An international team of scientists said Thursday that they have tracked down the origin of the mysterious "Oh-My-God" particle -- a cosmic ray bearing energies millions of times larger than the most powerful particle accelerator can produce on Earth.
Researchers at the Pierre Auger Observatory, a complex of detectors spread over a Rhode Island-sized slice of the South American prairie, said the most likely source for these ultra-high-energy particles is a type of black hole found at the center of some galaxies.
These violent phenomena are called active galactic nuclei because they both gobble up energy from the surrounding space and spit some out with tremendous force.
"This is a fundamental discovery. We have taken a big step forward in solving the mystery of the nature and origin of the highest-energy cosmic rays," said Nobel laureate James Cronin, a professor emeritus at the University of Chicago and coauthor of the paper published in the current issue of the journal Science.
Cosmic rays are charged particles that can strike with the force of a thrown baseball. Fortunately for humans, Earth's atmosphere serves as a shield, preventing the particles from reaching the surface.
The origin of cosmic rays, particularly the high-energy ones, has been a mystery since their discovery in 1938 by French scientist Pierre Auger. Scientists have only been able to speculate over what tumultuous processes could accelerate particles to energy levels 100 million times more powerful than anything produced at Fermi National Accelerator Laboratory in Illinois, Earth's most powerful accelerator.
One of the chief candidates was black holes at the center of many galaxies, mainly because these are some of the most energetic objects in the universe.
But tracking the source of the particles has been difficult, in part because of their rarity. Each century, only one of the highest-energy particles -- sometimes called Oh-My-God particles -- strikes the atmosphere over any particular spot on Earth.
To research them, "You either need very long-lived scientists or an instrument covering a very large area," said Henry Glass, a Fermilab particle physicist and a coauthor of the paper.
Besides being large, the detector had to be sophisticated enough to spot the cascade of billions of secondary particles that rain down when a high-energy cosmic ray hits the upper atmosphere.
