This story, well-handled in both the Guardian and the BBC, has been brewing for a while. Gerry Gilmore told us a little about the results back in November 2004 when we filmed the Chile program, but it’s still slightly shocking to see in print. The following all comes with the disclaimer that there doesn’t appear to be a paper yet available.
The fact that there is much missing mass in the Universe has been known for decades – the classic indicator is the fact the spiral galaxies don’t rotate as you’d expect if the luminous disk contained all the matter. For years the arguments raged between those who believed in MACHOs (MAssive Compact Halo Objects – ie many small, faint planets or other blobs of normal matter) and WIMPs (Weakly Interacting Massive Particles). Eventually, surveys suggested that there was no evidence for MACHOs and so we were left with a mysterious particle which had only been detected via its gravitational influence on the rest of the Universe. Large simulations, like these appeared to confirm that whatever the dark matter was, it had to be cold – able to form small clumps – otherwise it seemed impossible to produce a Universe that looks like the one around us.
That last result is what these new observations, of small galaxies surrounding the Milky Way, cast doubt on. By studying the way the stars in these structures are influenced by the dark matter, the researchers were able to build up an idea of where it is. They found that essentially it lies in large blocks, and refuses to split into smaller chunks in order to agree with the simulations. This isn’t a blow for dark matter per se – we’re just learning more about how it actually behaves, about its physics. But it is a problem for our nice models of how galaxies form around the dark matter, and it will be interesting to see how the modellers respond.
It’s also my feeling that this is a boost for those working on MOND – modifying the theory of gravity to get rid of the need for extra particles, although you’d need to be more of a MOND expert than I to comment on whether this result is reproducible by MOND.
In short – isn’t it great when the Universe isn’t as we expect?