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Study explains why most cosmic rays miss Earth

By Carlos Mayorga

During a visit to the U in 1966, physicist Kenneth Greisen of Cornell University predicted that most ultra-high energy cosmic ray particles rarely reach Earth at full strength, instead weakening after colliding with leftover radiation in the universe.

After nine years of investigation, researchers at the U say they have confirmed his 42-year-old prediction that many particles collide with radiation left over from the “big bang,” an explosion that some believe formed the universe about 14 billion years ago.

Cosmic rays are subatomic particles, which contain mostly hydrogen, helium and other elements. The sun and other stars are responsible for emitting low-energy cosmic rays while exploding stars are the source of medium-energy cosmic rays.

The prediction, which physicists call the Greisen-Zatsepin-Kuzmin, focuses on ultra-high energy cosmic rays. These rays carry more energy than any other known particle.

One subatomic particle has the energy comparable to a fast-pitched baseball.

“If you throw a baseball, like Nolan Ryan for example, and you measure the energy, it is about 50 joules,” said Pierre Sokolsky, a professor of physics. “With these cosmic rays, a single subatomic particle will have about the same energy.”

Although the ultra-high energy cosmic rays don’t reach Earth, lower and medium energy rays sometimes do, but are not harmful to humans. If the high-energy cosmic rays were to reach Earth, they could be a threat to humans, Sokolsky said.

Researchers from the U’s High Resolution Fly’s Eye cosmic ray observatory, located in Utah’s west desert, have teamed up with scientists from the University of Tokyo to learn more because physicists still don’t know a lot about these ultra-high energy cosmic rays.

“We still don’t know where the highest energy particles come from,” said Charlie Jui, a professor of physics at the U.

The most common theory is that these cosmic rays originate in active galactic nuclei, which is a super-massive black hole a billion times the mass of the sun. The black hole is so large that it sucks in matter spirally, like water going down a drain. Instead of the particles disappearing into the black hole, these cosmic rays are accelerated through jets and sent toward Earth.

“You would think that the particles would be absorbed into the black hole, but they are not,” Sokolsky said.

Greisen’s prediction in 1966 was that these ultra-high energy cosmic rays are unlikely to reach Earth because they lose energy en route when they collide with radiation left over from the birth of the universe.

The U is one of two places in the world studying these cosmic rays. Recent results from an unrelated study coming out of an observatory in Argentina show similar findings.

The results from the U’s study were published last week in the journal Physical Review Letters.

Sokolsky said that the next step is identifying the source of these cosmic rays and whether or not they are really coming from a black hole. Physicists are also using a new $17 million telescope in an observatory west of Delta, Utah, to confirm the theory that cosmic rays come from black holes.

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