Jan. 4, 1996
Vol. 15, No. 8

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    Schramm: cosmic rays may be signature of early universe

    Highest-energy cosmic rays may be signature of early universe

    A trio of rare and mysterious cosmic rays of the highest energy ever recorded may be the first signs of fundamental physical processes in deep space that have not been seen even in the most powerful particle accelerators.

    David Schramm, Louis Block Professor in Physical Sciences and Vice President for Research, and his co-authors report in the Dec. 22 issue of Science magazine that the three cosmic rays, which were detected in the early 1990s, may be fundamentally different from the trillions of ordinary cosmic rays that bombard Earth every day.

    "These three events are so much more energetic than any others that they really stick out like a sore thumb," Schramm said. "Our analysis suggests that they may be created in entirely different processes from ordinary cosmic rays. This would be very exciting because they would be the first evidence we have of the exotic processes of the Grand Unification Scale that created all the matter in the universe."

    Schramm said the ultra-high-energy particles may have been produced by defects in the "false vacuum" that preceded massive expansion in the early universe, by topological defects left over from the early universe or by the quantum-mechanical decay of supermassive elementary particles that are far beyond the reach of even the most powerful particle accelerators.

    The three events -- 10 times more energetic than the highest-energy cosmic rays ever before seen -- were detected in Russia in 1990, in Utah in 1991 and in Japan in 1992, but they were not announced until two years ago.

    Schramm explained that most cosmic rays are the nuclei of atoms that have been accelerated to very high velocities -- and therefore energies -- by exploding stars or powerful magnetic fields in space. Most cosmic rays have energies of millions or billions of electron volts, but some are much more energetic. These extremely energetic particles become ever more scarce at the highest energies, and except for the three ultra-high-energy particles, no cosmic rays have been seen with more energy than 30 or 40 million trillion electron volts. This is due in part to the scarcity of events that can accelerate particles to such enormous energies, and in part to the fact that these particles lose energy when they collide with the ever-present cosmic microwave background radiation.

    How then could the three mystery cosmic rays have managed to arrive at Earth without losing energy to collisions with the microwave background?

    "The point is that cosmic rays produced in the most exotic ways would start with a hundred thousand times more energy than any ordinary cosmic rays," he said. "The collisions would simply have lowered their energy to the ultra-high levels observed."

    Another argument in favor of an exotic origin for the ultra-high-energy rays is that they come from directions in space that contain no known sources of energetic particles. And unlike lower-energy particles, these are so energetic that they cannot be deflected by magnetic fields in space, so they give an accurate picture of their direction of origin.

    Because only three such ultra-high-energy cosmic rays have been detected, Schramm warned that he and his colleagues aren't sure they represent exciting new physics. But there is one way to find out.

    "The only way to test this idea is to observe more of these particles," he explained. "The best way to do that is to build the giant array detector proposed by Jim Cronin."

    Cronin, who shared the Nobel Prize in physics in 1980 for his work in particle physics, is University Professor in Physics. He has proposed two arrays of automobile-size detectors spaced every mile over areas the size of the state of Delaware that could detect the extremely rare, ultra-high-energy rays as often as every week or two. Because Cronin's detectors would take advantage of cellular telephone and global positioning technology, they could be constructed for far less than the cost of even a medium-sized particle accelerator, yet they would help study particles far beyond the range of any accelerator that could be built on Earth.

    "It's really a very elegant project," Schramm said. "And it could tell us whether we are seeing the first direct evidence of the universe's earliest moments. That would be very exciting."