the observed energies.
Yet astronomers have seen no plausible source within that distance. Exploding stars can propel particles up to only about 1 percent of the required energy. And the mightiest known cosmic slingshots--quasars and active galactic nuclei, the by-products of a massive black hole at lunch--are all too far away, as Jerome W. Elbert and Paul Sommers of the University of Utah showed in 1995. Researchers are forced to one of two equally bizarre conclusions: either the cosmic rays evade the cutoff, or their source is not a normal astronomical object.
In favor of the former, Glennys R. Farrar of New York University and Peter L. Biermann of the Max Planck Institute for Radioastronomy in Bonn recently matched the five most powerful rays with the directions of rare young quasars. The distance of these quasars ranges from four billion to 13 billion light-years. If cosmic rays traverse such lengths, they must be a type of particle that is barely affected by the cosmic microwave background. A neutral and heavier relative of the proton would do the trick. No such stable particle is predicted by the Standard Model, but enhanced theories--drawing on the concept of supersymmetry--do predict one: the so-called S0 particle.
Another idea, proposed by Thomas J. Weiler of Vanderbilt University, invokes energetic neutrinos that smack into other neutrinos milling about the Milky Way and spill debris particles in the earth's direction. The only requirement is that the neutrinos have a slight mass--which again extends the Standard Model. It is also conceivable that there is no Greisen-Zatsepin-Kuzmin cutoff after all, as Sidney Coleman and Sheldon L. Glashow of Harvard University speculated in August. But if so, special relativity does not apply at high energy.
What if the correlation seen by Farrar and Biermann turns out to be pure chance? Then cosmic rays must emanate from some unidentified celestial phenomenon. The enigmatic sources of gamma-ray bursts might be responsible. More exotic candidates include kinks in the fabric of space and time, such as monopoles and cosmic strings. Tucked within their folds is a sample of the hot early universe in which the forces of nature are unified. As they decay, a miniature big bang ensues, and particles are created with energies up to the unification scale of 1025 eV and names like crypton and vorton. The cosmic rays may be these particles or their decay products, as first suggested in 1987 by Christopher T. Hill and David N. Schramm of the Fermi National Accelerator Laboratory and Terrence P. Walker of Ohio State University.
It is probably not a very good sign that the number of models exceeds the number of data points. "When you have so many speculations," declares James W. Cronin of the University of Chicago, "it shows we really don't understand much at all."
To tilt the balance in favor of data, Cronin and Alan A. Watson of the University of Leeds are heading the Pierre Auger project, an international effort to build two huge cosmic-ray observatories, one south of Salt Lake City and the other near San Rafael, in the wine country of western Argentina. Each will have 50 times the sensitivity of AGASA and should detect rays at a proportionately greater rate. Meanwhile an upgraded version of the Utah experiment--the High Resolution Fly's Eye--should start scanning the skies later this year. Theorists will soon need to be more parsimonious.