Tag Archives: Cosmology (general)

Back at Cambridge

This week I’m back in Cambridge University, attending  a cosmology conference at  DAMTP, the famous Department of Applied Mathematics and Theoretical Physics. I’m delighted to be back – Cambridge is only a short hop from Dublin and it is such a great place to visit, with its beautiful colleges, bijou shops and lively student life. I arrived late in the afternoon, and walked to the town centre in a light rain; tourists everywhere were complaining about the English weather but I thought the rain and the falling light set the scene perfectly as I walked along past the ancient colleges.


St John’s College in the rain this evening

This time around I’m staying in Clare College, one of the oldest colleges in the university. Its beautiful front quad is just off Kings’ parade to the front, while the back of the college straddles the River Cam all the way back to the University Library. The rooms are lovely (no tv – wouldn’t have it otherwise). In fact, working at my little desk and watching the rain across the quad makes me feel quite nostalgic, like a student again – perhaps in another universe there is a younger me starting out in this fabulous university .


Clare College in the daytime

The conference, Infinities and Cosmology,  is not on theoretical or experimental cosmology, but on the philosophy of cosmology. It forms part of a new Oxford-Cambridge initiative  aimed at bringing physicists and philosophers together in order to improve our understanding  of the universe and its origins, from exploring the meaning of the initial singularity to the philosophical implications of theories such as cosmic inflation and the multiverse. This particular conference was organised by John Barrow , Jeremy Butterfield and David Sloan, names that carry a lot of weight in the intersection of physics and philosophy, and visiting speakers include other heavy hitters such as Anthony  Aguirre, Mihalis Dafermos, George  Ellis and Simon  Saunders. You can see the conference program here.

That said, mixing philosophy with physics is not an approach that meets with universal approval – Stephen Hawking once declared that  ‘philosophy is dead’, while Laurence Krauss has also been pretty scathing about the contribution of philosophers to physics.  Both are physicists I hugely respect, but I think this initiative is more about making physicists aware of their deepest assumptions than about  converting philosophers into cosmologists.  Also, those of us with an interest in the history of cosmology notice that scientific progress has often been hindered by unexamined philosophies – from Aristotle’s geocentric model of the solar system to Harlow Shapley’s faith in a single-galaxy model, from Einstein’s assumption of a static universe to the steady-state universe of Hoyle, Bondi and Gold. More recently, I have long suspected that some of the resistance to inflationary models arises from a simple dislike of the exceedingly large numbers involved – an objection that is understandable, but not really tenable from a philosophical point of view.

So I’m not expecting that philosophers will suddenly shine light on well-known problems in big bang physics – it’s more that we physicists can profit by examining the philosophical assumptions we operate under. In general,  scientists  are pretty good at being aware of underlying scientific assumptions, but sometimes a general philosophical viewpoint is often overlooked precisely because it is so widespread. Another  advantage is that philosophy gives us a useful language in which to articulate underlying assumptions.

To give one example, consider the following. The  ‘big bang ‘ model predicts a universe that was once in a hot, tiny, dense state,  expanding and cooling ever since. There is a great deal of evidence to support this model, but it runs into mathematical difficulties as time zero is approached (part of the problem is that we do not have a theory to describe gravity on the smallest or ‘quantum’ scales).  These are technical problems that every cosmologist battles with, but they might one day be resolved, leaving us with a consistent theory of a universe with a definite beginning. In that case, questions that few physicists ever consider become very important:

–          In a universe with a definite beginning, when did the laws of physics becomes the laws of physics?  Were they somehow ‘born’ with the universe, or did they come into being at a later stage. In other words are they emergent, rather than fundamental? If so, what entity or entities did they emerge from?

–          Could it be that space and time themselves are not fundamental but also emergent? In other words, is it possible that space and time were not born with the universe, but are made up of something more fundamental than either? (One clue here is Einstein’s discovery that space and time are not absolute but affected by motion and by gravity).  Could it be that they are non-fundamental as well as non-static?

–          If so, doesn’t this create problems of causality in the case of time?

This is just a flavour of the sort of questions one encounters in the philosophy of cosmology.  Right now, I’d better turn in so I’m wide awake for  tomorrow. In the first lecture, George Ellis, one of the world’s leading theoretical cosmologists, will give a talk ‘Infinites of age and size, including issues in global topology’ .  I suspect I’ll need my wits about me….


Filed under Cosmology (general), Travel

Cosmic fingerprints at Trinity College Dublin

I was back in my alma mater Trinity College Dublin on Monday evening in order to catch a superb public lecture, ‘ Fingerprinting the Universe’ , by Andrew Liddle, Professor of Astrophysics at the University of Edinburgh. The talk was presented by Astronomy Ireland, Ireland’s largest astronomy club and there was a capacity audience (despite the threat of snow) in the famous Schrödinger lecture theatre in the Fitzgerald Building, Trinity’s physics department.


Professor Liddle was introduced by David Moore, Chairman of Astronomy Ireland, who also presented an update of the club’s recent activities  (David and I participated in a discussion of the life and science of Sir Isaac Newton on NEWSTALK radio station the evening before, you can hear a podcast of the show here). Anyone with an interest in cosmology will be familiar with Andrew Liddle’s seminal textbook ‘ An Introduction to Modern Cosmology’, (not to mention several other books) and the ensuing lecture certainly didn’t disappoint.


Starting with a tribute to the work of both Schrödinger and Fitzgerald, Andrew gave a brief outline of today’s cosmology, showing how it has moved from a rather speculative subject to a mature field of study. He attributed this progress to key advances in three main areas: precision observations by satellite, sophisticated theoretical models and high performance computing for both analysis and simulation.

He then described five specific challenges that any successful model of the cosmos must address –  the expanding universe;  the formation of structure (galaxies etc);  the age of the universe; the composition of the universe (baryonic matter, radiation, neutrinos, dark matter and dark energy);  a consistent description of the very early universe (cosmic inflation or alternatives).

As ever, many in the audience were surprised to hear that, while dark energy is estimated to make up about 73% of the mass-energy content of the universe, we have very little idea of the nature of this phenomenon!

In the second part of the lecture, Andrew focused on the cosmic microwave background (CMB), explaining how the study of this ‘fossil radiation’  gives precious information on the early universe,  and in particular describing how tiny non-uniformities (or anisotropies) imprinted on the radiation formed the seeds of today’s galaxies (‘cosmic finger-printing’). There followed a swift description of results of CMB studies by the COBE and WMAP satellite missions, with a reminder that more recent measurements by the European Space Agency’s   PLANCK Satellite Observatory  will be announced next week. He also reminded us how, amongst other triumphs, the theory of inflation gives a very satisfactory explanation for the origin of the variations in the background radiation terms of quantum fluctuations in the very early universe. This link between inflation and galaxy formation is often under-stated in the popular literature; in answer to a query from me question time, Andrew confirmed that non-inflationary explanations for the origins of the observed variations in the microwave background have not been very successful. It’s pretty impressive that inflation can give an explanation for the origin of structure, given that this was not part of the original motivation for the theory.

ESA's Planck mission

The ESA’s PLANCK Satellite will report new measurements of the cosmic microwave background on March 21st this month

All in all, a fantastic talk, well worth the trip; afterwards, we all repaired to a nearby pub for sandwiches and further discussion of the universe over hot ports and Guinness…

P.S. In his discussion of the discovery of the expanding universe, I was pleased to see Professor Liddle refer to the work of Vesto Slipher; it seems that recent historical work on the important contribution of Slipher is finding its way into the mainstream community.


Filed under Cosmology (general), History and philosophy of science

VM Slipher and the expanding universe

In an earlier post, I mentioned an upcoming  conference in Arizona to celebrate the pioneering work of the American astronomer Vesto Slipher. As mentioned previously, 2012 marks the centenary of Slipher’s observation that light from the Andromeda nebula was Doppler shifted, a finding he interpreted as evidence of a radial velocity for the nebula. By 1917, he had established that the light from many of the distant nebulae is redshifted, i.e. shifted to lower frequency than normal. This was the first  indication that the most distant objects in the sky are moving away at significant speed, and it was an important step on the way to the discovery of the expanding universe.

Vesto Melvin Slipher (1875-1969)

The conference turned out to be very informative and enjoyable, with lots of interesting presentations from astronomers, historians and science writers. It’s hard to pick out particular talks from such a great lineup, but three highlights for me were Einstein, Eddington and the 1919 Eclipse Expedition by Peter Coles, Georges Lemaitre: A Personal Profile by John Farrell and Slipher’s redshifts as support for de Sitter’s universe? by Harry Nussbaumer. The latter compared the importance of the contributions of Slipher, Hubble, Einstein, De Sitter, Friedmann and Lemaitre (to mention but a few) and was a focal point for the conference. My own talk ‘Who discovered the expanding universe? – an open bus tour’ was quite similar to Harry’s , with some philosophy of science thrown in, while Micheal Way’s talk Dismantling Hubble’s Legacy? also touched on similar ground.  However, there was little danger of overlap since viewpoints and conclusions drawn from the material varied quite widely! You can see the conference program here.

A slide from Peter Cole’s talk on the Eddington eclipse experiment

A slide from John Farrell’s talk showing a postcard from Lemaitre to Slipher, announcing the former’s visit to the Lowell observatory

Harr Nussbaumer, author of ‘The Discovery of the Expanding Universe’,  in action

Front slide of my own presentation

The best aspect of the conference was the question and answer session after each talk. There was quite a divergence of opinion amongst the delegates concerning the relative importance of the various scientists in the story, which made for great discussions (though I suspect that much of the argument arises from differing views concerning the role of the theoretician vs the role of the experimentalist). You can see a list of speakers and abstracts for the talks here and the slides for my own talk are here.

There was plenty of material here for the relativist; indeed, quite a bit of discussion concerned the relative contributions of Friedmann and Lemaitre (told you it was a good conference). In particular, the Israeli mathematician Ari Belenkiy gave a defence of Friedmann’s work in his talk Alexander Friedmann and the Origin of Modern Cosmology, pointing out that the common assertion that Friedmann took no interest in practical matters is simply untrue, given his work in meteorology, and that the relevant astronomical data was not widely available to Europeans at the time. I must admit I share Ari’s view to some extent; I’m always somewhat in awe of a theoretician who describes all possible solutions to a problem (in this case the universe), as opposed to one solution that seems to chime with experiments of the day.

Title slide of Ari’s talk on Friedmann

The conference also included a trip to the Lowell observatory, including a view of the spectrograph used by Slipher for his groundbreaking measurements and a peep through the famous 24-inch Clark telescope which remains in operation to this day. We were also treated to a few scenes from Dava Sobel’s upcoming play based on her book on Copernicus, read by Dava herself and the eminent Harvard science historian Owen Gingerich.

The famous spectograph, perfectly preserved

Slipher’s telescope remains in use today

Dava Sobel and Professor Owen Gingerich reading from her new play at the Lowell observatory

All in all, a superb conference, definitely worth the long trip (Dublin-Chicago-Phoenix-Flagstaff). Earlier in the week, I gave a longer version of my talk at the BEYOND centre at Arizona State University in Phoenix; I was afraid some of the theoreticians in Larry Krauss’s  group might find it a bit equation-free, but they seemed to enjoy it. Larry and Paul Davies have a fantastic operation going on at the BEYOND centre, but I have to say the ambience and surroundings  at Flagstaff are probably more suitable for a European – much nicer weather!

Many thanks to Ari Belenkiy for the photographs. You can find more descriptions of the conference on John Farrell’s Forbes blog, and on Peter Coles’s  In The Dark blog.


Filed under Astronomy, Cosmology (general), Travel

Astronomy and cosmology at Birr Castle

Yesterday, I travelled to historic Birr Castle in the centre of Ireland in order to catch the end of the annual meeting of the Astronomical Science Group of Ireland. Birr Castle is a great setting for an astronomy meeting –  not only is it a beautiful castle with fantastic grounds, it is also an important landmark in the history of astronomy. The castle was the home of the famous Leviathan, a reflecting telescope that was the largest instrument of its kind in the world for many years. The telescope was built in the 1840s by Lord Parsons, the third Earl of Rosse, and featured  a 72-inch mirror, a marvel of engineering at the time.  He made many important discoveries with the instrument, not least the first observation of the spiral structure of some of the distant nebulae and the detection of stars within the nebulae. Indeed, the Earl was one of the first to propose that the nebulae were entire galaxies distinct from our own, a hypothesis that was not definitely established until Hubble’s measurements with the 100-inch Hooker telescope at Mt Wilson in California.

Birr Castle in Co.Offaly

The Leviathan telescope at Birr castle

There were a great many interesting talks over the two days of the meeting (see program here), but I was there to catch ‘The Search for Polarization Fluctuations in the Cosmic Microwave Background’ by Creidhe O’Sullivan of NUI Maynooth. Creidhe started with a basic overview of the cosmic microwave background (CMB), explaining its importance as evidence in support of the big bang model and describing the measurements of temperature fluctuations in the radiation by the COBE and WMAP satellites. (The CMB is the primordial radiation left over from the time that atoms first began to form. Cosmologists and astronomers spend a great deal of time studying the tiny temperature fluctuations imprinted in the CMB, as this gives information on the density and geometry of the early universe, see the Cosmology 101 section of this blog.)

Creidhe then moved on to explain the study of polarization in the background radiation. The CMB radiation is expected to be polarized because it comprises light that has been scattered by many particles; when light is scattered, it gets polarized into different planes of vibration. (Polaroid sunglasses operate on the same principle; they cut down on light by allowing only light polarised in one plane to pass through). Hence cosmologists search for fluctuations in polarization as well as temperature in the CMB, although the polarization fluctuations are much smaller. Mathematically speaking, the polarization is divided into two modes: electric (E –mode) and magnetic (B-mode) polarisation. E-modes have been detected since 2003; the analysis of these modes has become a major area of research in cosmology. Creidhe gave a superb overview of the instruments used to analyse the E- modes, including the work of her own group with the QuaD experiment at the South Pole.

The QUaD experiment at the South Pole

She finished the talk by explaining that the next big challenge in cosmology is the detection of B–mode polarization in the background radiation. B-modes present a great challenge as they are yet more difficult to detect. The great hope here is that the PlANCK satellite telescope, with its improved resolution. Just as the COBE satellite results were a watershed in our view of the early universe, the resolution of B-mode polarization in the CMB by PLANCK would give yet more support for the big bang model and cosmic inflation, and even offer evidence for the existence of gravity waves.

The Planck satellite telescope

That is not to say terrestrial experiments will not have their place. After Creidhe’s talk, another member of the Maynooth group, Stephen Scully, gave a brief overview of the team’s work on the QUBIC experiment. This is a new type of the bolometric interferometer that will be used in the next generation of terrestrial measurements at the South Pole.

All in all, a most informative afternoon. After the talks, we were shown the site in the castle grounds where a new radiotelescope is to be situated. This will form the Irish node of the international LOFAR astronomy project, bringing Birr castle up to date with modern astronomy – more on this in the next post.

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Filed under Astronomy, Cosmology (general)

A tribute to Stephen Hawking

RTE radio recorded an interview with me today on the subject of Stephen Hawking. I’m told it’s to have on file so I trust they don’t know something I don’t! Whatever the reason, it’s nice to have the opportunity to pay tribute to a living legend. Below is a script I prepared the interview; we only used a small part.


Q: Who is he?

Stephen Hawking is a famous English physicist at Cambridge University known for his work in cosmology, the study of the universe. In particular, he is admired for his work on black holes and on the big bang model of the origin of the universe.

Q: Why is he so famous?

Einstein used to be the only famous scientist of modern times, but Stephen Hawking has inherited that role. I like to think that one reason is his field of study; there seems to be a public fascination with scientific concepts such as the big bang and the nature of space and time (it’s hardly a coincidence that much of Einstein’s work was in this field).

Another reason may be Hawking’s disability. He was diagnosed with motor neuron disease (ALS) in his early 20s and given two years to live. The story of a brilliant mind trapped in a crippled body has universal appeal, and the wheelchair-bound figure communicating deep ideas by voice synthesizer has become an icon of science.

Then there’s the book. In the 1980s, Hawking published A Brief History of Time, a book on the big bang aimed at the general public  – it quickly became an unprecedented science bestseller and made him a household name. Since then, he has devoted a great deal of time to science outreach, unusual for a scientist at this level.

Q: Where is he from?

He was born in London in 1942, the son of two academics, and studied physics at Oxford. He wasn’t outstanding as an undergraduate but he did well enough to be accepted for postgraduate research in Cambridge. There, he became interested in cosmology, in particular in the battle being waged at Cambridge between the ‘big bang’ and ‘eternal universe’ theories. He showed early promise as a postgraduate when he demonstrated that Fred Hoyle, a famous cosmologist and prominent exponent of the eternal universe, had made a mathematical error in his work.

Q: Can you say a little about Hawking’s science?

His work is focused mainly on phenomena such as black holes and the big bang. Such phenomena are described by Einstein’s theory of relativity, which predicts that space and time are not fixed but affected by gravity. (In the case of black holes, relativity predicts that space is so distorted by gravity that energy,even light, cannot escape. In the case of the universe at large, relativity predicts that our universe started in a tiny, extremely hot state and has been expanding and cooling ever since; the so-called big bang model).

However, relativity does not work well on very small scales; this is the realm of quantum physics. Hawking’s lifelong work concerns the attempt to obtain a better picture of the universe by combining relativity (used to describe space and time) with quantum physics (used to describe the world of the very small).

He first established his reputation by defining the problem; with the mathematician Roger Penrose, he showed that relativity predicts that, under almost all conditions, an expanding universe such as our own must begin in a singularity i.e. a point of infinite density and temperature. This is not physically realistic and suggests that relativity on its own does not provide a true picture of the universe.

In later work, Hawking focused on black holes (a black hole is something like a big bang in reverse and may therefore offer clues to the puzzle of the origin of the universe). Successfully combining general relativity with quantum physics for this special case, Hawking was able to predict that black holes are not entirely black; instead they emit some energy in the form of radiation, now known as HawkingBekenstein radiation.  Most physicists are convinced by the logic and beauty of this result but Hawking radiation will be difficult to measure experimentally as it is predicted to be extremely weak.

My favourite Hawking contribution is the no-boundary universe. Working with James Hartle, he used a combination of relativity and quantum physics to predict that our universe may not have had a definite point of beginning because time itself may not be well-defined in the intense gravitational field of the infant universe!

Q: Is Hawking another Einstein?

No. Einstein made a great many contributions to diverse areas of physics. Also, relativity fundamentally changed our understanding of space and time, with profound implications for all of science and philosophy.(For example, the big bang model is merely one prediction of relativity). It’s hard for any scientist to compete with this.

Q: Why has Hawking not been awarded a Nobel prize?

He has received many prestigious awards, but not a Nobel. It’s quite difficult for a modern theoretician to win the prize because Nobel committees put great emphasis on experimental evidence. While we now have strong evidence that black holes exist, Hawking radiation will be very difficult to detect as it is predicted to be extremely weak.

Q; What is he working on these days?

At a conference in Dublin a few years ago, Hawking suggested a possible solution to the information paradox, a controversy over whether information is lost in black holes. The jury is still out on his solution. He is also involved with the theory of the cyclic universe, a theory that suggests there many have been many bangs.

Q: What lies in the future for Hawking?

Who knows. Last month, he celebrated his 70th birthday with a prestigious conference at Cambridge, 50 years after his terminal diagnosis. However, he was too ill to attend in person, reviving fears about his health. For now, he continues to work as ever, defying the predictions of modern medicine…

P.S. What’s all this about Hawking and God?

A Brief History of Time famously concludes with the phrase ‘‘..and then we would know the mind of God’’. At the time, many commentators interpreted this statement to mean that Hawking was religious. However, he was being mischievous – it is clear from other writings that he is not a believer in the normal sense. Indeed, his most recent book, The Grand Design, provoked controversy by stating that ‘‘It is not necessary to invoke God to set the universe going.” This statement was interpreted widely as a dismissal of God – in fact, it reflects standard cosmology (something can indeed arise from ‘nothing’) and says nothing about the existence of God.


Filed under Cosmology (general), Science and society

Current status of the concordance model

This week I’m studying a very nice article on the ArXiv by L.Perivolaropoulos on recent observational challenges to the ΛCDM model (thanks Bee).

The ΛCDM model is the technical name given to the concordance model of Big Bang cosmology (see final post in cosmology 101 series). Essentially, the model is the best attempt to account for the three main strands of observational evidence: the measurements of the cosmic microwave background, the measurements of the large scale structure of the universe by gravitational lensing, and the supernova measurements of the accelerated expansion of the universe. CDM stands for Cold Dark Matter, the postulate that much of the matter holding the galaxies and galaxy clusters together is unseen – i.e. does not couple with the electromagnetic interaction (see previous post on Dark Matter). Λ refers to the so-called cosmological constant –  i.e.  the ‘dark energy’ term thought to be responsible for the current acceleration of the universe expansion (see previous post on dark energy here).


The matter-energy composition of the universe according to ΛCDM

However, cosmologists are well aware that there is an alternative: the ΛCDM model could simply be wrong, and the postulates of dark matter and dark energy completely spurious, if our underlying theory of gravity – general relativity – does not apply at the largest scales. Both postulates arise from the attempt to shoehorn the observational data into gravitational theory, and it is always possible that the underlying theory is incomplete (after all, we know GR breaks down at the smallest scales). There is a very nice discussion of this in Perivolaropoulos’ s paper, in the context of six experimental observations that have emerged in the last few years that don’t seem to fit easily into the ΛCDM model.

Of course, given the spectacular success of general relativity in explaining so many aspects of our universe so far, the betting money is on relativity being correct, while the new observational data may modified as more measurements are made (this has happened countless times before). Either way, it’s a really nice update on the current state of play and shows how good science is done – not to mention the usefulness of the ArXiv database.


Over on the DiscoverScience blog, Sean Carroll also has very nice post on a specific challenge to the concordance model from measurements of the large scale structure of the universe by weak gravitational lensing. Again, both the post and the discussion afterwards are excellent and give a good idea of how this sort of science is done.

It”s worth mentioning that both dark matter and dark energy are favourite targets of skeptics, philosophers of science and other commentators. To be sure, they both probably seem like an obvious fix to an outsider, particularly given their postulated prevalence relative to ordinary matter (our universe is estimated to comprise 73% dark energy, 23%  dark matter and only 4% ordinary matter!). However, in this sort of debate, it’s important to listen to the experts. While keeping an open mind, most cosmologists seem convinced that dark matter almost certainly exists. The general line is that you can see it – by its gravitational effect, not electromagnetic. This is perfectly feasible if dark matter is made up of WIMPS (weakly interacting massive particles), a not unreasonable proposition. Such particles may even be detected at the LHC, which would be very exciting. It should also be remembered that the existence of dark matter is also invoked to account for the nucleosynthesis of the elements, a seperate plank of the big bang model. Finally, there are now strong experimental hints of the existence of dark matter from studies of galaxy collisions


Evidence for dark matter in the bullet cluster

As for dark energy, it is certainly true that this is a lot more speculative, and could turn out to be one of many different things  (see wiki for a good summary). However, it’s important to note that the postulate does not arise solely from the supernova measurements – there are also indirect evuidence of dark energy from measurements of the cosmic microwave background.


Filed under Cosmology (general)

Binary black holes, gravitational waves and numerical relativity

We had an excellent turn-out for yesterday’s superb Institute of Physics seminar even though we are in the last hectic week of the teaching semester (thanks to the organisational skills of the WIT maths/physics seminar group). The talk ‘Binary black holes, gravitational waves and numerical relativity’ was given by Dr Joan Centrella, head of the Gravitational Astrophysics Laboratory at NASA’s Goddard Space Flight Centre. Dr Centrella is a distinguished relativist, well known for her work in the simulation of black hole mergers and she certainly didn’t disappoint.

The lecture started with an overview of massive black holes, intermediate black holes and gravitational waves. Just as general relativity predicts that a large mass will curve spacetime, it predicts that moving mass will cause ripples in the curvature of spacetime – known as gravitational waves. Of course, such disturbances will be extremely difficult to detect due to the weakness of the gravitational interaction. Indeed, while many of the spectacular predictions of general relativity have been verified (the bending of light in a gravitational field, time dilation in a gravitational field, black holes and even the expanding universe) the direct detection of gravitational waves is possibly the last great test of relativity. The speaker explained that the best chance of seeing the phenomenon directly is by studying the most explosive events known: black hole mergers.

There was a brief description of the indirect observation of gravitational waves, in particular the Hulse-Taylor pulsar. This is a binary pulsar found in 1974, whose orbit has been observed to be gradually shrinking due to the radiation of energy by gravitational waves: the two stars will merge in about 300 million years. Interesting that Hulse got the Nobel for work done while still a postgraduate, while Jocelyn Bell was overlooked for her discovery of pulsars – see post on IoP meeting below.

Centrella then gave an overview of direct searches for gravitational waves, both earth-bound (LIGO) and space-based (LISA). LIGO, the Large Interferometer Gravitational Wave Observatory, is basically a huge Michelson interferometer, complete with laser source, beam splitter and mirrors – the arms of the interferometer are several kilometers in length! LISA, the Laser Interferometer Space Antenna, is an astounding project: a joint NASA/ESA mission, it will consist of three separate mini-spacecraft, each with its own laser source, maintained in an equilateral triangle that will form a giant Michelson interferometer in space. Minute disturbances in spacetime by a passing gravitational wave will be measured as tiny changes in relative arm length (having taken all other factors into account). A crucial difference between the two systems is the target: while LIGO searches for intermediate black hole events, LISA will search for massive BH events (a much stronger source in a different region of the spectrum).

LIGO (California)

LISA (artist’s impression)

Dr Centrella then described her own field: the use of numerical methods and algorithims to solve the equations of general relativity for the particular case of relativistic binary systems and their associated gravitational waves. She gave a great overview of historic problems in the area and recent breakthroughs in the field, from the puncture method to the Lazarus approach. I won’t attempt to summarize this part of the talk, but there is a nice overview of the field here and I should have a link to the slides from the talk in a day or two.

Dr Centrella with a scale model of one of the LISA spacecraft

All in all, this was a superb lecture, courtesy of the Institute of Physics. It was clear the audience enjoyed the lecture thoroughly and there were plenty of queries at question time – indeed the lecture would have continued for another hour had we not whisked the speaker off for dinner. In answer to my own question on the detection of gravitational waves from the Big Bang itself, Dr Centrella pointed out that one would certainly to see expect a signal from cosmic inflation – however these waves would be in a very different region of the spectrum from that studied by either LIGO or LISA. ..

Update: Joan has been in contact to say you can get a review article she wrote on the subject for the Scidac Review here; she has also done a podcast for Sky and Telescope with movies of the simulations here. She also has two comments and corrections to the text above; rather than paraphrase them I have put them verbatim in the comments section!

Update II: there is a wonderful article on gravitational waves and the early universe by Craig Hogan in the June 2007 edition of Physics World, which you can access here if you’re a member


Filed under Cosmology (general)

A brief history of cosmology

The last 12 posts described each of the main topics of modern cosmology: from the predictions of general relativity to the three main planks of evidence for the Big Bang, from the theory of inflation to the most recent measurements of the cosmic microwave background. All of this theory and experiment has been put together to form what is known as the Standard Model of cosmology. Before we describe the Standard Model, let’s briefly review the main discoveries in chronological order (you can read a seperate post on each starting here)


A Brief History of Cosmology

1.  The general theory of relativity (Einstein, 1916) and the prediction of a dynamic universe (Friedmann, 1919)

2. The observation of the expanding universe (Hubble’s Law, 1929)

3. Rewinding the Hubble graph: the ‘primeval atom’ and the calculation of the age of the universe (Lemaitre, 1929)

4. The Big Bang model and the problem of the singularity

5. The prediction of the abundances of H and He from the BB model (Gamow, 1945)

6. The predicton of the cosmic microwave background (CMB) from the BB model (Alphaer, Heuer and Gamow, 1949)

7. The detection of the cosmic microwave background (Penzias and Wilson, 1965)

8. The CMB puzzles of flatness, homogeneity and galaxy formation (1965 -)

9. The theory of inflation (Guth, Linde and Steinhardt, 1981-82)

10. The COBE study of the CMB –  support for the BB model and inflation (1992)

11. Dark energy – the supernova measurements of an accelerating universe (1998)

12. The WMAP study of the CMB – more support for inflation and dark energy (2005)


I gave a talk on the Big Bang to our astronomy class at WIT last night, describing most of the above topics. It was great fun, with a lengthy question and answer session afterwards, fielded by both me and well-known astronomer Emmet Mordaunt who normally takes the class. You can find the slides for the talk here.

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Filed under Cosmology (general), Cosmology 101

Dark Energy

In 1998, a totally unexpected result from astronomy caused a dramatic rethink of the Big Bang model. Measurements of the light emitted by a certain supernova suggested that it was further away than predicted by the Hubble constant. In other words, the exploding star did not lie on the straight line of the Hubble graph! This is a startling discovery as it implies that the expansion of the universe is not constant – instead the expansion is currently accelerating (see a description of the experiment here).

Skeptics at first suggested the result might arise from an error in the measurement of stellar distance – however, a similar observation was reported by a different group within two years. Further, independent support for the result soon emerged from measurements of the cosmic microwave background (CMB). In 2002, precision measurements of the CMB by the WMAP satellite suggested a universe with geometry that is flat to within 1%. This result is completely inexplicable in the context of the known density of the matter of the universe (both ordinary and dark). The known density of matter points to a universe with = 0.3, a long way from flatness ( =1). Hence the CMB measurements suggest that there is a great deal of matter/energy in the uiverse unaccounted for.

As a result, cosmologists now talk about a new phenomenon; a form of energy that is pushing the universe outward, causing the expansion to accelerate and the geometry to be flat. The phenomenon is labelled Dark Energy : the physical cause for dark energy is thought to be some sort of vacuum energy. However, it should be pointed out that the numbers don’t yet stack up – detailed calculations suggests that the postulated vacuum energy would cause an accelerated expansion many orders of magnitude greater than that observed…this is a major area of research at the moment.

Putting Dark Energy together with Dark Matter, cosmologists postulate that ordinary matter, dark matter and dark energy all add up to the critical density required for the geometry of the universe to be flat (as measured). In other words, the current model of the universe can be summed up by

Density ord matter (4%) + Dens dark matter (22%) + Dens dark energy (74%) = 100%

or M (0.04) + DM (0.22) + ΩDE (0.74) = 1


Technical notes:

1. Is Dark Energy compatible with relativity?

Yes, but note that an accelerating universe is not predicted by the Friedmann equation, i.e. does not feature in any of the Friedmann universes (see post on the expanding universe below). Going back to 1st principles, when one applies the equations of general relativity to the cosmos, an extra term must be added in order to account for the accelerating expansion. This is rather reminiscent of the manner in which Einstein himself , dismayed by the prediction of a dynamic universe, originally added a term to his equations in order to keep the universe static (the positive cosmological constant) . Now we apply a term to the other side of the equation for the opposite reason.

Revised Friedmann graphs of the evolution of the universe

2. Is Dark Energy compatible with inflation?

Yes, for two reasons:

1. The fact that the expansion is accelerating now makes the suggestion of an exponential expansion in the first instants a lot less fanciful.

2. While the current accleration is many orders of magnitude less than that of inflation, it may be that the cause is some energy left over from inflation – more on this later.


Filed under Cosmology (general), Cosmology 101

Dark Matter

The posts below constitute a brief introduction to the Big Bang model: the three planks of evidence, the problems of singularity, horizon and flatness, and the theory of inflation. Before we go on to discuss the Standard Model of cosmology (yes, there is such a thing), two further concepts are necessary: the old puzzle of Dark Matter and the new puzzle of Dark Energy.

Dark matter is thought to make up about 70% of the matter of the universe. Although we can’t see it, we presume it exists because of its gravitational effect on visible matter. Put differently, we don’t insist that all matter be ‘visible’ i.e. interact with the electromagnetic force. Instead, we include the possibility that some matter may be seen only by its gravitational effect on other matter.

DM was first postulated by Fritz Zwicky in the 1930s to account for a discrepancy between the calculated velocity of spiral galaxies and that observed. Nowadays, it has been proposed to account for the motion of many astrophysical phenomena from the smaller scales to the largest e.g. local stellar dynamics, galaxy rotation, galaxy cluster dynamics, X-ray halos, gravitational lensing and cluster streaming.

Calculations for galaxy rotation based on ordinary matter (curve A) and experimental points (curve B)

A second pointer of evidence for Dark Matter comes from cosmology, in particular from the cosmic microwave background (see post below). In order to to relate the miniscule variations in temperature seen in the CMB to galaxy formation in the early universe, all current models invoke the existence of DM. Even more importantly, the existence of DM is necessary to provide enough gravity to explain the flatness of the universe, as measured from the CMB (in conjunction with the postulate of dark energy – see next week).

It should be pointed out that not everyone agrees with the postulate of Dark Matter. Skeptics point to the possibility that our laws of gravity (both Newtonian and Einsteinian) may be failing at the largest scales – a theory known as modified Newtonian dynamics or MOND. However, most cosmologists now consider this possibility unlikely, due to the astrophysical and cosmological evidence above.

Best of all, the first hint of direct evidence DM was reported in a study of galaxy collision in 2007. If Dark Matter really exists, one might expect to observe ‘galaxy splitting’ in the case of galaxy collision. This is because the DM of each galaxy should interact little with the other, while the ordinary matter of each will interact strongly (just as a couple crossing a crowded room soon become separated if one is more social than the other!). Researchers at the University of Arizona are pretty sure this is exactly what they observed (see here). A similar result was reported by NASA in September 2008.

The famous bullet cluster collision (2007)

What could Dark Matter be made up of? Clearly, DM particles must be weakly interacting (otherwise we would see them) and possibly massive – i.e. weakly interacting massive particles or WIMPs. It is currently thought that the most likely candidates might be supersymmetric particles. (As we saw before, the theory of supersymmetry (SUSY) arises out of attempts to unify three of the fundamental forces – the theory postulates that every normal particle has a heavier supersymmetric partner). It turns out the most likely candidiate for DM is the neutralino, the lightest SUSY particle which cannot decay further.

Many groups around the world have been constructing experiments to look for particles that might be candidates for Dark Matter – you can find a post on a lecture on this subject by Tim Sumner of the Zeplin III experiment here and there is a very good overview of the Zeplin experiment itself here . However, this is straying into the area of particle physics; for cosmologists, establishing the existence of DM unequivocally is the real challenge.


Filed under Cosmology (general), Cosmology 101