Results tagged “CMB”

I was sitting in the lecture theatre of the Royal Institution at the Science Online London meeting (of which I hope to write more later, but you can retroactively follow the day’s tweets or just search for the day’s tags) when I realized I had missed the fifth anniversary of this blog this past July. So: thanks for your attention for over 400 posts on cosmology, astrophysics, Bayesian probability and probably too much politics and religion.

Today is also a much more important date: the 400th anniversary of Galileo’s telescope. Even Google is celebrating.

Right now, I’m at a meeting in Cambridge discussing Primordial Gravitational Waves — ripples in space and time that have been propagating since the first instants after the big bang. Despite my training as a theoretical physicist, I’m here to discuss the current state of the art of experiments measuring those waves using the polarization of the cosmic microwave background, which probes the effect of those gravitational waves on the constituents of the Universe about 400,000 years after the big bang. Better go finish writing my talk.

[Warning: this post will be fairly technical and political and may only be of interest to those in the field.]

I spent the first couple of days this week stuck in a room in Cambridge with about 40 of my colleagues pondering a very important question: what is the future of the study of the Cosmic Microwave Background in the UK?

Organized by Keith Grainge of Cambridge’s MRAO, and held at Cambridge’s new Kavli Institute for Cosmology, the workshop brought together a significant fraction of the UK CMB community, from Cambridge itself, Cardiff, Imperial, Manchester, Oxford and elsewhere.

With the recent cancellation of the Clover experiment by STFC, there is no major UK-led CMB experiment (I am making a distinction between CMB experiments per se and those with other primary purposes, such observing the Sunyaev-Zel’dovich effect with AMI, or astrophysical foregrounds with QUIJOTE.) However, there is a huge amount of CMB expertise in the UK, from the design of detectors and telescopes through to the analysis of CMB data.

In the short term, it seems there is some appetite for attempting to revive the Clover effort at some level, perhaps in collaboration with other experimental teams outside of the UK. The major driver — and the only way it makes any sense at all — is to get this done quickly, before the other experiments pursuing the same goals begin to gather data (in the interests of full disclosure, I should point out that I am involved in a couple of those other experiments: EBEX and PolarBear). This decision, I imagine, will be dominated by the politics and economics of the current STFC funding debacle fiasco debate as well as what I understand are the internal relationships of the Clover team.

So of more scientific interest is the question of what to do next. Right now, the UK astronomy and particle physics community is undertaking a series of consultations to figure out what it thinks are the most important topics, instruments and experiments to concentrate upon over the next few years. One very real possibility is that we could decide not to lead any new CMB experiments, but just to continue to lend our expertise to other efforts. This is cost-effective but unsatisfying, especially to experimentalists who want to take the lead in the design of new efforts. The only viable alternative, I think, is for the community to come together and, with apologies for the cliche, speak with a unified voice in support of a coherent plan. There is enough expertise in the UK to produce great CMB science over the next decade, but it is thinly spread. The basic design of any such experiment is clear: thousands of detectors observing the sky over as many frequencies as possible. But the details — exactly what sorts of detectors, flown from a balloon or stationary on the ground, or to wait for a future satellite — will be crucial to the success or otherwise of the experiment. Unfortunately, these decisions can often degenerate into “not-invented-here” syndrome and personality clashes between strong scientific egos. But as Ben Franklin said on signing the Declaration of Independence, “we must, indeed, all hang together, or assuredly we shall all hang separately.”

Another launch

Not all CMB (Cosmic Microwave Background) experiments get launched on a rocket.

There’s a long history of telescopes flown from balloons — huge mylar balloons floating over 100,000 feet in the air. MAXIMA and BOOMERaNG, the first experiments to map out the microwave sky on the sub-degree scales containing information about the detailed physical conditions in the Universe over the first few hundred thousand years after the Big Bang. The Planck Satellite will close out that era of CMB experiments, by giving us a complete picture of the microwave sky down to less than a tenth of a degree.

But there is still more to be done, even beyond what Planck is capable of. By measuring the polarization of the microwave background at even higher sensitivities than Planck, we hope to observe the effects of gravitational radiation in the early Universe.

Last week, EBEX, one of a new generation of balloon-borne experiments designed specifically with this goal, had its maiden flight from Fort Sumner, New Mexico.

EBEX Launch, 6/11/09 from asad137 on Vimeo.

It’s worth remembering, of course, that even with a parachute, these telescopes hit the ground pretty hard. But these things are amazingly well-built, and the EBEX crew have managed to recover most of the hardware and all of the data. So now the team have some time to get the hardware and software ready to fly for a couple of weeks over Antarctica next year.

And let’s not forget that New Mexico is also the home of Roswell, where conspiracy theorists and other wackjobs have been trying to find the government cover-up of UFO sightings. Indeed, the EBEX balloon was spotted, but at least in neighbouring Arizona, they can tell the difference.

Meanwhile, another CMB experiment, PolarBear, is about to start its first set of important tests. PolarBear is a ground-based telescope, which means it can watch the sky for far longer than a balloon, at the cost of being at the bottom of the atmosphere and all of the extra noise that adds to the signal. So despite some hard times (especially here in the UK), the next generation of CMB experiments are on the way, hoping to probe all the way back to the epoch of inflation.

Loading Planck

The Planck Surveyor Satellite has finished its assembly and testing in Liège, Belgium, and this week was loaded onto a Volga-Dnepr Antonov AN-124 plane, and sent to Kourou, French Guiana, location of the Centre Spatial Guyanais (one of the few places near the Equator politically connected to Europe). It’s due to be launched in tandem with Herschel on April 16. Here are some pictures of the “Planck Transport and Storage Container” making its way on a “Convoi Exceptionnel” to the airstrip. These photos came to me third-hand, so my apologies and thanks to the unknown (to me) photographer.
342-Planck entering container in CSL 344-Truck in position 347-Planck loaded on truck 352-Transfer on ramp 354-Planck inside the plane

Bayesian Inference in the NY Times

In today’s Sunday NY Times Magazine, there’s a long article by psychologist Steven Pinker, on “Personal Genomics”, the growing ability for individuals to get information about their genetic inheritance. He discusses the evolution of psychological traits versus intelligence, and highlights the complicated interaction amongst genes, and between genes and society.

But what caught my eye was this paragraph:

What should I make of the nonsensical news that I… have a “twofold risk of baldness”? … 40 percent of men with the C version of the rs2180439 SNP are bald, compared with 80 percent of men with the T version, and I have the T. But something strange happens when you take a number representing the proportion of people in a sample and apply it to a single individual…. Anyone who knows me can confirm that I’m not 80 percent bald, or even 80 percent likely to be bald; I’m 100 percent likely not to be bald. The most charitable interpretation of the number when applied to me is, “If you knew nothing else about me, your subjective confidence that I am bald, on a scale of 0 to 10, should be 8.” But that is a statement about your mental state, not my physical one. If you learned more clues about me (like seeing photographs of my father and grandfathers), that number would change, while not a hair on my head would be different. [Emphasis mine].

That “charitable interpretation” of the 80% likelihood to be bald is exactly Bayesian statistics (which I’ve talked about, possibly ad nauseum, before) : it’s the translation from some objective data about the world — the frequency of baldness in carriers of this gene — into a subjective statement about the top of Pinker’s head, in the absence of any other information. And that’s the point of probability: given enough of that objective data, scientists will come to agreement. But even in the state of uncertainty that most scientists find themselves, Bayesian probability forces us to enumerate the assumptions (usually called “prior probabilities”) that enter into our assignments reasoning along with the data. Hence, if you knew Pinker, your prior probability is that he’s fully hirsute (perhaps not 100% if you allow for the possibility of hair extensions and toupees); but if you didn’t then you’d probably be willing to take 4:1 odds on a bet about his baldness — and you would lose to someone with more information.

In science, of course, it usually isn’t about wagering, but just about coming to agreement about the state of the world: do the predictions of a theory fit the data, given the inevitable noise in our measurements, and the difficulty of working out the predictions of interesting theoretical ideas? In cosmology, this is particularly difficult: we can’t go out and do the equivalent of surveying a cross section of the population for their genes: we’ve got only one universe, and can only observe a small patch of it. So probabilities become even more subjective and difficult to tie uniquely to the data. Hence the information available to us on the very largest observable scales is scarce, and unlikely to improve much, despite tantalizing hints of data discrepant with our theories, such as the possibly mysterious alignment of patterns in the Cosmic Microwave Background on very large angles of the sky (discussed recently by Peter Coles here). Indeed, much of the data pointing to a possible problem was actually available from the COBE Satellite; results from the more recent and much more sensitive WMAP Satellite have only reinforced the original problems — we hope that the Planck Surveyor — to be launched in April! — will actually be able to shed light on the problem by providing genuinely new information about the polarization of the CMB on large scales to complement the temperature maps from COBE and WMAP.


A quick pointer to Initiative for Cosmology (iCosmo). The website brings together a bunch of useful calculations for physical cosmology — relatively simple quantities like the relationship between redshift and distance, and also more complicated ones like the power spectrum of density perturbations (which tells us the distribution of galaxies on the largest scales in the Universe) and quantities derived from that like the distortions in the shapes of galaxies due to gravitational lensing, when the path of light from galaxies is perturbed by intervening mass in the Universe. Combined with good documentation and tutorials (and downloadable source), it makes a good companion to sites such as LAMBDA’s CMB toolbox, which provides similar services targeted specifically at Cosmic Microwave Background science. iCosmo looks like it will be useful for researchers in the field as well as students, so thanks and congratulations to its creators (I’d like to point directly at the page listing them, but that doesn’t seem to be possible… instead, there’s a discussion forum at CosmoCoffee.).

OK, not a vacation in the true sense of the word: I’ve been in the US, attending meetings (in Berkeley), workshops (in Santa Fe), conferences (in Pasadena) and, because I can’t seem to escape them, teleconferences everywhere and all the time.


In Berkeley, I attended the first all-hands collaboration meeting for PolarBear, an experiment that will measure the polarization of the CMB from a telescope that will eventually be situated on the Atacama desert plain in Chile — one of the highest, driest, least accessible places on the earth, and one of the least contaminated with light or radio interference. (Despite the name of the experiment, there are no polar bears there.) First, we’ll test it at the somewhat less remote White Mountain facility in California, shake out all the bugs. PolarBear is one of a new generation of experiments that will measure the CMB using not just a few tens of detectors, but a few thousand, which brings with it all sorts of technical challenges. In hardware, the first challenge is simply making so many detectors and keeping their properties uniform each to each — these are among the most sensitive microwave detectors ever built, essentially as good as the constraints of quantum mechanics and thermodynamics allow. The second, related to the first, is to pack as many of these into a small space — the focal plane of the telescope — as possible. Traditionally, microwave detectors have used horns to guide the electromagnetic waves from the sky onto the detectors, but those horns are much wider than the detector hardware. For experiments like PolarBear, we put the detectors themselves right at the focus of the telescope and make each of them into a little antenna, receiving directly the focused light after passing through a hemispherical lens. The final hardware challenge is to get the information from these thousands of detectors off of the telescope and into our computers, which the PolarBear designers have solved with a new technique called “frequency-domain multiplexing”. Sort of like the way FM radio manages to convey the full spectrum of sound by modulating at a particular frequency, the very high-tech SQUIDs (Superconducting QUantum Interference Devices) can then amplify these tiny CMB signals into data we can analyze.

In fact, the data analysis and computing challenges are almost as significant as those faced in hardware. With thousands of detectors and a telescope that will run for the better part of several, we have many orders of magnitude more CMB data than we’ve ever dealt with before, combined with a sensitivity goal better than a millionth of a degree. By adding more and more detectors, we can make the raw experiment itself sensitive enough to do this. What we don’t know is whether we can eliminate everything else that can possibly contaminate our results: light may spill over our shield from the 300 degree ground or directly from the atmosphere; dust in our solar system or our galaxy also glows in the bands we want to measure, as do external galaxies millions of light-years away. So our task is to compress the terabytes of data into a few interesting numbers (like the energy scale of inflation) and to simultaneously separate the cosmic signal from the that produced by instrument and from the rest of the Universe (which may be much brighter!). Suffice to say, we have some good ideas but until we’re confronted with real data we won’t know how successful we’ll be.

Plus, I ate bagels (better than London; not as good as New York) and burritos, and bought shoes at cheap American prices (at least when I think in British Pounds).

Next up, Santa Fe

Gruber Cosmology Prize 2008: Dick Bond

Dick Bond, a friend, mentor and longtime collaborator has won the 2008 Gruber Cosmology prize. Dick’s work has been instrumental at bringing us into this age of “precision cosmology”. He has always concentrated on that interface between theory and observation, making predictions for what we would see in the Cosmic Microwave Background, and how we might best extract that information. The present industry in Cosmological Data Analysis is in no small part down to his ongoing work in the field. To quote the Gruber citation itself:

Professor Bond’s work has provided the theoretical framework to interpret the observed inhomogeneities in the fossil radiation left over from the early stages of expansion of the Universe—the Big Bang. Professor Bond’s research has helped us understand the transition from the nearly featureless early Universe to the wonderfully structured world of galaxies, stars and planets today.

Congratulations, Dick!

In its continuing bid to take over all aspects of science communication, Nature magazine (or more properly, an alliance between Nature Network and the Royal Institution) will be hosting a European Science Blogging conference in August or September.

Right now, however, I’m in Norway. In addition to discussing how we’re going to measure the CMB power spectrum with Planck, I’ve already eaten a slab of reindeer, ran for an hour up and down the snowy hills, and sweated in a sauna.

Mapping the Galaxy from Portugal

I spent the week before last in Portugal working with the team designing and building the GEM telescope: The Polarized Galactic Emission Mapping Project in Portugal. GEM (aka GEM-P or even P-GEM-P) aims to measure the emission of our Milky Way galaxy using light at a wavelength of 6 cm. Those frequencies are dominated by synchrotron emission, generated by electrons deflected by the magnetic field of the galaxy. These measurements will give us invaluable information about the structure of the galaxy. Moreover, this emission is an important contaminant for the cosmological maps that experiments like the Planck Surveyor and its successors like the BPol project that many of us have just proposed to the European Space Agency.

GEM-P is being built mostly by a small group at the Instituto de Telecomunicações and the Universidade de Aveiro. Here’s me with a large part of the Aveiro team:
GEM-P team small
That’s Rui Fonseca, Domingos Barbosa, me, Dinis Magalhães and Luis Cupido. This wasn’t taken at the GEM-P lab, by the way, but in Luis’s backyard, and that’s the 5.5 meter dish he keeps there to play with and do things like track NASA spacecraft in the outer reaches of the solar system!

Aveiro is a lovely town (even The New York Times agrees) experiencing a pretty remarkable building boom that manages to combine a high-tech University with an old-fashioned fishing village. For me, that meant the useful combination of pretty ubiquitous wifi and great seafood. (More pictures of Aveiro here).

(I also got to spend some time wandering around Porto, where I stayed right down the road from Rem Koolhaas’s new Casa de Musica, just shortlisted for the RIBA Stirling Prize. Photos of the older bits of Porto here.)
Casa da Musica

Scientific Illiteracy

The Observer featured a lengthy article by Tim Adams bemoaning the generic scientific illiteracy of society today, tracing a line from CP Snow’s “Two Cultures” through Natalie Angier’s new book, The Canon:A Whirligig Tour of the Beautiful Basics of Science. It concentrates a bit too heavily on uber-agent John Brockman’s somewhat pretentious “Third Culture, a marriage of physics and philosophy, astronomy and art,” as exemplified by his website, The Edge, but it does finger a real and disturbing (but not really new) trend. But to me the real howler was the following quote:

George Smoot, the Nobel-winning astrophysicist who first identified the background radiation of the Big Bang and thereby invented cosmology.
OK, first, George Smoot didn’t identify “the background radiation of the Big Bang”, he was the Principal Investigator of the DMR Instrument on the COBE satellite, which identified the fluctuations in the background radiation (aka the CMB), the seeds of structure that eventually grew into galaxies and cluster of galaxies in the Universe today. The CMB itself was first identified by Penzias and Wilson in the 1960s — for which they also won the Nobel Prize. That may give you a hint about the other problem here: George, although a pretty smart guy, certainly didn’t invent cosmology, which has been around as a legitimate scientific field at least since Einstein’s discovery of General Relativity, and as a human endeavor for thousands of years.

April First (and Second)

We take so much of the web for granted today, we often forget how very contingent it all is. Without the very specific work by Tim Berners-Lee inventing the http protocol, perhaps some sort of hypertext communication standard would have come along, but it’s hard to believe that it would be quite the same. Berners-Lee has always advocated a still more open “read/write” web, and about the closest we come to that is, of course, the weblog. Well, blogs were arguably launched ten years ago, on April 1, 1997, by Dave Winer. Scripting News was an outgrowth of his DaveNet emails, but had all the usual hallmarks of a blog: short items, lots of links, and, crucially, reverse chronological order. Dave has gone onto a career as a general computer pundit and curmudgeon — and also invented RSS (that orange “XML Feed” icon over at the side).

Of course, the first of April has another name. If you read this blog regularly, you know that we physicists have a fantastic collective sense of humor, as evidenced here, here and here. Funny, huh?

My own April Fool’s incident came a bit early, last Thursday night, unable to make my way from the arrival hall of Rome’s Fiumicino airport to the airport Hilton. I arrived around midnight, after the trains stop running into town. What they don’t tell you is that all the passageways between the airport buildings are also shut — without signs to tell you where to go. After conflicting information from three different sets of people, I found myself staggering around the deserted parking lots searching for the warm bed I had booked (I did eventually find it, and the front desk took pity on me in the form of an upgrade to the “executive suite” floor). The next day, although a bit sleepy, was at least a productive discussion of the next step in our proposal for a new mission to measure the polarization of the microwave background — in about 2015 or 2020.

But since today is really April 2, you can also read a real blog post by Amedeo Balbi, my cosmology colleague (on MAXIMA and Planck and probably more in the future) over in Tommaso Dorigo’s blog; he’s got one of his own, but it will only make sense if you read Italian.

Yesterday evening I attended the launch party for Nature Network London, a new site run by Nature magazine, which hopes to be a web home for science and scientists in London. There are articles, blogs, discussion forums and calendars of scientific events.

Perhaps unsurprisingly, I ended up meeting lots of people from Imperial — whom of course I had never met here on campus. I also met the site’s editor, Matt Brown, as well as blogger Jennifer Rohn, who also runs the science/culture site LabLit.

It’s an ambitious idea, and anything that gets us out of our offices and talking with other scientists is welcome. The formal barriers to entry are quite low, but to get working scientists to spend their time blogging, posting in discussion forums, and just taking this newfangled social web 2.0 thing seriously may be a hard sell. We’ll have to hook ‘em young. However, “science” in London is dominated by Medicine and biology — we physical scientists are a distinct minority, and our interests, academic lives and ways of working are often very different indeed (for example, the biologists last night spent a lot of time trying to decide whether to approach someone like Paul Smith for a design of a fashionable lab coat — I’ve never worn a lab coat in my life!). Anyway, if you’re a London-based scientist of any stripe reading this, sign up and join in!

Tonight I’m off on a 24-hour jaunt to Rome to discuss our proposal for a new Satellite, BPol, to measure the CMB polarization (and thereby discover if inflation could be responsible for getting our Universe into the shape we find it today). Unfortunately, this satellite wouldn’t be launched until the late 2010s, which means that the data wouldn’t flow for a staggering decade and a half.

Luckily, cosmology will remain interesting while we’re waiting — as Tommaso Dorigo’s ongoing reports from our Outstanding questions for the standard cosmological model meeting continue to attest.

I’m just back from a couple of days up in Edinburgh, one of my favorite cities in the UK. London is bigger, more intense, but Edinburgh is more beautiful, dominated by its landscape—London is New York to Edinburgh’s San Francisco.

I was up there to give the Edinburgh University Physics “General Interest Seminar”. Mostly, I talked about the physical theory behind and observations of the Cosmic Microwave Background, but I was also encouraged to make some philosophical excursions. Needless to say, I talked about Bayesian Probability, and this in turn gave me an excuse to talk about David Hume, my favorite philosopher, and son of Edinburgh. Hume was the first to pose the “problem of induction”: how can we justify our prediction of the future based on past events? How can we logically justify our idea that there is a set of principles that govern the workings of the Universe? The canonical version of this asks: how can we be sure that the sun will rise tomorrow? Yes, it’s done so every day up until now, but tomorrow’s sunrise doesn’t logically follow from that. One possible argument is that induction has always worked up until now, so we can expect it to work again in this case. But this seems to be a vicious circle (rather than a virtuous spiral). As I discussed a few weeks ago, I think this whole problem just grows out of a category error: one cannot make logical proofs of physical theories.

I also went down the dangerous road of discussing anthropic arguments in cosmology, to some extent rehashing the discussion in my review of Paul Davies’ “Goldilocks Enigma”.

But in between I talked about the current state of CMB data, our own efforts to constrain the topology of the Universe, and the satellites, balloons and telescopes that we hope will improve our knowledge even further over the coming few years.

You can find copies of my slides and podcast of the talk itself here.

Thanks to my host Alex Murphy and other Edinburgh physicists for showing me a good time. (By the way, I took the train.)

Next up, a more general talk on the topology of the Universe at next week’s Outstanding questions for the standard cosmological model meeting, and then a more general review of the CMB at the Institute of Physics Nuclear and Particle Physics Divisional Conference.

Planck scanning strategy

OK, this is going to be very technical. In his comment to my last post, my colleague Ned Wright asks a couple of important questions about the way that the Planck Surveyor satellite is going to observe the sky. In the spirit of Mark Trodden’s question about the use of blogs in the research process, let’s see if we can answer these questions in a way that will satisfy Ned (who knows more about observing the CMB than most people on the planet) and not be completely opaque to the rest of my readers. Ned asks:

What is the current plan for the Planck scan pattern? I see this quote from the 2005 Blue Book:

The spacecraft will spin at 1 rpm around an axis offset by 85 degrees from the telescope boresight, so that the observed sky patch will trace a large circle on the sky (Dupac and Tauber, Astronomy & Astrophysics 430, 363, 2005)….

As the spin axis follows the Sun, the circle observed by the instruments sweeps through the sky at a rate of 1 degree/day. The whole sky will be covered (by all feeds) in a little more than 6 months; this operation will be repeated twice, resulting in a mission lifetime of around 15 months.

This describes a terrible scan pattern that may ruin Planck’s ability to measure the low-ell polarization signal that is essential for deterimining tau.

And the claim of covering the whole sky is wrong since as described, a 5 degree radius about each ecliptic pole is left out. That’s most of the sky, but not all.

The inner quote describes the way Planck will scan the sky — every minute, the satellite will observe a circle with an opening of twice the 85 degree angle described in the quote (a great circle that goes through the poles would have twice 90 degrees). Here’s a picture (from Dupac and Tauber 2005) of Planck’s location and the way it will scan the sky:


Ned is worried about two things, but I’ll discuss them in the opposite order.

He points out that the scan strategy leaves “holes” about five degrees across in the North and South poles. Indeed, the so-called “nominal” scan strategy above does suffer from this (although this is somewhat ameliorated by the fact that Planck has many detectors all looking at spots up to eight degrees from one another on the sky, so in fact those holes are largely filled). A more realistic scan strategy, as described in the paper by Dupac and Tauber mentioned above, will dip up and down out of the plane defined by pointing away from the sun, earth and moon. The exact way we perform these dips (how quickly and with what pattern) remains to be decided, but in any event will fill in those holes. For one possible strategy, the coverage looks like the following, from the same paper (yellow and red areas are observed more than blue and green):


Second, and most important, are the effects of long-term drifts in our detectors. An instrument like Planck can’t just look at a point in the sky and measure the temperature directly. Instead, the background level coming out of our detector is drifting over time, and these drifts can actually be large compared to the tiny CMB signal we’re trying to measure (for aficionados, this is often known as 1/f noise, after the power spectrum of the noise often observed in cases like this). This means that it’s relatively easy to measure the relative temperature of points that are observed nearby in time — since the background hasn’t drifted by much. But it’s much more difficult to measure relative temperatures over long periods of time. Therefore (as Ned points out), it might be difficult to observe patterns on large angular scales, across many individual rings. This is “the low-ell polarization signal that is essential for deterimining tau”: only on these scales can we observe the effects on the CMB of the very first objects to “light up”, more than twelve billion years ago.

This difficulty can only be ameliorated by “cross-linking” — making sure that you observe the same point at many different times. This lets us recalibrate the baseline of the detector every time we revisit that point or, better, set of points. The experiments that Ned Wright himself has worked on, COBE/DMR and WMAP, cleverly achieve this by the very complicated way they observe the sky.

Planck will definitely have a harder time, since its cross-linking only occurs at those points near the poles with many repeated observations. This puts a strong constraint on our detectors: they can’t drift very much over the one minute it takes to make a single circular scan. In this article, my Planck colleagues Christopher Cantalupo, Julian Borrill & Radek Stompor show that we can indeed handle these problems for more-or-less realistic kinds of noise.

To be sure, the real world is always more complicated than our simulations. The hard part will be dealing with what we euphemistically call “systematic effects” — roughly speaking, those errors that we don’t know how to describe very well, or that we don’t know about when we first fly the satellite. The ability to find these systematic effects is another reason why we want both very small known sources of error and the greater redundancy afforded by cross-linking, by comparing the same signal seen under very different conditions at different times during the mission.

Undoubtedly, we will encounter such unexpected sources of noise when we confront real data from Planck late next year, but we hope that the quality of our detectors, combined with the design of our scan strategy, will give us enough extra information to account for these inevitable problems. (But I probably won’t be able to tell you for sure until about 2011 when we’re due to make our first release of Planck results!)

Update: Further comments from Ned below. Discretion being the better part of valor, I shan’t comment on why these decisions have been left until now (although it is certainly arguable that flexibility is a good thing), and why Ned himself wasn’t consulted (suffice to say I wasn’t a member of the team that far back). However I must certainly agree that Planck’s ability to measure the polarization of the CMB would certainly be better if, as he suggests, the scan strategy visited pixels from many different directions, rather than approximately along lines of “longitude”; the measurement of polarization depends on just those directions, and having many different such measurements at the same location would make it easier to account for the aforementioned 1/f noise and possible systematic effects. Indeed, the experience of the WMAP team teaches us the difficulties of the measurement of large-scale polarization. We do believe that our raw sensitivity will be such that we can recover this polarization sufficiently accurately, but the proof will be in our results, and not in any simulations we do beforehand.

Planck Press

With only [sic] about a year and a half to go before launch, The Observer has a story on the ESA Planck Surveyor mission that I’ve spending much of my time working on over the last several years. (In fact, I have to spend the day writing a program that will play a very small part in working out exactly where the satellite’s detectors are pointing while it’s spinning around in space.)

Update: The BBC has got an article that goes more in-depth (and with more Nobel prize-winners, but less of me…).

Après Café Sci

There was a pretty good turnout a last night’s Café Scientifique in London. Thanks to any and all who showed up to hear my spiel about the cosmos (and, crucially, to talk back). We talked about matter & antimatter, the Cosmic Microwave Background, and even more esoteric topics like the origins of time (about which I had nothing more interesting to say than many members of the audience).

Thanks especially to Daniel Glaser and Ashish Ranpura for running the Café Sci series, and to all the people at the Photographers’ Gallery.

And now a deomographic question: was anyone reading this in the audience? If so, did you read about it here on the blog, or via Café Sci? Please leave a comment and let me know.

Where I'm calling from

I know that nobody cares about the peregrinations of astrophysicists but there’s not much else to blog about when you’re on the road. So a quick explanation of my absence from the blogosphere:

Last week, I was in Taipei for the CoSPA meeting (at which website you can find a copy of my talk on Bayesian Analysis of CMB Data — although as I tell the students that I’m teaching about giving presentations, if you could get all the information from the slides, what we be the point of listening?). Aside from interesting talks on the early Universe, measurements of varying fundamental “constants”, and new experimental initiatives in Taiwan and elsewhere, the meeting was especially notable for a banquet which featured that venerable Taiwanese institution — the Beaujolais Nouveau!

After a 24-hour stopover back in London, this week I’m in Trieste at SISSA, where I’m giving a couple of talks, one on the analysis of Microwave Background data (highlighting work done with my students JZ and AN) and another one, later today, on gravitational waves from supermassive black hole binaries at the centers of galaxies, work largely done with another student, DD. (Of course when I say “work done with” a student, I really mean “work done by” that student — they deserve all of the credit!). I’ve also got to spend a few wonderful hours actually talking about science in front of a blackboard with collaborators and understand how gravitational waves evolve and propagate in the early Universe (at least that’s the idea).

Happy Thanksgiving!

News flash: John Mather and George Smoot, two of the scientists behind the COBE Satellite, have won the 2006 Nobel Prize in Physics for their measurements of the average temperature of the CMB and the fluctuations about that average. (Here’s one self-aggrandizing reason why I find this particularly exciting.)

The average, measured by the FIRAS instrument, proves that the CMB is a “black body” to an accuracy actually better than the instrument was capable of measuring, which in turn proves that the Universe started out in a hot, dense state and has been expanding ever since — the hot Big Bang.

The CMB Spectrum measured by COBE/FIRAS—with error bars!

The DMR instrument measured tiny — one part in 100,000 — fluctuations on top of that average. These hot and cold spots trace the tiny initial lumps and voids that eventually grew into the so-called large-scale structure of galaxies and clusters of galaxies that we see today.

CMB Fluctuations measured by COBE/DMR

Of course these measurements open up some very big questions: why is the Universe so very nearly homogeneous (i.e., why is the temperature the same in every direction?)? What caused those tiny inhomogeneities that became everything that we can see in the Universe today? Unfortunately I’ve got lectures to prepare, so I can’t tell you about cosmological inflation, currently our best idea for solving both of these problems in one swoop….

(Images courtesy Lambda and the COBE Team.)

More cosmology prizes

The Balzan prize, worth 1,000,000 Swiss Francs, was just awarded to Andrew Lange and Paolo de Bernardis for their work as the original Principal Investigators of Boomerang, which, in 2000, produced the first high-resolution maps of the Cosmic Microwave Background and allowed a definitive measurement of the curvature of the Universe, in the sense of Einstein’s General Relativity. In fact, Boomerang showed that the Universe is flat: parallel lines don’t ever cross, and triangles have 180 degrees. I am lucky enough to be part of the Boomerang team, and I remember being interviewed on Irish radio at the time of our first results, and explaining to a puzzled newscaster that, no, we didn’t contradict Einstein: a flat Universe was just one of the particular kinds of curvature allowed by Relativity. Since then, Boomerang has refined its measurements of the cosmological parameters and, most recently, measured the CMB’s polarization (as I discussed about a year ago, and are summarized in this week’s Science).

Congratulations to Andrew and Paolo and the rest of the team.