Category Archives: Cosmology (general)

Gruber prize at Cambridge

There was some excitement at the COSMO 2013 conference at Cambridge yesterday evening, with the presentation of this year’s Gruber prize for cosmology. The prize went to Viatcheslav Mukhanov and Alexei Starobinsky, two Russian theoreticians who made legendary contributions to our understanding of the formation of structure in the early universe.  After a very nice ceremony, we got a superb seminar from each; Starobinksy gave a talk on ‘Quantum Beginning of the Universe’, while Muhkanov gave a moving and often hilarious account of a scientist’s life in the old Soviet Union .

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Mukhanov (L) and Starobinsky (R) accepting the Gruber prize

During the day, we had many seminars on the cosmic microwave background, notably by George Efstathiou and Jo Dunkley, and a talk by John Kovac on attempts to detect B-mode polarization in the CMB from ground-based telescopes. You can see the conference programme here. The Gruber ceremony was followed by a reception, so I didn’t get home until 10 pm.  All in all, a pretty full day.

Today, the talks are on large scale structure in the universe and quite a bit more technical (at least for your humble correspondent). On the other hand, there is quite a frisson in the room as Stephen Hawking has just arrived to catch Andreas Ringwald’s talk on axions. This evening, Professor Hawking and Brian Cox will each give a public talk as part of the conference, I’m looking forward to it.

Update

We had three public lectures this evening. Andrew Liddle on cosmology and the Planck results, Brian Cox on the LHC and the Higgs boson, and Stephen Hawking on space and time or  ‘Fire in the Equations’. Andrew gave his usual tour de force (see here for a review of his recent Dublin lecture), Brian gave a surprisingly mathematical lecture on the standard Model of particle physics, and Stephen stole the show with a truly inspirational lecture on space, time, the meaning of it all and why scientists need to stay curious. Just the thing for a jaded conference delegate with a paper to finish before he goes home!

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Brian Cox in action

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Stephen Hawking musing on the meaning of the universe

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COSMO 2013 at Cambridge University

Today was the first day of the COSMO 2013 conference at Cambridge. Walking up the path to the hallowed Department of Applied Mathematics and  Theoretical Physics (DAMTP), I was gripped by my usual fear that I might meet with a frosty reception at the door; “No experimentalists, please!”

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The hallowed halls of DAMTP

But it’s not that sort of conference. COSMO 2013 is a very nice mix of cosmology and particle physics, theory and experiment. You can see the conference poster and programme here.

This morning started with two contrasting plenary talks on particle physics; an experimental talk by Lars Sonneschein, and a more general talk ‘From the Higgs boson to Cosmology’ by well-known CERN theoretician John Ellis.

In his talk ‘Recent Results from the LHC’, Professor Sonnenschein gave a brief overview of recent results at the LHC, from current production rates of top anti-top quarks to the famous discovery of the Higgs boson. Much of this probably wasn’t that new to the audience given the number of Higgs talks last year, but it was good to see up-to-date information on the decay modes and coupling constants for the Higgs.The main point was that with more and more accurate measurements, there is still no evidence yet of any physics beyond the Standard Model, whether one was searching for dark matter, microscopic black holes or indeed supersymmetry (SUSY). On the other hand, there were grounds for good cheer for the experimentalists given the projections Lars gave for increased luminosity at the LHC in the next few years.

John Ellis’s talk took a very different tack. He starting by explaining why a light Higgs mass and weak couplings is a good result for supersymmetry (SUSY can stabilize a light Higgs), giving theorists yet another reason to take the theory seriously, despite the ecent narrowing of windows of possibility at the LHC (at least for minimal models). Professor Ellis then made a connection with cosmology, remarking that basic Wess-Zumino SUSY models can be shown to fit very well with many generic models of inflation;in particular, adding supersymmetry to the mix can give models that fit very comfortably within the recent PLANCK results (some fall well within the dark blue region in the famous Planck figure below). A colleague of a certain age commented to me afterwards  that he isn’t quite reconciled  with the way inflation has become the dominant paradigm in today’s cosmology; for my part, I can never get used to today’s discussions of  supersymmetry in both cosmology and particle physics, having grown up thinking of it as an obscure theory practised only by my father and a few colleagues around the world! Science truly evolves…

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Prof Ellis wearing his Standard Model t-shirt

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Generic SUSY versions of inflation can give models that fall within the most probable region (dark blue)

At question time afterwards, I commented that I was struck by the contrast between the two talks, i.e. the strong motivation for SUSY from theory but the lack of results so far at the LHC, and asked Professor Ellis whether he thought the first evidence for SUSY might indeed come from the cosmic microwave background rather than particle accelerators (I made a mess of the question, nervous for once!). He responded by pointing out that it took 40 years to find the Higgs in particle accelerators, thus we should not be too impatient.  This answer makes a lot of sense to me, I’m a bit dismayed at the way SUSY scepticism has quickly become almost as popular a sport as string theory scepticism. After all, theory is often decades ahead of experiment, particularly in particle physics…

There were two other plenary lectures after coffee, an overview of Dark Matter by Malcolm Fairbairn and a talk on neutrino masses by Silvia Pascoli. They were both excellent talks but there is so much going on I just can’t keep up! Also, Stephen Hawking is sitting three tables away, also working away at a computer – I’m going to tidy myself off to the afternoon sessions before someone mistakes me for a journalist and chases me out of the canteen!

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Last day of Quantum Foundations conference at Oxford

Yesterday was the last day of the  Cosmology and Quantum Foundations  conference, a symposium that formed part of the  Establishing the Philosophy of Cosmology project at Cambridge and Oxford.

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There was no workshop in the  morning, but there were two weighty lectures in the afternoon, ‘Inflationary Cosmology as a Laboratory for Primordial Quantum Mechanics’ by Antony Valentini and ‘Relational Quantum Mechanics: Spinfoam Cosmology’  by Carlo Rovelli.

Antony Valentini’s talk was the second installment of his thesis that we should consider the possibility that the quantum equilibrium universe we experience is simply a subset of a much larger ensemble which is deterministic, not in equilibrium, and does not obey the standard rules of quantum probability. In this model, elements of the larger ensemble made a transition  by a process of relaxation on atomic timescales to the quantum equilibrium we see today. Antony hypothesized that observational cosmology might offer a test for his model because any non-equilibrium states remaining before cosmic inflation would have become frozen during this period,  feeding into the cosmic microwave background at the end of inflation. His analysis suggested one explanation for the well-known power deficit in the CMB at long wavelengths in the Planck and WMAP data.  I have no idea what the theoreticians thought of Antony’s hypothesis, but talks like this certainly give the lie to those who accuse physicists of groupthink and of being incapable of thinking outside the box!

Carlo Rovelli then gave the second installment of his talk on his relational view of quantum mechanics (see last post). The main point here was that adding gravity to the analysis is not a complication in the case of the relational interpretation of qt because the model is fundamentally relativistic in nature (gravity is simply a curvature of spacetime in relativity). He went on to describe how the theory leads to the ‘quantum loop’ view of quantum gravity. I am not qualified to comment on the theory, but what I took out of Carlo’s talks is that the only fundamental entities in relational theory are covariant quantum fields -the wavefunction has no physical significance, any more than a mathematical operator.

All this was followed by a round table discussion between, Carlo, Antony, Simon Saunders and David Wallace. For many of us, this was a major highlight of the conference. It was a privilege to hear major proponents of the many-worlds interpretation of quantum theory (Saunders and Wallace) arguing point-by point against the relational view (Rovelli), not to mention pointed interjections from heavyweights in the audience such as John Barrow, Julian Barbour and Joe Silk. During the course of the debate, it struck me that the discussion was in some ways a modern echo of the classic debate between the Heisenberg and Schroedinger interpretations of the quantum world. I could almost see Heisenberg behind Carlo Rovelli’s chair, applauding his emphasis on the discreteness as the key property of the quantum world and his dislike of the wavefunction. In the opposite corner, Schroedinger’s view had much in common with the many-worlds camp because of his dislike of collapsing wavefunctions. Indeed, it has recently been suggested by several authors that Schroedinger’ s later work on quantum interpretation somewhat anticipates the many-worlds view (will dig out references on this).

So a splendid finish to a splendid conference; an important debate on the meaning of quantum theory between leading proponents of alternate modern interpretations of the theory, with echoes of history throughout.

Update

It all happens at Oxford. Strolling past the Sheldonian this evening, I heard the familiar strains of Vivaldi’s Four Seasons. Turns out Nigel Kennedy was giving a concert with the Oxford Philomusica, so I sneaked into the foyer to hear the last few movements. You don’t hear much about Kennedy since he moved to Poland, but his performance was as electric as ever. I timed the applause at over 20 minutes, he certainly hasn’t lost his gift for communicating with the audience. However, the real surprise was the orchestra, it didn’t sound like any college orchestra to me – lovely crisp playing, fantastic articulation in the fast passages, and super pianissimos in the slow passages. Turns out the Oxford Philomusica is a relatively new initiative, a professional orchestra in residence at the university. What a great idea , I’m sure it gives a unique opportunity for the very best of the music students

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Nigel Kennedy at the Sheldonian

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Day II at Oxford

Today was the second day of the  Cosmology and Quantum Foundations  conference, a symposium that forms part of the  Establishing the Philosophy of Cosmology project at Cambridge and Oxford.

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The workshop this morning started with a fascinating talk by Douglas Spolyar  on a model of cosmic inflation that predicts that inflation could happen at relatively low energies. The big advantage of such models that they are testable at the TeV energies, i.e., at accelerators such as the LHC; I need to read the paper before I comment further, but all the talks will soon be available on the conference website.

Laura Mersini then gave a talk on evidence for the multiverse post-Planck. This was a discussion of her thesis that the multiverse should in principle be detectable in the cosmic microwave background because of the phenomena of quantum entanglement and decoherence. She then discussed how in her view the Planck data offers support for the model in terms of the cold spot, the dark flow and other effects. It was a good thorough lecture and I understood a lot more than I did at the Cambridge conference on the philosophy of cosmology last March.  Of course, not all cosmologists agree with her thesis and there was plenty of lively discussion from the audience – as an experimentalist, I really like the way theoreticians constantly challenge each other  during their talks, it’s very interactive!

In the afternoon , it was back to the conference proper for ‘Probability and the multiverse: an Everettian view’, the second installment of Simon Saunder’s discussion of the many-worlds interpretation of quantum theory. I found this a lot more challenging than Monday’s talk, I really need to brush up on my reading on many-worlds. Max Tegmark then gave a talk on ‘Thermodynamics, information and consciousness in a quantum multiverse’, a discussion that was  full of interesting insights and provocative ideas. A central theme of his is that entropy does not always increase, but can in fact decrease on observation. I have heard this idea before but I’ve never been clear whether it is an argument that pertains to entropy as a state of information about a system, or whether it is literally true of physical entropy.  I wanted to ask this at question time, and how one might test the hypothesis,  but time ran out.

[Update: I asked Max this question over coffee. I think the answer is yes to physical entropy and he suggested an experiment that could test the idea; unfortunately, I understood about 5% of what he said, I need to read up on this!]

The last speaker of the day was Carlo Rovelli, who spoke on a new interpretation of quantum theory known as the relationary view, a hypothesis  he put forward in the 1990s. This interpretation of qt  imports a lot of ideas from special relativity, in particular applying the idea of the reference frame of the observer to the measurement problem. Thus, instead of talking about wavefunctions that collapse into one state or another, one has to consider that measurements of systems are made relative to another system – it is the relation between the systems that counts. It was fascinating to hear a description of this intriguing new idea from its creator, and tomorrow he will explain how the new theory gives a description of  quantum gravity. [Writing this, I seem to remember that one of Schrodinger’s own objections to the notion of collapsing wavefunctions involved the problem of observations of the same object from different reference frames, must look this up]

After all that, it was time for the conference dinner. I was lucky enough to be at the same table as Carlo, who is also  the author of the highly regarded book ‘The First Scientist: Anaximander and His Legacy’ and we had a great discussion on the history of science. I have never met a physicist who is not interested in the history of our subject – how things were found out is almost as interesting as the things themselves!

As a bonus, the an after-dinner talk was given by Max Tegmark who posed an intriguing question; what if mathematics is a useful way of describing nature simply because nature *is* mathematics? This question was  first raised by Pythagoras, and Max gave an extremely interesting talk on the subject. So much so that I finally realised who he reminds me of – Richard Feynman!

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I had a quick walk under the Bridge of Sighs before dinner

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Oxford!

I’m at Oxford University this week, at the Cosmology and Quantum Foundations  conference, a symposium that forms part of the recent Establishing the Philosophy of Cosmology project at Cambridge and Oxford.

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Physicists don’t always accept the relevance of philosophy in the study of physics, but there is no question that quantum theory has long posed difficult questions of a philosophical nature, not least the interpretation of the quantum wavefunction. In addition, modern cosmology points towards a universe that was once in an extremely small and dense state,  that may or may not have had a finite beginning. As well as the familiar problems of quantum philosophy, this raises a host of other philosophical problems, such as ‘When did the laws of physics become the laws of physics?’ or ‘Were space and time always there or did emerge with the big bang ?’

The conference started at 9.30 this morning with a workshop on cosmic inflation. First up was Andrew Liddle ; after a brief review of the basic postulate of inflation, Andrew explained how the theory soon provided an explanation for the formation of galactical structure (in terms of quantum fluctuations in the early universe inflated to the perturbations observable in the cosmic background radiation). This explanation has since become a major motivation for the theory. Andrew then described new constraints imposed on inflationary models by the data from the Planck satellite.

Andrew’s talk was followed by a seminar by Douglas Spolyar on a new model describing how inflation might have ended (‘supercooled inflation’). I won’t describe it here as part II is due tomorrow. For lunch, we all trooped over to the famous Clarendon lab to hear well-known MIT physicist Max Tegmark give  a rather different sort of talk, ‘The future of life – a cosmic perspective’,  hosted by Oxford’s Future of Humanity Institute. Max’s main thesis was that a cosmological perspective renders existential problems more important, not less. Given that there is a finite chance that mankind is the only conscious life in the universe, if mankind were to die out there would be no-one to observe the universe! It’s a fascinating and provocative argument, and I was pleased to see climate change up there amongst the existentialist risks. However, I wasn’t entirely convinced by Max’s central theme ; apart from the philosophical debate concerning the role of the observer (is he/she really that important?) one wonders are there not more selfish reasons to tackle existential risks (what do I know,  it was an interesting take anyway).

After the Tegmark seminar, it was off to St Anne’s College for the opening of the conference proper. The afternoon session kicked off with a talk by well-known Oxford physicist Simon Saunders on the Many Worlds interpretation of quantum theory. Oxford have a major reputation in this area and I know no better physicist to give an introduction to this topic. It was a fascinating lecture and part II is tomorrow.  After coffee, Max Tegmark gave another excellent talk, this time on The cosmological interpretation of quantum mechanics – unifying the inflationary and quantum multiverses’ (more on this tomorrow).

After dinner with Andrew, I walked around Oxford and took a few photos.There is such fabulous  architecture everywhere you look, no wonder it produces great thinkers. Also, there’s a real thrill in seeing so many locations that are familiar from Inspector Morse , not to mention Lewis.

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The Bodlean (I think)

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The Bridge of Sighs

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The River Thames

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Last day at Cambridge Infinities Conference

Today was the third and last day of the ‘Infinities and Cosmology’ conference at Cambridge (there is also a workshop tomorrow, see website). Yesterday saw quite a heavy schedule, with part II of George Ellis’s ‘Infinites of Age and Size Including Global Topological Issues’, part II of Anthony Aguirre’s ‘Infinite and Finite State Spaces’ and part II of Michael Douglas’s ‘Can We Test the String Theory Landscape?’ (see previous post for an outline of these topics). We also had a fairly technical talk on ‘Singularities and Cosmic Censorship in General Relativity’ by the Cambridge mathematician Mihalis Dafermos: nuts-and-bolts talks like these are great for non-relativists like me because you get to see the mathematical tools used in GR research.

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The logo for the Infinities in Cosmology conference; an artist’s impression of small universes

Today saw part II of Mihalis’s talk and the lecture ‘Infinite Computations and Spacetime’ by Mark Hogarth, a fascinating exploration of new methods of computation by exploiting relativistic spacetime . I won’t attempt to summarize either, but the lectures should soon be available on the conference website.

For me, the highlight of the day was the talk ‘At Home and At Sea in an Infinite Universe: Newtonian and Machian Theories of Motion’ by Simon Saunders,  the well-known Oxford physicist and philosopher of physics. This was a superb discussion of Newton’s cosmology, in particular the paradox of gravitational instability in the Newtonian universe of infinite size and absolute, fixed space. Did Newton realize that our solar system might possess a net acceleration, or did he assume that external gravitational forces somehow cancel out? Drawing on material from Newton’s Principia and his ‘System of the World’,  Professor Saunders argued that Newton assumed the latter, though whether he attributed such a delicate cosmic balancing act to divine intervention or to unknown forces is not clear. (The possibility of a theological argument is not so fanciful as this work was the first mathematical attempt to try to describe the universe as a whole). Later, Professor Saunders suggested that it is likely Newton declined to spend too much time on the question simply because it was untestable.

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Newton’s famous Principia

There were many other interesting points in this fascinating lecture. Viewing the slides shown from Newton’s Principia, I was struck by the equivalence drawn again and again between bodies at rest and in uniform motion. This anticipates Einstein’s special theory of relativity and is again slightly in conflict with Newton’s assumption of a fixed, absolute space, as Simon pointed out. All this hints at a possible difference in Newton’s philosophy towards the universe at large versus motion on local scales – ironic as he was the first scientist to unite terrestrial and celestial motion in a single framework. I won’t comment further, but the lecture left one eager to read Simon’s recent paper on the subject.

All in all, a superb conference. It was interesting that, even with such distinguished speakers, moderators observed time limits strictly in order to allow plenty of time for questions and comments after the talks. In some ways, this was the best part; it’s not often one gets to hear to-and-fro arguments between scientists like John Barrow, George Ellis, Julian Babour and Simon Saunders, in the lecture theatre and over coffee.

Speaking of coffee, one of the best aspects of the conference was the venue. Cambridge’s Department of Applied and Theoretical Physics forms part of its Centre for Mathematical Sciences and is housed in a lovely modern open-plan building, with the smell of coffee and scones wafting throughout the atrium. What other mathematics institute can boast such a setup?  Not DIAS, I’m afraid. Indeed, I’m writing this post in the quiet atrium/canteen (no annoying background music – that wouldn’t be tolerated here). However, I’ve just realised that we are now finished for the day, so I’m off to do some sight-seeing at last.

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The main atrium in the Center for Mathematical Sciences is one big coffee shop, perfect for group discussions of physics, philosophy and mathematics

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The Department of Applied Mathematics and Theoretical Physics forms part of the Centre for Mathematical Sciences at Cambridge

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Infinities at Cambridge

The ‘Infinities and Cosmology’ conference  (see last post) got off to a great start here at Cambridge today. The first surprise was that DAMTP, Cambridge’s Department of Applied Mathematics and Theoretical Physics, is now housed in a beautiful modern building with lots of light, wide open spaces and a great canteen. The building forms part of the new Centre for Mathematical Sciences, most impressive. I couldn’t resist taking a few other photos after breakfast on my way to the conference, nearly missed registration!

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The Department of Applied Maths and Theoretical Physics at Cambridge

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 Clare College (where I’m staying) in the mist at breakfast this morning

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Walking through Clare College on the way to the conference

After registration and coffee, the conference started with a ‘brief introduction’ by John Barrow . This comprised a succinct but comprehensive overview of problems posed by infinities in mathematics, classical physics, quantum physics and particle physics, finishing with a discussion of specific problems in cosmology. There’s nothing quite like an overview like this by an expert, all sorts of connections between diverse phenomena become apparent. I took copious notes which will keep me busy over the next few days. Indeed, I suspect that if no other speaker had turned up, Prof Barrow could have expounded further on the topics he touched on for the duration of the conference.

George Ellis then took the podium for the first installment of his talk ‘Infinities of age and size, including global topology issues’. He set a no-nonsense tone by starting with a pet peeve – that physicists routinely confuse inconceivably large numbers with infinity, a very different beast. He expounded on this theme at length and then set about an interesting argument: that talk of infinities in physical systems is meaningless unless one can verify that they are truly infinite – which cannot be done, as pointed out by David Hilbert. Thus, the hypothesis of an infinite universe is dubious science and dubious philosophy. George then postulated a general test (the Ellis/Hilbert fork) for theories; any hypothesis that no longer works when infinite quantities are replaced by arbitrarily large numbers is bunk!

We were still pondering this opening salvo when Anthony Aguirre took the podium after coffee to talk about ‘Infinite and finite spacetimes’. This started with a succinct review of the ‘initial conditions’ problem in the big bang model, the theory of cosmic inflation and the main inflationary models of today. In particular, Anthony explained why inflation leads naturally to the concept of the multiverse  (essentially, quantum tunneling or equivalent processes are simply far too slow to compete with the still-inflating universe, leading to separate bubble universes). Personally, I once hoped that some mathematician would one day prove that inflation either happened to all or the universe or not at all, but this is looking increasingly unlikely. Anthony then went on to describe the model of eternal inflation and explained how Hoyle’s famous ‘steady-state universe’ could be right after all (at least on the global scale of the multiverse, as he explained in response to a silly question from yours truly).

After lunch, string theorist Michael Douglas presented the first installment of his talk ‘Can we test the sting theory landscape?’. This was the most technical talk so far, nothing less than a brief review of fundamental ideas in string theory and the famous problem of the landscape. A very basic argument Michael made chimed with me, namely that “almost all physical theories have a landscape of possible solutions” (there are dozens of example of this in solid-state physics). After some more general points, Michael went on to address the problem of dark energy, describing how his recent work on the flux vacua hypothesized by Bousso and Polchinski might deliver a mechanism for the cancellation necessary to reduce the quantum energy of the vacuum to the tiny ‘dark energy’ value we see today. I need to read around this area before Michael’s follow-up talk tomorrow so I’ll stop there!

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