Wednesday is my favourite day this year. The weekly STS seminar is over and discussed, the fellows group meeting is done and it’s too early to start next week’s readings. At 12.15, I give a solid-state physics class over the web to my hapless students in Ireland and then I’m finally free to catch up on what’s going on in the world…
One of the things going on this week is a terrific cover story in Scientific American on dark matter by particle cosmologists Jonathan Feng and Mark Trodden. As every schoolgirl knows, particle cosmology is one of the most exciting areas of physics today; the convergence of the study of the extremely small (particle physics) and the study of the extremely large (cosmology) has had some spectacular successes in recent years. For example, the theory of cosmic inflation arose from considerations of particle physics, see post on inflation here.
The article gives a great overview of the concept of dark matter, from a postulate in particle physics (Fermi’s beta decay – bit of a stretch here), to the postulate of dark matter in galaxy formation in the 1930s (Fritz Zwicki). Of course, the W and Z particles of ‘ordinary matter’ are now associated with the former, but it is thought that dark matter may play a role in their masses. Similarily, Zwicki’s proposal has now been extended to explain galaxy formation all scales, from galaxy clusters to halos. (Essentially, dark matter is thought to provide the inert scaffolding on which ordinary matter clustered to form galaxies during the expansion of the universe). The article goes on to describe the standard candidate for dark matter; hypothetical particles that feel only the gravitational and weak nuclear force (i.e. do not interact with the electromagnetic force, hence ‘dark’) they are known as known as weakly interacting massive particles or WIMPs. The authors do a great job of carefully describing the WIMP coincidence; the fact that the density of WIMPs postulated by particle physicists closely matches that postulated by cosmologists for the scaffolding necessary for the galaxy formation. The article also gives a useful overview of current searches for WIMPs in particle physics experiments.
What is unusual about the piece is that the authors then go on to explain the newer concept of super-WIMPs; the idea that the original WIMPs may have decayed into particles that do not feel even the weak nuclear force. Thus is a fascinating idea and leaves open the possibility that such particles may interact with ‘dark forces’ we are completely unaware of.
It’s a great overview, well worth reading – and unlike many such articles, it also includes a clear description of the famous bullet cluster i.e. the first tangible cosmological evidence for dark matter.
Galaxy collision: x-rays (pink) are emitted when the interstellar gas clouds collide, while the dark matter (blue) remains aligned with distant stars because it is unreactive .
Update
The first comment below makes me realise that I should have mentioned the counter-argument. Some physics groups suggest that dark matter does not exist – instead our current understanding of gravity is incomplete. This is a perfectly respectable area of research, known as MOND; however, sophisticated experimental tests of our law of gravity (GR) have come out strongly in favour of the current theory..so far. Meanwhile, there have been tantalizing hints of particles that could be candidates for dark matter in at least two of the particle experiments mentioned in the article.
I should also mention that dark matter is a favourite target of science skeptics. However, it is often overlooked that the central thesis of the postulate is about not making an assumption i.e. just because the ordinary matter that we are familiar with can be seen, we should not assume that all matter can be seen..and science is very much a game of making as few assumptions as possible
Antimatter 
