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Audio Video Disco

Posted in Biographical, The Universe and Stuff with tags , , on August 26, 2009 by telescoper

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This scary picture is taken from an interactive exhibit in the Weller Galleries of the Royal Observatory at Greenwich, which opened in 2007. The exhibit, I mean, not the Royal Observatory. I remember going down there to record the video segments, but had forgotten all about it until somebody found this image on the net and drew my attention to it.

The exhibit consists of a series of display screens with various astronomical and cosmological concepts and questions on them, along with appropriate images. Visitors touch the screens to bring up the video segments in which distinguished astronomers (or me) attempt to provide explanations.

The lady to the bottom right is probably providing a sign language translation of my contribution. Or she could simply be screaming and waving her hands in terror. Wouldn’t you?

PS. If you want an explanation of the title of this blog post, I’ll translate Audio Video Disco from the latin for you. It means “I hear, I see, I learn”. Since they have to touch the screen, I might have added “I touch” which would be Tango….

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Much Ado About a Null Result

Posted in Science Politics, The Universe and Stuff with tags , , , on August 20, 2009 by telescoper

In today’s Nature there’s an article outlining the current upper limits on the existence of a stochastic cosmological background of gravitational waves. The basis of the analysis presented in the paper is a combination of data from two larger international collaborations, called VIRGO and LIGO. Cardiff University is a member of the latter, so I suppose I should be careful about what I say…

These experiments have achieved incredible sensitivity – they can measure distortions that are a tiny fraction of an atomic nucleus in scale – but because gravity is such a very weak force they still haven’t managed to find direct evidence of gravitational waves. The next generation of these laser interferometers – Advanced LIGO – should get within hailing distance of a detection but in the meantime we have to do with upper limits. Since the sensitivity of the instruments is so well calibrated, the lack of a signal can yield interesting information. The Nature paper is quite interesting in that it summarizes the constraints that can be placed in such a way on some models of the early Universe. Mostly, though, these are “exotic” models that have already been excluded by other means. If I’ve got my sums right the stochastic gravitational wave background expected to be produced within the standard “concordance” cosmology, in which gravitational wave modes are excited by cosmic inflation, is at least three orders of magnitude lower than current experimental sensitivity.

I can’t resist including the following excerpts from a press release, produced by the Media Relations Department at Caltech whose spin doctors have apparently been hard at work.

Pasadena, Calif.—An investigation by the LIGO (Laser Interferometer Gravitational-Wave Observatory) Scientific Collaboration and the Virgo Collaboration has significantly advanced our understanding the early evolution of the universe.

Analysis of data taken over a two-year period, from 2005 to 2007, has set the most stringent limits yet on the amount of gravitational waves that could have come from the Big Bang in the gravitational wave frequency band where LIGO can observe. In doing so, the gravitational-wave scientists have put new constraints on the details of how the universe looked in its earliest moments.

Much like it produced the cosmic microwave background, the Big Bang is believed to have created a flood of gravitational waves—ripples in the fabric of space and time—that still fill the universe and carry information about the universe as it was immediately after the Big Bang. These waves would be observed as the “stochastic background,” analogous to a superposition of many waves of different sizes and directions on the surface of a pond. The amplitude of this background is directly related to the parameters that govern the behavior of the universe during the first minute after the Big Bang.

and

“Since we have not observed the stochastic background, some of these early-universe models that predict a relatively large stochastic background have been ruled out,” says Vuk Mandic, assistant professor at the University of Minnesota.

“We now know a bit more about parameters that describe the evolution of the universe when it was less than one minute old,” Mandic adds. “We also know that if cosmic strings or superstrings exist, their properties must conform with the measurements we made—that is, their properties, such as string tension, are more constrained than before.”

This is interesting, he says, “because such strings could also be so-called fundamental strings, appearing in string-theory models. So our measurement also offers a way of probing string-theory models, which is very rare today.”

“This result was one of the long-lasting milestones that LIGO was designed to achieve,” Mandic says. Once it goes online in 2014, Advanced LIGO, which will utilize the infrastructure of the LIGO observatories and be 10 times more sensitive than the current instrument, will allow scientists to detect cataclysmic events such as black-hole and neutron-star collisions at 10-times-greater distances.

“Advanced LIGO will go a long way in probing early universe models, cosmic-string models, and other models of the stochastic background. We can think of the current result as a hint of what is to come,” he adds.

“With Advanced LIGO, a major upgrade to our instruments, we will be sensitive to sources of extragalactic gravitational waves in a volume of the universe 1,000 times larger than we can see at the present time. This will mean that our sensitivity to gravitational waves from the Big Bang will be improved by orders of magnitude,” says Jay Marx of the California Institute of Technology, LIGO’s executive director.

“Gravitational waves are the only way to directly probe the universe at the moment of its birth; they’re absolutely unique in that regard. We simply can’t get this information from any other type of astronomy. This is what makes this result in particular, and gravitational-wave astronomy in general, so exciting,” says David Reitze, a professor of physics at the University of Florida and spokesperson for the LIGO Scientific Collaboration.

If hyperbole is what you’re looking for, go no further. There’s nothing wrong with presenting even null results in a positive light but, I don’t think this paints a very balanced picture of the field. For examples, early Universe models involving cosmic strings were already severely constrained before these results, so we know that they don’t have a significant effect on the evolution of cosmic structure anyway.

Clearly the political intention was to flag the importance of Advanced LIGO, although even that will probably be unable to detect the cosmological gravitational-wave background.  Overstatements contained in press releases of this type usually prove counterproductive in the long run.

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Beginning Again

Posted in Books, Talks and Reviews, The Universe and Stuff with tags , on August 19, 2009 by telescoper

I keep finding old forgotten bits and pieces – especially book reviews – on my computer. This one is about five years old but I thought I might as well put it on here to save having to think of anything else for today. It’s also a little bit topical because the author, Simon Singh, has recently been the subject of much discussion on this blog (here and here).

This piece was eventually published in an edited form as as Nature 432, 953-954 (23 December 2004) | doi:10.1038/432953b; Published online 22 December 2004.

BOOK REVIEWED–Big Bang: The Most Important Scientific Discovery of All Time and Why You Need to Know About It

by Simon Singh
Fourth Estate: 2004. 544 pp. £20, $27.95

When the British astrophysicist Fred Hoyle coined the phrase ‘Big Bang’ to describe the rival to his beloved ‘steady state’ theory of the Universe, he meant it to be disparaging. It was bad enough for Hoyle that his pet theory turned out to disagree with astronomical observations, but it must have been especially galling that his cosmological adversaries embraced his derisive name. The tag has since spread into the wider cultural domain — nowadays even politicians have heard of the Big Bang.

But what is the Big Bang? In a nutshell, it is the idea that our Universe — space, time and all its matter content — was born in a primordial fireball, from which the whole caboodle has been expanding and cooling ever since. Pioneering theorists such as Aleksander Friedmann and Georges Lemaître derived mathematical solutions of Einstein’s field equations that could be used to describe the evolution of a Big Bang Universe. These models involve a creation event, in which space-time and matter-energy sprang into existence to form our Universe. We are still in the dark about how this happened, but we think it took place about 14 billion years ago.

Edwin Hubble’s discovery of the recession of distant galaxies gave support to the idea that the Universe was expanding, but the notion that it might be evolving from a hot beginning was rejected by many theorists, including Hoyle. He favoured a model in which the origin of matter was not a single event but a continuous process in which atoms were created to fill in the gaps created by cosmic expansion. The battle between these competing views of creation raged until the accidental discovery in 1965 of the cosmic microwave background radiation, which marked the beginning of the end for the steady-state theory.

This conflict between the two theories plays a central role in Simon Singh’s book Big Bang. His previous books, Fermat’s Last Theorem and The Code Book, succeeded admirably in bringing difficult mathematical subjects to a popular readership, using a combination of accessible prose, a liberal sprinkling of jokes and a strong flavouring of biographical anecdotes. The recipe for his new book is similar.

In Big Bang, Singh uses the historical development of modern cosmological theory as a case study for how scientific theories are conceived, and how they win or lose acceptance. He rightly points out that science rarely proceeds in an objective, linear fashion. Correct theories are often favoured for the wrong reasons; observations and experiments are frequently misinterpreted; and sometimes force of personality holds sway over analytic reason. Because cosmology has such ambitious goals — to find a coherent explanation for the entire system of things and how it has evolved — these peculiarities are often exaggerated. In particular, cosmology has more than its fair share of eccentric characters, providing ample illustration of the role of personal creativity in scientific progress.

This very well written book conveys the ideas underpinning cosmological theory with great clarity. Taking nothing for granted of his readership, Singh delves into the background of every key scientific idea he discusses. This involves going into the history of astronomical observation, as well as explaining in non-technical language the principles of basic nuclear physics and relativity. The numerous snippets of biographical information are illuminating as well as amusing, and the narrative is driven along by the author’s own engaging personality.

However, even as a fan of Singh’s previous books, I have to admit that, although this one has many strengths, I found it ultimately rather disappointing. For one thing, there isn’t anything in this book that could be described as new. The book follows a roughly historical thread from pre-classical mythology to the middle of the twentieth century. This is a well-worn path for popular cosmology, and the whole thing is rather formulaic. Each chapter I read gave me the impression that I had read most of it somewhere before. It certainly lacks the ground-breaking character of Fermat’s Last Theorem.

The past ten years in cosmology have witnessed a revolution in observation that has, among many other things, convinced us of the existence of dark energy in the Universe. Theory has also changed radically over this period, largely through the introduction of ideas from high-energy physics, such as superstring theory. Indeed, some contemporary Big Bang models bear a remarkable resemblance to the steady-state universe, involving the continuous creation not of mere atoms, but of entire universes.

Frustratingly, virtually all the exciting recent developments are missing from this book, which leaves off just when things started to get interesting, with the COBE satellite in 1992. Readers who want to know what is going on now in this field should definitely look elsewhere. The processes of cosmic discovery and controversy are ongoing, not just relics of the past.

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The Cold Spot

Posted in Cosmic Anomalies, The Universe and Stuff with tags , , , , on August 16, 2009 by telescoper

Musing yesterday about the rapidly approaching restart of the academic year reminded me that I really ought to get on and finish the bunch of papers sitting on my desk and on various computers. I’ve also got a book to finish before October so I’d better get cracking with that too.

More importantly, however, it reminded me to congratulate my PhD student Rockhee Sung who has just had her first paper published (in the journal Classical and Quantum Gravity). The paper is available online here and it’s free to download for a month even if you don’t have a personal or institutional subscription to the journal.

The idea of this paper came a while ago but it has taken us a long time to get everything in place to start writing it up. In the meantime other papers have been written on the subject, but Rockhee and I have done this our own way – or rather she has, as she put most of the hard work into actually doing the calculations.

About four years ago, during the course of careful statistical analysis of data from the Wilkinson Microwave Anisotropy Probe (WMAP), a group based in Santander (Spain) published a paper drawing attention to the existence of an anomalous “Cold Spot” in the data. This phenomenon has now acquired its own Wikipedia entry (here), so I won’t repeat all the details except to say that it is about 5° across and that it is colder than one would expect if the temperature fluctuations are Gaussian, as is predicted in the simplest models of the early Universe involving cosmological inflation. The spot is to the bottom right, and is marked with an arrow on the picture below.

It’s worth digressing a little here to explain that a fluctuating field of course contains both hot spots and cold spots. Because there CMB temperature fluctuations comprise a wide range of wavelengths there are also spots on different scales. Assessing the statistical significance of a single isolated feature like the cold spot is not particularly easy. Based on the brute force method of simulating skies according to the Gaussian hypothesis and then repeating the approach that led to the original discovery, the result is that around 1% of Gaussian CMB skies have a cold spot as cold as that observed in the real data. Before the non-Bayesians among you get too excited, I’ll remind you that this means that the probability of a Cold spot given the standard model is about 1%, i.e. P(Cold Spot | Standard Model)=0.01. This is NOT the same as saying that the probability of the standard model being correct is 0.01…

A probability of 1% is an in-between kind of level: not too small to be decisive, and not too large to be instantly dismissed as just being a chance fluctuation. My personal opinion is that the Cold Spot is an interesting feature that deserves to be investigated further, but is not something that in itself should cause anyone to doubt the standard model. I include it among the list of cosmological anomalies that I’ve blogged about before (for example, here, here and here). I find them interesting but don’t lose sleep worrying that the standard model is about to fall to pieces. Not yet, anyway.

Not all theorists are as level-headed as me, however, and within weeks of the discovery of the cold spot suggestions were already being put forward as to how it could be “explained” theoretically. Some of these are described in the Wikipedia entry, so I won’t rehash the list. However, one suggestion not included there was the idea that the anomalous cold spot might be there because the Universe were not isotropic, i.e. if the Cosmological Principle were violated.

Way back when I was a lad doing my own PhD, my supervisor John Barrow had been interested in globally anisotropic (but nevertheless homogeneous) cosmologies. These are models in which any observer sees different things in different directions, but the pattern seen by observers in different places is always the same. I never worked on these at the time – they seemed a bit too esoteric even for me – but I remembered bits and pieces about them from conversations.

A complete classification of all the space-times  possessing this property was completed over a hundred years ago (before General Relativity was invented) by the Italian mathematician Luigi Bianchi, and cosmological models based on them are called the Bianchi models.

This isn’t the place to go into detail about the Bianchi models: the classification is based on the mathematical properties of Lie groups, which would take me ages to explain. However, it is worth pointing out that only five Bianchi types actually contain the cosmologically principled Friedmann-Lemaître-Robertson-Walker universe as a special case: I, V, VII0 ,VIIh and IX. If you really want to know what the classes are you’ll have to look them up! Since we know our Universe is very close to being homogeneous and isotropic, it seems reasonable to look at those models capable of describing small departures from that case so the above list provides a useful subset of the models to explore.

Rockhee’s PhD project was to explore  the patterns of cosmic microwave background  fluctuations that can arise in that set of Bianchi cosmologies, not just in the temperature (which had been done before) but also in polarization (which hadn’t). I’ve already posted some of the temperature patterns Rockhee computed here.

The reason for extending wanting to extend this work to include polarization was the following. The microwave background radiation is partly linearly polarized because of the way radiation is scattered by electrons. If an electron is immersed in a radiation bath which is isotropic there is no net polarization, but if the radiation field is anisotrpic – in particular if it varies on an angular scale of 90º (i.e. a quadrupole) – then the scattered radiation will be partly polarized. In the standard cosmology the variations in the radiation field are random fluctuations so each electron “sees” a different quadupole. The net polarization field is therefore produced incoherently, by adding stochastic contributions. In  a  Bianchi model the situation is different. Each electron in this case sees the same quadupole. The polarization pattern produced is therefore coherent. Not only do anisotropic universes produce characteristic radiation patterns, they also produce a corresponding pattern in polarization.

So what does this all have to do with the Cold Spot? Well, in anisotropic spaces that are also curved, it is possible for light rays to get focussed in such a way that the entire pattern of flucuations present at least-scattering winds up concentrated in a small patch of the sky as seen by a late-time observer. for this to happen the space has to be negatively curved. Only two of the Bianchi types can do this, as there are only two that are both near-FLRW and negatively curved: V and VIIh. Both of these models could, in principle, therefore produce a cold spot by geometrical, rather than stochastic means. In the little figure below, taken from our paper, you can see examples of Bianchi VIIh (top) and Bianchi V (bottom) showing the temperature (left) and polarization (right) in each case. We’ve oriented the model to put the cold spot in approximately the right location as the observed one.

 

cold

 

The point is that there is a pretty heavy price to be paid for producing the cold spot in this way: an enormous, coherent signal in the polarized radiation field.

As often happens in such situations, somebody else had the idea to investigate these models and we were scooped to a large extent by Andrew Pontzen and Anthony Challinor from Cambridge, who recently published a paper showing that the polarization produced in these models is already excluded by experimental upper limits. They concentrated on the Bianchi VIIh case, as this appears to have a more general structure than V and it was the model first advocated as an explanation of the cold spot. In this model the combined effect of vorticity and shear introduces a swirly pattern into the radiation field that you can see clearly in the top two panels of the figure as well as focussing it into a small patch. Bianchi V doesn’t produce the same kind of pattern either in temperature or polarization: it looks more like a simple quadrupole squeezed into a small part of the sky. A particularly interesting aspect of this is that the Bianchi VIIh case clearly has a definite “handedness” while the Bianchi V one doesn’t.

The moral of all this is that the polarization of the cosmic microwave background provides key additional information that could prove decisive in eliminating (or perhaps even confirming) models of the Universe more exotic than the standard one. That’s one of the areas in which  we expect Planck to produce the goods!

In the meantime Rockhee and I will be submitting a couple of much larger papers in due course, one containing a wider discussion of the possible pattern morphologies that can be produced in these models, and another about their detailed statistical properties.

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Upon Nothing

Posted in Poetry, The Universe and Stuff with tags , , on August 15, 2009 by telescoper

I used to live in Wilmot Street in Bethnal Green, in the East End of London. I’d been resident there quite a while before I realised that the street was named after John Wilmot, Earl of Rochester, one of the great metaphysical poets, who lived from 1647 to 1680 (although I doubt he ever lived in Bethnal Green).

John Wilmot was a lifelong atheist, bon viveur and generally dissolute individual who famously converted to Christianity on his deathbed, causing much debate about whether he actually meant it.

Much of Wilmot’s literary output is actually quite crude (and often pornographic). However, first published in 1679, Upon Nothing is  certainly among the cleverest of his works and is possibly the most important poem he wrote. It’s clearly a satire  on John Milton‘s Paradise Lost (especially Book II). Starting out with a dig at the vanity of man’s attempts to solve the problem of existence, it moves into a more general lampoon of fashion victims, pompous politicians and self-important persons generally.

Updated by a few hundred years, this poem could equally be applied to the programme of quantum cosmology advocated by, e.g. Alexander Vilenkin which tries to explain the existence of the Universe by quantum tunneling  ex nihilo.  I always have a problem understanding how the equations of quantum mechanics could exist, as it were, in advance of the material they try to describe. I suppose the point is that there’s really no such thing as nothing, but then I’m no metaphysicist…

Upon Nothing

Nothing, thou elder brother even to shade,
That hadst a being ere the world was made,
And (well fixed) art alone of ending not afraid.

Ere time and place were, time and place were not,
When primitive Nothing Something straight begot,
Then all proceeded from the great united—What?

Something, the general attribute of all,
Severed from thee, its sole original,
Into thy boundless self must undistinguished fall.

Yet Something did thy mighty power command,
And from thy fruitful emptiness’s hand,
Snatched men, beasts, birds, fire, air, and land.

Matter, the wickedest offspring of thy race,
By Form assisted, flew from thy embrace,
And rebel Light obscured thy reverend dusky face.

With Form and Matter, Time and Place did join,
Body, thy foe, with these did leagues combine
To spoil thy peaceful realm, and ruin all thy line.

But turncoat Time assists the foe in vain,
And, bribed by thee, assists thy short-lived reign,
And to thy hungry womb drives back thy slaves again.

Though mysteries are barred from laic eyes,
And the Divine alone with warrant pries
Into thy bosom, where thy truth in private lies,

Yet this of thee the wise may freely say,
Thou from the virtuous nothing takest away,
And to be part of thee the wicked wisely pray.

Great Negative, how vainly would the wise
Inquire, define, distinguish, teach, devise?
Didst thou not stand to point their dull philosophies.

Is, or is not, the two great ends of Fate,
And true or false, the subject of debate,
That perfects, or destroys, the vast designs of Fate,

When they have racked the politician’s breast,
Within thy bosom most securely rest,
And, when reduced to thee, are least unsafe and best.

But Nothing, why does Something still permit
That sacred monarchs should at council sit
With persons highly thought at best for nothing fit?

Whist weighty Something modestly abstains
From princes’ coffers, and from statesmen’s brains,
And Nothing there like stately Nothing reigns,

Nothing, who dwellest with fools in grave disguise,
For whom they reverend shapes and forms devise,
Lawn sleeves, and furs, and gowns, when they like thee look wise.

French truth, Dutch prowess, British policy,
Hibernian learning, Scotch civility,
Spaniard’s dispatch, Dane’s wit are mainly seen in thee.

The great man’s gratitude to his best friend,
King’s promises, whore’s vows, towards thee they bend,
Flow swiftly to thee, and in thee never end.

Incidentally, the first use of the word metaphysical to describe the particular “poetic style, characterized by wit, syntactic complexity, and the use of elaborate and intricate schemes of imagery to express abstract ideas and emotional states” was in 1693, by John Dryden who clearly meant it to be pejorative. Those whose philosophical inclination is in the direction of positivism would look down on the more orthodox meaning of the word metaphysical, i.e. meaning “of or relating to the  branch of philosophy that deals with the first principles of things or reality, including questions about being, substance, time and space, causation, change, and identity”. Dryden, however, was alluding to another meaning, now quite rare but prevalent in the 17th Century, that has something to do with magical or supernatural things beyond the bounds of respectable thought. Thomas More used the word “metaphysical” to attack William Tyndale for translating the Bible into English, for example.

You can find other examples of  metaphysical poetry in the collection I blogged about here.

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The Axle of Elvis

Posted in Cosmic Anomalies, The Universe and Stuff with tags , , , , , , on August 6, 2009 by telescoper

An interesting paper on the arXiv yesterday gave me a prod to expand a little on one of the cosmic anomalies I’ve blogged about before.

Before explaining what this is all about, let me just briefly introduce a bit of lingo. The pattern of variations fluctuations in the temperature of the cosmic microwave background (CMB) across the sky, such as is revealed by the Wilkinson Microwave Anisotropy Probe (WMAP), is usually presented in terms of the behaviour of its spherical harmonic components. The temperature as a function of position is represented as a superposition of spherical harmonic modes labelled by two numbers, the degree l and the order m. The degree basically sets the characteristic angular scale of the mode (large  scales have low l, and small scales have high l). For example the dipole mode has l=1 and it corresponds to variation across the sky on a scale of 180 degrees; the quadrupole (l=2) has a scale of 90 degrees, and so on. For a fixed l the order m runs from -l to +l and each order represents a particular pattern with that given scale.

The spherical harmonic coefficients that tell you how much of each mode is present in the signal are generally  complex numbers having real and imaginary parts or, equivalently, an amplitude and a phase.  The exception to this are the modes with m=0, the zonal modes, which have no azimuthal variation: they vary only with latitude, not longitude. These have no imaginary part so don’t really have a phase. For the other modes, the phase controls the variation with azimuthal angle around the axis of the chosen coordinate system, which in the case of the CMB is usually taken to be the Galactic one.

In the simplest versions of cosmic inflation, each of the spherical harmonic modes should be statistically independent and randomly distributed in both amplitude and phase. What this really means is that the harmonic modes are in a state of maximum statistical disorder or entropy. This property also guarantees that the temperature fluctuations over the sky should be described by  a Gaussian distribution.

That was perhaps a bit technical but the key idea is that if you decompose the overall pattern of fluctuations into its spherical harmonic components the individual mode patterns should look completely different. The quadrupole and octopole, for example, shouldn’t line up in any particular way.

Evidence that this wasn’t the case started to emerge when WMAP released its first set of data in 2003 with indications of an alignment between the modes of low degree. In their  analysis, Kate Land and Joao Magueijo dubbed this feature The Axis of Evil; the name has stuck.They concluded that there was a statistically significant alignment (at 99.9% confidence) between the multipoles of low degree (l=2 and 3), meaning that the measured alignment is only expected to arise by chance in one in a thousand simulated skies. More recently, further investigation of this effect using subsequent releases of data from the WMAP experiment and a more detailed treatment of the analysis (including its stability with respect to Galactic cuts) suggested that the result is not quite as robust as had originally been claimed. .

Here are the low-l modes of the WMAP data so you see what we’re talking about. The top row of the picture contains the modes for l=2 (quadrupole) and l=3 (octopole) and the bottom shows l=4 and l=5.

 

The two small red blobs mark the two ends of the preferred axis of each mode. The orientation of this axis is consistent across all the modes shown but the statistical significance is much stronger for the ones with lower l.

It’s probably worth mentioning a couple of neglected aspects of this phenomenon. One is that the observed quadrupole and octopole appear not only to be aligned with each other but also appear to be dominated by sectoral orders, i.e those with m=l. These are the modes which are, in a sense, opposite to the zonal modes in that they vary only with longitude and not with latitude. Here’s what the sectoral mode of the quadrupole looks like:

map22

Changing the phase of this mode would result in the pattern moving to the left or right, i.e. changing its origin, but wouldn’t change the orientation. Which brings me to the other remarkable thing, namely that the two lowest modes also have  correlated phases. The blue patch to the right of Galactic centre is in the same place for both these modes. You can see the same feature in the full-resolution map (which involves modes up to l~700 or so):

I don’t know whether there is really anything anomalous about the low degree multipoles, but I hope this is a question that Planck (with its extra sensitivity, better frequency coverage and different experimental strategy) will hopefully shed some light on. It could be some sort of artifact of the measurement process or it could be an indication of something beyond the standard cosmology. It could also just be a fluke. Or even the result of an over-active imagination, like seeing Elvis in your local Tesco.

On its own I don’t think this is going to overthrow the standard model of cosmology. Introducing extra parameters to a model in order to explain a result with a likelihood that is only marginally low in a simpler model does not make sense, at least not to a proper Bayesian who knows about model selection…

However, it is worth mentioning that the Axis of Evil isn’t the only cosmic anomaly to have been reported. If an explanation is found with relatively few parameters that can account for all of these curiosities in one fell swoop then it would stand a good chance of convincing us all that there is more to the Universe than we thought. And that would be fun.

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Return of the Clerihews!

Posted in Poetry, The Universe and Stuff with tags , , on August 2, 2009 by telescoper

As a result of an after-dinner discussion at the meeting I attended last week, I’ve decided to put a revised cosmological clerihew collection back online. I’ve removed or edited those that caused the greatest offence, and added a few new ones.

Bernard Carr
Has gone a bit far:
His Anthropic Principle
Makes theories invincible

Sean Carroll
Has me over a barrel
Because the only plausible rhyme
Plugs his new book on Time

The mind of John Barrow
Is not very narrow:
He’s more open than me
To a variable c

Stephen Hawking
Lets a machine do the talking
But even  he can’t vocalize in-
side a black hole horizon.

Joe Silk
Is one of that ilk
Who writes far more articles
Than there are elementary particles

Matt Griffin
Has healthy salad for tiffin
But he’d probably expire
If something went wrong with SPIRE.

Peter Ade
Would never be afraid
To enter his name
In the citation game

Andy Lawrence
Would shed tears in torrents
If they finally got rid
Of the Astrogrid

Steve Maddox
Never eats haddocks
But he’s quite a dab hand
In the optical band

Ofer Lahav
Is awfully suave
But must be getting nervy
About the cancellation of funding for the Dark Energy Survey

Joao Magueijo
Was on the Today Show
Talking some shite
About travelling faster than light

Keith Mason
Said to Lord Drayson
“Can we have some more money?”
He replied “Don’t try to be funny…”

Andrei Linde
Felt rather windy
A peculiar sensation:
The result of internal inflation?

To rhyme Carlos Frenck
I’ve drawn a complete blenk
But I found in the lexicon
A good one for Mexican

When Andrew Jaffe
Plots a new graph he
Thinks fits his theory he’ll
Tell everyone at Imperial

Paul Steinhardt
Said “Lust not after beauty in thine heart”
But why he did so
I really don’t know

Feel free to offer your own through the comments box, after consulting the rules, although I remind you I don’t accept anonymous comments, even if they’re funny.

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A Mountain of Truth

Posted in Bad Statistics, The Universe and Stuff with tags , , , , on August 1, 2009 by telescoper

I spent the last week at a conference in a beautiful setting amidst the hills overlooking the small town of Ascona by Lake Maggiore in the canton of Ticino, the Italian-speaking part of Switzerland. To be more precise we were located in a conference centre called the Centro Stefano Franscini on  Monte Verità. The meeting was COSMOSTATS which aimed

… to bring together world-class leading figures in cosmology and particle physics, as well as renowned statisticians, in order to exchange knowledge and experience in dealing with large and complex data sets, and to meet the challenge of upcoming large cosmological surveys.

Although I didn’t know much about the location beforehand it turns out to have an extremely interesting history, going back about a hundred years. The first people to settle there, around the end of the 19th Century,  were anarchists who had sought refuge there during times of political upheaval. The Locarno region had long been a popular place for people with “alternative” lifestyles. Monte Verità (“The Mountain of Truth”) was eventually bought by Henri Oedenkoven, the son of a rich industrialist, and he  set up a sort of commune there at  which the residents practised vegetarianism, naturism, free love  and other forms of behaviour that were intended as a reaction against the scientific and technological progress of the time.  From about 1904 onward the centre became a sanatorium where the discipline of psychoanalysis flourished and it later attracted many artists. In 1927,   Baron Eduard Von dey Heydt took the place over. He was a great connoisseur of Oriental philosophy and art collector and he established  a large collection at Monte Verità, much of which is still there because when the Baron died in 1956 he left Monte Verità to the local Canton.

Given the bizarre collection of anarchists, naturists, theosophists (and even vegetarians) that used to live in Monte Verità, it is by no means out of keeping with the tradition that it should eventually play host to a conference of cosmologists and statisticians.

The  conference itself was interesting, and I was lucky enough to get to chair a session with three particularly interesting talks in it. In general, though, these dialogues between statisticians and physicists don’t seem to be as productive as one might have hoped. I’ve been to a few now, and although there’s a lot of enjoyable polemic they don’t work too well at changing anyone’s opinion or providing new insights.

We may now have mountains of new data in cosmology in particle physics but that hasn’t always translated into a corresponding mountain of truth. Intervening between our theories and observations lies the vexed question of how best to analyse the data and what the results actually mean. As always, lurking in the background, was the long-running conflict between adherents of the Bayesian and frequentist interpretations of probability. It appears that cosmologists -at least those represented at this meeting – tend to be Bayesian while particle physicists are almost exclusively frequentist. I’ll refrain from commenting on what this might mean. However, I was perplexed by various comments made during the conference about the issue of coverage. which is discussed rather nicely in some detail here. To me the question of of whether a Bayesian method has good frequentist coverage properties  is completely irrelevant. Bayesian methods ask different questions (actually, ones to which scientists want to know the answer) so it is not surprising that they give different answers. Measuring a Bayesian method according to  a frequentist criterion is completely pointless whichever camp you belong to.

The irrelevance of coverage was one thing that the previous residents knew better than some of the conference guests:

mvtanz3

I’d like to thank  Uros Seljak, Roberto Trotta and Martin Kunz for organizing the meeting in such a  picturesque and intriguing place.

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Astronomy or Astrophysics?

Posted in The Universe and Stuff with tags , , , , , on July 25, 2009 by telescoper

A chance encounter with the parent of a prospective student the other day led eventually to the question What’s the difference between Astronomy and Astrophysics? This is something I’m asked quite often so I thought I’d comment on here for those who might stumble across it. I teach a first-year course module entitled “Astrophysical Concepts”. One of the things I try to do in the first lecture is explain that difference. The Oxford English Dictionary gives the following primary definition for astronomy:

The science which treats of the constitution, relative positions, and motions of the heavenly bodies; that is, of all the bodies in the material universe outside of the earth, as well as of the earth itself in its relations to them.

Astrophysics, on the other hand, is described as

That branch of astronomy which treats of the physical or chemical properties of the celestial bodies.

So astrophysics is regarded as a subset of astronomy which is primarily concerned with understanding the properties of stars and galaxies, rather than just measuring their positions and motions. It is possible to assign a fairly precise date when astrophysics first came into use in English because, at least in the early years of the subject, it was almost exclusively associated with astronomical spectroscopy. Indeed the OED gives the following text as the first occurence of astrophysics, in 1869:

As a subject for the investigations of the astro-physicist, the examination of the luminous spectras of the heavenly bodies has proved a remarkably fruitful one

The scientific analysis of astronomical spectra began with a paper  William Hyde Wollaston in the Philosophical Transactions of the Royal Society Vol. 102, p. 378, 1802. He was the first person to notice the presence of dark bands in the optical spectrum of the Sun. These bands were subsequently analysed in great detail by Joseph von Fraunhofer in a paper published in 1814 and are now usually known as Fraunhofer lines.  Technical difficulties  made it impossible to obtain spectra of stars other than the Sun for a considerable time, but  William Huggins finally succeeded in 1864. A drawing of his pioneering spectroscope is shown below.

Meanwhile, fundamental work by Gustav Kirchoff and Robert Bunsen had been helping  to establish an understanding the spectra produced by hot gases.  The identification of features in the Sun’s spectrum  with similar lines produced in laboratory experiments led to a breakthrough in our understanding of the Universe whose importance shouldn’t be underestimated. The Sun and stars were inaccessible to direct experimental test during the 19th Century (as they are now). But spectroscopy now made it possible to gather evidence about their chemical composition as well as physical properties. Most importantly, spectroscopy provided definitive evidence that the Sun wasn’t made of some kind of exotic unknowable celestial material, but of the same kind of stuff (mainly Hydrogen) that could be studied on Earth.  This realization opened the possibility of applying the physical understanding gained from small-scale experiments to the largest scale phenomena that could be seen. The science of astrophysics was born. One of the leading journals in which professional astronomers and astrophysicists publish their research is called the Astrophysical Journal, which was founded in 1895 and is still going strong. The central importance of the (still) young field of spectroscopy can be appreciated from the subtitle given to the journal: Initially the branch of physics most important to astrophysics was atomic physics since the lines in optical spectra are produced by electrons jumping between different atomic energy levels. Spectroscopy of course remains a key weapon in the astrophysicist’s arsenal but nowadays the term is taken to mean any application of physical laws to astronomical objects. Over the years, astrophysics has gradually incorporated nuclear and particle physics as well as thermodynamics, relativity and just about every other branch of physics you can think of. I realise, however, that this  isn’t really the answer to the question that potential students want to ask. What they (probably) want to know is what is the difference between undergraduate courses called Astronomy and those called Astrophysics? The answer to this one depends very much on where you want to study. Generally speaking the differences are in fact quite minimal. You probably do a bit more theory in an Astrophysics course than an Astronomy course, for example. Your final-year project might have to be observational or instrumental if you do Astronomy, but might be theoretical in Astrophysics.  If you compare the complete list of modules to be taken, however, the difference will be very small.

Over the last twenty years or so, most Physics departments in the United Kingdom have acquired some form of research group in astronomy or astrophysics and have started to offer undergraduate degrees with some astronomical or astrophysical content. My only advice to prospective students wanting to find which course is for them is to look at the list of modules and projects likely to be offered. You’re unlikely to find the name of the course itself to be very helpful in making a choice. One of the things that drew me into astrophysics as a discipline (my current position is Professor of Theoretical Astrophysics) is that it involves such a wide range of techniques and applications, putting apparently esoteric things together in interesting ways to develop a theoretical understanding of a complicated phenomenon. I only had a very limited opportunity to study astrophysics during my first degree as I specialised in Theoretical Physics.  This wasn’t just a feature of Cambridge. The attitude in most Universities in those days was that you had to learn all the physics before applying it to astronomy. Over the years this has changed, and most departments offer some astronomy right from Year 1. I think this change has been for the better because I think the astronomical setting provides a very exciting context to learn physics. If you want to understand, say, the structure of the Sun you have to include atomic physics, nuclear physics, gravity, thermodynamics, radiative transfer and hydrostatics all at the same time. This sort of thing makes astrophysics a good subject for developing synthetic skills while more traditional physics teaching focusses almost exclusively on analytical skills. Indeed, my first-year Astrophysical Concepts course is really a course about modelling and problem-solving in physics.

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In a Galaxy, Faraday…

Posted in The Universe and Stuff with tags , , , , on July 21, 2009 by telescoper

I was finishing off the draft of a paper the other day and remembered a little paper I did some time ago with a former PhD student of mine, Patrick Dineen. I thought it would be fun to put the pictures up here because it was one of those occasions when a little idea turns out much nicer than you expected…

What we had to start with was a collection of Faraday Rotation measurements of extragalactic radio sources dotted around the sky. Their distribution is fairly uniform but I hasten to add that it was not a controlled sample so it would be not possible to take the sources as representative of anything for statistical purposes.

Faraday rotation occurs because left and right-handed polarizations of electromagnetic radiation travel at different speeds along a magnetic field line. The effect of this is for the polarization vector to be rotated as light waves travel and the net rotation angle (which can be either positive or negative) is related to the line integral of the component of the magnetic field along the line of sight travelled by the waves. The picture below shows the distribution of sources, plotted in Galactic coordinates and coded black for negative and white for positive.

rotation

Some radio galaxies have enormously large Faraday rotation measures because light reaches us through regions of the source that have strong magnetic fields. However, for most sources in our sample the rotation measures are smaller and are thought to be determined largely by the propagation of light not through the emitting galaxy, near the start of its journey towards us, but through our own Galaxy, the Milky Way, which is near the end of its path.

If this is true then the distribution of rotation measures across the sky should contain information about the magnetic field distribution inside our own Galaxy. Looking at the above picture doesn’t give much of a hint of what this structure might be, however.

What Patrick and I decided to do was to try to make a map of the rotation measure distribution across the sky based only on the information given at the positions where we had radio sources. This is like looking at the sky through a mask full of little holes at the source positions. Using a nifty (but actually rather simple) trick of decomposing into spherical harmonics and transforming to a new set of functions that are orthogonal on the masked sky we obtained the following map:

uni_16_rmjoint

(The technical details are in the paper, if you’re interested.) You probably think it looks a bit ropey but, as far as I’m concerned, this turned out stunningly well. The most obvious features are a big blue blob to the left and a big red blob to the right, both in the Galactic plane. What you’re seeing in those regions is almost certainly the local spur (sometimes called the Orion Spur; see below), which is a small piece of spiral arm in which the local Galactic magnetic field is confined. The blobs show the field coming towards the observer on one side and receding on the other. The structure seen is relatively local, i.e. within a kiloparsec or so of the observer.

I was very pleased to see this come out so clearly from an apparently unpromising data set, although we had to confine ourselves to large-scale features because of instabilities in the reconstruction of high-frequency components.

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