Archive for supernovae

Another Nobel Prize for Cosmology!

Posted in The Universe and Stuff with tags , , , , , , , on October 4, 2011 by telescoper

Just time in between teaching and meetings for a quick post on today’s announcement that the 2011 Nobel Prize for Physics has gone to Saul Perlmutter, Brian P. Schmidt and Adam G. Riess “for the discovery of the accelerating expansion of the Universe through observations of distant supernovae.”

I’ve taken the liberty of copying the following text from the press release on the Nobel Foundation website

In 1998, cosmology was shaken at its foundations as two research teams presented their findings. Headed by Saul Perlmutter, one of the teams had set to work in 1988. Brian Schmidt headed another team, launched at the end of 1994, where Adam Riess was to play a crucial role.

The research teams raced to map the Universe by locating the most distant supernovae. More sophisticated telescopes on the ground and in space, as well as more powerful computers and new digital imaging sensors (CCD, Nobel Prize in Physics in 2009), opened the possibility in the 1990s to add more pieces to the cosmological puzzle.

The teams used a particular kind of supernova, called type Ia supernova. It is an explosion of an old compact star that is as heavy as the Sun but as small as the Earth. A single such supernova can emit as much light as a whole galaxy. All in all, the two research teams found over 50 distant supernovae whose light was weaker than expected – this was a sign that the expansion of the Universe was accelerating. The potential pitfalls had been numerous, and the scientists found reassurance in the fact that both groups had reached the same astonishing conclusion.

For almost a century, the Universe has been known to be expanding as a consequence of the Big Bang about 14 billion years ago. However, the discovery that this expansion is accelerating is astounding. If the expansion will continue to speed up the Universe will end in ice.

The acceleration is thought to be driven by dark energy, but what that dark energy is remains an enigma – perhaps the greatest in physics today. What is known is that dark energy constitutes about three quarters of the Universe. Therefore the findings of the 2011 Nobel Laureates in Physics have helped to unveil a Universe that to a large extent is unknown to science. And everything is possible again.

I’m definitely among the skeptics when it comes to the standard interpretation of the supernova measurements, and more recent complementary data, in terms of dark energy. However this doesn’t diminish in any way my delight that these three scientists have been rewarded for their sterling observational efforts. The two groups involved in the Supernova Cosmology Project on the one hand, and the High Z Supernova Search, on the other, are both supreme examples of excellence in observational astronomy, taking on and overcoming what were previously thought to be insurmountable observational challenges. This award has been in the air for a few years now, and I’m delighted for all three scientists that their time has come at last. To my mind their discovery is all the more exciting because nobody really knows precisely what it is that they have discovered!

I know that Brian Schmidt is an occasional reader and commenter on this blog. I suspect he might be a little busy right now with the rest of the world’s media right to read this, let alone comment on here, but that won’t stop me congratulating him and the other winners on their achievement. I’m sure they’ll enjoy their visit to Stockholm!

Meanwhile the rest of us can bask in their reflected glory. There’s also been a huge amount of press interest in this announcement which has kept my phone ringing this morning. It’s only been five years since a Nobel Prize in physics went to cosmology, which says something for how exciting a field this is to work in!

UPDATE: There’s an interesting collection of quotes and reactions on the Guardian website, updated live.

UPDATE on the UPDATE: Yours truly gets a quote on the Nature News article about this!


Nobel Predictions

Posted in The Universe and Stuff with tags , , , on September 24, 2010 by telescoper

I was quite interested to see, in this week’s Times Higher, a set of predictions of the winners of this years Nobel Prizes. I’ve taken the liberty of publishing the table here, although for reasons of taste I’ve removed the column pertaining to Economics.

Year Medicine Chemistry Physics
2010 D. L. Coleman, J. M. Friedman (leptin)
E. A. McCulloch, J. E. Till (stem cells)
and S. Yamanaka (iPS cells)
R. M. Steinman (dendritic cells)
P. O. Brown (DNA microarrays)
S. Kitagawa, O. M. Yaghi (metal-organic frameworks)
S. J. Lippard (metallointercalators)
C. L. Bennett, L. A. Page,
D. N. Spergel (WMAP)
T. W. Ebbesen (surface plasmon photonics)
S. Perlmutter, A. G. Riess, B. P. Schmidt (dark energy)
2009 E.H. Blackburn, C. W. Greider, J.W. Szostak (telomeres) (won in 2009)
J.E. Rothman, R. Schekman (vesicle transport)
S. Ogawa (fMRI)
M. Grätzel (solar cells)
J.K. Barton, B. Giese, G.B. Schuster (charge transfer in DNA)
B. List (organic asymmetric catalysis)
Y. Aharonov, M.V. Berry (Aharonov-Bohm effect and Berry phase)
J.I. Cirac, P. Zoller (quantum optics)
J.B. Pendry, S. Schultz, D.R. Smith (negative refraction)
2008 S. Akira, B.A. Beutler, J. Hoffmann (toll-like receptors)
V.R. Ambros, G. Ruvkun (miRNAs)
R. Collins, R. Peto (meta-analysis)
Roger Y. Tsien (green fluorescent protein)
C.M. Lieber (nanomaterials)
K. Matyjaszewski (ATRP)
A.K. Geim, K. Novoselov (graphene)
V.C. Rubin (dark matter)
R. Penrose, D. Schechtman (Penrose tilings, quasicrystals)
2007 F.H. Gage (neurogenesis)
R.J. Ellis, F.U. Hartl, A.L. Horwich (chaperones)
J. Massagué (TGF-beta)
S.J. Danishefsky (epothilones)
D. Seebach (synthetic organic methods)
B.M. Trost (organometallic and bio-organic chemistry)
S. Iijima (nanotubes)
A.B. McDonald (neutrino mass)
M.J. Rees (cosmology)
2006 Mario Capecchi, Martin J. Evans and Oliver Smithies (gene targeting) (won in 2007)
P. Chambon, R.M. Evans, E.V. Jensen (hormone receptors)
A.J. Jeffreys (DNA profiling)
G.R. Crabtree, S.L. Schreiber (small molecule chembio)
T.J. Marks (organometallic)
D.A. Evans, S.V. Ley (natural products)
Albert Fert and Peter Grünberg (GMR) (won in 2007)
A.H. Guth, A. Linde, P.J. Steinhardt (inflation)
E. Desurvire, M. Nakazawa, D.N. Payne (erbium-doped fibre amplifiers)
2002-05 M.J. Berridge (cell signalling)
A.G. Knudson, B. Vogelstein, R.A. Weinberg (tumour suppressor genes)
F.S. Collins, E.S. Lander, J.C. Venter (gene sequencing)
Robert H. Grubbs (metathesis method) (predicted and won in 2005)
A. Bax (NMR and proteins)
K.C. Nicolaou (total synthesis, taxol)
G.M. Whitesides, S. Shinkai, J.F. Stoddart (nano self-assembly)
M.B. Green, J.H. Schwarz, E. Witten (string theory)
Y. Tokura (condensed matter)
S. Nakamura (gallium nitride-based LEDs)

It’s quite interesting to see two sets of contenders from the field of cosmology, one from the Wilkinson Microwave Anisotropy Probe (WMAP) and another from the two groups studying high-redshift supernovae whose studies have led to the inference that the universe is accelerating thus indicating the presence of dark energy. Although both these studies are immensely important, I’d actually be surprised if either is the winner of the physics prize. In the case of WMAP I think it’s probably a bit too soon after the 2006 award for COBE for the microwave background to collect another prize. In the case of the supernovae searches I think it’s still too early to say that we actually know what is going on with the apparent accelerated expansion.

You never know, though, and I’d personally be delighted if either of these groups found themselves invited to Stockholm this December.

Interested to see how these predictions were made I had a quick look at the link the Times Higher kindly provided for further explanation, at which point my heart sank. I should have realised that it would be the dreaded Thomson Reuters, purveyors of unreliable numerology to the unwary. They base their predictions on the kind of bibliometric flummery of which they are expert peddlers, but which is not at all similar to the way the Nobel Foundation does its selections. No wonder, then, that their track-record in predicting Nobel prizes is so utterly abysmal…


Science as a Religion

Posted in Books, Talks and Reviews, Science Politics, The Universe and Stuff with tags , , , , , , , on July 6, 2010 by telescoper

With the reaction to Simon Jenkins’ rant about science being just a kind of religion gradually abating, I suddenly remembered that I ended a book I wrote in 1998 with a discussion of the image of science as a kind of priesthood. The book was about the famous eclipse expedition of 1919 that provided some degree of experimental confirmation of Einstein’s general theory of relativity and which I blogged about at some length last year, on its 90th anniversary.

I decided to post the last few paragraphs here to show that I do think there is a valuable point that Simon Jenkins could have made out of the scientist-as-priest idea. It’s to do with the responsibility scientists have to be honest about the limitations of their research and the uncertainties that surround any new discovery. Science has done great things for humanity, but it is fallible. Too many scientists are too certain about things that are far from proven. This can be damaging to science itself, as well as to the public perception of it. Bandwagons proliferate, stifling original ideas and leading to the construction of self-serving cartels. This is a fertile environment for conspiracy theories to flourish.

To my mind the thing  that really separates science from religion is that science is an investigative process, not a collection of truths. Each answer simply opens up more questions.  The public tends to see science as a collection of “facts” rather than a process of investigation. The scientific method has taught us a great deal about the way our Universe works, not through the exercise of blind faith but through the painstaking interplay of theory, experiment and observation.

This is what I wrote in 1998:

Science does not deal with ‘rights’ and ‘wrongs’. It deals instead with descriptions of reality that are either ‘useful’ or ‘not useful’. Newton’s theory of gravity was not shown to be ‘wrong’ by the eclipse expedition. It was merely shown that there were some phenomena it could not describe, and for which a more sophisticated theory was required. But Newton’s theory still yields perfectly reliable predictions in many situations, including, for example, the timing of total solar eclipses. When a theory is shown to be useful in a wide range of situations, it becomes part of our standard model of the world. But this doesn’t make it true, because we will never know whether future experiments may supersede it. It may well be the case that physical situations will be found where general relativity is supplanted by another theory of gravity. Indeed, physicists already know that Einstein’s theory breaks down when matter is so dense that quantum effects become important. Einstein himself realised that this would probably happen to his theory.

Putting together the material for this book, I was struck by the many parallels between the events of 1919 and coverage of similar topics in the newspapers of 1999. One of the hot topics for the media in January 1999, for example, has been the discovery by an international team of astronomers that distant exploding stars called supernovae are much fainter than had been predicted. To cut a long story short, this means that these objects are thought to be much further away than expected. The inference then is that not only is the Universe expanding, but it is doing so at a faster and faster rate as time passes. In other words, the Universe is accelerating. The only way that modern theories can account for this acceleration is to suggest that there is an additional source of energy pervading the very vacuum of space. These observations therefore hold profound implications for fundamental physics.

As always seems to be the case, the press present these observations as bald facts. As an astrophysicist, I know very well that they are far from unchallenged by the astronomical community. Lively debates about these results occur regularly at scientific meetings, and their status is far from established. In fact, only a year or two ago, precisely the same team was arguing for exactly the opposite conclusion based on their earlier data. But the media don’t seem to like representing science the way it actually is, as an arena in which ideas are vigorously debated and each result is presented with caveats and careful analysis of possible error. They prefer instead to portray scientists as priests, laying down the law without equivocation. The more esoteric the theory, the further it is beyond the grasp of the non-specialist, the more exalted is the priest. It is not that the public want to know – they want not to know but to believe.

Things seem to have been the same in 1919. Although the results from Sobral and Principe had then not received independent confirmation from other experiments, just as the new supernova experiments have not, they were still presented to the public at large as being definitive proof of something very profound. That the eclipse measurements later received confirmation is not the point. This kind of reporting can elevate scientists, at least temporarily, to the priesthood, but does nothing to bridge the ever-widening gap between what scientists do and what the public think they do.

As we enter a new Millennium, science continues to expand into areas still further beyond the comprehension of the general public. Particle physicists want to understand the structure of matter on tinier and tinier scales of length and time. Astronomers want to know how stars, galaxies  and life itself came into being. But not only is the theoretical ambition of science getting bigger. Experimental tests of modern particle theories require methods capable of probing objects a tiny fraction of the size of the nucleus of an atom. With devices such as the Hubble Space Telescope, astronomers can gather light that comes from sources so distant that it has taken most of the age of the Universe to reach us from them. But extending these experimental methods still further will require yet more money to be spent. At the same time that science reaches further and further beyond the general public, the more it relies on their taxes.

Many modern scientists themselves play a dangerous game with the truth, pushing their results one-sidedly into the media as part of the cut-throat battle for a share of scarce research funding. There may be short-term rewards, in grants and TV appearances, but in the long run the impact on the relationship between science and society can only be bad. The public responded to Einstein with unqualified admiration, but Big Science later gave the world nuclear weapons. The distorted image of scientist-as-priest is likely to lead only to alienation and further loss of public respect. Science is not a religion, and should not pretend to be one.

PS. You will note that I was voicing doubts about the interpretation of the early results from supernovae  in 1998 that suggested the universe might be accelerating and that dark energy might be the reason for its behaviour. Although more evidence supporting this interpretation has since emerged from WMAP and other sources, I remain skeptical that we cosmologists are on the right track about this. Don’t get me wrong – I think the standard cosmological model is the best working hypothesis we have _ I just think we’re probably missing some important pieces of the puzzle. I don’t apologise for that. I think skeptical is what a scientist should be.


Posted in Politics, The Universe and Stuff with tags , , , , , , on May 1, 2010 by telescoper

This past week was the final week of proper teaching at Cardiff University, so I’ve done my last full lectures, tutorials and exercise classes of the academic year. Yesterday I assessed a bunch of 3rd-year project talks, and soon those students will be handing in their written reports for marking.  Next week will be a revision week, shortly after that the examinations begin. And so the cycle of academic life continues, in a curious parallel to the  football league season – the other routine that provides me with important markers for the passage of the year.

Anyway, this week I gave the last lecture to my first-year class on Astrophysical Concepts. This is a beginning-level course that tries to introduce some of the theory behind astronomy, focussing on the role of gravity. I cover orbits in newtonian gravity, gravity and hydrostatic equilibrium in extended bodies, a bit about stellar structure, gravitational collapse, and so on. In the last part I do a bit of cosmology. I decided to end this time with a lecture about dark energy as, according to the standard model, this accounts for about 75% of the energy budget of the Universe. It’s also something we don’t understand very well at all.

To make a point, I usually show the following picture (credit to the High-z supernova search team).

 What is plotted is the redshift of each supernova (along the x-axis), which relates to the factor by which the universe has expanded since light set out from it. A redshift of 0.5 means the universe was compressed by a factor 1.5 in all dimensions at the time when that particular supernova went bang. The y-axis shows the really hard bit to get right. It’s the estimated distance (in terms of distance modulus) of the supernovae. In effect, this is a measure of how faint the sources are. The theoretical curves show the faintness expected of a standard source observed at a given redshift in various cosmological models. The bottom panel shows these plotted with a reference curve taken out so the trend is easier to see.

The argument from this data is that the high redshift supernovae are fainter than one would expect in models without dark energy (represented by the \Omega_{\Lambda}  in the diagram. If this is true then it means the luminosity distance of these sources is greater than it would be in a decelerating universe. They can be accounted for, however, if the universe’s expansion rate has been accelerating since light set out from the supernovae. In the bog standard cosmological models we all like to work with, acceleration requires that \rho + 3p/c^2 be negative. The “vacuum” equation of state p=-\rho c^2 provides a simple way of achieving this but there are many other forms of energy that could do it also, and we don’t know which one is present or why…

This plot contains the principal evidence that has led to most cosmologists accepting that the Universe is accelerating.  However, when I show it to first-year undergraduates (or even to members of the public at popular talks), they tend to stare in disbelief. The errors are huge, they say, and there are so  few data points. It just doesn’t look all that convincing. Moreover, there are other possible explanations. Maybe supernovae were different beasties back when the universe was young. Maybe something has absorbed their light making them look fainter rather than being further away. Maybe we’ve got the cosmological models wrong.

The reason I show this diagram is precisely because it isn’t superficially convincing. When they see it, students probably form the opinion that all cosmologists are gullible idiots. I’m actually pleased by that.  In fact, it’s the responsibility of scientists to be skeptical about new discoveries. However, it’s not good enough just to say “it’s not convincing so I think it’s rubbish”. What you have to do is test it, combine it with other evidence, seek alternative explanations and test those. In short you subject it to rigorous scrutiny and debate. It’s called the scientific method.

Some of my colleagues express doubts about me talking about dark energy in first-year lectures when the students haven’t learned general relativity. But I stick to my guns. Too many people think science has to be taught as great stacks of received wisdom, of theories that are unquestionably “right”. Frontier sciences such as cosmology give us the chance to demonstrate the process by which we find out about the answers to big questions, not by believing everything we’re told but by questioning it.

My attitude to dark energy is that, given our limited understanding of the constituents of the universe and the laws of matter, it’s the best explanation we have of what’s going on. There is corroborating evidence of missing energy, from the cosmic microwave background and measurements of galaxy clustering, so it does have explanatory power. I’d say it was quite reasonable to believe in dark energy on the basis of what we know (or think we know) about the Universe.  In other words, as a good Bayesian, I’d say it was the most probable explanation. However, just because it’s the best explanation we have now doesn’t mean it’s a fact. It’s a credible hypothesis that deserves further work, but I wouldn’t bet much against it turning out to be wrong when we learn more.

I have to say that too many cosmologists seem to accept the reality of dark energy  with the unquestioning fervour of a religious zealot.  Influential gurus have turned the dark energy business into an industrial-sized bandwagon that sometimes makes it difficult, especially for younger scientists, to develop independent theories. On the other hand, it is clearly a question of fundamental importance to physics, so I’m not arguing that such projects should be axed. I just wish the culture of skepticism ran a little deeper.

Another context in which the word “skeptic” crops up frequently nowadays is  in connection with climate change although it has come to mean “denier” rather than “doubter”. I’m not an expert on climate change, so I’m not going to pretend that I understand all the details. However, there is an interesting point to be made in comparing climate change with cosmology. To make the point, here’s another figure.

There’s obviously a lot of noise and it’s only the relatively few points at the far right that show a clear increase (just as in the first Figure, in fact). However, looking at the graph I’d say that, assuming the historical data points are accurate,  it looks very convincing that the global mean temperature is rising with alarming rapidity. Modelling the Earth’s climate is very difficult and we have to leave it to the experts to assess the effects of human activity on this curve. There is a strong consensus from scientific experts, as monitored by the Intergovernmental Panel on Climate Change, that it is “very likely” that the increasing temperatures are due to increased atmospheric concentrations of greenhouse gas emissions.

There is, of course, a bandwagon effect going on in the field of climatology, just as there is in cosmology. This tends to stifle debate, make things difficult for dissenting views to be heard and evaluated rationally,  and generally hinders the proper progress of science. It also leads to accusations of – and no doubt temptations leading to – fiddling of the data to fit the prevailing paradigm. In both fields, though, the general consensus has been established by an honest and rational evaluation of data and theory.

I would say that any scientist worthy of the name should be skeptical about the human-based interpretation of these data and that, as in cosmology (or any scientific discipline), alternative theories should be developed and additional measurements made. However, this situation in climatology is very different to cosmology in one important respect. The Universe will still be here in 100 years time. We might not.

The big issue relating to climate change is not just whether we understand what’s going on in the Earth’s atmosphere, it’s the risk to our civilisation of not doing anything about it. This is a great example where the probability of being right isn’t the sole factor in making a decision. Sure, there’s a chance that humans aren’t responsible for global warming. But if we carry on as we are for decades until we prove conclusively that we are, then it will be too late. The penalty for being wrong will be unbearable. On the other hand, if we tackle climate change by adopting greener technologies, burning less fossil fuels, wasting less energy and so on, these changes may cost us a bit of money in the short term but  frankly we’ll be better off anyway whether we did it for the right reasons or not. Of course those whose personal livelihoods depend on the status quo are the ones who challenge the scientific consensus most vociferously. They would, wouldn’t they? Moreover, as Andy Lawrence pointed out on his blog recently, the oil is going to run out soon anyway…

This is a good example of a decision that can be made on the basis of a  judgement of the probability of being right. In that respect , the issue of how likely it is that the scientists are correct on this one is almost irrelevant. Even if you’re a complete disbeliever in science you should know  how to respond to this issue, following the logic of Blaise Pascal. He argued that there’s no rational argument for the existence or non-existence of God but that the consequences of not believing if God does exist (eternal damnation) were much worse than those of behaving as if you believe in God when he doesn’t. For “God” read “climate change” and let Pascal’s wager be your guide….

Deterministic Chaos

Posted in Biographical with tags , on April 25, 2009 by telescoper

Yesterday was the occasion of the Annual Ball of the Cardiff  University School of Physics & Astronomy‘s Student Society Chaos held in the Cardiff Arms Suite of the Millennium Stadium. I had reservations about going because things like this always make me feel very old, but having been persuaded I was determined to have a good time. It turned out to be very enjoyable, so much so that I ended up moving on with some others to a nightclub to continue the party into the small hours. I think I kept up with the youngsters quite well, although I was well and truly knackered when I got home.

I’m also glad I didn’t disgrace myself too much, or if I did I don’t remember…

There was about a hundred people at the Chaos Ball, the vast majority of them students in the department. Not many staff members went along, although those that did all seemed to have a good time. These social events are quite tricky to pull off for a number of reasons. One is that there’s an inevitable “distance” between students and staff, not just in terms of age but also in the sense that the staff have positions of responsibility for the students. Students are not children, of course, so we’re not legally  in loco parentis, but something of that kind of relationship is definitely there. Although it doesn’t stop either side letting their hair down once in a while, I always find there’s a little bit of tension especially if the revels get a bit out of hand.

To help occasions like this I think it’s the responsibility of the staff members present to drink heavily in order to put the students at ease. United by a common bond of inebriation, the staff-student divide crumbles and a good time is had by all.

A couple of other incidents that happened this week serve to illustrate related issues. On Thursday we had to evacuate the building because the fire alarm went off. It turned out that some work being done on the roof had triggered a smoke detector. Although it wasn’t a real emergency, four fire engines arrived and we all stood outside for the best part of an hour while they figured out what had happened and, curiously, how to switch the alarm off.

The fire alarm had gone off, the fire brigade had turned out, but there was no fire to be seen. I joked that the only possible explanation of this state of affairs was that there must be a dark fire…

Standing outside, staff and students chatted casually while waiting to be let back into the building. It was sunny, which added to the conviviality. I realised, though, that I’d  never really spoken to many of my students like that before, i.e. outside the lecture  or tutorial. I see the same faces in my lectures day in, day out but all I do is talk to them about physics. I don’t know them at all. It’s strange.

The other thing was yesterday morning where I was giving one of my first year lectures on Astrophysical Concepts, a course which I really enjoy teaching. The topic was supernovae and it’s a lecture which I always end by doing an impersonation of a supernova explosion. If you want to see it, you’ll have to sign up for the course.

I was doing my PhD in 1987 when a supernova (SN1987A) went off in the Large Magellanic Cloud. It was  a hot topic for a while and I mentioned in my talk. I started to say “Some of you will remember…” then I suddenly realised to my horror that in 1987  nobody in my class had yet been born…