Dark Matter: Warm or Cold?

This morning I woke to find that most of the UK media outlets (like the Guardian and the BBC) were carrying a story that astronomers, in particular a team lead by Gerry Gilmore of Cambridge’s Institute of Astronomy, had unlocked some of the secrets of Dark Matter, the mysterious stuff that makes up, we think, 30% of the total mass of the universe, a substantially greater proportion than the atoms and molecules that we interact with every day.

It seems that the team has measured the velocity dispersion of stars in dwarf galaxies in the local group — small galaxies near our own Milky Way that, we think, are dominated not by the stars and gas that we can see, but by the dark matter. The motions of these stars are caused by their gravitational attraction to that dark matter, however it is spread through the galaxy, and so by measuring the speeds of these stars we can map out the distribution of matter.

It was originally expected that the matter would be “cuspy” — it would get denser and denser towards the center. This is the signature of “cold dark matter”, heavy particles moving so slowly that they could be compressed to very high densities. However, observations seem to show (and indeed have been showing for several years) that, instead, the distribution shows a “core” — it reaches a maximum density and cannot be compressed further. This is expected if the dark matter is, instead, “warm” — moving at a few kilometers per second.

Unfortunately, I have to keep using the word “seems” here, since, in fact, all the available information as I write this is from the mainstream media, rather than in journals or even unrefereed preprints.

Update: Steinn has a bit more detail.

Update 2: And this conference proceedings seems to actually be what the fuss is about.