Dark matter may be source of exotic space particles, physicist says
NASA handout/AFP/GETTY IMAGES – This NASA image shows the Alpha Magnetic Spectrometer aboard the International Space Station. The AMS may have detected the first hints of dark matter in the universe.
By Joel Achenbach, Wednesday, April 3, 1:38 PM
The first results from a $2 billion instrument aboard the international space station have offered tentative support for the theory that exotic dark matter, invisible but abundant, permeates the galaxy.
The instrument, the Alpha Magnetic Spectrometer (AMS), has not seen dark matter directly — by definition, the stuff is invisible — and the results announced so far do not lend themselves to a slam-dunk conclusion. But the 7.5-ton device, which rides a truss of the space station like a bell on a bicycle’s handlebars, has detected hundreds of thousands of exotic particles called positrons that may be the debris from dark matter particles colliding with each other.
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“We, of course, have a feeling what is happening,” said Nobel Laureate physicist Samuel Ting of the Massachusetts Institute of Technology, speaking in a packed auditorium in Geneva at CERN, the European particle physics laboratory. But Ting, despite being pressed by audience members to reveal more, stuck to a set of tentative conclusions.
“It took us 18 years to build this experiment. We want to do it very accurately,” he said.
He added: “This is a very difficult experiment.”
The AMS is also a very costly experiment, which has been a source of controversy in recent years. The detector, which is funded through an international collaboration, including money from NASA, was once canceled outright. After delays and some redesign, the instrument rode to orbit in 2011 on the last flight of the shuttle Endeavour.
The instrument had to be designed to withstand the rigors of space and to operate without the benefit of repair or recalibration. It has functioned splendidly, Ting said. It detects cosmic rays, which are particles moving at extraordinary velocity and coming from all over the galaxy. The AMS sorts through these particles, measuring their momentum and charge.
The device can discern whether a particle is a positron, which is a form of antimatter. It is the antiparticle of the electron, identical but with the opposite charge. Our universe has largely been swept clean of antimatter, because when matter and antimatter collide, they are mutually annihilated. (For reasons that remain the subject of much theorizing, the universe from the beginning had a bit more matter than antimatter.) But new bits of antimatter can be created within our galaxy in certain kinds of events. One way is if two particles of dark matter collide.
Dark matter emits and absorbs no light, and interacts with ordinary matter in a ghostly fashion, primarily through gravity. Dark matter affects the way galaxies move; they rotate in a manner that suggests that they are carrying some unseen load of matter.
In the past two decades, other experiments and detectors have bolstered the notion that dark matter is far more abundant than ordinary matter. What it is, exactly, remains unclear. The AMS, however, has picked up signals from more than 400,000 positrons, and they came into the detector from all directions, with no variation over time. That is consistent with the idea that they are coming from collisions of particles of dark matter that saturate the galaxy.
The other possible origin is that positrons are emitted by rapidly rotated, ultra-dense stars called pulsars, which are scattered throughout the galaxy. Ting said Wednesday that his results cannot rule out that possibility.
But his results appear to be more consistent with the dark matter origin, even if he was hesitant to go that far. He said that, with further data collection in the coming months, he may be able to make a more definitive statement.