* Astronomy

A team of astrophysicists claims to have identified evidence that space is six-dimensional.

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

Silk and his colleagues looked at how dark matter behaves differently in small galaxies and large clusters of galaxies. In the smaller ones, dark matter seems to be attracted to itself quite strongly. But in the large galactic clusters this doesn't seem 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. Lisa Randall, a Harvard physicist who has explored the possibility of extra spatial dimensions, says "Even if their idea works, which it probably does, it may be an overstatement to use these observations as evidence of extra dimensions."

Silk himself acknowledges that the proposal is "extremely speculative".

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

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. Physicists such as Randall, 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 '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.


This variation in gravity, says Silk, could be why dark matter behaves differently in different galactic environments.

According to one interpretation of the astronomical observations, dark matter, which is thought to account for 85% of all the mass in the Universe but not to be made from the known fundamental particles, seems to attract itself through some unknown force. And this attraction seems to be stronger in dwarf galaxies than in galactic clusters. This is very odd: it is rather as if apples were to fall faster from single trees than from trees in an orchard.

But the attraction isn't due to an unknown force, argue Silk and his colleagues, but to the effect of extra dimensions on gravity. And because dark matter particles are accelerated to higher speeds in massive galactic clusters than in dwarf galaxies, they spend less time close to each other, so the effects of these extra dimensions are felt less.

There are other ways of explaining the puzzling dark-matter distributions, admits Silk's colleague Ue-Li Pen of the University of Toronto in Canada. For example, one could assume that the rate at which stars explode, as supernovae, was quite different in the past.

"Personally, I think changing the supernovae rate is more conservative than changing the number of spatial dimensions" - Ue-Li Pen.
He thinks that invoking extra dimensions is such an exciting idea that it is worth investigating, "even if it is a long shot" - Ue-Li Pen.

The most popular versions of string theory suggest that there are as many as eight extra dimensions, not just three. But thankfully this needn't be a problem. There's no reason why, in addition to the three large extra dimensions predicted by Silk and colleagues, there might not be several other small ones too.

Bosonic string theories are 26-dimensional, while classic superstring and M-theories turn out to involve 10 or 11 dimensions. However new sting theories now have reduced that number to 6 dimensions using Penroses twister space.


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superstring theory is a theory of all fundamental physical interactions. While in principle able to explain all phenomena, it has in practice been very challenging to find any observable predictions. One generic prediction is the existence of extra dimensions in addition to our familiar 3-dimensional space. These extra dimensions had been thought to be extremely tiny (of order the Planck length about 10^-33cm), until in recent years the idea of large extra dimensions was proposed to address the “hierarchy problem”—a problem associated with the factor of the 10^17 huge difference between the Planck scale and the weak interaction scale.
In the Arkani-Hamed, Dimopoulos and Dvali (ADD) scenario, the electro-weak and Planck energies are the same, and the large extra dimensions explain the apparent discrepancy in strength when measured on macroscopic scales.
In the context of string cosmology, the fundamental scale would be about 1TeV, corresponding to 10^-17cm. There are 3+1 cosmological size dimensions, n large extra dimensions, and 6 - n fundamental scale dimensions.
The size R of these large extra dimensions depends on the number of large extra dimensions n. Gravity would start to deviate from Newton’s inverse square law at small distance scales r less than R. For n = 2, R is about 1mm.
This opens a new window for experimental tests of string theory and searches for extra dimensions, by precise measurements of the gravitational force at sub-mm scales.
Tremendous efforts have been made in the past few years in testing Newton’s inverse square law at small scales and searching for evidence of the large extra dimensions. Currently, the measurements are reaching micron scales and no deviation from Newton’s law has been found from about 1cm down to about 10^-3cm.
While string scenarios and extra dimensions have not yet been tested experimentally so far in the laboratory, recent astronomical observations of dark matter, on the other hand, may shed light on the issue. It should be noted that any connection of string scenarios to observable phenomena would be an exciting possibility deserving further investigation.
The existence of dark matter was first realized by Zwicky nearly 70 years ago, and it is now well established that dark matter constitutes the major matter component of the universe. Although its nature remains a mystery, most cosmologists and particle physicists believe that dark matter is likely to be a new species of elementary particle that is neutral, long-lived, cold (or nonrelativistic), and collisionless (i.e. dark matter particles have very little interactions with themselves as well as with ordinary matter). This “standard” picture of collisionless cold dark matter (CCDM) has gained great success in explaining the origin and evolution of cosmic structures on large scales.
However, the CCDM model is facing a potential challenge in recent years from observations on galactic and sub-galactic scales. Numerical simulations of the CCDM model predict that the density profiles of dark matter halos should exhibit a cuspy core in which the density rises sharply as the distance from the centre decreases. In contrast, observations of systems of dark matter, ranging from dwarf galaxies, low surface brightness galaxies [17, 18] to galaxies comparable in mass with the Milky Way, indicate that the central density profiles are probably much less cuspy than predicted. Clusters of galaxies also seem to reveal a near isothermal core, although there is considerable scatter.
The plausible discrepancies between theory and observations, although still vigorously debated, have stimulated many attempts to understand the nature of dark matter and to modify the CCDM model, among which one of the more popular schemes is the self-interacting cold dark matter model. As proposed by Spergel and Steinhardt, the above conflicts can be readily resolved if the cold dark matter particles are self-interacting with a large scattering cross section Oxx/md = 8 × 10^-(25-22)cm^2/GeV, where md is the mass of dark matter particles.