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.

Almost Nothing

February 25, 2011

More astronomical hyperbole. I just remembered a story about Glenn White from days of yore at QMW. Glenn, as many of you will know, was and is Mister Molecular Cloud. He was getting fed up with us extragalactic types stealing the limelight. I think we’d just been in the Daily Mirror with the Most Luminous Object in the  Universe, aka IRAS F10214+4724. So when he came back from Hawaii with some pretty random CO data, he put out a press release saying he’d found a cloud with enough Carbon to make all the pencils on the planet. By golly it worked ! Snaffled up.

We are used to thinking of everything in astronomy as bigger and bolder than our humdrum earthly existence. Recently I heard someone talking with passion about the enormous amount of obscuring material towards some object, with an extinction of hundreds of magnitudes… But hang on there. At a normal gas-to-dust ratio, thats a hydrogen column of maybe 10^24 atoms per cm^2. But thats peanuts; the column through the Earth’s atmosphere is much more. If it wasn’t, we’d be able to do X-ray astronomy without those expensive rockets. That obscuring muck is pretty pathetic.

I was expressing these thoughts to Eric Tittley at lunchtime. Indeed, quoth he, there is more stuff in the 10km above our heads than there is all the rest of the way to the edge of the universe. That just made me go quiet for a bit. What a thought.

One of the most striking things about the universe is that it is transparent. And how nice that is; otherwise we couldn’t study those lovely high redshift galaxies. Every X-ray astronomer knows that you have to include the Galactic Column in your spectral fit – but do you add the Intergalactic Column ? Nope.

Ahh, the learned among you cry, this is because the IGM is ionised. However, the WMAP polarisation measurements of the microwave background tell us that the average electron scattering optical depth from here to the re-ionisation epoch is around tau=0.1. So we know how many free electrons, and so how many protons, there are in a typical line of sight from here to redshift ten-ish. If you drill a tube about a thumbnail across from here to eternity, you get about 3 grams of stuff. About as much as a teaspoon of water.

Almost nothing.

Guest post : Over for Clover

April 1, 2009

You may have heard rumours that the Clover experiment has been cancelled, and indeed Peter C has just written about it. My colleague Alan Heavens had some observations on this turn of events, so I invited him to write about it. Here is his piece :

A few days ago STFC cancelled the Clover project – a casualty of the financial crisis in the council, and perhaps not the last.  What should we make of this?

First, some background. Clover was a project to detect B-mode polarisation in the Cosmic Microwave Background, would have detected the gravitational lensing effect of intervening structure on the CMB E-mode polarisation, but much more excitingly it was designed to seek evidence for inflation, which in the standard cosmological model is assumed to have driven the expansion soon after the Big Bang.  This is noble and potentially Nobel science.

Clover’s cancellation raises questions about which projects the UK should be funding, especially in lean times.  STFC’s stance on this at the beginning of the crisis was set out very persuasively by John Womersley in a talk in Edinburgh and elsewhere – the UK should be pursuing projects of high scientific importance, with big international impact, and with UK leadership.   The message was clear – we should do fewer, more important things, and lead them.  Absolutely right, I thought – this is what our research councils should be saying.

Clover ticks all these boxes – it was purely a UK project; a positive finding would have been of colossal scientific importance, and the impact would have been enormous.   So what went wrong?  The bare answer is that the costs went up, and STFC Council reconsidered it, and cut it.  The project certainly faced challenges, in detector technology, and in the team moving from two institutes to four, but the project was well advanced.  Unlike the VSA, where an opportunity to fund a seminal experiment well in advance of competition was missed, Clover had been funded early and boldly.  The delays meant that it might not have been the first to an accurate detection, as other experiments were catching up, but it still might have won the race. The fear of competition can be damaging; other projects are not immune to delays and difficulties and one might miss an opportunity.  A salutary lesson comes from 2dF and SDSS: had PPARC taken the same view of 2dF in the face of the published schedule of SDSS, the important scientific results made by 2dF in large-scale structure would have been lost to the USA.

As always the case when finance is the issue, the decision is a matter of scientific judgement and strategic priority.  On finance, it would be interesting to know if a minority partner was sought.   On scientific priority, then the advisory panels must be allowed to advise and the Science Council allowed to decide, but it would be interesting to know what advice informed the decision.

The Clover shortfall is, incidentally, the same figure as is rumoured on this blog to have been allocated to MoonLITE.

Alan Heavens

Obama and the last scattering surface

January 21, 2009

What a fine day. I saw Obama’s inauguration and attended a colloquium by Rashid Sunyaev.

The three hour East-West time difference was just right. I was able to catch the inauguration speech on TV and still get in to work before coffee. It was a beautifully crafted and well delivered speech. Emotional but not mawkish. Had just enough content to avoid being vacuous, but not so much that his freedom of action is blocked once real world compromises are needed. And very promising from a rationalist point of view .. he said “we will restore science to its rightful place”, mentioned “curiosity” as one of the “values on which our success depends”, and even mentioning “nonbelievers” is very unnusual in America. You can see Andrew Jaffe’s take here, and read the speech here.

Sunyaev’s talk was fascinating. He is of course just a tad well known for things like inventing the “alpha disk” paradigm for accretion disks, and predicting the Sunyaev-Zeldovich effect; but in 1970 he also predicted acoustic oscillations in the CMB power spectrum. I didn’t know this. Its in this paper, published in Russian in 1970, and in translation in 1972.  He said that Zeldovich almost stopped him stating that such observations could measure Omega, because obviously these fluctuations could never be measured ! So now he has a new wacky prediction : seeing beyond the surface of last scattering by looking for the imprint of emission lines produced during the preceding recombination phase. These should produce tiny wiggles in the high frequency end of the CMB spectrum; measuring them could for example tell us the pre-stellar abundance of Helium. Sunyaev said that expert opinion is divided on whether these wiggles can be measured, but the twinkle in his eye let us know the right answer…