Some mysterious source from beyond the galaxy periodically zaps Earth with high-energy cosmic rays from all directions. So far, the two leading experiments devoted to the detection and understanding of these rays have yielded contradictory results.
"Right now theorists are having a lot of fun because we have two possible outcomes, and maybe a third one, that both experiments are incorrect and there's a third explanation," said Angela Olinto, Associate Professor in Astronomy & Astrophysics at the University of Chicago.
A new experiment will likely pierce the secrets of these high-energy cosmic rays and resolve the data conflict within a few years, said Olinto, who will discuss the mystery in a session on cosmic-ray astrophysics on Sunday, Feb. 16, at the American Association for the Advancement of Science annual meeting in Denver.
High-energy cosmic rays are subatomic scraps of matter that fly through the universe at nearly the speed of light. These particles hit Earth's atmosphere with the energy of a tennis ball traveling at 167 miles an hour, Olinto said.
The Auger experiment is operated in Argentina by a collaboration of more than 250 scientists in 16 nations. The project was initiated by James Cronin, a Nobel laureate and University Professor Emeritus in Physics at the University of Chicago, and Alan Watson of the University of Leeds. Cronin and Watson named the project after Pierre Auger, the scientist who discovered cosmic rays in 1938. Auger conducted research at Chicago in 1942, launching hot-air balloon experiments from the University's Stagg Field to study cosmic rays.
High-energy cosmic rays are relatively rare. They occur only once a century over a given square kilometer patch of land-thus the large size of high-energy cosmic ray experiments. When complete, the Auger experiment will consist of a grid of electronic instruments that covers 3,000 square kilometers, an area more than half the size of the state of Delaware.
When a cosmic ray strikes Earth, it reacts with atoms in the atmosphere to create a cascade of a billion particles that shower the ground. Utah's existing High Resolution Fly's Eye detects cosmic rays by observing the fluorescent light they cause when they strike the atmosphere. Japan's current Akeno Giant Air Shower Array detects the cascade of secondary particles when they strike the ground. Auger will use both techniques on a larger scale and should resolve the data discrepancy between HiRes and AGASA.
For years it seemed that cosmic rays emanated from all over the sky, but that is no indication of where they originally came from. That is because the rays are electrically charged and magnetic fields deflect their paths as they travel through the universe. But at the very highest energies, the rays will travel a direct line from their source to Earth, enabling scientists to pinpoint their origin.
AGASA has begun to see a hint of clusters of particles coming from the same region. AGASA also sees cosmic rays at unexpectedly high energies.
"That is really exciting, because if AGASA is really finding particles above the energy where we thought they wouldn't be, then there's a whole new class of cosmic particle accelerators that nobody has predicted," Olinto said.
HiRes, meanwhile, has detected neither clustering nor cosmic rays at unexpectedly high energies. But Auger should be able to settle the discrepancy as it grows from its current 30 detectors to 1,600 detectors in the coming years. "I believe AGASA has a better case for being correct," Olinto said. "I also hope AGASA is correct-it will be a lot more fun."
Some scientists have proposed that the high-energy cosmic rays are produced by jets of matter emitted by supermassive black holes or by gamma-ray bursts, which are the most powerful explosions in the universe. Another possibility is topological defects, stresses and strains comparable to faults and folds in the Earth's crust that periodically release tremendous energies generated early in the history of the universe. Spinning neutron stars within the Milky Way galaxy are yet another explanation, according to a proposal put forward by Olinto, Pasquale Blasi of the Astrophysical Observatory in Arcetri, Italy, and Richard Epstein of Los Alamos National Laboratory.
"A very young neutron star could be spinning so fast, 3,000 times a second, that its strong magnetic fields could hit these particles, almost like a baseball bat, to incredible energies," Olinto said.
Only time will tell which theory, or another one entirely, proves correct.
"Right now we have a lot of fun things to debate," she said.
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