Dark Matter Decrypted

October 30, 2007

I am starting to believe in dark matter. I don’t mean “believe” as in “rationally favour” – thats been true for a long while. I mean it feels right. Genuinely convincing evidence is starting to pile up. But what IS dark matter ?

Last week we had a workshop at the ROE whose title was Decrypting the Universe. The poster seemed to be a pun on “decrypting”. Matrix-like strings of numbers are descending from the top, but it looks like green slime dripping into a dark background, giving the whole thing a sort of Monster Mash-ish flavour. The workshop outing was a trip to the spooky Mary Kings Close – the street that Edinburgh buried during the plague, so I guess this was a deliberate theme. (Peder, are you a Hammer Film fan ??). Anyhoo, the workshop itself was really good. As usual these days, I was nipping in and out of very boring University meetings and so only got to half the talks, but I caught some excellent stuff.

Cosmology consistency checks

David Spergel of WMAP fame summarised the overall state of cosmology. As he emphasised, if the concordance cosmology came only from fitting the CMB, you’d be worried we were all having a communal fantasy. However the constraints from several independent techniques – CMB spectrum, large scale galaxy clustering, supernova redshifts, cluster counts – intersect at the same spot – ordinary matter 4%, dark matter 21%, dark energy 75%.

Mapping the dark matter

Nowadays we can map out the invisible dark matter by tracing the distortions in background galaxies caused the gravitational lensing effect of the foreground matter. A few months back there was a lot of publicity about the COSMOS maps and how they showed the galaxies and dark matter tracing the same patterns. But this wouldn’t convince a sceptic – two things that look the sameish derived from the same dataset … hmmm. Enter the Bullet Cluster, stage left, and a lovely paper by Clowe et al. Here they have both X-ray and optical data on two clusters merging together. The galaxies and the dark matter go together, but the X-rays, which trace the hot gas representing most of the baryonic matter, looks different. This makes sense as the gas can kinda slop about, but the starlight is essentially a tracer for the dark matter halos, where the galaxies form. More importantly, modified gravity theories like MOND, which try to dispense with dark matter, can’t explain all this.

Hot or cold ? Galactic Archaeology

Next up, what type of dark matter ? Many moons back, the rivals were light fast moving particles like neutrinos (hot dark matter, HDM) or weakly interacting massive particles – WIMPS, otherwise known as cold dark matter, CDM. From the 1980s on, galaxy clustering studies have argued that it can’t be HDM, as this would make too much large scale structure and not enough small scale structure. But this is a rather indirect and statistical argument.

A consequence of CDM is that galaxies build themselves bottom up, by the gradual merging of smaller things. Clearly, galaxy mergers happen occasionally today, but what about lots of merging way back in the past ? This history of ancient merging should be encoded in the structure of the Galaxy. Looking for this is known as Galactic Archaeology. At the workshop, Amina Helmi reviewed the evidence and showed stunning pictures from 2MASS and SDSS, where very old, metal poor stars have been picked out, to look at the structure of the halo, the oldest part of the Galaxy. The halo of the Galaxy is not smooth – it has lumps and streams and ripples. The fossil history of the Galaxy just stares out at you.

There was a problem – when Majewski et al 2003 studied the Sagittarius stream with 2MASS, they found it to lie on a great circle, suggesting the gravitational potential of the Galaxy is pretty much spherical, whereas CDM predicts distinctly non-spherical halos. However looking at a picture projected in 2D can hide a lot of stuff… Helmi showed new data from RAVE, which collects velocities of those halo stars; this showed very clear departure from the spherical potential prediction. The best fit model has a dark matter halo thats a prolate ellipsoid with axial ratio 5/3. Just about on the money for CDM.

Dark matter on Planet Earth

So CDM is looking pretty solid. But the snag is nobody has ever actually seen one of these particles. The theoretical suggestions for what could fit the bill all come from unproven speculations in particle physics, like supersymmetry. But this is just what excites folk of course – its not just astronomy, there’s PHYSICS in them thar hills. If dark matter is universal, it should be streaming past the earth, and we should be able to detect the efffect of these particles colliding with regular earthly particles.

At the workshop, Alex Murphy reviewed the attempts to do this, which normally involve putting detectors way underground, to avoid confusion with plain ole cosmic rays, and re-deploying nuclear physicists who would otherwise be doing all that dull stuff about exotic nuclei and hadron structure. Oops, sorry Alex. Anyway. The British Effort is in Yorkshire, at the bottom of the Boulby mine. Despite a false alarm from DAMA, so far nowt has been seen. But our intrepid scientists are building bigger detectors with better discrimination. Alex reckons we will be in detection land in 5-6 years. That would be fantastic. Watch – this – space.

Err.. what about dark energy then ?

I’m tired now. Perhaps another day.