When I was in China, Joe Silk told me about an interesting paper he was just finishing, and one of his collaborators, Bo Qin, from Beijing University, gave a nice talk about the work. The paper has now appeared, and has been discussed in a recent Nature news item. I've written before on the basic idea of large extra dimensions, and some variants on which I've worked. I won't repeat the motivations for considering them here, but will repeat the main idea. You can imagine the strength of gravity as being a bit like the force due to a steady stream of water emerging from the nozzle of a hosepipe. Suppose that the water is confined, by some fancy nozzle, say, to emerge in a stream that is essentially one dimensional - a very fine stream. If you've ever fitted a tight nozzle to a hosepipe, then you'll know that such a stream is very powerful, and the force it exerts on you, if pointed your way, is very high. Now imagine that, instead, the water is spread out to emerge in a plane (OK, that would require one fancy nozzle, but I'm sure you can imagine it). In this case, if your body is in the way of the water in some direction (the same distance from the nozzle as in the first case), it will still exert a force on you, but less than when you were being hit by the one-dimensional stream. This is because you're not being hit by all the water, but instead by a portion of it - there are other directions available for the rest of the water to go. If we now fit a nozzle that allows the water to spread out in a spherically-symmetric three-dimensional pattern, then the force on your body will be yet weaker because there are still further directions for the water to spread. The analogy I'm drawing here is with lines of gravitational flux, the density of which, in Newtonian gravity, describes the strength of gravity. The more directions (think dimensions) available for the water (think gravitational flux) to spread, the weaker is the force experienced. Silk, Qin and their collaborator, Ue-Li Pen, from the Canadian Institute for Theoretical Astrophysics (CITA), suggested that this change in the behavior of gravity due to large extra dimensions (three, in fact) might explain why the dark matter cores of large galactic clusters are smaller than those expected from current theoretical models. It's a fun idea, albeit a speculative one, and it's refreshing that they aren't over-hyping the paper, as you can see from
Silk himself acknowledges that the proposal is "extremely speculative".
and
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," Pen confesses. But he thinks that invoking extra dimensions is such an exciting idea that it is worth investigating, "even if it is a long shot".
I can't say I think it's likely that this is how the universe works. Nevertheless, this paper caught my eye partly because I'd discussed it with Joe, but also because it is the kind of particle physics/cosmology idea that I find most attractive - namely one that takes a model (large extra dimensions) that was developed to address an existing particle physics problem (the hierarchy problem) and derives interesting cosmological implications (in this case involving dark matter). Take a look at the brief description in Nature if you'd like to learn more, or if you're up to it, dive right into the paper.
