Shedding some light on dark matter:
The $2bn experiment in space that may be
about to turn the world of physics on its head
· Dark matter is believed to provide the gravity which binds the cosmos
· But if proved to exist it would challenge the conventional view of physics
· Scientists have attempted to track it down with spectrometer bolted to ISS
· First set of results from the instrument to be published in two weeks
By Damien Gayle
PUBLISHED: 10:52 GMT, 19 February 2013 | UPDATED: 11:39 GMT, 19 February 2013
It is still not proven that dark matter even exists, but it is speculated that its gravity holds the universe together.
However, if dark matter is real, its existence would defy conventional notions of the cosmos.
In two weeks scientists will publish the first set of results from the Alpha Magnetic Spectrometer, a $2billion particle collector mounted on the outside of the International Space Station in a bid to find dark matter.
Leading the experiment is Nobel Laureate Samuel Ting, a physicist with the Massachusetts Institute of Technology, who said: 'It will not be a minor paper.'
Professor Ting would not be drawn on what exactly the AMS has found, but he told the American Association for the Advancement of Science (AAAS) its results will represent a 'small step' in understanding what dark matter is.
'We've waited 18 years to write this paper, and we're now making the final check,' the BBC quoted him as saying. 'I would imagine in two or three weeks, we should be able to make an announcement.
'We have six analysis groups to analyse the same results. With physicists, as you know, everybody has their own interpretations, and we're now making sure everyone agrees with each other. And this is pretty much done now.'
Dark matter poses a serious challenge to the conventional view of physics, referred to as the Standard Model.
A particular problem with the Standard Model is that it can't explain how gravity holds the universe together.
Because the gravitational pull of objects is directly linked to their mass, according to the conventional view there would not be enough mass to hold the universe together.
This has led scientists to calculate that a further 23 per cent of the universe must be composed of dark matter.
The rest, they say, is composed of dark energy, an even more mysterious force that seems to drive the expansion of the cosmos.
Michael Turner, director of the Kavli Institute for Cosmological Physics at the University of Chicago, told AFP: 'On the cosmology side we now understand that this mysterious dark matter holds together our galaxy and the rest of the Universe.
'And the tantalising thing on the cosmology side is that we have an airtight case that the dark matter is made of something new… there is no particle in the Standard Model that can account for dark matter.'
Researchers don't yet know what dark matter is made of, but there are some theories which suggest it is composed of what physicists have dubbed weakly interacting massive particles (WIMPs for short).
These exotic particles are believed to be their own antimatter particles so, if two collided, they would be destroyed to release an electron and its antimatter equivalent, a positron.
That's where AMS comes in. The $2billion, seven-ton machine, which was installed on the ISS in May 2011, is able to detect the positrons and electrons left behind by dark matter collisions in the Milky Way.
According to Professor Ting, the spectrometer in its first 18 months witnessed 25billion particle events, including eight billion fast-moving electrons and positrons.
The forthcoming paper will report how many of each it found and what their energies were, he said.
By calculating the ratio of electrons to positrons and analysing the behaviour of any excess across the energy spectrum, researchers hope to edge their way towards an understanding of dark matter.
'The smoking gun signature in the positron to electron ratio is a rise and then a dramatic fall,' said Professor Turner. 'That is the key signature for the dark matter annihilation in our galaxy's halo.'
Another key piece of evidence will be whether positrons appear to be flowing in just one direction, or coming in from all around.
Professor Ting told the BBC: 'Dark matter is supposed to be everywhere. So if we see the positrons coming from a particular direction, it means astrophysics like a pulsar (a type of neutron star) is responsible for the signal, not dark matter.'
Whether or not the AMS has detected dark matter, scientists are looking forward to the publication of its findings.
'We are so excited because we believe we are on the threshold of a major discovery,' said Professor Turner. 'We believe this will be the decade of the wimp.'