New light on dark matter

The Hubble image shows the Bullet Cluster, studies of which have given the best evidence of dark matter.

The Hubble image shows the Bullet Cluster, studies of which have given the best evidence of dark matter.  

Dark matter is as mysterious as it sounds – very little is known about it, save that it makes up about 85 per cent of all the matter in the universe. Now, German and Hungarian scientists have thrown some light on a type of dark matter particle that has been postulated, known as the axion. They have established that axions can have a mass between 50 and 1500 micro electron volts, making them some ten billion times lighter than the electron. This computation has been published in the journal Nature. An interesting fact is that these calculations were done numerically using a (Bluegen/Q) super computer, JuQueen, housed in the Julich Supercomputer Centre in Germany.

Dark matter is so known because it interacts weakly with matter and so is notoriously difficult to detect. Yet, indirect proof of its existence comes from observation of rapidly rotating galaxies, which cannot be held together merely by the gravitational pull of the matter they contain – there has to be a lot of invisible stuff known as “dark matter” to prevent them from flying apart with the force of their own energies. Such inferences imply that nearly 85 per cent of the universe is made of dark matter, the known matter only contributes 15 per cent.

Several candidate particles have been postulated that may constitute dark matter – both highly massive and lightweight – but none of the experiments have detected any such particle so far, directly. Axions are particles proposed by extending quantum chromodynamics (QCD) the theory that describes “strong interactions,” the way quarks and gluons bond to form matter particles such as protons, neutrons etc. Though they have been proposed and there are experiments to study them (for instance, the Axion Dark Matter Experiment, ADMX), there has been no real handle on these until now. The present work sets a mass bound on the axions, between 50 and 1500 micro electron volts, as mentioned earlier. This would require that there exist ten million such particles for every cubic centimetre of the universe. Also, because dark matter is not evenly spread out, but occurs in clumps, there should be nearly a trillion axions per cubic centimetre in the Milky Way – our galaxy.

Knowing the expected mass range of the axion not only gives a better understanding of the particle itself, but also can serve as a guideline for doing experiments. Instead of firing in the dark, ADMX, for instance, now has a definite range to study keenly.

The work sets a bound on the

axion mass which can guide experiments

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