A $2 billion cosmic ray detector aboard the International Space Station has detected tantalizing signs of the elusive "dark matter" particles that scientists say fill space, an international astrophysics team reports.
At a briefing Wednesday at Europe's CERN laboratory, experiment chief Samuel Ting of MIT, a Nobel-prize winning physicist, reported that first measurements made by the Alpha Magnetic Spectrometer detector aboard the space station indicate that cosmic rays, which fill the universe as the fallout from exploding stars, have left evidence of collisions with dark matter particles in deep space.
"The data agrees very well with the simplest model," Ting said. He cautioned "more statistics" are needed to be certain of the result.
Dark matter, as the name suggests, is a form of matter that appears invisible. Astronomers say they think it must fill space because of observations of its gravitational tugs on galaxies. Dark matter may be mostly undiscovered exotic physics particles (amusingly called "WIMPS" for "weakly interacting massive particles"), and scientists say dark matter outweighs normal matter -- the stuff of stars, planets and people -- by more than five times throughout the universe, based on observations of stars' gravitational pull on one another.
But they don't know for certain that it's real. To detect more direct signs of this elusive stuff and determine if it really exists, astronauts installed the AMS detector aboard the space station two years ago. In space, the detector can record cosmic rays, outbursts of charged particles delivered from deep space, otherwise absorbed by Earth's atmosphere. Over the past 18 months, the detector has recorded 25 billion cosmic ray signals, Ting says.
Scientists say that when cosmic rays bang into dark matter, the collision throws off antimatter particles called positrons -- a positively charged mirror image of an electron. The spectrometer can record these positrons. Cosmic rays have different energy levels, depending on where they originate in the cosmos. When the highest-energy versions hit dark matter, scientists say, they no longer generate as many positrons, but rather cast off other particles more often. The detection of positrons in that case would drop off sharply. If dark matter doesn't exist, those positron readings would drop off slowly instead.
The numbers do point to dark matter existing as physicists predict, Ting says, but they do not rule out a competing explanation for the antimatter particles. Alternately, they could result from cosmic rays originating instead from distant compact stars called "pulsars" ringing the Milky Way galaxy.
"Beautiful results, but we are not there yet in terms of identifying the dark matter," says astrophysicist Michael Turner of the University of Chicago. Turner first described the antimatter drop-off effect tested by the AMS experiment with Nobel prize-winning physicist Frank Wilczek of MIT in 1990. "(T)he instrument is working beautifully, and the results that will come in the future should be able to test the WIMP hypothesis," Turner says by e-mail.
Ting agrees that over the next two decades aboard the space station, the experiment's smooth operation indicates it should produce enough data to settle the dark matter mystery. "We have a feeling what is happening, but it probably is too early to discuss," he says.