Spring & Summer Science

As the academic year winds to a close, scientists’ thoughts turn towards all of the warm-weather travel ahead (in order to avoid thinking about exam marking). Mostly, that means attending scientific conferences, like the upcoming IAU Symposium, Statistical Challenges in 21st Century Cosmology in Lisbon next month, and (for me and my collaborators) the usual series of meetings to prepare for the 2014 release of Planck data. But there are also opportunities for us to interact with people outside of our technical fields: public lectures and festivals.

Next month, parallel to the famous Hay Festival of Literature & the Arts, the town of Hay-on-Wye also hosts How The Light Gets In, concentrating on the also-important disciplines of philosophy and music, with a strong strand of science thrown in. This year, along with comic book writer Warren Ellis, cringe-inducing politicians like Michael Howard and George Galloway, ubiquitous semi-intellectuals like Joan Bakewell, there will be quite a few scientists, with a skew towards the crowd-friendly and controversial. I’m not sure that I want to hear Rupert Sheldrake talk about the efficacy of science and the scientific method, although it might be interesting to hear Julian Barbour, Huw Price, and Lee Smolin talk about the arrow of time. Some of the descriptions are inscrutable enough to pique my interest: Nancy Cartwright and George Ellis will discuss “Ultimate Proof” — I can’t quite figure out if that means physics or epistemology. Perhaps similarly, chemist Peter Atkins will ask “Can science explain all of existence” (and apparently answer in the affirmative). Closer to my own wheelhouse, Roger Penrose, Laura Mersini-Houghton, and John Ellis will discuss whether it is “just possible the Big Bang will turn out to be a mistake”. Penrose was and is one of the smartest people to work out the consequences of Einstein’s general theory of relativity, though in the last few years his cosmological musings have proven to be, well, just plain wrong — but, as I said, controversial and crowd-pleasing… (Disclosure: someone from the festival called me up and asked me to write about it here.)

Alas, I’ll likely be in Lisbon, instead of Hay. But if you want to hear me speak, you can make your way up North to Grantham, where Isaac Newton was educated, for this year’s Gravity Fields festival in late September. The line-up isn’t set yet, but I’ll be there, as will my fellow astronomers Chris Lintott and Catherine Heymans and particle physicist Val Gibson, alongside musicians, dancers, and lots of opportunities to explore the wilds of Lincolnshire. Or if you want to see me before then (and prefer to stay in London), you can come to Imperial for my much-delayed Inaugural Professorial Lecture on May 21, details TBC…

Gravitational Waves?

[Uh oh, this is sort of disastrously long, practically unedited, and a mixture of tutorial- and expert-level text. Good luck. Send corrections.]

It’s been almost exactly a year since the release of the first Planck cosmology results (which I discussed in some depth at the time). On this auspicious anniversary, we in the cosmology community found ourselves with yet more tantalising results to ponder, this time from a ground-based telescope called BICEP2. While Planck’s results were measurements of the temperature of the cosmic microwave background (CMB), this year’s concerned its polarisation.

Background

Polarisation is essentially a headless arrow that can come attached to the photons coming from any direction on the sky — if you’ve worn polarised sunglasses, and noticed how what you see changes as you rotate them around, you’ve seen polarisation. The same physics responsible for the temperature also generates polarisation. But more importantly for these new results, polarisation is a sensitive probe of some of the processes that are normally mixed in, and so hard to distinguish, in the temperature.

Technical aside (you can ignore the details of this paragraph). Actually, it’s a bit more complicated than that: we can think of the those headless arrows on the sky as the sum of two separate kinds of patterns. We call the first of these the “E-mode”, and it represents patterns consisting of either radial spikes or circles around a point. The other patterns are called the “B-mode” and look like patterns that swirl around, either to the left or the right. The important difference between them is that the E modes don’t change if you reflect them in a mirror, while the B modes do — we say that they have a handedness, or parity, in somewhat more mathematical terms. I’ve discussed the CMB a lot in the past but can’t do the theory of the CMB justice here, but my colleague Wayne Hu has an excellent, if somewhat dated, set of web pages explaining the physics (probably at a physics-major level).

EBfig

The excitement comes because these B-mode patterns can only arise in a few ways. The most exciting is that they can come from gravitational waves (GWs) in the early Universe. Gravitational waves (sometimes incorrectly called “gravity waves” which historically refers to unrelated phenomena!) are propagating ripples in space-time, predicted in Einstein’s general relativistic theory of gravitation. Because the CMB is generated about 400,000 years after the big bang, it’s only sensitive to gravitational radiation from the early Universe, not astrophysical sources like spiralling neutron stars or — from where we have other, circumstantial, evidence for gravitational waves, and which are the sources for which experiments like LIGO and eLISA will be searching. These early Universe gravitational waves move matter around in a specific way, which in turn induce those specific B-mode polarization pattern.

In the early Universe, there aren’t a lot of ways to generate gravitational waves. The most important one is inflation, an early period of expansion which blows up a subatomically-sized region by something like a billion-billion-billion times in each direction — inflation seems to be the most well thought-out idea for getting a Universe that looks like the one in which we live, flat (in the sense of Einstein’s relativity and the curvature of space-time), more or less uniform, but with small perturbations to the density that have grown to become the galaxies and clusters of galaxies in the Universe today. Those fluctuations arise because the rapid expansion takes minuscule quantum fluctuations and blows them up to finite size. This is essentially the same physics as the famous Hawking radiation from black holes. The fluctuations that eventually create the galaxies are accompanied by a separate set of fluctuations in the gravitational field itself: these are the ones that become gravitational radiation observable in the CMB. We characterise the background of gravitational radiation through the number r, which stands for the ratio of these two kinds of fluctuations — gravitational radiation divided by the density fluctuations.

Important caveat: there are other ways of producing gravitational radiation in the early Universe, although they don’t necessarily make exactly the same predictions; some of these issues have been discussed by my colleagues in various technical papers (Brandenberger 2011; Hindmarsh et al 2008; Lizarraga et al 2014 — the latter paper from just today!).

However, there are other ways to generate B modes. First, lots of astrophysical objects emit polarised light, and they generally don’t preferentially create E or B patterns. In particular, clouds of gas and dust in our galaxy will generally give us polarised light, and as we’re sitting inside our galaxy, it’s hard to avoid these. Luckily, we’re towards the outskirts of the Milky Way, so there are some clean areas of sky, but it’s hard to be sure that we’re not seeing some such light — and there are very few previous experiments to compare with.

We also know that large masses along the line of sight — clusters of galaxies and even bigger — distort the path of the light and can move those polarisation arrows around. This, in turn, can convert what started out as E into B and vice versa. But we know a lot about that intervening matter, and about the E-mode pattern that we started with, so we have a pretty good handle on this. There are some angular scales over which this is larger than the gravitational wave signal, and some scales that the gravitational wave signal is dominant.

So, if we can observe B-modes, and we are convinced that they are primordial, and that they are not due to lensing or astrophysical sources, and they have the properties expected from inflation, then (and only then!) we have direct evidence for inflation!

Data

Here’s a plot, courtesy the BICEP2 team, with the current state of the data targeting these B modes: Almost all BB limits

The figure shows the so-called power spectrum of the B-mode data — the horizontal “multipole” axis corresponds to angular sizes (θ) on the sky: very roughly, multipole ℓ ~ 180°/θ. The vertical axis gives the amount of “power” at those scales: it is larger if there are more structures of that particular size. The downward pointing arrows are all upper limits; the error bars labeled BICEP2 and Polarbear are actual detections. The solid red curve is the expected signal from the lensing effect discussed above; the long-dashed red curve is the effect of gravitational radiation (with a particular amplitude), and the short-dashed red curve is the total B-mode signal from the two effects.

The Polarbear results were announced on 11 March (disclosure: I am a member of the Polarbear team). These give a detection of the gravitational lensing signal. It was expected, and has been observed in other ways both in temperature and polarisation, but this was the first time it’s been seen directly in this sort of B-mode power spectrum, a crucial advance in the field, letting us really see lensing unblurred by the presence of other effects. We looked at very “clean” areas of the sky, in an effort to minimise the possible contamination from those astrophjysical foregrounds.

The BICEP2 results were announced with a big press conference on 17 March. There are two papers so far, one giving the scientific results, another discussing the experimental techniques used — more papers discussing the data processing and other aspects of the analysis are forthcoming. But there is no doubt from the results that they have presented so far that this is an amazing, careful, and beautiful experiment.

Taken at face value, the BICEP2 results give a pretty strong detection of gravitational radiation from the early Universe, with the ratio parameter r=0.20, with error bars +0.07 and -0.05 (they are different in the two different directions, so you can’t write it with the usual “±”).

This is why there has been such an amazing amount of interest in both the press and the scientific community about these results — if true, they are a first semi-direct detection of gravitational radiation, strong evidence that inflation happened in the early Universe, and therefore a first look at waves which were created in the first tiny fraction of a second after the big bang, and have been propagating unimpeded in the Universe ever since. If we can measure more of the properties of these waves, we can learn more about the way inflation happened, which may in turn give us a handle on the particle physics of the early Universe and ultimately on a so-called “theory of everything” joining up quantum mechanics and gravity.

Taken at face value, the BICEP2 results imply that the very simplest theories of inflation may be right: the so-called “single-field slow-roll” theories that postulate a very simple addition to the particle physics of the Universe. In the other direction, scientists working on string theory have begun to make predictions about the character of inflation in their models, and many of these models are strongly constrained — perhaps even ruled out — by these data.

Skepticism

This is great. But scientists are skeptical by nature, and many of us have spent the last few days happily trying to poke holes in these results. My colleagues Peter Coles and Ted Bunn have blogged their own worries over the last couple of days, and Antony Lewis has already done some heroic work looking at the data.

The first worry is raised by their headline result: r=0.20. On its face, this conflicts with last year’s Planck result, which says that r<0.11 (of course, both of these numbers really represent probability distributions, so there is no absolute contradiction between these numbers, but rather they should be seen to be as a very unlikely combination). How can we ameliorate the “tension” (a word that has come into vogue in cosmology lately: a wimpy way — that I’ve used, too — of talking about apparent contradictions!) between these numbers?

PlanckCl lowFirst, how does Planck measure r to begin with? Above, I wrote about how B modes show only gravitational radiation (and lensing, and astrophysical foregrounds). But the same gravitational radiation also contributes to the CMB temperature, albeit at a comparatively low level, and at large angular scales — the very left-most points of the temperature equivalent of a plot like the above — I reproduce one from last year’s Planck release at right. In fact, those left-most data points are a bit low compared to the most favoured theory (the smooth curve), which pushes the Planck limit down a bit.

But Planck and BICEP2 measure r at somewhat different angular scales, and so we can “ameliorate the tension” by making the theory a bit more complicated: the gravitational radiation isn’t described by just one number, but by a curve. If both data are to be believed, the curve slopes up from the Planck regime toward the BICEP2 regime. In fact, such a new parameter is already present in the theory, and goes by the name “tensor tilt”. The problem is that the required amount of tilt is somewhat larger than the simplest ideas — such as the single-field slow-roll theories — prefer.

If we want to keep the theories simple, we need to make the data more complicated: bluntly, we need to find mistakes in either Planck or BICEP2. The large-scale CMB temperature sky has been scrutinised for the last 20 years or so, from COBE through WMAP and now Planck. Throughout this time, the community has been building up a catalog of “anomalies” (another term of art we use to describe things we’re uncomfortable with), many of which do seem to affect those large scales. The problem is that no one quite figure out if these things are statistically significant: we look at so many possible ways that the sky could be weird, but we only publish the ones that look significant. As my Imperial colleague Professor David Hand would point out, “Coincidences, Miracles, and Rare Events Happen Every Day”. Nonetheless, there seems to be some evidence that something interesting/unusual/anomalous is happening at large scales, and perhaps if we understood this correctly, the Planck limits on r would go up.

But perhaps not: those results have been solid for a long while without an alternative explanation. So maybe the problem is with BICEP2? There are certainly lots of ways they could have made mistakes. Perhaps most importantly, it is very difficult for them to distinguish between primordial perturbations and astrophysical foregrounds, as their main results use only data from a single frequency (like a single colour in the spectrum, but down closer to radio wavelengths). They do compare with some older data at a different frequency, but the comparison does not strongly rule out contamination. They also rely on models for possible contamination, which give a very small contribution, but these models are very poorly constrained by current data.

Another way they could go wrong is that they may misattribute some of their temperature measurement, or their E mode polarisation, to their B mode detection. Because the temperature and E mode are so much larger than the B they are seeing, only a very small amount of such contamination could change their results by a large amount. They do their best to control this “leakage”, and argue that its residual effect is tiny, but it’s very hard to get absolutely right.

And there is some internal evidence within the BICEP2 results that things are not perfect. The most obvious one comes from the figure above: the points around ℓ=200 — where the lensing contributions begins to dominate — are a bit higher than the model. Is this just a statistical fluctuation, or is it evidence of a broader problem? Their paper show some somewhat discrepant points in their E polarisation measurements, as well. None of these are very statistically significant, and some may be confirmed by other measurements, but there are enough of these that caution makes sense. From only a few days thinking about the results (and not yet really sitting down and going through the papers in great depth), it’s hard to make detailed judgements. It seems like the team have been careful that it’s hard to imagine the results going away completely, but easy to imagine lots of ways in which it could be wrong in detail.

But this skepticism from me and others is a good thing, even for the BICEP2 team: they will want their results scrutinised by the community. And the rest of us in the community will want the opportunity to reproduce the results. First, we’ll try to dig into the BICEP2 results themselves, making sure that they’ve done everything as well as possible. But over the next months and years, we’ll want to reproduce them with other experiments.

First, of course, will be Planck. Since I’m on Planck, there’s not much I can say here, except that we expect to release our own polarisation data and cosmological results later this year. This paper (Efstathiou and Gratton 2009) may be of interest….

Next, there are a bunch of ground- and balloon-based CMB experiments gathering data and/or looking for funding right now. The aforementioned Polarbear will continue, and I’m also involved with the EBEX team which hopes to fly a new balloon to probe the CMB polarisation again in a few years. In the meantime, there’s also ACT, SPIDER, SPT, and indeed the successor to BICEP itself, called the Keck array, and many others besides. Eventually, we may even get a new CMB satellite, but don’t hold your breath…

Rumour-mongering

I first heard about the coming BICEP2 results in the middle of last week, when I was up in Edinburgh and received an email from a colleague just saying “r=0.2?!!?” I quickly called to ask what he meant, and he transmitted the rumour of a coming BICEP detection, perhaps bolstered by some confirmation from their successor experiment, the Keck Array (which does in fact appear in their paper). Indeed, such a rumour had been floating around the community for a year or so, but most of thought it would turn out to be spurious. But very quickly last week, we realised that this was for real. It became most solid when I had a call from a Guardian journalist, who managed to elicit some inane comments from me, before anything was known for sure.

By the weekend, it became clear that there would be an astronomy-related press conference at Harvard on Monday, and we were all pretty sure that it would be the BICEP2 news. The number r=0.20 was most commonly cited, and we all figured it would have an error bar around 0.06 or so — small enough to be a real detection, but large enough to leave room for error (but I also heard rumours of r=0.075).

By Monday morning, things had reached whatever passes for a fever pitch in the cosmology community: twitter and Facebook conversations, a mention on BBC Radio 4’s Today programme, all before the official title of the press conference was even announced: “First Direct Evidence for Cosmic Inflation”. Apparently, other BBC journalists had already had embargoed confirmation of some of the details from the BICEP2 team, but the embargo meant they couldn’t participate in the rumour-spreading.

I was traveling during most of this time, fielding occasional call from journalists (there aren’t that many CMB-specialists within within easy of the London-based media), though, unfortunately for my ego, I wasn’t able to make it onto any of Monday night’s choice tv spots.

By the time of the press conference itself, the cosmology community had self-organised: there was a Facebook group organised by Fermilab’s Scott Dodelson, which pretty quickly started dissecting the papers and was able to follow along with the press conference as it happened (despite the fact that most of us couldn’t get onto the website — one of the first times that the popularity of cosmology has brought down a server).

At the time, I was on a series of trains from Loch Lomond to Glasgow, Edinburgh and finally on to London, but the facebook group made (from a tech standpoint, it’s surprising that we didn’t do this on the supposedly more capable Google Plus platform, but the sociological fact is that more of us are on, and use, Facebook). It was great to be able to watch, and participate in, the real-time discussion of the papers (which continues on Facebook as of now). Cosmologists have been teasing out possible inconsistencies (some of which I alluded to above), trying to understand the implications of the results if they’re right — and thinking about the next steps. IRL, Now that I’m back at Imperial, we’ve been poring over the papers in yet more detail, trying to work exactly how they’ve gathered and analysed their data, and seeing what parts we want to try to reproduce.

Aftermath

Physics moves fast nowadays: as of this writing, about 72 hours after the announcement, there are 16 papers mentioning the BICEP2 results on the physics ArXiV (it’s a live search, so the number will undoubtedly grow). Most of them attempt to constrain various early-Universe models in the light of the r=0.20 results — some of them with some amount of statistical rigour, others just pointing out various models in which that is more or less easy to get. (I’ve obviously spent too much time on this post and not enough writing papers.)

It’s also worth collecting, if only for my own future reference, some of the media coverage of the results:

For more background, you can check out

Around Asia in search of a meal

I’m recently back from my mammoth trip through Asia (though in fact I’m up in Edinburgh as I write this, visiting as a fellow of the Higgs Centre For Theoretical Physics).

I’ve already written a little about the middle week of my voyage, observing at the James Clerk Maxwell Telescope, and I hope to get back to that soon — at least to post some pictures of and from Mauna Kea. But even more than telescopes, or mountains, or spectacular vistas, I seemed to have spent much of the trip thinking about and eating food. (Even at the telescope, food was important — and the chefs at Halu Pohaku do some amazing things for us sleep-deprived astronomers, though I was too tired to record it except as a vague memory.) But down at sea level, I ate some amazing meals.

When I first arrived in Taipei, my old colleague Proty Wu picked me up at the airport, and took me to meet my fellow speakers and other Taiwanese astronomers at the amazing Din Tai Fung, a world-famous chain of dumpling restaurants. (There are branches in North America but alas none in the UK.) As a scientist, I particularly appreciated the clean room they use to prepare the dumplings to their exacting standards: Dumpling lab at Din Tai Fung, Taipei

Later in the week, a few of us went to a branch of another famous Taipei-based chain, Shin Yeh, for a somewhat traditional Taiwanese restaurant meal. It was amazing, and I wish I could remember some of the specifics. Alas, I’ve only recorded the aftermath: Shin Yeh (Nanxi) restaurant

From Taipei, I was off to Hawaii. Before and after my observing trip, I spent a few days in Honolulu, where I managed to find a nice plate of sushi at Doraku — good, but not too much better than I’ve had in London or New York, despite the proximity to Japan. Doraku Sushi, Waikiki, Honolulu, Hawaii

From Hawaii, I had to fly back for a transfer in Taipei, where I was happy to find plenty more dumplings (as well as pleasantly sweet Taiwanese pineapple cake). Certainly some of the best airport food I’ve had (for the record, my other favourites are sausages in Munich, and sushi at the Ebisu counter at San Francisco): Taipei airport dumplings

From there, my last stop was 40 hours in Beijing. Much more to say about that visit, but the culinary part of the trip had a couple of highlights. After a morning spent wandering around the Forbidden City (aka the Palace Museum), I was getting tired and hungry. I tried to find Tian Di Yi Jia, supposedly “An Incredible Imperial-Style Restaurant”. Alas, some combination of not having a website, not having Roman-lettered signs, and the likelihood that it had closed down meant an hour’s wandering Beijing’s streets was in vain. Instead, I ended up at this hole in the wall: Restaurant near the Forbidden City And was very happy indeed, in particular with the amazing slithery, tangy eggplant: Lunch near the Forbidden City That night, I ended up at The Grandma’s, an outpost of yet another chain, seemingly a different chain than Grandma’s Kitchen, which apparently serves American food. Definitely not American food. Note especially the “thousand-year egg” at left (I was happy to see from wikipedia that the idea they’re cured in horse urine is only a myth!): Grandma's Restaurant, Beijing

It was a very tasty trip. I think there was science, too.

After a couple of days of lousy weather, the sky cleared up and dried out Wednesday. Eventually, we got down to τ<0.08 — not quite the best possible conditions, but good enough for almost anything we might want to do. We started out slightly worse than that, but that meant we got to observe more interesting things: nearby bright, big galaxies. Unfortunately, a galaxy that is bright and big in visible light is still just a blob in the submillimetre (submm). Our first one was NGC 3034, aka M82, exciting for two reasons. First, it’s the prototypical starburst galaxy, a galaxy undergoing a rapid period of star formation, gobbling up gas and dust and turning them into stars, which in turn heat up the remaining dust, making the galaxy glow brightly in the infrared and submm. Second, M82 is the home of a recent supernova explosion, the nearest one since 2004, and the nearest one of the particularly important type Ia since 1972. And it was first discovered by students at University College London, right across town.

So, I am sure that you are very excited to see a beautiful picture of the galaxy, at right. The elongated blob in the center isn’t even the whole galaxy: M82 3that’s the bright nucleus glowing from the concentration of star formation there. I think — and my proper observational-astronomer friends will correct me if I’m wrong — that some of the dark fuzz around the nucleus is really part of the galaxy, which would take up most of this picture, about 15 arc minutes from top to bottom.

After M82, we observed another nearby galaxy, the somewhat less famous NGC 4559, and then conditions improved enough that we could do observations as part of the SCUBA-2 Cosmology Legacy Survey (CLS), which is officially why I’m here. But that’s a lot less fun, as it’s just observing more or less blank patches, again and again, building up a deep submm survey of large areas of sky (where for these purposes, “large” just means about 35 square degrees, out of about 41,000 on the whole sky). We repeat each small patch dozens of times, adding them up and building up pictures so dense with galaxies that they are said to be “confusion limited” — the main source of noise is just the population of galaxies themselves, individually too faint to see, but contributing to the infrared background light everywhere we look (this depends on both the wavelength of the light and the resolution of the telescope — that is, the size of the smallest object that you can make out.).

For the rest of the night, through until dawn, we kept on observing the CLS fields, and have started back onto them today, in even better conditions than yesterday.

So far, I’ve been pleasantly surprised about life at 14,000 feet: there is definitely less oxygen than down at sea level (or even than Hale Pohaku at 9,000 feet where I sleep and spend the days), but I’ve been spared the worse symptoms of altitude sickness. And jet-lag, combined with strong and good coffee provided by the excellent Telescope System Specialist (TSS) has meant that staying up through until 7 or 8am hasn’t been too bad. (On the other hand, re-reading this post leaves the impression that my ability to string a sentence together has been somewhat impaired by the lack of sleep and oxygen…)

In fact, the TSS is really the one doing — quite literally — all of the work here. Because we are observing as part of the JCMT Legacy Survey, there’s nothing for the “observer” (i.e., me) to do. Later on, the survey team will collate the data that have been gathered and make the final images and catalogs, but that’s a slow and painstaking process, not one that happens on the night the data are taken. And taking the data is such a complicated task that only the Specialist really has the expertise to do it. He keeps me informed of what’s going on, but I don’t really get much of a say in what happens.

You may ask why someone spends the money to send us astronomer/observers across an ocean or two to stay up at night, drink coffee, and not really do any science. Gift-horses aside, so do I.

But it certainly is a gift and a privilege to be here:

JCMT

P1020552

Observing, days 1-2

I am sitting in the control room of the James Clerk Maxwell Telescope (JCMT), 14,000 feet up Mauna Kea, on Hawaii’s Big Island. I’m here to do observations for the SCUBA-2 Cosmology Legacy Survey (CLS).

I’m not really an observer — this is really my first time at a full-sized, modern telescope. But much of JCMT’s observing time is taken up with a series of so-called Legacy Surveys (JLS) — large projects, observing large amounts of sky or large numbers of stars or galaxies.

JCMT is a submillimeter telescope: it detects light with wavelength at or just below one millimeter. This is a difficult regime for astronomy: the atmosphere itself glows very strongly in the infrared, mostly because of water vapour. That’s why I’m sitting at the cold and dry top of an active volcano (albeit one that hasn’t erupted in thousands of years).

Unfortunately, “cold and dry” doesn’t mean there is no precipitation. Here is yesterday’s view, from JCMT over to the CSO telescope:

Snowy view of the CSO from JCMT

This is Hawaii, not Hoth, or even Antarctica.

Tonight seems more promising: we measure the overall quality as an optical depth, denoted by the symbol τ, essentially the probability that a photon you care out will get scattered by the atmosphere before it reaches your telescope. The JLS survey overall requires τ<0.2, and the CLS that I’m actually here for needs even better conditions, τ<0.10. So far we’re just above 0.20 — good enough for some projects, but not the JLS. I’m up here with a JCMT Telescope System Specialist — who actually knows how to run the telescope — and he’s been calibrating the instrument, observing a few sources, and we’re waiting for the optical depth to dip into the JLS band. If that happens, we can fire up SCUBA-2, the instrument (camera) that records the light from the sky. SCUBA-2 uses bolometers (like HFI on Planck), very sensitive thermometers cooled down to superconducting temperatures.

(You can keep track of the conditions here, and specifically monitor the optical depth here. News flash: as I type this, τ=0.199, less than 0.2!)

Later this week, I’ll try to talk about why these are called “Legacy” surveys — and why that’s bad news.

 

meTube

Some time last year, Physics World magazine asked some of us to record videos discussing scientific topics in 100 seconds. Among others, I made one on cosmic inflation and another on what scientists can gain from blogging, which for some reason has just been posted to YouTube, and then tweeted about by FQXi (without which I would have forgotten the whole thing). There are a few other videos of me, although it turns out that there are lots of people called “Andrew Jaffe” on YouTube.

I’m posting this not (only) for the usual purposes of self-aggrandizement, but to force — or at least encourage — myself to actually do some more of that blogging which I claim is a good thing for us scientists. With any luck, you’ll be able to read about my experiences teaching last term, and the trip I’m about to take to observe at a telescope (a proper one, at the top of a high mountain, with a really big mirror).

[On a much more entertaining note, here’s a song from a former Imperial undergraduate recounting “A Brief History of the Universe”. Give it a listen!]

Academic Blogging Still Dangerous?

Nearly a decade ago, blogging was young, and its place in the academic world wasn’t clear. Back in 2005, I wrote about an anonymous article in the Chronicle of Higher Education, a so-called “advice” column admonishing academic job seekers to avoid blogging, mostly because it let the hiring committee find out things that had nothing whatever to do with their academic job, and reject them on those (inappropriate) grounds.

I thought things had changed. Many academics have blogs, and indeed many institutions encourage it (here at Imperial, there’s a College-wide list of blogs written by people at all levels, and I’ve helped teach a course on blogging for young academics). More generally, outreach has become an important component of academic life (that is, it’s at least necessary to pay it lip service when applying for funding or promotions) and blogging is usually seen as a useful way to reach a wide audience outside of one’s field.

So I was distressed to see the lament — from an academic blogger — “Want an academic job? Hold your tongue”. Things haven’t changed as much as I thought:

… [A senior academic said that] the blog, while it was to be commended for its forthright tone, was so informal and laced with profanity that the professor could not help but hold the blog against the potential faculty member…. It was the consensus that aspiring young scientists should steer clear of such activities.

Depending on the content of the blog in question, this seems somewhere between a disregard for academic freedom and a judgment of the candidate on completely irrelevant grounds. Of course, it is natural to want the personalities of our colleagues to mesh well with our own, and almost impossible to completely ignore supposedly extraneous information. But we are hiring for academic jobs, and what should matter are research and teaching ability.

Of course, I’ve been lucky: I already had a permanent job when I started blogging, and I work in the UK system which doesn’t have a tenure review process. And I admit this blog has steered clear of truly controversial topics (depending on what you think of Bayesian probability, at least).

Teaching mistakes

The academic year has begun, and I’m teaching our second-year Quantum Mechanics course again. I was pretty happy with last year’s version, and the students didn’t completely disagree.

This year, there have been a few changes to the structure of the course — although not as much to the content as I might have liked (“if it ain’t broke, don’t fix it”, although I’d still love to use more of the elegant Dirac notation and perhaps discuss quantum information a bit more). We’ve moved some of the material to the first year, so the students should already come into the course with at least some exposure to the famous Schrödinger Equation which describes the evolution of the quantum wave function. But of course all lecturers treat this material slightly differently, so I’ve tried to revisit some of that material in my own language, although perhaps a bit too quickly.

Perhaps more importantly, we’ve also changed the tutorial system. We used to attempt an imperfect rendition of the Oxbridge small-group tutorial system, but we’ve moved to something with larger groups and (we hope) a more consistent presentation of the material. We’re only on the second term with this new system, so the jury is still out, both in terms of the students’ reactions, and our own. Perhaps surprisingly, they do like the fact that there is more assessed (i.e., explicitly graded, counting towards the final mark in the course) material — coming from the US system, I would like to see yet more of this, while those brought up on the UK system prefer the final exam to carry most (ideally all!) the weight.

So far I’ve given three lectures, including a last-minute swap yesterday. The first lecture — mostly content-free — went pretty well, but I’m not too happy with my performance on the last two: I’ve made a mistake in each of the last two lectures. I’ve heard people say that the students don’t mind a few (corrected) mistakes; it humanises the teachers. But I suspect that the students would, on the whole, prefer less-human, more perfect, lecturing…

Yesterday, we were talking about a particle trapped in a finite potential well — that is, a particle confined to be in a box, but (because of the weirdness of quantum mechanics) with some probability of being found outside. That probability depends upon the energy of the particle, and because of the details of the way I defined that energy (starting at a negative number, instead of the more natural value of zero), I got confused about the signs of some of the quantities I was dealing with. I explained the concepts (I think) completely correctly, but with mistakes in the math behind them, the students (and me) got confused about the details. But many, many thanks to the students who kept pressing me on the issue and helped us puzzle out the problems.

Today’s mistake was less conceptual, but no less annoying — I wrote (and said) “cotangent” when I meant “tangent” (and vice versa). In my notes, this was all completely correct, but when you’re standing up in front of 200 or so students, sometimes you miss the detail on the page in front of you. Again, this was in some sense just a mathematical detail, but (as we always stress) without the right math, you can’t really understand the concepts. So, thanks to the students who saw that I was making a mistake, and my apologies to the whole class.

Songs about f*&%ing

First, my apologies that I couldn’t resist the almost not-safe-for-work title, especially to those expecting posts about astrophysics and cosmology rather than a reference to a 1987 record by Big Black (which it’s worth pointing out can be found in its entirety on YouTube). But this is not a post about Big Black.

Rather, it’s a brief reminiscence of another album with a similar subject matter and a very different style, Liz Phair’s Exile in Guyville, which I was shocked to discover is about to have its 20th anniversary, also commemorated with an article and interview in the Chicago Tribune.

I lived in Chicago in the early 90s when Exile In Guyville was released, although I don’t think I heard it until I left town and moved to Toronto a few months later. But she was already a presence on the scene when Chicago was taking its place in the world of post-Nirvana indie-rock (led by the Smashing Pumpkins, along with Urge Overkill, who never quite capitalised on the marquee placement of their “Girl, You’ll Be A Woman Soon” cover on the Pulp Fiction soundtrack, and my favourite, Eleventh Dream Day). It was a record full of great songs about fucking and love and being a lonely twenty-something hipster in a big city, and was a sort of homage to the Rolling Stones’ own Exile on Main Street, all of which was enough to make rock critics (and wannabes like me) wet their pants — although by now I’m sure the Stones reference is irrelevant to record’s brilliance. “Guyville” was code (surfacing first in an Urge Overkill song) for the Wicker Park neighbourhood which was the center of the Chicago rock scene, and home to my second-favourite Chicago bar, the still-going-strong Rainbo Club (alas, my favourite, Ciral’s House of Tiki, closed in 2000).

And the title of this post also covers The Book of Mormon, which I went to see in London’s West End last week, the filthy and wonderful musical comedy from the creators of South Park. Despite songs about sex with amphibians (and worse), a character named “General Butt Fucking Naked” (sort of named after a real Liberian warlord), and being self-consciously suffused with coarse stereotyping of Africans and the eponymous Mormons, manages to be old-fashioned, warm-hearted and strangely, uncynically, affirming of the ability of individuals to actually make a difference in each other’s lives.

Today was the deadline for submitting so-called “White Papers” proposing the next generation of the European Space Agency satellite missions. Because of the long lead times for these sorts of complicated technical achievements, this call is for launches in the faraway years of 2028 or 2034. (These dates would be harder to wrap my head around if I weren’t writing this on the same weekend that I’m attending the 25th reunion of my university graduation, an event about which it’s difficult to avoid the clichéd thought that May, 1988 feels like the day before yesterday.)

At least two of the ideas are particularly close to my scientific heart.

The Polarized Radiation Imaging and Spectroscopy Mission (PRISM) is a cosmic microwave background (CMB) telescope, following on from Planck and the current generation of sub-orbital telescopes like EBEX and PolarBear: whereas Planck has 72 detectors observing the sky over nine frequencies on the sky, PRISM would have more than 7000 detectors working in a similar way to Planck over 32 frequencies, along with another set observing 300 narrow frequency bands, and another instrument dedicated to measuring the spectrum of the CMB in even more detail. Combined, these instruments allow a wide variety of cosmological and astrophysical goals, concentrating on more direct observations of early Universe physics than possible with current instruments, in particular the possible background of gravitational waves from inflation, and the small correlations induced by the physics of inflation and other physical processes in the history of the Universe.

The eLISA mission is the latest attempt to build a gravitational radiation observatory in space, observing astrophysical sources rather than the primordial background affecting the CMB, using giant lasers to measure the distance between three separate free-floating satellites a million kilometres apart from one another. As a gravitational wave passes through the triangle, it bends space and effectively changes the distance between them. The trio would thereby be sensitive to the gravitational waves produced by small, dense objects orbiting one another, objects like white dwarfs, neutron stars and, most excitingly, black holes. This would give us a probe of physics in locations we can’t see with ordinary light, and in regimes that we can’t reproduce on earth or anywhere nearby.

In the selection process, ESA is supposed to take into account the interests of the community. Hence both of these missions are soliciting support, of active and interested scientists and also the more general public: check out the sites for PRISM and eLISA. It’s a tough call. Both cases would be more convincing with a detection of gravitational radiation in their respective regimes, but the process requires putting down a marker early on. In the long term, a CMB mission like PRISM seems inevitable — there are unlikely to be any technical showstoppers — it’s just a big telescope in a slightly unusual range of frequencies. eLISA is more technically challenging: the LISA Pathfinder effort has shown just how hard it is to keep and monitor a free-floating mass in space, and the lack of a detection so far from the ground-based LIGO observatory, although completely consistent with expectations, has kept the community’s enthusiasm lower. (This will likely change with Advanced LIGO, expected to see many hundreds of sources as soon as it comes online in 2015 or thereabouts.)

Full disclosure: although I’ve signed up to support both, I’m directly involved in the PRISM white paper.