Fun Times with Cosmic Rays
by Christopher Wanjek
High-energy cosmic rays could be pointing to dead quasars, and in some cases, new physics.
Who would have thought cosmic rays could be so hip? Although discovered 90 years ago on death-defying manned balloon flights ? hip even by twenty-first-century extremesport standards ? cosmic rays quickly lost popularity as way-cool telescopes were finding way-too-cool phenomena across the electromagnetic spectrum.
Yet cosmic rays are back in vogue, boasting their own set of superlatives. Scientists are tracking them down with new resolve from the Arctic to Antarctica and even on the high western plains of Argentina. Theorists, too, now see cosmic rays as harbingers of funky physics.
Cosmic rays are atomic and subatomic particles ? the fastest moving bits of matter in the universe and the only sample of matter we have from outside the solar system (with the exception of interstellar dust grains). Lower-energy cosmic rays come from the Sun. Mid-energy particles come from stellar explosions ? either spewed directly from the star like shrapnel, or perhaps accelerated to nearly the speed of light by shock waves.
The highest-energy cosmic rays, whose unequivocal existence remains one of astronomy's greatest mysteries, clock in at a staggering 1019 to 1022 electron volts. This is the energy carried in a baseball pitch; seeing as how there are as many atomic particles in a baseball as there are baseballs in the Moon, that's one powerful toss. No simple stellar explosion could produce them.
At a recent conference in Albuquerque, scientists presented the first observational evidence of a possible origin for the highest-energy variety. A team led by Elihu Boldt at NASA's Goddard Space Flight Center found that five of these very rare cosmic rays (there are only a few dozen confirmed events) come from the direction of four "retired" quasar host galaxies just above the arm of the Big Dipper, all visible with backyard telescopes: NGC 3610, NGC 3613, NGC 4589, and NGC 5322. These galaxies are billions of years past their glory days as the brightest beacons in the universe. Yet they still harbor central, supermassive black holes, which could generate energetic particles if they are spinning.
"The very fact that these four giant elliptical galaxies are apparently inactive makes them viable candidates for generating ultrahigh-energy cosmic rays," says Boldt. Drenching radiation from an active quasar dampens cosmic-ray acceleration, sapping most of their energy. No one had thought to look to a so-called inactive galaxy for cosmic rays before.
Scientists need better statistics to confirm this scenario. The highest-energy particles strike Earth at a rate of one per square kilometer per decade. The Auger Observatory is a vast system of detectors that will span 3000 square kilometers in Argentina. Auger hopes to detect up to 100 ultrahigh-energy events a year when completed in 2004. A proposed mission called OWL (Orbiting Wide-angle Light-collectors) could detect perhaps a thousand ultrahigh-energy cosmic rays per year by looking down on the atmosphere from space.
NASA theorist Demos Kazanas thinks cosmic rays may trigger funky, new physics. He says that when cosmic rays smack into the atmosphere, part of the energy released seemingly disappears, entering a realm not measurable by current detectors. This energy possibly forms miniature black holes or is transferred to "particles" of gravity, called gravitons, which might leak into other dimensions.
Kazanas points to the abrupt change in the sampling rate of cosmic rays collected above a specific energy range, known as the "cosmic-ray spectrum knee." For every tenfold increase in their energy, cosmic rays become roughly 60 times rarer. Yet above a very specific energy level, 1015.5 eV, cosmic rays suddenly become even rarer, roughly 100 times for every tenfold increase in energy.
It is above this energy level that the effects of new physics may occur, leading to the disappearance of energy. Cosmic rays create showers of secondary particles when they collide with atoms in the atmosphere. If part of the collision energy is channeled into another form of matter or energy, or into other dimensions, then the inferred energy of a given cosmic ray will seem to be lower than it really is.
The well-recorded knee in the cosmic-ray spectrum at 1015.5 eV corresponds to a particle collision energy of 1012 eV. This turns out to also be the energy at which, according to some theories, the signatures of new physics beyond the so-called Standard Model should reveal themselves. "This seems to be a little more than a coincidence," says Kazanas.
CERN will install a new particle accelerator by 2006 capable of producing energies above 1012 eV, which could test Kazanas' theory. In the meantime, restless 20- somethings longing for adventure and a taste of funky physics can trek to the most extreme locations on Earth to collect the most extreme particles in the universe. Or, you can look out over the Big Dipper for a Randy Johnson fastball of cosmic proportions.
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