The race to discover the particles that constitute dark matter has begun after a supercomputer worked out the mass of one of the most promising candidates – axions. The computer calculations have placed constraints on the weight of axions, providing scientists with a concrete range in which to carry out experiments.
Dark matter makes up most of the universe, yet we know virtually nothing about it. Understanding it would vastly increase our knowledge of the cosmos and everything in it. But because it is invisible, it is undetectable.
As a result, scientists have to make predictions on what it is made of then conduct experiments to work out if they are right. At present there are two main theories about what dark matter is made of – either it is composted of a few but very heavy particles, or lots of extremely light ones.
To find out if the first is correct, scientists are currently using large underground detectors to search for dark matter. An example of this is the IceCube Neutrino Observatory in the South Pole. As of yet, there has been no sign of dark matter.
Looking for very light particles is more problematic, however. Andreas Ringwald, from DESY, Germany's largest particle accelerator centre, said: "It would be extremely helpful to know what kind of mass we are looking for. Otherwise the search could take decades, because one would have to scan far too large a range."
How do we know dark matter exists?
Dark matter makes up about a quarter of the mass of the universe. Unlike normal matter, which accounts for around 5% of the universe, dark matter does not interact with electromagnetic forces. As a result, it does not reflect, absorb or emit light – rendering it invisible. So how do we know it is there?
Galaxies rotate at such a fast speed that the gravity generated by normal matter would mean they were torn apart. However, as they still exist, there must be something else at play, holding them together. Dark matter, researchers believe, gives galaxies extra mass generating the extra gravity needed to exist.
Ringwald proposed the idea of using a supercomputer to come up with an outline of the mass of axions. These particles are predicted to exist through the quantum theory governing strong interaction – quantum chromodynamics (QCD).
Using a supercomputer, the team was able to calculate temperature ranges of axions. Results showed that axions (if they exist) have a mass of between 50 and 1500 micro-electronvolts – about 10 billion times lighter than electrons. If axions do make up matter, every cubic centimetre of the universe would contain 10 million axions, on average. Results are published in the journal Nature.
Now they have a range in which to search for axions, physicists can start conducting experiments to try to find them. If successful, this would solve the mystery of dark matter and answer many more questions about how the universe works. The team says we should be able to confirm or rule out the existence of axions within just a few years. "The results we are presenting will probably lead to a race to discover these particles," said study leader Zoltán Fodor.