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Newsflash: Direct Detection of B-mode Polarization

Posted in The Universe and Stuff with tags , , , , , on July 23, 2013 by telescoper

I’m not meant to be blogging these days but I thought I’d break radio silence to draw attention to a new paper on the arXiv by Hanson et al. from SPTpol, an experiment which aims to measure the polarization of the cosmic microwave background using the South Pole Telescope. One of the main aims of experiments such as this is to measure the so-called “B-mode” of polarization (the “curl” component of the polarization signal, which possesses a handedness) because this holds the key to direct detection of a number of interesting cosmological phenomena such as the existence of primordial gravitational waves.  However, primordial effects aren’t  the only way to generate B-mode polarization. Other “foreground” effects can do the job too, especially gravitational lensing can also generate a signal of this form. These “late-time” effects have to be understood before the primordial contribution can be isolated.

Before today there was no direct measurement of B-mode polarization at all, primordial nor not.

The abstract basically says it all:

Gravitational lensing of the cosmic microwave background generates a curl pattern in the observed polarization. This “B-mode” signal provides a measure of the projected mass distribution over the entire observable Universe and also acts as a contaminant for the measurement of primordial gravity-wave signals. In this letter we present the first detection of gravitational lensing B modes, using first-season data from the polarization-sensitive receiver on the South Pole Telescope (SPTpol). We construct a template for the lensing B-mode signal by combining E-mode polarization measured by SPTpol with estimates of the lensing potential from a Herschel-SPIRE map of the cosmic infrared background. We compare this template to the B modes measured directly by SPTpol, finding a non-zero correlation at 7.7 sigma significance. The correlation has an amplitude and scale-dependence consistent with theoretical expectations, is robust with respect to analysis choices, and constitutes the first measurement of a powerful cosmological observable.

This measurement is not unexpected. Indeed, the B-mode contribution from lensing by the known distribution of galaxies can be calculated fairly straightforwardly because the physics is well understood; failure to find the expected signal would therefore have been somewhat embarrassing.  It’s a different story for the primordial B-mode because that depends strongly on what is going on in the very early universe, and that is much less certain. Although the new result doesn’t itself tell us anything new about the very early Universe it is definitely an important step on the way, and it’s a fairly safe prediction that there will be a great deal of activity and interest in CMB polarization over the next few years, including next year’s planned release of polarization data from Planck.

I’ll also note the use of Herschel-SPIRE images in tracing the galaxy images, in deference to my former colleagues in Cardiff who played a key role in developing that instrument!

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2013 Gruber Prize in Cosmology

Posted in The Universe and Stuff with tags , , , on July 11, 2013 by telescoper

The latest session at this Summer School began with a nice announcement, that one of the organizers (and lecturers) Viatcheslav Mukhanov has, together with Alexei Starobinsky, been awarded the prestigious Gruber Prize for cosmology.

The press release linked above states:

According to the Prize citation, their theoretical work “changed our views on the origin of our universe and on the mechanism of its formation of structure.” Thanks to their contributions, scientists have provided a compelling solution to two of the essential questions of cosmology:  Why is the structure of the universe so uniform on the largest scales?  Where did the departures from uniformity—such as galaxies, planets, and people—come from?

Mukhanov, full professor of physics at the Ludwig-Maximilians-Universität in Munich, and Starobinsky, the main research scientist at the Landau Institute for Theoretical Physics in Moscow, will share the $500,000 award, which will be presented on September 3 as part of the COSMO2013 conference at the Stephen Hawking Centre for Theoretical Cosmology in Cambridge, UK.

The work for which they are being honored began in the late 1970s and early 1980s, during a period of fertile, even fervid, theoretical investigations into the earliest moments of the universe.  In 1965 astronomers had discovered the cosmic microwave background—relic radiation dating to an era 13.8 billion years ago, when the universe was approximately 380,000 years old, during which hydrogen atoms and photons (packets of light) decoupled, causing a kind of “flashbulb” image that pervades the universe to this day.  This discovery validated a key prediction of the Big Bang theory and inspired a generation of theorists.

Among them was Starobinsky, then a senior research scientist at the Landau Institute.  His approach was to use quantum mechanics and general relativity to try to address how an expanding universe might have originated.  While he did not resolve that issue, his calculations made in 1979 – 1980 did indicate that the universe could have gone through an extraordinarily rapid exponential expansion in the first moments of its existence.

The following year Mukhanov (Moscow Physical-Technical Institute) and G. V. Chibisov (Lebedev Physical Institute, Moscow; he passed away several years ago), began working on the implications of quantum fluctuations within the Starobinsky model.  Quantum fluctuations—disturbances in the fabric of space predicted by Heisenberg’s uncertainty principle—are always present in the universe.  But in an extremely small, extremely dense, and extremely energetic newborn universe they would have had an outsized presence.  What’s more, the kind of exponential expansion that Starobinsky was proposing would have stretched those fluctuations beyond the quantum scale.  In 1981 Mukhanov and Chibisov discovered that these fluctuations could play the role of the seeds that eventually bloomed into the present large-scale web-like structure of the universe:  galaxies, clusters of galaxies, and superclusters of galaxies.

When this mechanism was first proposed, it looked like a piece of science fiction. Indeed, usually quantum fluctuations appear only on tiny subatomic scales, so the idea that galaxies have been born from quantum fluctuations seemed totally outlandish. And yet the subsequent developments in theoretical and observational cosmology strongly favored this possibility.

Shortly after the Starobinsky work, the American physicist Alan Guth proposed a brilliant idea that an exponential expansion stage of the early universe, which he called “inflation,” could explain the incredible uniformity of our universe and resolve many other outstanding problems of the Big Bang cosmology. However, Guth immediately recognized that his proposal had a flaw: the world described by his scenario would become either empty or very non-uniform at the end of inflation. This problem was solved by Andrei Linde, who introduced several major modifications of inflationary theory, such as “new inflation” (later also developed by Albrecht and Steinhardt), “chaotic inflation”, and “eternal chaotic inflation.” A new cosmological paradigm was born. In 2004, Guth and Linde received the Gruber Prize for the development of inflationary theory.

The original goals of the Starobinsky model were quite different from the goals of inflationary theory. Instead of trying to explain the uniformity of the universe, he assumed that the universe was absolutely homogeneous from the very beginning. However, it was soon realized that the mathematical structure of his model was very similar to that of new inflation, and therefore it naturally merged into the rapidly growing field of inflationary cosmology.

In 1982, several scientists, including Starobinsky, outlined a theory of quantum fluctuations generated in new inflation. This theory was very similar to the theory developed by Mukhanov and Chibisov in the context of the Starobinsky model. Investigation of inflationary fluctuations culminated in 1985in work by Mukhanov, who developed a rigorous theory of these fluctuations applicable to a broad class of inflationary models, including new and chaotic inflation.

This theory predicted that inflationary perturbations have nearly equal amplitude on all length scales. An equally important conclusion was that this scale invariance is close, but not exact: the amplitude of the fluctuations should slightly grow with the distance. These fluctuations would have equal amplitudes for all forms of matter and energy (called adiabatic fluctuations). The theory also predicted a specific statistical form of the fluctuations, known as Gaussian statistics.

Since then, increasingly precise observations of the cosmic microwave background radiation (CMB) have provided decisive matches for theoretical predictions of how those initial quantum fluctuations would look after the universe had been expanding for 380,000 years.  Those observations include all-sky maps produced by the Cosmic Microwave Background Explorer (COBE), the Wilkinson Microwave Anisotropy Probe (WMAP), and the Planck satellite.  John Mather and the COBE team received the Gruber Cosmology Prize in 2006; Charles Bennett and the WMAP team received theirs in 2012.

Back in 1979, Starobinsky also found that exponential expansion of the universe should produce gravitational waves — a quantum by-product of general relativity, and a target for the new generation of instruments expected over the next decade.

This year’s Gruber Cosmology Prize citation credits Starobinsky and Mukhanov with a profound contribution to inflationary cosmology and the theory of the inflationary perturbations of the metric of space-time. This theory, explaining the quantum origin of the structure of our universe, is one of the most spectacular manifestations of the laws of quantum mechanics on cosmologically large scales.

Congratulations to them both! Sadly, Slava Mukhanov left Bad Honnef yesterday evening in order to return to Munich so he’s unable to use a small part of his share of the $500,000 prize to buy celebratory drinks for all the participants, but I’m sure we’ll have some sort of  celebration in his absence. But that will have to wait until this evening. We wouldn’t want to interrupt the lectures, would we?

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The Inflationary Bubble

Posted in The Universe and Stuff with tags , , , , on July 9, 2013 by telescoper

The Summer School I’m attending on Inflation and the CMB got under way yesterday morning with a couple of lectures (90 minutes each) by Andrei Linde, one of the pioneers of the theory of cosmic inflation. I enjoyed the first part of the session, but then he went off into the technical details of a specific model for which there seemed previous little in the way of physical motivation or testable consequences. There’s an occupational hazard for people working on inflation which is that they become so absorbed by their calculations that they forget that they’re supposed to be doing science. It sometimes appears that this field has reached a critical density of activity which means that it’s in danger of forming a closed universe completely incapable of communicating with the world outside and perhaps of collapsing in on itself.

The other thing I didn’t like was the evangelism about the multiverse, which is widespread amongst theorists these days. I’ve stated my position about this before so I won’t repeat my objections here. I will, however, lodge an objection to the way Prof. Linde answered a question about whether the multiverse theory was a testable of various fine-tuning problems in cosmology by saying

Ihe multiverse is the only known explanation so in a sense it has already been tested.

I don’t mind particularly if theories are not testable with current technology. New ideas often have to wait a very long time before equipment and techniques are developed to test them, but Linde’s response is rather symptomatic of a frame of mind that does not consider testability important at all. The worst offenders in this regard are certain string theorists who seem to thing string theory is so compelling in its own right that it just has to be the one true description of how the Universe works, even if the framework it provides is unable to make any predictions at all.

IMG-20130708-00145

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Germany Calling…

Posted in Biographical, Books, Talks and Reviews, The Universe and Stuff with tags , , , , on July 7, 2013 by telescoper

Just a quick post to break radio silence and announce my arrival in the picturesque town of Bad Honnef, spa town in Germany near Bonn in the Rhein-Sieg district of North Rhine-Westphalia. We’re right on the banks of the Rhine actually, and there are some fine views of castles and hills to be had all round.

To get here I took my life in my hands and flew with a German budget airline called Germanwings from Heathrow to nearby Bonn-Cologne airport. I mean it’s near to Bad Honnef, not to Heathrow. Apart from the fact that I had to queue for an hour at check-in because the staff apparently didn’t know how to operate the computer system, and the flight was delayed leaving because it was delayed on the way in, it wasn’t actually too bad; we arrived only about 25 minutes late and I was able to have a few beers and some food when I arrived at my destination.

The reason for this expedition is that I’m giving two lectures at the Deutschen Physikalischen Gesellschaft (henceforth DPG) Summer School on Inflation and the CMB. The list of other speakers is very impressive so I assume that some form of administrative error is responsible for my invitation, and especially for the fact that I’ve got to give two lectures while everyone else is just giving one…

Anyway, it’s lovely weather here – although a little on the toasty side for my cold English blood – and I hope to get the chance to take a few pictures as well as some updates from the meeting. I also hope to find out why this place is called Bad Honnef. I know I’ve only been here a few hours, but it seems to me that, as Honnefs go, it’s really not bad at all…

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Physics Proverbs

Posted in The Universe and Stuff with tags , on July 2, 2013 by telescoper

I was a bit bored on the bus this morning, as it got stuck in a traffic jam, so decided to amuse myself (and probably nobody else) by thinking up physics-related versions of traditional proverbs and tweeting them (hashtag #physicsproverbs). I thought it might be fun to use them to indulge in a bit of audience participation, by asking the blogosphere to contribute their own through  the comments box below.

Here are some of my offerings:

  • Never mind the Q-factor, feel the FWHM
  • Don’t throw stones if there are periodic boundary conditions
  • A stitch in time may violate causality
  • A thing of beauty is now generally known as a bottom
  • No amplifier, no gain
  • Nothing is certain, except death and deterministic processes
  • Blood is thicker than dark matter
  • May the Devil take the Hindmarsh
  • Don’t change potentials in mid streamline
  • Angular momentum makes the world go round
  • Many a micro makes a mega
  • When the cat’s away the mice will annoy Dr Schrödinger
  • Ask a silly question, and you might well get a research grant
  • Discreteness is the greater part of granularity
  • There’s no time like t=0
  • The course of a random walk never did run smooth
  • Many hadrons make very few Higgs Bosons at CERN
  • Actions speak louder than differential equations
  • Radiation pressure makes light work
  • Don’t cast your PRLs before swine
  • Nature abhors most of the papers submitted there
  • Photons should be seen and not heard. As opposed to phonons.
  • Power corrupts. Absolute power has exactly the same effect because power is always positive.

You can see all the tweets resulting from the Twitter version of this game here.

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The Local Universe

Posted in The Universe and Stuff with tags , , , , on July 2, 2013 by telescoper

I just stumbled across this on Amanda Bauer’s blog  and thought I’d post it here because it’s so nice. The film is by Hélène Courtois, Daniel Pomarède, R. Brent Tully, Yehuda Hoffman, and Denis Courtois and it describes the Cosmography – like geography, only more cosmic – of the Local Universe. I’m not sure there’s a consensus among cosmologists about what exactly “local” means, but I’d say it probably means out to a few hundred Megaparsecs from the observer (say up to about a billion light years) or, alternatively, with redshifts much less than unity.  That may not sound very nearby at all, but even on such scales the look-back time is sufficiently short that the effect of cosmic evolution and/or the expansion of the Universe is negligible, so when we look at objects at such distances we’re seeing them as they are “now” rather than as they were in the past, which is the case when we study extremely distant objects.

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Slow Progress for Female Physics Professors

Posted in Biographical, The Universe and Stuff with tags , , , , on July 1, 2013 by telescoper

One of the more pleasant jobs I have to do these days is to congratulate staff in the School of Mathematical and Physical Sciences at the University of Sussex when they get promoted, whether it be to Senior Lecturer, Reader or Professor. There has been quite a crop of promotions at all levels in the School recently, owing to the excellent contributions made by so many people to teaching, research and other aspects of the work we do.

One of the successful promotion candidates in the latest round was the Head of our Experimental Particle Physics group, Antonella de Santo, whose promotion to Professor of Physics makes her the first ever female Professor of Physics at the University of Sussex. I’m rather embarrassed to admit that, actually, as the University has existed for 51 years, but at least I can say better late then never!

Anyway, Antonella’s well-deserved success prompted me to look into the statistics of female physics & astronomy professors. I’ve already posted about how the proportion of female undergraduates studying physics as been stuck at around the 20% mark for a decade despite strenuous efforts to widen participation. A recent (2012) study by the Institute of Physics contains a wealth of statistical information about staff in Physics departments, which I encourage people to read if they’re interested in the overall issue with equality and diversity in physics. Here I’ll just pull out the figure (based on a 2010 survey) that out of a total of 650 Professors of Physics (and/or Astronomy) in the UK, just 5.5% were female. At that date about 20 physics departments had no female professors at all; that would have included Sussex, of course.

Another University, Liverpool, also recently appointed its first female Professor of Physics in the person of Tara Shears, another particle physicist. The current  Head of the  Department of Physics at Imperial Collge is Joanna Haigh, (who I thought was the first to occupy such a position until corrected by the comment below) so there are signs that career prospects are improving for female physicists, but progress is painfully slow. The first ever female Professor of Physics in the United Kingdom was Daphne Jackson, a nuclear physicist, who took up her Chair at the University of Surrey way back in 1971. It’s interesting to note that when Daphne Jackson studied physics as an undergraduate at Imperial College she was one of only two women among the 88 undergraduates in her year.

I don’t personally think that there’s a significant gender bias when it comes to the consideration of promotion cases at the University of Sussex (or at any other institution I’ve worked at), but there’s plenty of anecdotal evidence that women are much more reluctant than men to put themselves forward for consideration at any level. I hope that recent successes in MPS, such as Antonella’s Professorship and Readerships for astronomer Kathy Romer and mathematician Vanessa Styles, will provide the necessary encouragement.

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Punch and Judy meet Quantum Technology

Posted in The Universe and Stuff with tags , , , , , , on June 28, 2013 by telescoper

It’s an Open Day here on campus, and there’s quite a crowd of potential students and parents gathering in the School of Mathematical and Physical Sciences here at the University of Sussex to find out a bit more about the School in advance of making decisions about where to apply next year.

I noticed the other day that quite a few of these have appeared on campus over the last few days:

IMG-20130627-00139

Apparently they’re information points manned by various helpers to help visitors find their way around the place. When I first saw this one, I thought it was a Punch and Judy box, so assumed that there was some sort of conference of Punch and Judy performers going on. That wouldn’t be inappropriate for a University campus, actually, because the traditional name for a Punch & Judy puppeteer is a “Professor”. Not a lot of people know that.

Anyway, none of that is really relevant to what I wanted to post today. I stumbled across this video featuring Winfried Hensinger (one of my colleagues from the Department of Physics & Astronomy within the School of Mathematical and Physical Sciences). I thought it would be fun to share it here, just to give an idea of some of the work that’s going on here outside my own speciality of astrophysics. I hope this will complement the real open day with a mini virtual open day on the blog.

Winfried is Reader in Quantum, Atomic and Optical Physics at the University of Sussex and he works in the group we generally call “AMO” (Atomic, Molecular and Optical). In this TEDX lecture he talks about the future of quantum computers and the role the team he is part of, at Sussex University, plays as they develop large scale quantum computers using ions cooled to extremely low temperatures using lasers. Enjoy!

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Universality in Space Plasmas?

Posted in Astrohype, The Universe and Stuff with tags , , , , , , , , on June 16, 2013 by telescoper

It’s been a while since I posted anything reasonably technical, largely because I’ve been too busy, so I thought I’d spend a bit of time today on a paper (by Livadiotis & McComas in the journal Entropy) that provoked a Nature News item a couple of weeks ago and caused a mild flutter around the internet.

Here’s the abstract of the paper:

In plasmas, Debye screening structures the possible correlations between particles. We identify a phase space minimum h* in non-equilibrium space plasmas that connects the energy of particles in a Debye sphere to an equivalent wave frequency. In particular, while there is no a priori reason to expect a single value of h* across plasmas, we find a very similar value of h* ≈ (7.5 ± 2.4)×10−22 J·s using four independent methods: (1) Ulysses solar wind measurements, (2) space plasmas that typically reside in stationary states out of thermal equilibrium and spanning a broad range of physical properties, (3) an entropic limit emerging from statistical mechanics, (4) waiting-time distributions of explosive events in space plasmas. Finding a quasi-constant value for the phase space minimum in a variety of different plasmas, similar to the classical Planck constant but 12 orders of magnitude larger may be revealing a new type of quantization in many plasmas and correlated systems more generally.

It looks an interesting claim, so I thought I’d have a look at the paper in a little more detail to see whether it holds up, and perhaps to explain a little to others who haven’t got time to wade through it themselves. I will assume a basic background knowledge of plasma physics, though, so turn away now if that puts you off!

For a start it’s probably a good idea to explain what this mysterious h* is. The authors define it via ½h*=εctc, where εc is defined to be “the smallest particle energy that can transfer information” and tc is “the correlation lifetime of Debye Sphere (i.e. volumes of radius the Debye Length for the plasma in question). The second of these can be straightforwardly defined in terms of the ratio between the Debye Length and the thermal sound speed; the authors argue that the first is given by εc=½(mi+me)u2, involving the electron and ion masses in the plasma and the information speed u which is taken to be the speed of a magnetosonic wave.

You might wonder why the authors decided to call their baby h*. Perhaps it’s because the definition looks a bit like the energy-time version of Heisenberg’s Uncertainty Principle, but I can’t be sure of that. In any case the resulting quantity has the same dimensions as Planck’s constant and is therefore measured in the same units (Js in the SI system).

Anyway, the claim is that h* is constant across a wide range of astrophysical plasmas. I’ve taken the liberty of copying the relevant Figure here:

constant_h

I have to say at this point I had the distinct sense of damp squib going off. The panel on the right purports to show the constancy of h* (y-axis) for plasmas of a wide range of number-densities (x-axis). However, but are shown on logarithmic scales and have enormously large error bars. To be sure, the behaviour looks roughly constant but to use this as a basis for claims of universality is, in my opinion, rather unjustified, especially since there may also be some sort of selection effect arising from the specific observational data used.

One of the authors is quoted in the Nature piece:

“We went into this thinking we’d find one value in one plasma, and another value in another plasma,” says McComas. “We were shocked and slightly horrified to find the same value across all of them. This is really a major deal.”

Perhaps it will turn out to be a major deal. But I’d like to see a lot more evidence first.

Plasma (astro)physics is a fascinating but very difficult subject, not because the underlying requations governing plasmas are especially complicated, but because the resulting behaviour is so sensitively dependent on small details; plasma therefore provide an excellent exemplar of what we mean by a complex physical system. As is the case in other situations where we lack the ability to do detailed calculations at the microscopic level, we do have to rely on more coarse=grained descriptions, so looking for patterns like this is a good thing to do, but I think the Jury is out.

Finally, I have to say I don’t approve of the authors talking about this in terms of “quantization”. Plasma physics is confusing enough as classical physics without confusing it with quantum theory. Opening the door to that is a big mistake, in my view. Who knows what sort of new age crankery might result?

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A Time for Honours

Posted in Education, Politics, Science Politics, The Universe and Stuff with tags , , , on June 15, 2013 by telescoper

The word “honour” provides a (tenuous) link between yesterday’s post and this one. After our recent preoccupation with the classification of honours for graduating students (i.e. first class, second class, and so on), today’s news included the Queen’s Birthday Honours List for 2013, which you can download in full here. To make up for the lack of recycling going on in Brighton these days because of the strike that started yesterday, I thought I’d recycle my thoughts from previous years.

The honours system must appear extremely curious to people from outside the United Kingdom. It certainly seems so to me. On the one hand, I am glad that the government has a mechanism for recognising the exceptional contributions made to society by certain individuals. Musicians, writers, sportsmen, entertainers and the like generally receive handsome financial rewards, of course, but that’s no reason to begrudge a medal or two in recognition of the special place they occupy in our cultural life.  It’s  good to see scientists recognized too, although they tend not to get noticed so much by the press.

The name that stood out for me in this year’s list is Professor Jim Hough, who gets an OBE. Jim is Professor of Experimental Physics at the University of Glasgow, and his speciality is in the detection of gravitational waves.  Gravitational waves haven’t actually been detected yet, of course, but the experimental techniques designed to find them have increased their sensitivity by many orders of magnitude in recent years, Jim having played a large part in those improvements. I imagine he will be absolutely thrilled in February 2016, when gravitational waves are finally detected. Jim is also Chief Executive of the Scottish University Physics Alliance, which does so much to nurture Physics and Astronomy North of the Border.

Although I’m of course more than happy to see recognition given to such people, as I did  a couple of years ago I can’t resist stating my objections to the honours system again. One is that the list of recipients  of certain categories of award is overwhelmingly dominated by career civil servants, for whom an “honour”  goes automatically with a given rank. If an honour is considered an entitlement in this way then it is no honour at all, and in fact devalues those awards that are  given on merit to people outside the Civil Service. Civil servants get paid for doing their job, so they should have no more expectation of an additional reward than anyone else. There’s much more honour in a  student who earns a First Class degree than for a career civil servant who gets a knighthood.

Honours have relatively little monetary value on their own, of course so this is not question of financial corruption. An honour does, however, confer status and prestige on the recipient so what we have is a much more subtle form of sleaze. One wonders how many names listed in the current roll of honours are there because of political donations, for example.

I wouldn’t accept an honour myself, but that’s easy to say because I’m sure I’ll never be nominated for one; hopefully this post will dissuade anyone from even thinking of nominating me for a gong. However, I imagine that even people like me who are against the whole system are probably still tempted to accept such awards when offered, as they generate good publicity for one’s field, institution and colleagues.It’s a very personal decision and I have no criticism to make of people who think differently from me about whether to accept an honour.

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