Sen— A $2 billion experiment on the outside of the International Space Station may have found fresh clues to the existence of an invisible substance called dark matter.
The costly detector, called the Alpha Magnetic Spectrometer, was ferried into orbit by the space shuttle Endeavour in May 2011. Since then, its seven instruments have already recorded more than 30 billion cosmic ray strikes - high speed particles from the depths of space.
Results based on the study of 25 billion of these events included the detection of 400,000 positrons - antimatter equivalents of the electron. That is the largest collection of antimatter particles recorded in space.
An international team of scientists says the new data is consistent with the positrons originating from the annihilation of dark matter particles in space. However the evidence is not strong enough yet to rule out other explanations such as pulsating stars.
So what is dark matter? Strange as it may seem, cosmologists' models suggest that ordinary matter which we can see or observe directly in other wavelengths makes up only one twentieth of the mass and energy of the Universe. Recent results from the European Space Agency's Planck satellite put the figure at 4.9 per cent.
There appears to be vastly more dark matter with a figure more than five times greater of 26.8 per cent. And more than two thirds - 68.3 per cent - is made up of a mysterious but unrelated force called dark energy which is thought responsible for causing the expansion of the Universe to speed up.
The proportions of ordinary matter, dark matter and dark energy thought to exist in the Universe before and after Planck's studies. Credit: ESA
Dark matter is only observable indirectly, by effects such as its gravitational pull on ordinary matter. As well as the research in space, attempts to prove its existence are being made at CERN's Large Hadron Collider in Switzerland and other underground detectors.
An excess of antimatter in the battering of the Earth by cosmic rays was first noticed about 20 years ago. Though its origin remains unexplained, theory suggests the positrons could be produced when two particles of dark matter collide and annihilate.
The new results from AMS were revealed at a NASA press conference and CERN webcast yesterday and it is fair to say that many who watched found them underwhelming due to the lack of firm evidence displayed.
But American physicist Professor Samuel Ting, the project leader, believes confirmation or otherwise will not be long in coming. He said: “Over the coming months, AMS will be able to tell us conclusively whether these positrons are a signal for dark matter, or whether they have some other origin.”
NASA Administrator Charles Bolden commented: “The AMS cosmic ray particle results announced today could help foster a new understanding of the fields of fundamental physics and astrophysics. I am confident that this is only the first of many scientific discoveries enabled by the station that will change our understanding of the universe.”
The results will be published in the journal Physical Review Letters.
The AMS from assembly to fitting in space. Credit: Widlab