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 universe.
The instrument, the Alpha Magnetic Spectrometer (AMS), has not seen dark matter directly — by definition, the stuff is invisible — and results announced so far do not lend themselves to a slam-dunk conclusion that dark matter is a fact of the cosmos and not merely a theoretical construct.
But the 7.5-ton device, which rides a truss on the space station like a bell on a bicycle’s handlebars, has detected hundreds of thousands of particles that have features suggesting that they are debris from collisions of dark matter particles.
“We, of course, have a feeling what is happening,” said Nobel-winning physicist Samuel Ting of the Massachusetts Institute of Technology, speaking in a packed auditorium at CERN, the Geneva-based European particle physics laboratory. But Ting, pressed by audience members to reveal more of his data and give a stronger conclusion, stuck to a modulated message.
“It took us 18 years to build this experiment. We want to do it very accurately,” he said.
The AMS operates at the nexus of subatomic nature and Big Science. The project’s $2 billion cost has been a source of controversy. The detector, funded through an international collaboration, including money from NASA, overcame delays and redesigns and one outright cancellation before riding to orbit in 2011 on the last flight of the space 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 the particles, measuring their momentum and charge.
A small percentage of the particles that hit the detector are unusual things called positrons, which are like electrons but with the opposite charge. They’re in the class of particles known as antimatter.
There’s not much antimatter in our universe, and there hasn’t been for many billions of years. When matter and antimatter collide, they are mutually annihilated. The universe early in its history had a bit more matter than antimatter, an asymmetry that, from the human standpoint, is fortuitous, because matter and antimatter in precisely equal amounts would have obliterated each other and left a starless, planetless, uninhabitable cosmos.
Physicists say rare antimatter particles, such as positrons, can be created in certain violent, high-energy environments. For example, positrons might have been flung into space from the atmosphere of a pulsar, an ultra-dense, rapidly rotating star with a powerful magnetic field.
Another theorized source of positrons is dark matter. If antimatter seems exotic, dark matter is even more so. No one has ever seen the stuff, and its existence has never been nailed down definitely. Dark matter emits and absorbs no light, and interacts with ordinary matter in a ghostly fashion, primarily through gravity. Dark matter is thought to affect the way galaxies move; they rotate in a manner that suggests that they are carrying some unseen load. In the past two decades, other experiments and detectors have bolstered the idea that dark matter is far more abundant than ordinary matter.
The surprising abundance of positrons has been established by earlier experiments. But the AMS has “unprecedented accuracy and sensitivity,” Ting said when questioned by a reporter about whether the mission was worth the cost.
Although most of his statements were cautious, under questioning, he said his data “support” the dark matter origin of the positrons. He reiterated that he cannot rule out the pulsar origin.
One reason he and his collaborators lean toward a dark matter origin is that the detector gathered positrons from all directions, evenly, without pause. That suggests that they came from something that is omnipresent, such as the theorized dark matter.
A key question is whether the detector finds many positrons at very-high-energy levels. For theoretical reasons, a sudden drying up of positrons at the high-energy end of the scale would be consistent with a dark matter origin. Ting told the scientists that he wasn’t ready to release the high-energy data and that they should be patient.
Ting said in a later NASA teleconference that the AMS will collect data through the lifetime of the space station and that he expects to be able to solve, with finality, the mystery of the positrons — “hopefully, quickly.”
This will not, however, end the mystery of dark matter. Even if the AMS is a smashing success, it has no ability to discern what dark matter is, fundamentally, or how much of it is out there, or why it is dark.