A $2bn experiment on the space station has made observations that could prove to be the first signs of dark matter, a mysterious component of the Universe. The Alpha Magnetic Spectrometer (AMS) surveys the sky for high-energy particles, or cosmic rays.It has seen evidence for what could be dark matter colliding with itself in a process known as “annihilation”. But scientists stress that a precise description of this enigmatic cosmic constituent is still some way off.
“It could take a few more years,” said AMS deputy spokesman Roberto Battiston, a professor of physics at the University of Perugia, Italy. “But the accuracy that AMS is displaying is far greater than past experiments, so we’re getting closer to unveiling the cause of the particle events we’re detecting,” he told BBC News. Dark matter accounts for most of the mass in the Universe.
It cannot be seen directly with telescopes, but astronomers know it to be out there because of the gravitational effects it has on the matter we can see.
Galaxies, for example, could not rotate the way they do and hold their shape without the presence of dark matter.
AMS – a particle physics machine nicknamed the “Space LHC” in reference to the Large Hadron Collider here on Earth – has been hunting for some indirect measures of dark matter’s properties. It counts the numbers of electrons and their anti-matter counterparts – known as positrons – falling on to a battery of detectors. Theory suggests that showers of these particles should be produced when dark-matter particles collide somewhere in space and destroy each other.
In a paper in the journal Physical Review Letters, the AMS team reports the observation of a slight excess of positrons in the positron-electron count – an outcome expected of these dark matter annihilations.
Confusingly, particle physicists speak of both particle masses and their energies in terms of the energy unit called an electronvolt For example, the accelerator called the Large Hadron Collider speeds protons to energies of trillions of electronvolts. Yet, when the particle called the Higgs boson was discovered there, scientists said it had a mass of about 125 billion electronvolts. This is actually physicists’ shorthand, which arises from “mass-energy equivalence” – Albert Einstein’s most famous equation. The actual mass is found by dividing by the square of the speed of light – for the Higgs boson, that is about 0.0000000000000000000003 g
The group also says the positrons fall on the AMS from all directions in the sky with no particular variation over time. This is important because specific locations or timing variations in the signal could indicate a more conventional source for the particles, such as a pulsar (a type of neutron star).
AMS was placed on the International Space Station in 2011. The longer it operates, the better its statistics will be and the more definitive scientists can be in their statements.