Cosmology Conundrum

March 26, 2013

Some colleagues have suggested that my posting on Planck Day was overly frivolous, given the huge importance of what Planck has achieved. (Nicely written up by Andrew Jaffe.) Other colleagues have suggested that Planck Day was bad press, as it was such a huge public non-event, with a big fuss about mild parameter adjustment. I find both these things true, leaving a weird sense of tension and excitement. I’d better explain myself.

First, lets be clear about the technical achievement. Planck is an absolute triumph of technology, engineering, management, and organisation. An amazing machine that has worked beautifully. It also represents a stunning scientific achievement. The lamda-CDM model, and its beautifully articulated engine of prediction, is a conceptual and analytic triumph. Of course this triumph belongs not just to the Planck team, but represents the accumulated achievement of many scientists all over the world over a number of years.

Lets just look at that power spectrum fit. It is not just a question of the theoretical curve going vaguely up and down, in more or less the same way as the data. The detailed agreement is gob-smacking – multiple peaks, their positions, their sizes, their widths, their second and third order curvature. Any scientist will look at this and think “no way is this a fluke”. Don’t let doubters trot out that coffee-time stuff about being able to fit anything with enough parameters. Firstly, that old chestnut is largely nonsense, and secondly, the fit quality is way beyond that.

Parfait. Everybody love parfait

And yet – outside the world of the CMB, the CDM paradigm has problems, as we were reminded here at ROE the day after Planck Day, in a nice wee coffee talk by Jorge Penarrubia.The best known problems are that CDM predicts far too many dwarf galaxies, and galaxy profiles that are much cuspier than observed,  but there are other claims, such as the existence of a very unlikely polar structure of dwarfs surrounding the Milky Way,  and of suspicious uniformities in galaxy rotation curves. All these problems may still get patched up by astrophysical fixes to do with feedback, IMF games, etc… but we don’t know yet.

Its common to hear people say that Particle Physics and Cosmology are in a similar situation – a model rather than a theory – a perfect fit but with no explanation. Why do all those parameters have those particular values? It seems so arbitrary. And what are dark matter and dark energy anyway? In this view, even the boringly successful fit is tantalising because it tells us there must be a deeper theory in waiting that will explain the perfect fit.

That may describe particle physics, but it doesn’t describe cosmology. Rather, what we have is perfect but fishy. How can the model be so perfect in some places and so poor in others? Have we missed something simple? It may well be that the astrophysical fixes do the trick, and then cosmology does look indeed like particle physics – successful but unsatisfying. Or it could be that some conceptual change is needed, and a revolution is waiting.

I’d give the revolution 2:1 against. But thats good enough odds that your eyes are glued to the table… Mesdames et Messieurs, faites vos jeux.


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.”


Dark Matter Damping

May 8, 2009

There is an interesting discussion over at Sarah’s blog, about astronomers being driven by ambition and greed just like everybody else. I remember this concept shocking Donald Pettie, the ex Chief Engineer at ROE, who moved there from Ferranti back in the eighties. He thought academics would be gentle, polite people. In some ways this was true, but he told me that he soon learned that the average astronomer would be prepared to sell his grandmother for an extra night on a big telescope.

There was something of an unseemly stampede last year when the ATIC results were announced – a claim of a hump in the cosmic ray electron energy spectrum, that could be a feature caused by the decay of dark matter particles. Gold !!!!!!! At a conference where the pre-publication results were presented, someone apparently photographed the screen, measured the points, and rushed a paper out. Of course this was only one of a gazillion papers with different interpretations. But now Fermi has made this all seem a bit daft : a paper just out in Phys Rev Lett has made an absolutely exquisite measurement of the CR energy spectrum from ten to a thousand GeV, which slides neatly underneath the ATIC data. Bump not there. See below.

Cosmic Ray electron energy spectrum measured by Fermi satellite

Cosmic Ray electron energy spectrum measured by Fermi satellite

Well this result has already caused a splash in the popular press and astroblogs (here is Sean Carrol’s take), but I had it in mind because I just attended a SLAC seminar explaining the result, given by Luca Patronico. The particle physicists are still seeing the glitter of gold in the river; the beautiful Fermi data disagree not only with the ATIC hump, but also with the “conventional diffusive model” … there are already papers out on ArXiv with revised DM fits…

However I am glad to report that the astronomers in the audience were somewhat saner. Not only is it possible that nearby pulsars could produce this “excess”,  at least one older wiser head said “errr… you aren’t taking that conventional diffusive model too seriously are you ? We don’t really understand the transport of cosmic rays through the ISM in that level of detail. Its kinda tricky. I can give you any curve you want”.

Maybe its relevant that the person who said this is already mature and respected … no need to grab at every chance of fame….