January 17, 2006
It takes something spectacular to throw an astrophysicist off their guard, but that's just what happened at an observatory in Utah in 1991. What scientists saw back then was so amazing that they named it the Oh-My-God Particle.
In their year-end edition for 2005, the editors of Science named these ultra-high-energy cosmic particles (also known as rays) as a topic to watch in the New Year. That's because the completion of a humongous new observatory in Argentina might mean a solution to the mystery of where Oh-My-God Particles come from.
Every now and then, cosmic ray detectors like the one in Utah are struck by subatomic particles with huge amounts of energy; way too much energy to be caused by supernovae explosions or by matter falling into black holes, which is how regular cosmic rays are created. These particles have so much energy that they must have come from somewhere near Earth. These are ultra-high-energy cosmic rays, and though physicists have been aware of them for decades, no one knows how to explain them.
"They have about the amount of energy that a baseball has that comes at a speed of 100 kilometres per hour," said Rainer Dick, an astrophysicist at the University of Saskatchewan. That is an amazingly huge amount of energy to be packed into something as tiny as a proton or an atomic nucleus.
Dick says that there are three mysteries surrounding ultra-high-energy cosmic rays. The first is where they are coming from: "They seem to come literally out of the blue," he said. "If one maps the arrival direction of these cosmic rays on a celestial globe, for example, or on a map of the sky, they are not correlated with any know cosmic accelerators, they are not correlated with known supernovae or known super-massive black holes."
Although their origins appear random, the second puzzle is that ultra-high-energy cosmic rays seem to be coming from close by. "Outer space is empty to us, but for a charged particle that is traveling extremely fast, almost at the speed of light, the universe is pretty much a thick wall, because there are photons in outer space," said Dick. A subatomic particle would scatter off of those photons, thus losing more energy the further they travel.
The final unknown is what could possibly accelerate these particles to such high energies. "We don't know really of any acceleration process for charged particles that could really accelerate sub-atomic particles to energies of 1020 electron volts," Dick said.
The new Pierre Auger Observatory in Argentina will be equipped with 1500 water tanks spread out over four to six kilometres to detect ultra-high-energy cosmic rays. When an Oh-My-God Particle hits the atmosphere, it will strike the molecules there and create a secondary shower of particles that should reach the water tanks on the ground. The detectors at the Observatory should be able to detect between 30 and 100 ultra-high-energy rays per year, compared to about one per year now.
For Dick, development of this observatory means that he will be able to test his own theory for the origin of these ultra-high-energy particles; It involves that peculiar dark matter that we can't see and can only detect through its gravitational pull, but which makes up 90% of the universe.
"These dark matter particles are very heavy, so they have a lot of energy stored in them," said Dick. "And if two of these dark matter particles collide, they can convert the whole rest-mass energy into radiation, into cosmic rays."
If Dick is right, then most of the cosmic rays captured at the Pierre Auger Observatory should be traceable to the galactic centre of the Milky Way, where the dark matter density is highest. If not, then it's back to the drawing board.
But if Dick is correct, he believes that, "ultra-high-energy cosmic rays would be a window to the very first moments of the universe, right after the big bang." That's when the universe was expanding at its fastest, and when super-heavy dark matter particles were created.
"BREAKTHROUGH OF THE YEAR: Areas to Watch in 2006" Science 23(310): p.1885 http://sciencemag.org/cgi/content/full/310/5756/1885
Pierre Auger Observatory: http://www.auger.org/
Bethany Lindsay has a degree in Biology from Queen's University, where she spent her time chasing lizards and stealing their DNA. She later developed an insatiable need to see her name in print, so she moved to Vancouver to become a journalism student. Bethany writes a science news blog called "The Flycatcher" (www.livejournal.com/users/theflycatcher/).