Dark Matter heats up

June 13, 2011

As we all know, the non-existence of pancakes tells us that Dark Matter must be cold, not hot. This is why D, E, F, and W can now pay off their mortgages. (Back in them days, I dimly recall George telling me that he had trained his toddler to say “CDM Daddy !”). Here I refer not to the temperature of the mysterious particles, but to the temperature of the almost equally mysterious debate. Two rather fascinating but controversial results have come out in the last few weeks. They cannot both be right. Of course they could both be wrong. But if either one is right, its really really really important.

First up is a paper on arXiv by Hernandez, Jimenez and Allen. They examine the relative proper motions of very wide binaries from a new SDSS sample of such beasts, and find that, just like the rotation of galaxies and the random motions in clusters, they are moving faster than they ought to be, and in pretty much the same acceleration regime as those other examples. Its all baryons here squire, so dark matter ain’t the answer. This is the most interesting evidence yet for a modified law of gravity. Over at Cosmic Variance, Sean Carroll already wrote a short  post. There is as ever an interesting comment stream, but mostly a bit off base I think. Sean already made the key point : on the face of it, this is a crucial result, but statistical analyses of this kind can have systematic biases you just haven’t thought of yet. You can place a bet this will go away somehow. But then again thats what I thought when I first heard of the supernova acceleration results…

The Hernandez paper is a tad proselytising for my taste, which leaves me a little uncomfortable. Here is a quote from the opening :

Direct detectionof the elusive (illusive?) dark matter particles, in spite of decades of extensive and dedicated searches, remains lacking.

Ah … but maybe it doesn’t ! This week the CoGENT team posted a paper on arXiv confirming their claim from last year of direct detection of dark matter particles, and showing that the detection rate shows an annual modulation – just like those DAMA claims that everybody else has poured scorn on for a decade. In fact, their confidence contours for the mass and cross-section of the DM particles seems to be nicely consistent with DAMA, showing a rather light particle which will make many theorists uncomfortable. However – the much bigger CDMS and XENON experiments claim to have already ruled out this region of parameter space. The situation is summarised in the plots below, which I have taken from an independent paper analysing the CoGENT data, by Hooper and Kelso. The Hooper and Kelso paper appeared on arXiv THREE DAYS after the Aalseth paper. No slouches these guys. I have to confess I found the Hooper and Kelso paper a bit easier to understand.

Hooper-Kelso Figs 6-7

Figs 6 and 7 from Hooper and Kelso paper, comparing CoGENt and DAMA constraints (on left) and CoGENT, CDMS, and XENON constraints (on right)

The CoGENT team are being fairly cautious. They don’t claim they have a proved detection of DM, but rather a signal which is consistent with the predictions of DM. Removing the backgrounds and systematic effects from these experiments is very tricky, which is why most people will still be sceptical, even with CoGENT and DAMA agreeing. Of course the XENON and CDMS guys could get it wrong too. Either CoGENT and DAMA are reading too much into the inkblots, or CDMS and XENON are throwing the baby out with the bathwater. Improved metaphor contributions welcome.

I know Alex Murphy sometimes reads this blog so I am hoping he will give us the lowdown. Which is right ? Result A, Result B, or neither ?

Finally, a wee sociological note. None of these papers have been refereed yet. But the world might already have decided by then. Over at Cosmic Variance there was a string of people saying either “well, this is wrong because..” or “Hem. My theory about the dinosaurs, by Anne Elk. Hem.”