Around twenty to twenty-five per cent of all matter is dark, while less than one per cent emits light. However, we do not know what this dark matter consists of. The current paradigm is that of ‘cold' dark matter: heavy particles that were already collecting together before the primordial plasma turned into hydrogen and helium. These would then move towards the collections of dark matter, which explains the current structures of galaxies and clusters. However, despite many attempts, the particle in question has never been identified.

The influence of neutrinos

Nieuwenhuizen suggests that neutrinos could be the missing link. Neutrinos are uncharged particles, such as those created during the nuclear fusion processes in the sun. Their role in 'cold' dark matter is considered to be negligible, partly because the neutrino mass has never been determined. Nieuwenhuizen now concludes that this is indeed light, but not super-light: 1.5 electron volts or three millionths of the electron mass. He obtained this information by studying data from a cluster of galaxies, where there is a great deal of dark matter present, as well as a large amount of hot gas. Nieuwenhuizen formulated a new theory for this purpose, based on Newton's laws and quantum mechanics, but also virial equilibrium (a state in which all speeds are approximately the same). He then used this theory to determine the mass of the dark matter particle and even the temperature of the gas. The dark matter forms a quantum structure in the centre of the cluster that is a couple of light years in diameter: the largest known.

Nieuwenhuizen goes on to claim that neutrinos, as single particles, can explain the dark matter. Up until a few years ago, we believed that neutrinos were left-handed (like a top that spins to the left), and that anti-neutrinos were only right-handed. This theory leads to 9.5 percent dark matter; much more than is anticipated from neutrinos, but less than the estimated twenty to twenty-five per cent. It is possible to explain twice as much dark matter if right-handed neutrinos and left-handed antineutrinos are also normally present. However, this assumption requires changes to be made to the standard model of elementary particles. The lepton number (an indication of the number of subatomic, elementary particles) is therefore violated. Nieuwenhuizen claims that this means that it must be possible for two neutrons to disappear simultaneously without the release of neutrinos.

This therefore leads to nineteen per cent ‘hot' dark matter and also the need for a new explanation for structure formation in the early universe. A definite answer on the theory will be obtained in 2012, when neutrino mass will be measured in Karlsruhe.

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