If asked to describe the universe, how would you go about it? Perhaps we could start with all forms of life, then everything on Earth, our galaxy, every other galaxy we can see and finally everything that is not in our observable universe. Until around 1932, most scientists would probably have agreed with you, but in that unassuming year something rather ground-breaking happened. Astrophysicists started to notice discrepancies between the mass of large astronomical objects when determined by their gravitational effects and when calculated from the masses of the observable matter that they contain. Scientists developed the idea of ‘dark matter’ to compensate for this inconsistency, along with a range of subsequent discrepancies, such as orbital and rotational velocities of galaxies.
By its nature, dark matter is almost impossible to define. It is thought to be composed of some sort of unidentified sub-atomic particle, but it does not absorb or emit any electromagnetic radiation at any significant level, so can’t be detected by telescopes. It is also thought to make up for 25% of the mass of the universe, so its elusiveness is no mean feat. Until recently there was no generally agreed way in which scientists could directly detect dark matter, in fact, many believed that it was entirely undetectable. However, on April 3rd 2013, NASA reported that the Alpha Magnetic Spectrometer, AMS, on the International Space Station had detected hints of dark matter.
The AMS is a particle physics module that has been docked on the International Space Station since 19th May 2011. It is by no means a telescope, as one might have expected, which usually measure photons that are both visible to the naked eye and beyond our spectrum of detection. The AMS instead studies charged particles, caused by exploding stars and other cosmic phenomena which produce cosmic rays. These cosmic rays are composed of charged particles which reach our planet but are absorbed by our atmosphere so they are impossible to study on Earth.
The AMS, however, is floating in a blissfully atmosphere-free vacuum, aka space, so it does not suffer from this problem. A particle will freely pass into the AMS and detectors will identify its physical properties such as its mass, velocity and energy. In the heart of the AMS there is a magnet that controls the path taken by any particle that enters the module. This magnet causes particles classed as ‘matter’ to curve a different way from those classed as ‘anti-matter’, and therefore separating any incident particles into these categories. The AMS is hit by 25,000 cosmic rays per second and thus produces a huge amount of data that is then analysed back on Earth.
So far, the AMS has been incident to more than 30 million cosmic rays, 6.8 billion of which have been identified as electrons and their anti-matter counterparts, positrons. It is the proportion of electrons to positrons that have caused scientists to become suspicious of dark matter lurking. The proportion of positrons detected is much higher than expected, which fits with the theory that when dark matter particles collide they annihilate each other and produce positrons.
Although there are no concrete answer yet as to the definitive nature of dark matter, Nobel laureate and AMS project leader Dr Sam Ting is certain that such an answer is just around the corner, stating that “there is no question we’re going to solve this problem”.
Until then, we will have to wait for a little more light to be shed upon this dark shard of the universe’s mysterious composition.
Image by Sweetie187