Dark matter and spatial extra dimensions

Brussels, 06 Sep 2005

An international team of astrophysicists claims to have found evidence that space is six dimensional. These three new spatial directions could explain a longstanding astronomical puzzle. The discovery has been reported in Nature magazine.

Joseph Silk from the University of Oxford, UK, and colleagues from Canada and China, say that these extra spatial dimensions can be inferred from the perplexing behaviour of dark matter. This mysterious material cannot be seen, but its presence in galaxies is betrayed by the gravitational tug that it exerts on visible stars.

Physicists have suspected for years that 'hidden' dimensions exist, largely because they seem to be predicted by string theory, the current favourite theory of fundamental subatomic particles.

Silk and his colleagues looked at how dark matter behaves differently in small galaxies and large clusters of galaxies. In the small galaxies, dark matter seems to be attracted to itself quite strongly, but in larger galactic clusters this doesn't appear to be the case: strongly interacting dark matter should produce cores of dark material bigger than those that are actually there, as deduced from the way the cluster spins.

One explanation, they say, is that three extra dimensions, in addition to the three spatial ones to which we are accustomed, are altering the effects of gravity over very short distances of about a nanometre. The team argues that such astronomical observations of dark matter provide the first potential evidence for extra dimensions. Others are supportive, but unconvinced. Silk himself acknowledges that the proposal is 'extremely speculative'.

These extra dimensions are generally thought to be tiny: many billions of times smaller than atoms. This would make these dimensions very hard to detect, explaining why the Universe looks as if it has just three. Other physicists, however, have proposed that some extra dimensions might be relatively big, but inaccessible to us.

The extra dimensions that Silk and colleagues say they have identified are likewise relatively 'big', at about a nanometre across. In other words, they say, the Universe is only about a nanometre wide in these three 'directions'. They argue that the force of gravity does not obey Isaac Newton's famous laws over small distances, where these dimensions come into play. This has never been tested experimentally: no one has measured how gravity behaves over distances below about a hundredth of a millimetre.

The most popular versions of string theory suggest that there are as many as eight extra dimensions. The fact that only three extra dimensions have been predicted by Silk and colleagues does not imply, though, there might not be several other small ones too.

To download the original paper, please consult the following web address:
http:///xxx.arxiv.org/PS_cache/astro-p h/p df/0508/0508572.pdf
Remarks: Reference document: Observational Evidence for Extra Dimensions from Dark Matter Bo Qin, Ue-Li Pen and Joseph Silk, Arxiv.Org Astrophysics

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