Archive for January, 2012

Academic Interactions

Posted in The Universe and Stuff with tags , on January 31, 2012 by telescoper

I’ve spent nearly all day getting my notes ready to start teaching Accidental Raunchy Slippers  Nuclear and Particle Physics tomorrow to the massed ranks of Third-Year Physics students here in the School of Physics & Astronomy at Cardiff University. I’ve drawn so many Feynman diagrams in the last couple of days that I’ve started to see them everywhere I look, even in entirely unexpected contexts, as in this example from the excellent PHD Comics

Out, Mad Colleague!

Posted in Uncategorized with tags , on January 30, 2012 by telescoper

In order to develop further the problem-solving skills of students in the School of Physics & Astronomy at Cardiff University, it has been decided to list the entries in the Spring Semester module catalogue in the form of anagrams.

For example, here is the list for third year students doing basic courses in physics SHY PICS







while those taking courses involving ROMAN TOYS also have



Students doing  SCUM I also get to do


and DECIMAL students have


Students also have to do their JET CROP, of course…

Oh, and  I forgot that 3rd year students can also take


I hope this clarifies the situation.

Take a stand against Elsevier

Posted in Open Access with tags , , , on January 29, 2012 by telescoper

My views about the academic journal racket are on record. Of all the profiteering outfits out there, the commercial publisher Elsevier  is one of the worst offenders, for the following reasons:

  1. They charge exorbitantly high prices for their journals.
  2. They sell journals in very large “bundles,” so libraries must buy a large set with many unwanted journals, or none at all. Elsevier thus makes huge profits by exploiting their essential titles, at the expense of other journals.
  3. They support measures such as SOPA, PIPA and the Research Works Act, that aim to restrict the free exchange of information.

I believe the business practices of Elsevier are detrimental to the open exchange of information on which scientific progress depends, so I have added my name to the list here of academics who refuse to publish in, referee for, or do editorial work on behalf of any Elsevier journal. If you wish to add your name to the list you can do so here.

A list of journals published by Elsevier can be found here.

I Don’t Like Chocolate

Posted in Uncategorized on January 29, 2012 by telescoper

I was beginning to think I’m the only person in the Universe who doesn’t like chocolate, so I’m grateful to this blogger for showing me I’m not alone!

Yesterday Cadbury were “promoting” their revolting “Creme Eggs” on Twitter. These are particularly vile: sickly sugar-soaked globules of a mixture of pus and mucus, encased in solidified baby poo. Eat one and puke.

I don’t like them, you see.

There's a Story Inside

I often wonder what chocolate tastes like to other people, because so many are so over the moon about it. My boss has a bowl of chocolate on his desk for public consumption, and people are constantly stopping by. Even if we’re in the office having a meeting, they’ll open the door and duck and grab, with a “Sorry, just needed chocolate.” It’s worse in the afternoon, and particularly on Wednesdays. It usually derails my train of thought, because I have to wonder why these people who would otherwise never be rude, in this case intrude just because they need chocolate. I look at the bowl and feel nothing.


It’s a burden sometimes, to be an anomaly. What, not like chocolate? If I had a dime for every time someone asked me why not, I could quit my job and never look at that chocolate bowl again. I have grown to…

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The H-index is Redundant…

Posted in Bad Statistics, Science Politics with tags , , , , , on January 28, 2012 by telescoper

An interesting paper appeared on the arXiv last week by astrophysicist Henk Spruit on the subject of bibliometric indicators, and specifically the Hirsch index (or H-index) which has been the subject of a number of previous blog posts on here. The author’s surname is pronounced “sprout”, by the way.

The H-index is defined to be the largest number H such that the author has written at least H papers having H citations. It can easily be calculated by looking up all papers by a given author on a database such as NASA/ADS, sorting them by (decreasing) number of citations, and working down the list to the point where the number of citations of a paper falls below the number representing position in the list. Normalized quantities – obtained by dividing the number of citations a paper receives by the number of authors of that paper for each paper – can be used to form an alternative measure.

Here is the abstract of the paper:

Here are a couple of graphs which back up the claim of a near-perfect correlation between H-index and total citations:

The figure shows both total citations (right) and normalized citations (left); the latter, in my view, a much more sensible measure of individual contributions. The basic problem of course is that people don’t get citations, papers do. Apportioning appropriate credit for a multi-author paper is therefore extremely difficult. Does each author of a 100-author paper that gets 100 citations really deserve the same credit as a single author of a paper that also gets 100 citations? Clearly not, yet that’s what happens if you count total citations.

The correlation between H index and the square root of total citation numbers has been remarked upon before, but it is good to see it confirmed for the particular field of astrophysics.

Although I’m a bit unclear as to how the “sample” was selected I think this paper is a valuable contribution to the discussion, and I hope it helps counter the growing, and in my opinion already excessive, reliance on the H-index by grants panels and the like. Trying to condense all the available information about an applicant into a single number is clearly a futile task, and this paper shows that using H-index and total numbers doesn’t add anything as they are both measuring exactly the same thing.

A very interesting question emerges from this, however, which is why the relationship between total citation numbers and h-index has the form it does: the latter is always roughly half of the square-root of the former. This suggests to me that there might be some sort of scaling law describing onto which the distribution of cites-per-paper can be mapped for any individual. It would be interesting to construct a mathematical model of citation behaviour that could reproduce this apparently universal property….

Gershwin, Adams & Rachmaninov

Posted in Music with tags , , , , , , , , on January 28, 2012 by telescoper

Yesterday (Friday) being the last day of (relative) freedom before teaching resumes on Monday I took the opportunity to go to a concert by the Orchestra of Welsh National Opera at the splendid St David’s Hall in Cardiff. I had been looking forward to it for some time, as the programme featured two favourite pieces of mine, George Gershwin’s Rhapsody in Blue and John Adams’ The Chairman Dances (A Foxtrot for Orchestra), plus one longer piece that I’ve never heard live before, Symphony No. 2 (in E minor) by Sergei Rachmaninov.

There was a good crowd in St David’s last night, not surprisingly given the popularity of the pieces being performed. Conductor for the evening was Frédéric Chaslin, who led the orchestra from the piano during the opening number, Rhapsody in Blue. This is a very famous piece, and is played so often that it is in danger of becoming a bit of a cliché, especially when classical orchestras try too hard to sound like jazz musician; the piece was originally written for Paul Whiteman’s Orchestra. A case in point is the opening clarinet solo, which is often played like a ham-fisted parody. Not last night, though. Principal clarinettist of WNO Leslie Craven gave a very characterful rendition of the notoriously tricky opening, which seemed to inspire the orchestra into an excellent all-round performance. I particularly enjoyed seeing the cello section slapping the strings of their instruments much as a jazz-era double-bass player would.

Chaslin gave an idiosyncratic account of the piano part, to the extent that in the final solo passage before the finale he departed from the script entirely and interpolated an improvised section all of his own. Not everyone in the audience approved – there were a few tuts behind me – but it’s a piece undoubtedly inspired by jazz, so I don’t see anything wrong with doing this. I thought his ad-libbing was charming, and very witty. What I wasn’t so happy about were the changes in tempo, which were too exaggerated. I suppose conducting from the piano means you can do whatever you want, but I think he took the rubato too far. Some sections rely on strict rhythm for their sense of urgency, and I felt he got bogged down a bit in places. Still, on balance, it was very refreshing to hear an orchestra trying to do something different. Nothing hackneyed about last night’s performance, that’s for sure.

Next one up was The Chairman Dances by John Adams. This isn’t actually in the opera Nixon in China, which is what a lot of people seem to think. It was composed at the same time, but cut out and developed as a standalone concert piece. I posted a recording of this yesterday, so won’t say too much today, except that I thoroughly enjoyed my first live experience of this work. So did the orchestra by the look of it! It’s a hugely entertaining piece and had many in the audience tapping their feet along with it. As a matter of fact, I wouldn’t have minded getting up and dancing along myself..

Special mention has to go the percussion section of the orchestra for doing such an excellent job. The four xylophones were  a delight to listen to, and the drums, temple blocks, triangles and assorted ironmongery coped brilliantly with the intricate polyrhythms.

Then it was the interval, and a glass of wine before returing to savour the main piece of the evening, Rachmaninov’s Second Symphony. It’s a remarkable work because it’s not only a “proper” symphony in its construction and development but also the best part of an hour of one glorious melody after another. Rachmaninov’s music is not really very much like Mozart, but they certainly had a similar ear for the Big Tune! I particularly loved the third movement (Adagio), but I thought it was a magnificent performance throughout, not least because you could see how much both conductor and orchestra were enjoying themselves.

The end of the concert was met with rapturous applause from the (normally rather reticent) St Davids audience. Now I have to find the best recording I can of Rachmaninov’s Second Symphony so I can enjoy it again. Any suggestions?

Foxtrot for Orchestra

Posted in Music with tags , on January 27, 2012 by telescoper

Sitting in the office at the end of a long week, and looking forward to going to an interesting-sounding concert at St David’s Hall later on. I may get the chance to review it over the weekend, but in the meantime I thought I’d put up this version of one of the pieces I’m going to hear later. I think it’s great but I’ve never heard it live…

Galaxies from the Past

Posted in The Universe and Stuff with tags , , , on January 27, 2012 by telescoper

If you were wondering where I got yesterday’s piece from, the answer is that I fired up my old laptop and found it among a lot of old papers there. And by “old laptop”, I mean really old laptop: I bought it in 1995! Anyway, since I haven’t got time to write anything today here is another piece I wrote a long time ago but have only recently unearthed. This one is about Galaxies. It’s a lot longer than yesterday’s effort, but like that one I can’t remember what it was for. Still, some of you might find it interesting. The piece ends with a reference to galaxies observed as they were in the distant past, rather like the article itself!


A galaxy is a collection of stars, held together by their mutual gravitational attraction and orbiting around their common centre.  Galaxies range in size from dwarf systems of perhaps a few million stars, to giants containing up to a thousand billion. The Sun and all the stars visible in the night sky to the naked eye belong to one such galaxy, our own Galaxy the Milky Way. Although principally recognized through the light given off by their component stars, galaxies also contain other material such as clouds of gas and dust, and significant quantities of dark matter whose nature is not yet understood.

Only stars inside our Galaxy can be resolved with the naked eye; these stars have been studied and catalogued since antiquity. Ancient astronomers  also knew of the existence of a diffuse band of light crossing the sky they could not resolve into individual stars; we now call this the Milky Way. The word galaxy is derived from the Greek galaktos, meaning “milk”. The existence of galaxies other than our own is a much more recent discovery. While even relatively nearby stars appear as point sources of light, the light from other galaxies appears as cloudy and diffuse much like small fragments of the Milky Way. The generic term for a such sources is nebula, the latin word for “mist”.

A Persian astronomer, al-Sufi, in the 10th century AD described such a faint patch of light in the constellation Andromeda which is now known to be another galaxy, but it was only in the 18th Century that a systematic catalogue of  nebulae was compiled, by the French astronomer Charles Messier. Not all the objects he found were other galaxies – some were clouds of dust and gas inside our own – but the Messier catalogue contained 32 objects that we now know to be galaxies, including al-Sufi’s object, which was number 31 in his list. The Andromeda nebula is known to this day as M31. With the increasing power of astronomical telescopes, the list of known nebulae grew to thousands even before the use of astronomical photography became widespread. William and Caroline Herschel and, later, their son John played a leading role in identifying and cataloguing such objects in the early 19th century.

While the existence of large numbers of these nebulae was well established by the start of the 20th Century, their nature remained controversial. Since their distances could not be directly measured, it was possible that they could be inside our own galaxy. Many astronomers believed that the spiral structure seen in some of them, for example M31, suggested that they represented the formative stages of planetary systems like our own Solar System inside the Milky Way. Others argued that the nebulae were very much more distant than that, and were “island universes” on a much larger scale. This debate was only resolved in the 1920s, when Edwin Hubble was able to measure the distances to some nebulae using variable stars called Cepheids. He found them to be far too distant to be inside the Milky Way. This discovery established galaxies as the basic building-blocks of the Universe and gave rise to the field of extragalactic astronomy. Astronomers now estimate that there are as many galaxies in our observable Universe as there are individual stars in our own galaxy, i.e. around a hundred billion.

Galaxies come in a rich variety of shapes and sizes, but there are three basic types: Galaxies come in three basic types: spiral (or disk), elliptical and irregular. Hubble proposed a morphological classification, or taxonomy, for galaxies in which he envisaged the three basic types (spiral, elliptical and irregular) as forming a sequence which in the past was often assumed to represent various evolutionary stages of a galaxy . Although it is now not thought the interpretation as an evolutionary sequence is correct, Hubble’s nomenclature is still commonly used.

Spiral galaxies account for more than half the galaxies observed  in our neighbourhood.  These contain a bright central nucleus surrounded by a flattened disk that sometimes contains beautiful spiral arms. Hubble divided these galaxies into classes labelled as normal (S) or barred (SB) depending on whether the prominent spiral arms emerge directly the nucleus, or originate at the ends of a luminous bar projecting symmetrically through it . Spirals often contain copious amounts of dust, and the spiral arms containing many young stars givin them a noticeably blue colour.  The normal and barred spirals S and SB are further subdivided into a, b or c depending on how tightly the spiral arms are wound up.

The elliptical galaxies (E), which account for only around 10% of observed bright galaxies, are elliptical in shape and have no discernible spiral structure. They are usually red in colour, have very little dust and show no signs of active star formation. The further classification of elliptical galaxies into En depends on the degree of elongation of the galaxy: E0 is nearly spherical; E7 is cigar-shaped. Ellipticals tend to occur in regions of space where there are many other galaxies, giving rise to the idea that they might originally have been spiral galaxies but have lost their spiral structure through mergers or interactions with other galaxies.

The shapes and colours of elliptical galaxies resemble the corresponding properties of spiral nuclei. Elliptical galaxies cover a broad range in mass, from a few hundred thousand to a thousand billion times the mass of the Sun. Spiral galaxies seem to have a smaller spread in mass, typically weighing in at about a hundred billion times the mass of the Sun.

Lenticular, or S0 galaxies, were added later by Hubble to bridge the gap between normal spirals and ellipticals. Around 20% of galaxies we see have this morphology. They are more elongated than elliptical galaxies but have neither bars nor spiral structure.

Irregular galaxies have no apparent structure. They are relatively rare, and are often faint and small so are consequently very hard to see. Their irregularity may stem from the fact that they are have such small masses that the material within them is relatively loosely bound and may have been disturbed by the environment in which they sit.

The classification of galaxies proposed by Hubble applies to “normal” galaxies whose light output is dominated by radiation their constituent population of stars. Stars predominantly emit visible light, which occupies a relatively narrow part of the spectrum of electromagnetic radiation. Spiral galaxies also contain dust which is heated by starlight and radiates in the infra-red. Active galaxies are characterized by the prodigious amounts of energy they emit in regions of the spectrum normal galaxies cannot reach, particularly in radio and X-rays. Much of the energy broadcast by active galaxies is associated with the relatively small nucleus of the galaxy, so the term Active Galactic Nuclei (AGN) is often used to describe these regions. Sometimes the central nucleus is accompanied by a jet of material being ejected at high velocity into the surrounding intergalactic medium. The different types of active galaxy include Seyfert galaxies, radio galaxies, BL Lac objects, and quasars.

Seyfert galaxies are usually spiral galaxies with no radio emission and no evidence of jets. They do, however, emit radiation over a continuous range of frequencies from infra-red to X-rays. Splitting their optical light up into its characteristic spectrum reveals the presence of strong and variable emission lines.  One can see such lines in ordinary stellar spectra and consequently in the spectra of normal galaxies, but they are much more prominent in active galaxies. Radio galaxies, on the other hand, are more commonly elliptical galaxies. These objects are extremely dramatic in their appearance, frequently having two lobes of  radio-emitting material extending far away from the central compact nucleus. There is also sometimes the appearance of a jet of material, extending from the core into the radio lobes. It appears that material is ejected from the nucleus along the jet, eventually being slowed  down by its interaction with the intergalactic medium and forming the radio lobes. The central parts of radio galaxies seem to have properties similar to those of Seyfert galaxies.

BL Lac objects have spectra with no emission lines, but they emit strongly in all wavebands from radio to X-ray frequencies. Their main characteristic, however, is their extremely strong and rapid variability. It is thought that a possible explanation for these objects is that the observer is seeing a jet of material travelling head-on at close to the velocity of light.

The first quasars to be found were detected by their strong radio emission, but they were found to be so small that, like stars but unlike other galaxies, they could not be resolved with optical telescopes. For this reason they became known as quasi-stellar radio sources, or quasars for short. Later on, other such objects were found which did not emit radio waves at all, so the name was changed to quasi-stellar object or QSO, but the name quasar has in any case stuck. It seems that only one in about two hundred quasars is actually radio-loud, but the quasars are still the most powerful of all the active galaxy types.

These different kinds of objects were discovered at different times by different people and were originally thought to be entirely different phenomena. Now, however, there is a unified model in which these structures are all interpreted as having basically similar structure but a different orientation to the observer’s line-of-sight. The engine which powers the activity is thought to be a supermassive black hole, with a mass up to about 100 million solar masses. This seems very large, but is actually just a small fraction of the mass of the host galaxy, which may be a thousand times larger. Material surrounding the black hole is attracted towards it and undergoes a process of accretion, gradually spiralling in and being swallowed. As it spirals in, it forms an accretion disk around the black hole. This disk can be very hot, producing the X-ray radiation frequently seen in AGN, but its presence prevents radiation being transmitted through it. Radiation tends therefore to be beamed out of the poles of the nucleus and does not appear from the equatorial regions which are obscured by the disk. When the beamed radiation interacts with material inside the host galaxy or in the surrounding medium, it forms jets or radio lobes. Depending on the thickness of the disk, the size of the `host’ galaxy ,the amount of gas and dust surrounding the nucleus and the orientation at which the whole system is viewed one can, at least qualitatively, account for the variety of properties listed above.

It is not known what fraction of normal galaxies undergoes activity at some stage in their careers. Although active galaxies are relatively uncommon in our neighbourhood, this may simply be because the active phase lasts for a very short time compared to the total life of a galaxy. For example, if activity only lasts only one-thousandth of the total lifetime, we would expect only to see one in a thousand galaxies at any one time displaying the symptoms. It is perfectly possible, therefore, that the kind of extreme activity displayed by these galaxies is merely a phase through which all galaxies pass. If so, this would suggest that all galaxies should possess a massive black hole at their centre, which is no longer powering an accretion disk because there is insufficient gas left in the surrounding regions. Recent studies using the ultra-high resolution available on the Hubble Space Telescope suggest that most normal galaxies may indeed have black holes in their centres.

A somewhat milder form of activity is displayed by starburst galaxies which, as their name suggests are galaxies undergoing a vigorous period of star formation. Such activity is not thought to involve an active galactic nucleus, but is probably triggered by a tidal interaction between two galaxies moving closely past each other.

The stars in a galaxy exert gravitational forces on each other. This not only holds the galaxy together, it also causes the stars to move. The internal dynamical properties of galaxies are extremely important because they allow astronomers to work out how much matter is there.

In spiral galaxies, the component stars orbit roughly in a plane about the central nucleus. It is this bulk rotation that is responsible for the flattened shape of these systems. Much the same state of affairs applies in the Solar System, with all the planets moving in roughly circular orbits about the Sun. In the case of a disk galaxy that lies edge-on to the observer, stars on one side will be approaching while those on the other will be receding. These motions cause a Doppler shift in the light from different parts of the disk: one side will have a spectrum that is shifted towards blue colours, while the other side will be shifted to the red. One can therefore use spectroscopic methods to plot a graph showing how the rotation speed of material  varies with distance from the centre of rotation. Such a curve is called a rotation curve.  These curves show that the matter in spiral galaxies has a roughly constant velocity out to tens of thousands of light years from the centre. This is surprising because the planets of the Solar System have orbital speeds that fall off quite rapidly with distance from the Sun. Most of the mass of the Solar System lies in the Sun, which is near the centre of motion. Most of the light produced in a galaxy is likewise produced in the central regions. If all the mass in a galaxy were where the stars are, i.e. in the middle, the rotation speed should fall off the further out from the centre one looked. The simplest interpretation of this strange behaviour is that galaxies contain a large amount of material that does not produce starlight and which is not as concentrated in the centre of the galaxy as the stars. To make this work requires galaxies to be embedded in a diffuse halo of dark matter that is about ten times as large as the luminous part of the disk and containing perhaps ten times as much matter.

Dynamical studies of elliptical galaxies are more complicated because the stellar motions within them are not those of simple rotation. Nevertheless, these objects too reveal evidence for dark matter in similar quantity to that in spiral galaxies.

It is thought that less than 10 per cent of the total mass of a galaxy is in visible stars, but the form of the mysterious dark matter is not at all understood. The best candidate at the moment is some form of exotic particle left over from the Big Bang, usually called a WIMP (Weakly Interacting Massive Particle), although no such particle has yet been directly detected.

Galaxies are the basic building blocks of the Universe. They are not, however, the largest structures one can see. They tend not to be isolated, but cluster together. The distribution of nebulae on the sky was thought to be non-uniform even in the days of the Herschels, but it is only in the 20th century that it has become possible to map their three-dimensional positions in a systematic fashion.

The technique used to explore the large-scale distribution of galaxies is based on the discovery of the expanding universe usually attributed to Edwin Hubble, who built  on earlier work by Vesto Slipher. Slipher had discovered that lines in the optical spectra of galaxies were systematically shifted towards the longer wavelength, red end of the electromagnetic spectrum. Hubble extended this study by looking at these redshifts in tandem with the distances he had estimated for the galaxies. He found, to his surprise, that the redshift of a galaxy came out to be proportional to its distance. Contrary to popular belief, Hubble never really interpreted this himself as the result of cosmic expansion but the empirical correlation between redshift and distance now known as Hubble’s Law is the cornerstone of the big-bang cosmology. It is now accepted that the redshift of the galaxies arises from their motion away from the observer, similar to the Doppler shift that causes a change of pitch in a receding police siren. While the accurate determination of extragalactic distances remains difficult, measuring redshifts is rather straightforward. Hubble’s law has been used to chart the pattern traced out by millions of individual galaxies from their spectral shifts.

The general term used to describe a physical  aggregation of many galaxies is a cluster of galaxies, or galaxy cluster. Clusters can be systems of greatly varying size and richness. Our galaxy, the Milky Way,  is a member of the Local Group of galaxies which is a rather small cluster of galaxies of which the only other large member is the Andromeda galaxy (M31). On the other extreme, there are the so-called rich clusters of galaxies, also known as Abell clusters, which contain many hundreds or even thousands of galaxies in a region just few million light years across: prominent nearby examples of such entities are the Virgo and Coma clusters. In between these two extremes, galaxies appear to be distributed in systems of varying density.

Individual galaxy clusters are not the largest structures in the Universe. The distribution of galaxies on scales larger than around 30 million light years also reveals a wealth of complexity. Galaxies are not simply distributed in blobs, like the Abell clusters, but often lie in extended linear structures called filaments, such as the Perseus-Pisces chain, or flattened sheet-like structures like the Great Wall. The latter object is roughly two-dimensional concentration of galaxies, discovered in 1988 by astronomers from the Harvard-Smithsonian Center for Astrophysics. This structure is at least 200 million light years by 600 million light years in size, but is less than 20 million light years thick. It contains many thousands of galaxies and has a mass of at least 1016 solar masses.  The interconnecting network of filaments and sheets is aptly called the “cosmic web”, with rich clusters appearing where the parts of the web join together.

Rich clusters are clustered into enormous loosely-bound agglomerations called superclusters, containing anything from around ten rich clusters to more than 50. The most prominent known supercluster is called the Shapley concentration, while the most nearby is the Local Supercluster, a flattened structure in the plane of which the Local Group is moving. Superclustering is known to exist on scales up to 300 million light years, and superclusters may contain as much as 1017 solar masses of material or more.

These overdense structures are complemented by vast underdense regions known as voids, many of which appear to be roughly spherical.  These regions containing very many fewer galaxies than average, or even no galaxies at all. Voids with density less than 10% of the average density on scales of up to 200 million light years have been found in large-scale redshift surveys.

The existence of galaxies, clusters of galaxies and the overall complexity of large-scale structure in the Universe around us must be contrasted with the extreme simplicity of the very early Universe. Observations of the cosmic microwave background, relic radiation left over from the early stages of the Big Bang, suggest that the initial state of the Universe was almost featureless, with variations in density from place to place of less than one part in a hundred thousand.

The process that is thought to have transformed these smooth beginnings into the clumpiness we see today is called gravitational instability. If the universe were initially exactly smooth, it would have remained so as it expanded and cooled. But if there were small initial variations in density, these would become amplified. A small patch of the Universe that was more dense than average would exert a slightly greater gravitational pull on its surroundings than an average patch. This would cause material to flow in, making it even denser. This, in turn, would make it pull even more than average. This starts a runaway process by which small initial ripples can turn into dense clumps.

This basic idea has been around since it was first suggested by Sir James Jeans more than a hundred years ago, but it is only in the last ten years or so that a convincing picture has been put together explaining how it works in the expanding Universe. According the modern theories, most of the matter in the Universe is in the form of exotic particles left over from the primordial fireball phase that was the Big Bang. These particles are thought to be very slow-moving and are consequently called Cold Dark Matter (CDM). These particles cluster together via the process of gravitational instability, first forming small objects with the mass of a very small dwarf galaxy (around one hundred thousand solar masses). These small seed objects then progressively merge into larger objects in a hierarchical fashion, eventually forming galaxy-sized and cluster-sized dark matter clumps. These form gravitational wells into which gaseous matter falls and becomes trapped. Stars  form as gas clouds cool and fragment in the dark matter clumps. All this happens within a continuous sequence of interaction, disruption and merging. The whole process is extremely complicated, but extensive computer simulations show that the structure produced is very similar to the cosmic web revealed by observations, at least in the essential details.

Further support for these theoretical ideas is provided by observations of galaxies so distant that it has taken their light a large fraction of the age of the Universe to reach us. Looking at such objects allows astronomers to see galaxies in the process of formation.

A Potted Prehistory of Cosmology

Posted in History, The Universe and Stuff with tags , , , , , , , , , , , , , , , , , , , , , on January 26, 2012 by telescoper

A few years ago I was asked to provide a short description of the history of cosmology, from the dawn of civilisation up to the establishment of the Big Bang model, in less than 1200 words. This is what I came up with. Who and what have I left out that you would have included?


 Is the Universe infinite? What is it made of? Has it been around forever?  Will it all come to an end? Since prehistoric times, humans have sought to build some kind of conceptual framework for answering questions such as these. The first such theories were myths. But however naïve or meaningless they may seem to us now, these speculations demonstrate the importance that we as a species have always attached to thinking about life, the Universe and everything.

Cosmology began to emerge as a recognisable scientific discipline with the Greeks, notably Thales (625-547 BC) and Anaximander (610-540 BC). The word itself is derived from the Greek “cosmos”, meaning the world as an ordered system or whole. In Greek, the opposite of “cosmos” is “chaos”. The Pythagoreans of the 6th century BC regarded numbers and geometry as the basis of all natural things. The advent of mathematical reasoning, and the idea that one can learn about the physical world using logic and reason marked the beginning of the scientific era. Plato (427-348 BC) expounded a complete account of the creation of the Universe, in which a divine Demiurge creates, in the physical world, imperfect representations of the structures of pure being that exist only in the world of ideas. The physical world is subject to change, whereas the world of ideas is eternal and immutable. Aristotle (384-322 BC), a pupil of Plato, built on these ideas to present a picture of the world in which the distant stars and planets execute perfect circular motions, circles being a manifestation of “divine” geometry. Aristotle’s Universe is a sphere centred on the Earth. The part of this sphere that extends as far as the Moon is the domain of change, the imperfect reality of Plato, but beyond this the heavenly bodies execute their idealised circular motions. This view of the Universe was to dominate western European thought throughout the Middle Ages, but its perfect circular motions did not match the growing quantities of astronomical data being gathered by the Greeks from the astronomical archives made by the Babylonians and Egyptians. Although Aristotle had emphasised the possibility of learning about the Universe by observation as well as pure thought, it was not until Ptolemy’s Almagest, compiled in the 2nd Century AD, that a complete mathematical model for the Universe was assembled that agreed with all the data available.

Much of the knowledge acquired by the Greeks was lost to Christian culture during the dark ages, but it survived in the Islamic world. As a result, cosmological thinking during the Middle Ages of Europe was rather backward. Thomas Aquinas (1225-74) seized on Aristotle’s ideas, which were available in Latin translation at the time while the Almagest was not, to forge a synthesis of pagan cosmology with Christian theology which was to dominated Western thought until the 16th and 17th centuries.

The dismantling of the Aristotelian world view is usually credited to Nicolaus Copernicus (1473-1543).  Ptolemy’s Almagest  was a complete theory, but it involved applying a different mathematical formula for the motion of each planet and therefore did not really represent an overall unifying system. In a sense, it described the phenomena of heavenly motion but did not explain them. Copernicus wanted to derive a single universal theory that treated everything on the same footing. He achieved this only partially, but did succeed in displacing the Earth from the centre of the scheme of things. It was not until Johannes Kepler (1571-1630) that a completely successful demolition of the Aristotelian system was achieved. Driven by the need to explain the highly accurate observations of planetary motion made by Tycho Brahe (1546-1601), Kepler replaced Aristotle’s divine circular orbits with more mundane ellipses.

The next great development on the road to modern cosmological thinking was the arrival on the scene of Isaac Newton (1642-1727). Newton was able to show, in his monumental Principia (1687), that the elliptical motions devised by Kepler were the natural outcome of a universal law of gravitation. Newton therefore re-established a kind of Platonic level on reality, the idealised world of universal laws of motion. The Universe, in Newton’s picture, behaves as a giant machine, enacting the regular motions demanded by the divine Creator and both time and space are absolute manifestations of an internal and omnipresent God.

Newton’s ideas dominated scientific thinking until the beginning of the 20th century, but by the 19th century the cosmic machine had developed imperfections. The mechanistic world-view had emerged alongside the first stirrings of technology. During the subsequent Industrial Revolution scientists had become preoccupied with the theory of engines and heat. These laws of thermodynamics had shown that no engine could work perfectly forever without running down. In this time there arose a widespread belief in the “Heat Death of the Universe”, the idea that the cosmos as a whole would eventually fizzle out just as a bouncing ball gradually dissipates its energy and comes to rest.

Another spanner was thrown into the works of Newton’s cosmic engine by Heinrich Olbers (1758-1840), who formulated in 1826 a paradox that still bears his name, although it was discussed by many before him, including Kepler. Olbers’ Paradox emerges from considering why the night sky is dark. In an infinite and unchanging Universe, every line of sight from an observer should hit a star, in much the same way as a line of sight through an infinite forest will eventually hit a tree. The consequence of this is that the night sky should be as bright as a typical star. The observed darkness at night is sufficient to prove the Universe cannot both infinite and eternal.

Whether the Universe is infinite or not, the part of it accessible to rational explanation has steadily increased. For Aristotle, the Moon’s orbit (a mere 400,000 km) marked a fundamental barrier, to Copernicus and Kepler the limit was the edge of the Solar System (billions of kilometres away). In the 18th and 19th centuries, it was being suggested that the Milky Way (a structure now known to be at least a billion times larger than the Solar System) to be was the entire Universe. Now it is known, thanks largely to Edwin Hubble (1889-1953), that the Milky Way is only one among hundreds of billions of similar galaxies.

The modern era of cosmology began in the early years of the 20th century, with a complete re-write of the laws of Nature. Albert Einstein (1879-1955) introduced the principle of relativity in 1905 and thus demolished Newton’s conception of space and time. Later, his general theory of relativity, also supplanted Newton’s law of universal gravitation. The first great works on relativistic cosmology by Alexander Friedmann (1888-1925), George Lemaître (1894-1966) and Wilhem de Sitter (1872-1934) formulated a new and complex language for the mathematical description of the Universe.

But while these conceptual developments paved the way, the final steps towards the modern era were taken by observers, not theorists. In 1929, Edwin Hubble, who had only recently shown that the Universe contained many galaxies like the Milky way, published the observations that led to the realisation that our Universe is expanding. That left the field open for two rival theories, one (“The Steady State”, with no beginning and no end)  in which matter is continuously created to fill in the gaps caused by the cosmic expansion and the other in which the whole shebang was created, in one go, in a primordial fireball we now call the Big Bang.

Eventually, in 1965, Arno Penzias and Robert  Wilson discovered the cosmic microwave background radiation, proof (or as near to proof as you’re likely to see) that our Universe began in a  Big Bang…

Sonnet No. 60

Posted in Poetry with tags , , on January 26, 2012 by telescoper

Like as the waves make towards the pebbled shore,
So do our minutes hasten to their end;
Each changing place with that which goes before,
In sequent toil all forwards do contend.
Nativity, once in the main of light,
Crawls to maturity, wherewith being crown’d,
Crooked eclipses ‘gainst his glory fight,
And Time that gave doth now his gift confound.
Time doth transfix the flourish set on youth
And delves the parallels in beauty’s brow,
Feeds on the rarities of nature’s truth,
And nothing stands but for his scythe to mow:
And yet to times in hope my verse shall stand,
Praising thy worth, despite his cruel hand.

Sonnet No. 60, by William Shakespeare (1564-1616)