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Throwing a Fit

Posted in Bad Statistics, The Universe and Stuff with tags , on February 18, 2009 by telescoper

I’ve just been to a very interesting and stimulating seminar by Subir Sarkar from Oxford, who spoke about Cosmology Beyond the Standard Model, a talk into which he packed a huge number of provocative comments and interesting arguments. His abstract is here:

Precision observations of the cosmic microwave backround and of the large-scale clustering of galaxies have supposedly confirmed the indication from the Hubble diagram of Type Ia supernovae that the universe is dominated by some form of dark energy which is causing the expansion rate to accelerate. Although hailed as having established a ‘standard model’ for cosmology, this raises a profound problem for fundamental physics. I will discuss whether the observations can be equally well explained in alternative inhomogeneous cosmological models that do not require dark energy and will be tested by forthcoming observations.

He made no attempt to be balanced and objective, but it was a thoroughly enjoyable polemic making the point that it is possible that the dark energy whose presence we infer from cosmological observations might just be an artifact of using an oversimplified model to interpret the data. I actually agreed with quite a lot of what he said, and certainly think the subject needs people willing to question the somewhat shaky foundations on which the standard concordance cosmology is built.

But near the end, Subir almost spoiled the whole thing by making a comment that made me decide to make  another entry in my Room 101 of statistical horrors.  He was talking about the  spectrum of fluctuations in the temperature of the Cosmic Microwave Background as measured by the Wilkinson Microwave Anisotropy Probe (WMAP):

 

 

I’ve mentioned the importance of this plot in previous posts. In his talk, Subir wanted to point out that the measured spectrum isn’t actually fit all that well by the concordance cosmology prediction shown by the solid line.

A simple way of measuring goodness-of-fit is to work out the value of chi-squared which relates to the sum of the squares of the residuals between the data and the fit. If you do this with the WMAP data you will find that the value of chi-squared is actually a bit high, so high indeed that there is only a 7 per cent chance of such a value arising in a concordance Universe.  The reason is probably to do with the behaviour at low harmonics (i.e. large scales) where there are some points that do appear to lie off the model curve. This means that the best fit concordance model  isn’t a really brilliant fit, but it is acceptable at the usual 5% significance level.

I won’t quibble with this number, although strictly speaking the data points aren’t entirely independent so the translation of chi-squared into a probability is not quite as easy as it may seem.  I’d also stress that I think it is valuable to show that the concordance model isn’t by any means perfect.  However, in Subir’s talk the chi-squared result morphed into a statement that the  probability of the concordance model being right is only 7 per cent.

No! The probability of chi-squared given the model is 7%, but that’s quite different to the probability of the model given the value of chi-squared…

This is a thinly disguised example of the prosecutor’s fallacy which came up in my post about Sir Roy Meadow and his testimony in the case against Sally Clark that resulted in a wrongful conviction for the murder of her two children.

Of course the consequences of this polemicist’s fallacy aren’t so drastic. The Universe won’t go to prison. And it didn’t really spoil what was a fascinating talk. But it did confirm in my mind that statistics is like alcohol. It makes clever people say very silly things.

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Ecliptic Anomalies

Posted in Cosmic Anomalies, The Universe and Stuff with tags , , , , on February 12, 2009 by telescoper

Once a week the small band of cosmologists at Cardiff University have a little discussion group during which we look at an interesting and topical subject. Today my PhD student Rockhee chose an interesting paper by Diego et al entitled “WMAP anomalous signal in the ecliptic plane”. I thought I’d mention it here because it relates to an ongoing theme of mine, and I’ll refrain from commenting on the poor grammatical construction of the title.

The WMAP referred to is of course the Wilkinson Microwave Anisotropy Probe and I’ve blogged before about the tantalising evidence it suggests of some departures from the standard cosmological theory. These authors do something very simple and the result is extremely interesting.

In order to isolate the cosmic microwave background from foreground radiation produced in our own Galaxy, the WMAP satellite is equipped with receivers working at different frequencies. Galactic dust and free-free emission dominate the microwave sky temperature at high frequencies and Galactic synchotron takes over at low frequencies. The cosmic microwave background has the same temperature at all frequencies (i.e. it has a thermal spectrum) so it should be what’s left when the frequency-dependent bits are cleaned out.

What Diego et al. did was to make a map by combining the cleaned sky maps obtained at different frequencies obtained by WMAP in such a way as to try to eliminate the thermal (CMB) component. What is left when this is done should be just residual noise, as it should contain neither known foreground or CMB. The map they get is shown here.ecliptic

What is interesting is that the residual map doesn’t look like noise that is uniformly distributed over the sky: there are two distinct peaks close to the Ecliptic plane delineated by the black tramlines. Why the residuals look like this is a mystery. The peaks are both very far from the Galactic plane so it doesn’t look like they are produced by Galactic foregrounds.

One suggestion is that the anomalous signal is like an infra-red extension of the Zodiacal light (which is produced inside the Solar System and therefore is too local to be confined to the Galactic plane). The authors show, however, that a straightforward extrapolation of the known Zodiacal emission (primarily measured by the IRAS satellite) does not account for the signal seen in WMAP. If this is the explanation, then, there has to be a new source of Zodiacal emission that is not seen by IRAS but kicks in at WMAP frequencies.

Another possibility is that it is extragalactic. This is difficult to exclude, but is disfavoured in my mind because there is no a priori reason why it should be concentrated in the Ecliptic plane. Coincidences like this make me a bit uncomfortable. Some turn out to be real coincidences, but more often than not they are clues to something important. Agatha Christie would have agreed:

“Any coincidence,” said Miss Marple to herself, “is always worth noting. You can throw it away later if it is only a coincidence.”

On the other hand, the dipole asymmetry of the CMB (thought to be caused by our motion through a frame in which it is isotropic) is also lined up in roughly the same direction:

The dipole has a hot region and a cold region in places where the residual map has two hot regions and anyway it’s also a very large scale feature so the chances of it lining up by accident with the ecliptic plane to the accuracy seen is actually not small. Coincidences definitely do happen, and the rougher they are the more commonly they occur.

Obviously, I don’t know what’s going on, but  I will mention another explanation that might fit. As I have already blogged, the WMAP satellite scans the sky in a way that is oriented exactly at right angles to the Ecliptic plane. If there is an as yet unknown systematic error in the WMAP measurements, which is related in some way to the motion of the satellite, it could at least in principle produce an effect with a definite orientation with respect to the Ecliptic.

The only way we can rule out this (admittedly rather dull) explanation is by making a map using a different experiment. It’s good, then, that the Planck satellite is going to be launched in only a few weeks’ time (April 16th 2009). Fingers crossed that we can solve this riddle soon.

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From Here to Eternity

Posted in Books, Talks and Reviews, The Universe and Stuff with tags , , , , on February 3, 2009 by telescoper

I posted an item about astronomy and poetry a couple of days ago that used a phrase I vaguely remember having used somewhere else before. I’ve only just remembered where. It was in this book review I did for Nature some time ago. Since I’m quite keen on recycling, I’d thought I’d put it on here.

How do physicists cope with the concept of infinity in an expanding Universe?

BOOK REVIEWED – The Infinite Cosmos: Questions from the Frontiers of Cosmology

by Joseph Silk

Oxford University Press: 2006. 256 pp. £18.99, $29.95

Scientists usually have an uncomfortable time coping with the concept of infinity. Over the past century, physicists have had a particularly difficult relationship with the notion of boundlessness. In most cases this has been symptomatic of deficiencies in the theoretical foundations of the subject. Think of the ‘ultraviolet catastrophe’ of classical statistical mechanics, in which the electromagnetic radiation produced by a black body at a finite temperature is calculated to be infinitely intense at infinitely short wavelengths; this signalled the failure of classical statistical mechanics and ushered in the era of quantum mechanics about a hundred years ago. Quantum field theories have other forms of pathological behaviour, with mathematical components of the theory tending to run out of control to infinity unless they are healed using the technique of renormalization. The general theory of relativity predicts that singularities in which physical properties become infinite occur in the centre of black holes and in the Big Bang that kicked our Universe into existence. But even these are regarded as indications that we are missing a piece of the puzzle, rather than implying that somehow infinity is a part of nature itself.

The exception to this rule is the field of cosmology. Somehow it seems natural at least to consider the possibility that our cosmos might be infinite in extent or duration. If the Universe is defined as everything that exists, why should it necessarily be finite? Why should there be some underlying principle that restricts it to a size our human brains can cope with?

But even if cosmologists are prepared to ponder the reality of endlessness, and to describe it mathematically, they still have problems finding words to express these thoughts. Physics is fundamentally prosaic, but physicists have to resort to poetry when faced with the measureless grandeur of the heavens.

In The Infinite Cosmos, Joe Silk takes us on a whistle-stop tour of modern cosmology, focusing on what we have learned about the size and age of the Universe, how it might have begun, and how it may or may not end. This is a good time to write this book, because these most basic questions may have been answered by a combination of measurements from satellites gathering the static buzz of microwaves left over from the Big Bang, from telescopes finding and monitoring the behaviour of immensely distant supernova explosions, and from painstaking surveys of galaxy positions yielding quantitative information about the fallout from the primordial fireball. Unless we are missing something of fundamental importance, these observations indicate that our expanding Universe is about 14 billion years old, contains copious quantities of dark matter in some unidentified form, and is expanding at an accelerating rate.

According to the standard model of cosmology that emerges, the Universe has a finite past and (perhaps) an infinite future. But is our observable Universe (our ‘Hubble bubble’) typical of all there is? Perhaps there is much more to the cosmos than will ever meet our eyes. Our local patch of space-time may have its origin in just one of an infinite and timeless collection of Big Bangs, so the inferences we draw from observations of our immediate neighbourhood may never tell us anything much about the whole thing, even if we correctly interpret all the data available to us.

What is exciting about this book is not so much that it is anchored by the ramifications of infinity, but that it packs so much into a decidedly finite space. Silk covers everything you might hope to find in a book by one of the world’s leading cosmologists, and much more besides. Black holes, galaxy formation, dark matter, time travel, string theory and the cosmic microwave background all get a mention.

The style is accessible and informative. The book also benefits from having a flexible structure, free from the restrictions of the traditional historical narrative. Instead there are 20 short chapters arranged in a way that brings out the universality of the underlying physical concepts without having too much of a textbook feel. The explanations are nicely illustrated and do not involve any mathematics, so the book is suitable for the non-specialist.

If I have any criticisms of this book at all, they are only slight ones. The conflation of the ‘expanding Universe’ concept with the Big Bang theory, as opposed to its old ‘steady state’ rival, is both surprising and confusing. The steady-state model also describes an expanding Universe, but one in which there is continuous creation of matter to maintain a constant density against the diluting effect of the expansion. In the Big Bang, there is only one creation event, so the density of the expanding Universe changes with time. I also found the chapter about God in cosmology to be rather trite, but then my heart always sinks when I find myself lured into theological territory in which I am ill-equipped to survive.

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Poems of Space

Posted in Books, Talks and Reviews, Poetry, The Universe and Stuff with tags , , , , , , , on February 1, 2009 by telescoper

A couple of weeks ago I bought a copy of Dark Matter: Poems of Space, an anthology of poems old and new with astronomical connections edited by Maurice Riordan and Dame Jocelyn Bell Burnell.

I quite like having anthologies because if you open one randomly you’re not absolutely sure what’s going to crop up, which can lead to pleasant surprises. But they’re also unsatisfactory to read through from cover to cover because there are huge differences in style and substance that are difficult to adjust to on a poem-by-poem basis. Random access is definitely better than sequential for this type of thing, so rather than attempt to study it all, over the last fortnight or so I’ve been taking regular dips into this particular collection, and very interesting it has been too.

The book contains over 200 poems mostly by different authors, although there is more than one contribution from a few (including Shelley and Auden). It’s a mixture of the familiar and the brand new, including some commissioned especially for this book. I couldn’t possibly write about the whole, but a few things struck me as I sampled various tidbits.

The first is that while many of these poems celebrate the beauty and majesty of the heavens, and some even embrace the wonder of scientific discovery, quite a few are quite anti-scientific. Two examples spring to mind (both of them paradoxically by favourite poets of mine!). This excerpt from The Song of the Happy Shepherd, a very early poem by WB Yeats is a good example

………………………………Seek, then,
No learning from the starry men,
Who follow with the optic glass
The whirling ways of stars that pass –
Seek, then, for this is also sooth,
No word of theirs – the cold star-bane
Has cloven and rent their hearts in twain,
And dead is all their human truth.

Hardly a ringing endorsement of observational astronomy, although strictly speaking it only refers to optical techniques so I suppose those working in radio-, X-ray and other types of astronomy are off the hook.

Incidentally, if I’d been given the task of picking a poem by Yeats for this collection it would have been this:

HAD I the heavens’ embroidered cloths,
Enwrought with gold and silver light,
The blue and the dim and the dark cloths
Of night and light and the half light,
I would spread the cloths under your feet:
But I, being poor, have only my dreams;
I have spread my dreams under your feet;
Tread softly because you tread on my dreams.

It’s not really much to do with astronomy or space but it’s one of his most beautiful lyrical verses, with a wonderful use of repetition (e.g. light, dreams, spread, tread) and assonance (light/night, spread/tread).

Anyway, another example of this kind of attitude displayed by Yeats Happy Shepherd is provided by Walt Whitman:

WHEN I heard the learn’d astronomer;
When the proofs, the figures, were ranged in columns before me;
When I was shown the charts and the diagrams, to add, divide, and measure them;
When I, sitting, heard the astronomer, where he lectured with much applause in the lecture-room,
How soon, unaccountable, I became tired and sick;
Till rising and gliding out, I wander’d off by myself,
In the mystical moist night-air, and from time to time,
Look’d up in perfect silence at the stars.

I think I’ve been to enough boring seminars to understand how he feels, but the theme of both these poems is that  studying the stars or applying science to them somehow robs them of their wonder. I think many non-scientists probably go along with this view: it’s beautiful to gaze at the sky but reducing it to measurements and graphs somehow ruins it.

Andromeda_gendler_smOf course I don’t agree.  Without professional astronomers we would never have discovered that, say, the Andromeda Nebula (shown above) was a galaxy just like our own Milky Way containing thousands of millions of stars like our Sun  and that it is rotating about its axis with a timescale of hundreds of millions of years. Knowing things like this surely increases the sense of wonder rather than decreasing it?

On the other hand it is true that the nature of science makes it rather prosaic. When scientists try to write for a popular readership they often spice up their accounts with quotations from poems, even if the quotes aren’t really all that appropriate. Perhaps some will turn to this collection for a source of such snippets. I know I will!

Another thing that struck me was that I always tended to think that engagement between science and poetry was a relatively recent thing, typified by WH Auden’s humorously perplexed After Reading a Child’s Guide to Modern Physics:

Our eyes prefer to suppose
That a habitable place
Has a geocentric view,
That architects enclose
A quiet Euclidian space:
Exploded myths – but who
Could feel at home astraddle
An ever expanding saddle?

But in fact the metaphysical poets of the 17th century also grappled with such issues. Consider this fragment from John Donne’s An Anatomy of the World:

We think the Heavens enjoy their spherical,
Their round proportion embracing all.
But yet their various and perplexed course,
Observed in divers ages, doth enforce
Men to find out so many eccentric parts,
Such divers down-right lines, such overthwarts,
As disproportion that pure form….

That could almost have been written about the possibility of a lop-sided universe that I’ve blogged about here and there, and which is a major topic of current cosmological research.

Other reactions I had were more personal. There is a poem in the collection by Fleur Adcock, who visited the Royal Grammar School in Newcastle when I was there. She judged a poetry reading competition (which I didn’t win) for which the test piece was Stevie Smith’s Not Waving but Drowning. I remember that she was quite a glamorous-looking lady, but she got everybody’s name wrong in her presentation address. She must be getting on a bit by now.

I have also met one of the other poets represented here too, Gwyneth Lewis, who was elected the first national poet for Wales and also spent some time as poet-in-residence in the School of Physics & Astronomy at Cardiff University where I now work. She wrote a number of poems about science but is probably most famous for writing the words “In These Stones Horizons Sing” which are incorporated in the design of the facade of the Wales Millennium Centre.

Anyway, I thoroughly recommend this book which is a rich treasury of verse ancient and modern. Some of the lovely things in it are quite new to me and I am definitely going to read more by some of the poets represented in it. That’s the way to use an anthology: go and read more systematically whoever catches your eye.

Being an old-fashioned romantic I think I’ll finish off with an excerpt from William Wordsworth‘s epic The Prelude. Regular readers (both of you) will know that I greatly admire Wordsworth and, for me, The Prelude is one of the highest pinnacles in all of English literature.

The universal spectacle throughout
Was shaped for admiration and delight,
Grand in itself alone, but in that breach
Through which the homeless voice of waters rose,
That dark deep thoroughfare, had Nature lodged
The Soul, the Imagination of the whole.

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A New Theory of the Universe

Posted in The Universe and Stuff with tags , , , , on January 24, 2009 by telescoper

Yesterday I went on the train to London to visit my old friends in Mile End. I worked at the place that is now called Queen Mary, University of London for nearly a decade and missed it quite a lot when I moved to Nottingham. More recently I’ve had a bit more time and plausible excuses to visit London, including yesterday’s invitation to give a seminar at the Astronomy Unit. Although we were a bit late starting, owing to extremely slow service in the restaurant where we had lunch before the talk, it all seemed to go quite well. Afterwards we had a few beers and a nice chat before I took the train back to Cardiff again.

In the pub (which was the Half Moon, formerly the Half Moon Theatre,  a place of great historical interest) I remembered a joke I sometimes make during cosmology talks but had forgotten to do in the one I had just given.  I’m not sure it will work in written form, but here goes anyway.

I’ve blogged before about the current state of cosmology, but it’s probably a good idea to give a quick reminder before going any further. We have a standard cosmological model, known as the concordance cosmology, which accounts for most relevant observations in a pretty convincing way and is based on the idea that the Universe began with a Big Bang.  However, there are a few things about this model that are curious, to say the least.

First, there is the spatial geometry of the Universe. According to Einstein’s general theory of relativity, universes come in three basic shapes: closed, open and flat. These are illustrated to the right. The flat space has “normal” geometry in which the interior angles of a triangle add up to 180 degrees. In a closed space the sum of the angles is greater than 180 degrees, and  in an open space it is less. Of course the space we live in is three-dimensional but the pictures show two-dimensional surfaces.

But you get the idea.

The point is that the flat space is very special. The two curved spaces are much more general because they can be described by a parameter called their curvature which could in principle take any value (either positive for a closed space, or negative for an open space). In other words the sphere at the top could have any radius from very small (large curvature) to very large (small curvature). Likewise with the “saddle” representing an open space. The flat space must have exactly zero curvature. There are many ways to be curved, but only one way to be flat.

Yet, as near as dammit, our Universe appears to be flat. So why, with all the other options theoretically available to it, did the Universe decide to choose the most special one, which also happens in my opinion to be also the most boring?

Then there is the way the Universe is put together. In order to be flat there must be an exact balance between the energy contained in the expansion of the Universe (positive kinetic energy) and the energy involved in the gravitational interactions between everything in it (negative potential energy). In general relativity, you see, the curvature relates to the total amount of energy.

On the left you can see the breakdown of the various components involved in the standard model with the whole pie representing a flat Universe. You see there’s a vary strange mixture dominated by dark energy (which we don’t understand) and dark mattter (which we don’t understand). The bit we understand a little bit better (because we can sometimes see it directly) is only 4% of the whole thing. The proportions look very peculiar.

And then finally, there is the issue that I talked about in my seminar in London and have actually blogged about (here and there) previously, which is why the Universe appears to be a bit lop-sided and asymmetrical when we’d like it to be a bit more aesthetically pleasing.

All these curiosities are naturally accounted for in my New Theory of the Universe, which asserts that the Divine Creator actually bought  the entire Cosmos  in IKEA.

This hypothesis immediately explains why the Universe is flat. Absolutely everything in IKEA comes in flat packs. Curvature is not allowed.

But this is not the only success of my theory. When God got home he obviously opened the flat pack, found the instructions and read the dreaded words “EASY SELF-ASSEMBLY”. Even the omnipotent would struggle to follow the bizarre set of cartoons and diagrams that accompany even the simplest IKEA furniture. The result is therefore predictable: strange pieces that don’t seem to fit together, bits left over whose purpose is not at all clear, and an overall appearance that is not at all like one would have expected.

It’s clear  where the lop-sidedness comes in too. Probably some of the parts were left out so the whole thing isn’t  held together properly and is probably completely unstable. This sort of thing happens all the time with IKEA stuff. And why is it you can never find the right size Allen Key to sort it out?

So there you have it. My new Theory of the Universe. Some details need to be worked out, but it is as good an explanation of these issues as I have heard. I claim my Nobel Prize.

If anything will ever get me a trip to Sweden, this will.

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What’s all the Noise?

Posted in Science Politics, The Universe and Stuff with tags , , , , on January 18, 2009 by telescoper

Now there’s a funny thing…

I’ve just come across a news item from last week which I followed up by looking at the official NASA press release. I’m very slow to pick up on things these days, but I thought I’d mention it anyway.

The experiment concerned is called ARCADE 2, which is an somewhat contrived acronym derived from Absolute Radiometer for Cosmology, Astrophysics and Diffuse Emission. It is essentially a balloon-borne detector designed to analyse radio waves with frequencies in the range 3 to 90 Ghz. The experiment actually flew in 2006, so it has clearly taken considerable time to analyse the resulting data.

Being on a balloon that flies for a relatively short time (2.5 hours in this case) means that only a part of the sky was mapped, amounting to about 7% of the whole celestial sphere but that is enough to map a sizeable piece of the Galaxy as well as a fairly representative chunk of deep space.

There are four science papers on the arXiv about this mission: one describes the instrument itself; another discusses radio emission from our own galaxy, the Milky Way; the third discusses the overall contribution of extragalactic origin in the frequency range covered by the instrument; the last discusses the implications about extragalactic sources of radio emission.

The thing that jumps out from this collection of very interesting science papers is that there is an unexplained, roughly isotropic, background of radio noise, consistent with a power-law spectrum. Of course to isolate this component requires removing known radio emission from our Galaxy and from identified extragalactic sources, as well as understanding the systematics of the radiometer during its flight. But after a careful analysis of these the authors present strong evidence of excess emission over and above known sources. The spectrum of this radio buzz falls quite steeply with frequency so appears in the two long-wavelength channels at 3 and 8 GHz.

So where does this come from? Well, we just don’t know.

The problem is that no sensible extrapolation of known radio sources to high redshift appears to be able to generate an integrated flux equivalent to that observed. Here is a bit of the discussion from the paper:

It is possible to imagine that an unknown population of discrete sources exist below the flux limit of existing surveys. We argue earlier that these cannot be a simple extension of the source counts of star-forming galaxies. As a toy model, we consider a population of sources distributed with a delta function in flux a factor of 10 fainter than the 8.4 GHz survey limit of Fomalont et al. (2002). At a flux of 0.75 μJy, it would take over 1100 such sources per square arcmin to produce the unexplained emission we see at 3.20 GHz, assuming a frequency index of −2.56. This source density is more than two orders of magnitude higher than expected from extrapolation to the same flux limit of the known source population. It is, however, only modestly greater than the surface density of objects revealed in the faintest optical surveys, e.g., the Hubble Ultra Deep Field (Beckwith et al. 2006).  The unexplained emission might result from an early population of non thermal emission from low-luminosity AGN; such a source would evade the constraint implied by the far-IR measurements.

The point is that ordinary galaxies produce a broad spectrum of radiation and it is difficult to boost the flux at one frequency without violating limits imposed at others. It might be able to invoke Active Galactic Nuclei (AGN) to do the trick, but I’m not sure. I am sure there’ll be a lot work going on trying to see how this might fit in with all the other things we know about galaxy formation and evolution but for the time being it’s a mystery.

I’m equally sure that these results will spawn a plethora of more esoteric theoretical explanations, inevitably including the ridiculous as well as perhaps the sublime. Charged dark matter springs to mind.

Or maybe it’s not even extragalactic. Could it be from an unknown source inside the Milky Way? If so, it might shed some light on the curiosities we find in the cosmic microwave background that I’ve mentioned here and there, but it seems to peak at too low a frequency to account for much of the overall microwave sky temperature.

But it does have a lesson for astronomy funders. ARCADE 2 is a very cheap experiment (by NASA standards). Moreover, the science goals of the experiment did not include “discovering a new cosmic background”. It just goes to show that even in these times of big, expensive and narrowly targetted missions there is still space for serendipity.

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Job Advertisement

Posted in The Universe and Stuff with tags , , on January 14, 2009 by telescoper

This may not be a conventional use for a blog, but I thought I’d give it a go.

After receiving the news a while ago that I had a new research grant, I subsequently got official approval to advertise for a new postdoc and the advert has now been submitted to various places. I thought I might as well put the advertisement on here as well as the usual outlets. It will be on the AAS Jobs Register next month.

Research Associate
Cardiff School of Physics and Astronomy

Applications are invited for a position as Research Associate in Theoretical Cosmology in the School of Physics & Astronomy at Cardiff University. You will undertake research into departures from the standard “concordance” cosmological model and methods for extracting relevant evidence from observations of the cosmic microwave background and large-scale structure in the galaxy distribution. This position is funded by a grant from the Science & Technology Facilities Council.

You will have (or expect to obtain very soon) a PhD or have equivalent research experience in astronomy, astrophysics, cosmology or a closely related subject. A strong theoretical background and experience in the analysis of cosmological data are essential.

The School of Physics & Astronomy at Cardiff University hosts a broad and stimulating research program in astrophysics, cosmology and gravitational physics, encompassing theory, observation and instrumentation. In particular, it is involved in a large number of important cosmological experiments, including Planck, Quad and Clover.

See http://www.astro.cf.ac.uk/ for more information.

This post is fixed-term for 3 years.

Salary: £29704 – £35469 per annum.

Informal enquiries can be made to Professor Coles (Peter.Coles@astro.cf.ac.uk)

For an application pack and details of all Cardiff vacancies, visit www.cardiff.ac.uk/jobs alternatively email vacancies@cardiff.ac.uk or telephone +44 (0) 29 2087 4017 quoting vacancy number 2009/034.

For specific information on this particular vacancy, please go here.

Closing date: Monday, 02 March 2009.

I’ll take this post offline after the deadline passes.

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Maps, Territories and Landscapes

Posted in The Universe and Stuff with tags , , , , , , , , on January 10, 2009 by telescoper

I was looking through recent posts on cosmic variance and came across an interesting item featuring a map from another blog (run by Samuel Arbesman) which portrays the Milky Way in the style of  a public transport map:

mwta

This is just a bit of fun, of course, but I think maps like this are quite fascinating, not just as practical guides to navigating a transport system but also because they often stand up very well as works of art. It’s also interesting how they evolve with time  because of changes to the network and also changing ideas about stylistic matters.

A familiar example is the London Underground or Tube map. There is a fascinating website depicting the evolutionary history of this famous piece of graphic design. Early versions simply portrayed the railway lines inset into a normal geographical map which made them rather complicated, as the real layout of the lines is far from regular. A geographically accurate depiction of the modern tube network is shown here which makes the point:

tubegeo

A revolution occurred in 1933 when Harry Beck compiled the first “modern” version of the map. His great idea was to simplify the representation of the network around a single unifying feature. To this end he turned the Central Line (in red) into a straight line travelling left to right across the centre of the page, only changing direction at the extremities. All other lines were also distorted to run basically either North-South or East-West and produce a much more regular pattern, abandoning any attempt to represent the “real” geometry of the system but preserving its topology (i.e. its connectivity).  Here is an early version of his beautiful construction:

Note that although this a “modern” map in terms of how it represents the layout, it does look rather dated in terms of other design elements such as the border and typefaces used. We tend not to notice how much we surround the essential things with embellishments that date very quickly.

More modern versions of this map that you can get at tube stations and the like rather spoil the idea by introducing a kink in the central line to accommodate the complexity of the interchange between Bank and Monument stations as well as generally buggering about with the predominantly  rectilinear arrangement of the previous design:

I quite often use this map when I’m giving popular talks about physics. I think it illustrates quite nicely some of the philosophical issues related with theoretical representations of nature. I think of theories as being like maps, i.e. as attempts to make a useful representation of some  aspects of external reality. By useful, I mean the things we can use to make tests. However, there is a persistent tendency for some scientists to confuse the theory and the reality it is supposed to describe, especially a tendency to assert there is a one-to-one relationship between all elements of reality and the corresponding elements in the theoretical picture. This confusion was stated most succintly by the Polish scientist Alfred Korzybski in his memorable aphorism :

The map is not the territory.

I see this problem written particularly large with those physicists who persistently identify the landscape of string-theoretical possibilities with a multiverse of physically existing domains in which all these are realised. Of course, the Universe might be like that but it’s by no means clear to me that it has to be. I think we just don’t know what we’re doing well enough to know as much as we like to think we do.

A theory is also surrounded by a penumbra of non-testable elements, including those concepts that we use to translate the mathematical language of physics into everday words. We shouldn’t forget that many equations of physics have survived for a long time, but their interpretation has changed radically over the years.

The inevitable gap that lies between theory and reality does not mean that physics is a useless waste of time, it just means that its scope is limited. The Tube  map is not complete or accurate in all respects, but it’s excellent for what it was made for. Physics goes down the tubes when it loses sight of its key requirement: to be testable.

In any case, an attempt to make a grand unified theory of the London Underground system would no doubt produce a monstrous thing so unwieldly that it would be useless in practice. I think there’s a lesson there for string theorists too…

Now, anyone for a game of Mornington Crescent?

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Particle Physics – The Opera

Posted in Opera, The Universe and Stuff with tags , , , , on January 8, 2009 by telescoper

A new season is about to start at English National Opera and I’ve been spending a lot of time and money recently getting tickets for some of the operas, as well as organizing the logistics of getting to and from London. Among the forthcoming productions is a revival of Nicholas Hytner’s production of Mozart’s The Magic Flute (Die Zauberflöte, K. 620).

I can’t remember how many times I have seen this opera performed nor in how many different productions. It’s a wonderful creation because it manages to combine being utterly daft with being somehow immensely profound. The plot makes no sense at all, the settings are ridiculous (e.g. “rocks with water and a cavern of fire”), and the whole thing appears to be little more than a pantomime. Since it’s Mozart, though, there is one ingredient you can’t quibble with: a seemingly unending sequence of gorgeous music.

When I first saw The Magic Flute I thought it was just a silly but sublime piece of entertainment not worth digging into too deeply. I wondered why so many pompous people seemed to take it so terribly seriously. Real life doesn’t really make much sense, so why would anyone demand that an opera be any less ridiculous? Nevertheless, there is a vast industry devoted to unravelling the supposed “mystery” of this opera, with all its references to magic and freemasonry.

But now I can unveil the true solution of problem contained within the riddle encoded in the conundrum that surrounds the enigma that has puzzled so many Opera fans for so long. I have definitive proof that this opera is not about freemasons or magic or revolutionary politics.

Actually it is about particle physics.

To see how I arrived at this conclusion note the following figure which shows the principal elementary particles contained within the standard model of particle physics:

To the left of this picture are the fermions, divided into two sets of particles labelled “quarks” and “leptons”. Each of these consists of three pairs (“isospin doublets”), each pair defining a “generation”. This structure of twos and threes is perfectly represented in The Magic Flute.

Let’s consider the leptons first. These can be clearly identified with the three ladies who lust after the hero Tamino in Act 1. This emotional charge is clearly analogous to the electromagnetic charge carried by the massive leptons (the electron, muon and tauon, lying along the bottom of the diagram). The other components in the leptonic sector must be the three boys who pop up every now and again to help Papageno with useful advice about when to jangle his magic bells. These must therefore be the neutrinos, which are less massive than the ladies, and are also neutral (although I hesitate to suggest that this means they should be castrati). They don’t play a very big part in the show because they participate only in weak interactions.

Next we have the quarks, also arrayed in three generations of pairs. These interact more strongly than the leptons and are also more colourful. The first generation is easy to identify, from the phenomenology of the Opera, as consisting of the hero Tamino (d for down) and his beloved Pamina (u for up); her voice is higher than his, hence the identification. The second generation must comprise the crazy birdcatcher Papageno (s for strange) and his alluring madchen who is called Papagena (c for charmed). That just leaves the final pairing which clearly is the basso profundo and fount of all wisdom Sarastro (b for bass bottom) and my favourite character and role model the Queen of the Night (t for top).

To provide corroboration of the identification of the Queen of the Night with the “top” quark, here is a clip from Youtube of a bevy of famous operatic sopranos having a go at the immensely different coloratura passage from the Act 1 aria “O Zittre Nicht, mein leiber Sohn” culminating in a spectacular top F that lies beyond the range of most particle accelerators, never mind singers.

There’s some splendid frocks in there too.

The Queen of the Night isn’t actually in the Opera very much. After this aria in Act 1 she disappears until the middle of Act 2, probably because she needs to have a lie down. When she comes back on she sings another glass-shattering aria (Der Hölle Rache kocht in meinem Herzen), which I like to listen to when I’m writing referee reports. The first line translates as “The rage of hell is boiling in my heart”.

The remaining members of the cast – The Speaker and Monostatos, as well as sundry priests, slaves, enchanted animals and the chorus – must make up the so-called Force carriers at the left of the table, which are bosons, but I haven’t had time to go through the identifications in detail. They’re just the supporting cast anyway. And there is one particle missing from the picture, the Higgs boson. This accounts for the masses of other particles by exerting a kind of drag on them so it clearly must be the Dragon from Act 1.

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Professor Who?

Posted in Biographical, Music, Television, The Universe and Stuff with tags , , , , , on January 7, 2009 by telescoper

As a Professor of Astrophysics I am often asked “Why on Earth did you take up such a crazy subject?”

I guess many astronomers, physicists and other scientists have to answer this sort of question. For many of them there is probably a romantic reason, such as seeing the rings of Saturn or the majesty of the Milky Way on a dark night. Others will probably have been inspired by TV documentary series such as The Sky at Night, Carl Sagan’s Cosmos or even Horizon which, believe it or not, actually used to be quite good but which is nowadays uniformly dire. Or it could have been something a bit more mundane but no less stimulating such as a very good science teacher at school.

When I’m asked this question I’d love to be able to put my hand on my heart and give an answer of that sort but the truth is really quite a long way from those possibilities. The thing that probably did more than anything else to get me interested in science was a Science Fiction TV series or rather not exactly the series but the opening titles.

The first episode of Doctor Who was broadcast in the year of my birth, so I don’t remember it at all, but I do remember the astonishing effect the credits had on my imagination when I saw later episodes as a small child. Here are some tests for the sequence as it appeared in the very first series featuring William Hartnell as the first Doctor.

To a younger audience it probably all seems quite tame, but I think there’s a haunting, unearthly beauty to the shapes conjured up by Bernard Lodge. Having virtually no budget for graphics, he experimented in a darkened studio with an old-fashioned TV camera and a piece of black card with Doctor Who written on it in white. He created the spooky kaleidoscopic patterns you see by simply pointing the camera so it could see into its own monitor, thus producing a sort of electronic hall of mirrors.

What is so fascinating to me is how a relatively simple underlying concept could produce a rich assortment of patterns, particularly how they seem to take on an almost organic aspect as they merge and transform. I’ve continued to be struck by the idea that complexity could be produced by relatively simple natural laws which is one of the essential features of astrophysics and cosmology. As a practical demonstration of the universality of physics this sequence takes some beating.

As well as these strange and wonderful images, the titles also featured a pioneering piece of electronic music. Officially the composer was Ron Grainer, but he wasn’t very interested in the commission and simply scribbled the theme down and left it to the BBC to turn it into something useable. In stepped the wonderful Delia Derbyshire, unsung heroine of the BBC Radiophonic Workshop who, with only the crudest electronic equipment available, turned it into a little masterpiece. Ethereal yet propulsive, the original theme from Doctor Who is definitely one of my absolute favourite pieces of music and I’m glad to see that Delia Derbyshire is now receiving the acclaim she deserves from serious music critics.

It’s ironic that I’ve now moved to Cardiff where new programmes of Doctor Who and its spin-off, the anagrammatic Torchwood, are made. One of the great things about the early episodes of Doctor Who was that the technology simply didn’t exist to do very good special effects. The scripts were consequently very careful to let the viewers’ imagination do all the work. That’s what made it so good. I’m pleased that the more recent incarnations of this show also don’t go overboard on the visuals. Perhaps thats a conscious attempt to appeal to people who saw the old ones as well as those too young to have done so. It’s just a pity the modern opening title music is so bad…

Anyway, I still love Doctor Who after all these years. It must sound daft to say that it inspired me to take up astrophysics, but it’s truer than any other explanation I can think of. Of course the career path is slightly different from a Timelord, but only slightly.

At any rate I think The Doctor is overdue for promotion. How about Professor Who?

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