Archive for Joe Silk

More Cosmic Tension?

Posted in The Universe and Stuff with tags , , , , , , , , , on November 12, 2019 by telescoper

Quite a lot of fuss was being made in cosmological circles while I was away last week concerning a paper that had just been published in Nature Astronomy by Eleonora Di Valentino, Alessandro Melchiorri and Joe Silk that claims evidence from the Planck Cosmic Microwave background and other data that the Universe might be closed (or at least have positive spatial curvature) in contrast to the standard cosmological model in which the spatial geometry is Euclidean. Nature Astronomy is behind a paywall but the paper is available for free on the arXiv here. The abstract reads:

The recent Planck Legacy 2018 release has confirmed the presence of an enhanced lensing amplitude in CMB power spectra compared to that predicted in the standard ΛCDM model. A closed universe can provide a physical explanation for this effect, with the Planck CMB spectra now preferring a positive curvature at more than 99% C.L. Here we further investigate the evidence for a closed universe from Planck, showing that positive curvature naturally explains the anomalous lensing amplitude and demonstrating that it also removes a well-known tension within the Planck data set concerning the values of cosmological parameters derived at different angular scales. We show that since the Planck power spectra prefer a closed universe, discordances higher than generally estimated arise for most of the local cosmological observables, including BAO. The assumption of a flat universe could, therefore, mask a cosmological crisis where disparate observed properties of the Universe appear to be mutually inconsistent. Future measurements are needed to clarify whether the observed discordances are due to undetected systematics, or to new physics, or simply are a statistical fluctuation.

I think the important point to take from this study is that estimates of cosmological parameters obtained from Planck are relatively indirect, in that they involve the simultaneous determination of several parameters some of which are almost degenerate. For example, the `anomalous’ lensing amplitude discussed in this paper is degenerate with the curvature so that changing one could mimic the effect on observables of changing the other; see Figure 2 in the paper.

It’s worth mentioning another (and, in my opinion, better argued) paper on a similar topic by Will Handley of Cambridge which is on the arXiv here. The abstract of this one reads:

The curvature parameter tension between Planck 2018, cosmic microwave background lensing, and baryon acoustic oscillation data is measured using the suspiciousness statistic to be 2.5 to 3σ. Conclusions regarding the spatial curvature of the universe which stem from the combination of these data should therefore be viewed with suspicion. Without CMB lensing or BAO, Planck 2018 has a moderate preference for closed universes, with Bayesian betting odds of over 50:1 against a flat universe, and over 2000:1 against an open universe.

Figure 1 makes a rather neat point that the combination of Planck and Baryon Acoustic Oscillations does not separately give consistent values for the Hubble constant and the curvature and neither does the combination of Planck and direct Hubble constant estimates:

I don’t know what the resolution of these tensions is, but I think it is a bit dangerous to dismiss them simply as statistical flukes. They might be that, of course, but they also might not be. By shrugging one’s shoulders and ignoring such indications one might miss something very fundamental. On the other hand, in my opinion, there is nothing here that definitely points the finger at spatial curvature either: it is possible that there is something else missing from the standard model that, if included, would resolve these tensions. But what is the missing link?

Answers on a postcard, or through the comments box.

The 2019 Gruber Prize for Cosmology: Nick Kaiser and Joe Silk

Posted in The Universe and Stuff with tags , , , , , , , on May 9, 2019 by telescoper

I’ve just heard that the Gruber Foundation has announced the winners of this year’s Gruber Prize for cosmology, namely Nick Kaiser and Joe Silk. Worthy winners the both of them! Congratulations!

Here’s some text taken from the press release:

The recipients of the 2019 prize are Nicholas Kaiser and Joseph Silk, both of whom have made seminal contributions to the theory of cosmological structure formation and to the creation of new probes of dark matter. Though they have worked mostly independently of each other, the two theorists’ results are complementary in these major areas, and have transformed modern cosmology — not once but twice.

The two recipients will share the $500,000 award, and each will be presented with a gold medal at a ceremony that will take place on 28 June at the CosmoGold conference at the Institut d’Astrophysique de Paris in France.

The physicists’ independent contributions to the theory of cosmological structure formation have been instrumental in building a more complete picture of how the early Universe evolved into the Universe as astronomers observe it today. In 1967 and 1968, Silk predicted that density fluctuations below a critical size in the Cosmic Microwave Background, the remnant radiation “echoing” the Big Bang, would have dissipated. This phenomenon, later verified by increasingly high precision measurements of the CMB, is now called “Silk Damping”.

In the meantime, ongoing observations of the large-scale structure of the Universe, which evolved from the larger CMB fluctuations, were subject to conflicting interpretations. In a series of papers beginning in 1984, Kaiser helped to resolve these debates by providing statistical tools that would allow astronomers to separate “noise” from data, reducing ambiguity in the observations.

Kaiser’s statistical methodology was also influential in dark matter research; the DEFW collaboration (Marc Davis, George Efstathiou, Carlos Frenk, and Simon D. M. White) utilised it to determine the distribution and velocity of dark matter in the Universe, and discovered its non-relativistic nature (moving at a velocity not approaching the speed of light). Furthermore, Kaiser devised an additional statistical methodology to detect dark matter distribution through weak lensing — an effect by which foreground matter distorts the light of background galaxies, providing a measure of the mass of both. Today weak lensing is among cosmology’s most prevalent tools.

Silk has also been impactful in dark matter research, having proposed in 1984 a method of investigating dark matter particles by exploring the possibilities of their self-annihilations into particles that we can identify (photons, positrons and antiprotons). This strategy continues to drive research worldwide.

Both Kaiser and Silk are currently affiliated with institutions in Paris, Kaiser as a professor at the École Normale Supérieure, and Silk as an emeritus professor and a research scientist at the Institut d’Astrophysique de Paris (in addition to a one-quarter appointment at The John Hopkins University). Among their numerous significant contributions to their field, their work on the CMB and dark matter has truly revolutionised our understanding of the Universe.

I haven’t worked directly with either Nick Kaiser or Joe Silk but both had an enormous influence on me, especially early on in my career. When I was doing my PhD, Nick was in Cambridge and Joe was in Berkeley. In fact I think Nick was the first person ever to ask me a question during a conference talk – which terrified the hell out of me because I didn’t know him except by scientific reputation and didn’t realize what a nice guy he is! Anyway his 1984 paper on cluster correlations was the direct motivation for my very first publication (in 1986).

I don’t suppose either will be reading this but heartiest congratulations to both, and if they follow my advice they won’t spend all the money in the same shop!

P.S. Both Nick and Joe are so distinguished that each has appeared in my Astronomy Lookalikes gallery (here and here).

SPT and the CMB

Posted in The Universe and Stuff with tags , , , , , , , , , on November 30, 2012 by telescoper

I’ve been remiss in not yet passing on news  from the South Pole Telescope, which has recently produced a number of breakthrough scientific results, including:  improved cosmological constraints from the SPT-SZ cluster survey (preprint here); a new catalogue of 224 SZ-selected cluster candidates from the first 720 square-degrees of the survey (preprint here); the first measurement of galaxy bias from the gravitational lensing of the CMB (preprint here); the first CMB-based constraint on the evolution of the ionized fraction during the epoch of reionization (preprint here); the most-significant detection of non-Gaussianity induced from the gravitational lensing of the CMB (preprint here); and the most precise measurement of the CMB damping tail and improved constraints on models of Inflation (preprint here).

Here’s the graph that drew my eye (from this paper). It shows the (angular) power spectrum of the cosmic microwave for very high (angular) frequency spherical harmonics; the resolution of SPT allows it to probe finer details of the spectrum that WMAP (also shown, at lower l).


This is an amazing graph, especially for oldies like me who remember being so impressed by the emergence of the first “acoustic peak” at around l=200 way back in the days of Boomerang and Maxima and gobsmacked by WMAP’s revelation of the second and third. Now there are at least six acoustic peaks, although of progressively lower amplitude. The attenuation of the CMB fluctuations at high frequencies is the result of diffusion damping – similar to the way high-frequency sound waves are attenuated when they pass through a diffusive medium (e.g. a gas).  The phenomenon in this case is usually called Silk Damping, as it was first worked out back in the 1960s by Joe Damping Silk.

Anyway, there’ll be a lot more CMB news early (?) next year from Planck which will demonstrate yet again that cosmic microwave background physics has certainly come a long way from pigeon shit

Research Opportunities in the Philosophy of Cosmology

Posted in The Universe and Stuff with tags , , , , , , on March 16, 2012 by telescoper

I got an email this morning telling me about the following interesting opportunities for research fellowships. They are in quite an unusual area – the philosophy of cosmology – and one I’m quite interested in myself so I thought it might ahieve wider circulation if I posted the advertisement on here.


Applications are invited for two postdoctoral fellowships in the area of philosophy of cosmology, one to be held at Cambridge University and one to be held at Oxford University, starting 1 Jan 2013 to run until 31 Aug 2014. The two positions have similar job-descriptions and the deadline for applications is the same: 18 April 2012.

For more details, see here, for the Cambridge fellowship and  here for the Oxford fellowship.

Applicants are encouraged to apply for both positions. The Oxford group is led by Joe Silk, Simon Saunders and David Wallace, and that at Cambridge by John Barrow and Jeremy Butterfield.

These appointments are part of the initiative ‘establishing the philosophy of cosmology’, involving a consortium of universities in the UK and USA, funded by the John Templeton Foundation. Its aim is to identify, define and explore new foundational questions in cosmology. Key questions already identified concern:

  • The issue of measure, including potential uses of anthropic reasoning
  • Space-time structure, both at very large and very small scales
  • The cosmological constant problem
  • Entropy, time and complexity, in understanding the various arrows of time
  • Symmetries and invariants, and the nature of the description of the universe as a whole

Applicants with philosophical interests in cosmology outside these areas will also be considered.

For more background on the initiative, see here and the project website (still under construction).

Astronomy Look-alikes, No. 72

Posted in Astronomy Lookalikes with tags , , on December 7, 2011 by telescoper

Have you noticed the remarkable resemblance between esteemed cosmologist Joe Silk and renowned character actor Alastair Sim? If it weren’t for Prof. Silk’s unusual taste in headgear, it would be difficult to tell them apart….

From Here to Eternity

Posted in Books, Talks and Reviews, The Universe and Stuff with tags , , , , on February 3, 2009 by telescoper

I posted an item about astronomy and poetry a couple of days ago that used a phrase I vaguely remember having used somewhere else before. I’ve only just remembered where. It was in this book review I did for Nature some time ago. Since I’m quite keen on recycling, I’d thought I’d put it on here.

How do physicists cope with the concept of infinity in an expanding Universe?

BOOK REVIEWED – The Infinite Cosmos: Questions from the Frontiers of Cosmology

by Joseph Silk

Oxford University Press: 2006. 256 pp. £18.99, $29.95

Scientists usually have an uncomfortable time coping with the concept of infinity. Over the past century, physicists have had a particularly difficult relationship with the notion of boundlessness. In most cases this has been symptomatic of deficiencies in the theoretical foundations of the subject. Think of the ‘ultraviolet catastrophe’ of classical statistical mechanics, in which the electromagnetic radiation produced by a black body at a finite temperature is calculated to be infinitely intense at infinitely short wavelengths; this signalled the failure of classical statistical mechanics and ushered in the era of quantum mechanics about a hundred years ago. Quantum field theories have other forms of pathological behaviour, with mathematical components of the theory tending to run out of control to infinity unless they are healed using the technique of renormalization. The general theory of relativity predicts that singularities in which physical properties become infinite occur in the centre of black holes and in the Big Bang that kicked our Universe into existence. But even these are regarded as indications that we are missing a piece of the puzzle, rather than implying that somehow infinity is a part of nature itself.

The exception to this rule is the field of cosmology. Somehow it seems natural at least to consider the possibility that our cosmos might be infinite in extent or duration. If the Universe is defined as everything that exists, why should it necessarily be finite? Why should there be some underlying principle that restricts it to a size our human brains can cope with?

But even if cosmologists are prepared to ponder the reality of endlessness, and to describe it mathematically, they still have problems finding words to express these thoughts. Physics is fundamentally prosaic, but physicists have to resort to poetry when faced with the measureless grandeur of the heavens.

In The Infinite Cosmos, Joe Silk takes us on a whistle-stop tour of modern cosmology, focusing on what we have learned about the size and age of the Universe, how it might have begun, and how it may or may not end. This is a good time to write this book, because these most basic questions may have been answered by a combination of measurements from satellites gathering the static buzz of microwaves left over from the Big Bang, from telescopes finding and monitoring the behaviour of immensely distant supernova explosions, and from painstaking surveys of galaxy positions yielding quantitative information about the fallout from the primordial fireball. Unless we are missing something of fundamental importance, these observations indicate that our expanding Universe is about 14 billion years old, contains copious quantities of dark matter in some unidentified form, and is expanding at an accelerating rate.

According to the standard model of cosmology that emerges, the Universe has a finite past and (perhaps) an infinite future. But is our observable Universe (our ‘Hubble bubble’) typical of all there is? Perhaps there is much more to the cosmos than will ever meet our eyes. Our local patch of space-time may have its origin in just one of an infinite and timeless collection of Big Bangs, so the inferences we draw from observations of our immediate neighbourhood may never tell us anything much about the whole thing, even if we correctly interpret all the data available to us.

What is exciting about this book is not so much that it is anchored by the ramifications of infinity, but that it packs so much into a decidedly finite space. Silk covers everything you might hope to find in a book by one of the world’s leading cosmologists, and much more besides. Black holes, galaxy formation, dark matter, time travel, string theory and the cosmic microwave background all get a mention.

The style is accessible and informative. The book also benefits from having a flexible structure, free from the restrictions of the traditional historical narrative. Instead there are 20 short chapters arranged in a way that brings out the universality of the underlying physical concepts without having too much of a textbook feel. The explanations are nicely illustrated and do not involve any mathematics, so the book is suitable for the non-specialist.

If I have any criticisms of this book at all, they are only slight ones. The conflation of the ‘expanding Universe’ concept with the Big Bang theory, as opposed to its old ‘steady state’ rival, is both surprising and confusing. The steady-state model also describes an expanding Universe, but one in which there is continuous creation of matter to maintain a constant density against the diluting effect of the expansion. In the Big Bang, there is only one creation event, so the density of the expanding Universe changes with time. I also found the chapter about God in cosmology to be rather trite, but then my heart always sinks when I find myself lured into theological territory in which I am ill-equipped to survive.