March 2006 Archives

My mystery snow chimera is no more. The last of our snowpack melted away this week, and I think that this time, it's yielded for good to the oncoming forces of April. We've even had some good thunderstorms. The evenings have been glorious, almost early summer, which I can get behind since I've never been big on spring anyway.

There's still a few big piles of ice left over where the ploughs built up massive drifts, but that's about it. My creature was nearly the last unploughed ice to remain, though. It underwent a most curious process of de-evolution, too. Here, I caught it at a stage resembling either some primitive lungfish, or perhaps a scorpion, depending on your perspective. Although at the last, it was doing a good impression of the fuzzy protrusions and wakes often claimed to be photographs of the Loch Ness monster, just the crest of some large dream barely breaking the surface of the lawn.

My snow creature, now composed of melting ice, impersonating some different and more primative creature Monday last. 2006:03:27 11:43:41

Speaking of mind games.

I very rarely remember my dreams, but when I do they uniformly screw with my head in all sorts of ways. This morning I woke up from an entirely surreal dream that nevertheless evoked highly vivid memories. Or so I thought -- so much so that I immediately fired up my computer and used Google to check my sneaking suspicion that the places my dream-self had remembered did not in fact exist. (While still dreaming, said I to myself: That doesn't seem right. When I wake up I'll have to Google that.) Not until I went to write the thing down did I get around to noticing that they, in fact, could not physically exist.

Here's where things get fuzzy. The dream was a Gaiman-worthy visit to old haunts on the U. of Chicago campus, in which I noticed that certain aspects (completely uncorrelated to what's really there) were different than I had recalled. I think, in fact, that they were different from the vaguely similar setting of one or more dreams that I had many years ago, and had quite forgotten. The common element being the existence of a Labyrinth, half Jim Henson, half London Below, half Atuan, running through the hidden spaces of campus -- tunnels and battlements and foundations, accessed through forgotten stairwells and disused doors. The changes I noted were associated with settings connected to the Labyrinth, although they are settings I am not convinced actually occurred in the previous dream(s), so it is possible that last night's dream fabricated clearer memories than I actually had regarding an earlier dream.

So clear, in fact, that the first thing I did upon waking up was to search the web and make sure that the UofC Labyrinth does not, in fact, exist. Which it doesn't.

Although the stacks in the Seminary Co-op come close. And the odd connections between buildings on the main quad are infintely more intricate than anything the Gopher tunnels here can dream up, and the dusty passageways under the physics laboratories past ancient hulking equipment exude vastly more atmosphere. So perhaps my old dream is not so disconnected from reality after all.

"Sunlight Room", a print from Chris Appelhans' Alice in Underworld series, available at Froghat Studios. I thought it nicely captured the atmosphere I've in mind.

The always-intriguing This American Life managed to be particularly awesome this weekend in an episode that dwelled heavily upon the antics and consequences of New York-based Improv Everywhere. This is a group that considers life their stage, and with that motto takes the art of the mind game to a high point of absurdist refinement.

I am massively tickled by the idea of putting a Starbucks into a time loop. Even more so because it took most people three or four repetitions to notice.

Speaking of unusual blog traffic, hello and welcome to Tangled Bank readers. Tangled Bank #50 is out, and as usual it's got some excellent science reading. Since astronomy and physics are generally well-outnumbered by biology types, I submitted my series of WMAP posts this week.

Yesterday I jokingly asked my lab if anyone cared to hop an overnight flight to Turkey. No takers, I'm afraid, but there's a goodly number of tourists and astronomers alike camped on that country's southern coast just now. It's solar eclipse season again, and this year we get a good one. If you're in west or north Africa, Turkey, the Caucasus, or Mongolia, that is. For some reason the tourists are flocking to the Mediterranean instead of to Chechnyia, I notice.

As an aside, I also noticed a larger than normal number of visitors to my site yesterday. Glancing through the logs, it seems to be driven by people Googling for various permutations of "solar eclipse" and the Mideast. After I would assume scrolling through a great many pages of results, some of them appear to have landed here, where I last talked about eclipses.

The voting's over and the ballots nearly counted. The results are mostly unsurprising, but Political Arithmetik has a neat visualization of exactly where the surprises came. The diagonal line indicates equality between pre-election polling and post-election exit polls and official tallys. So when a party (notably Kadima and Likud) falls below the line, it did worse than expected; when above the line (the Pensioners party, by a startling margin) it has exceeded expectations.

At TPMCafe, Israel observer Jo-Ann Mort writes about the implications for Israeli politics, while the local newspapers are also hard at work on the question. Besides commanding performances by Kadima and Labor, there were unexpectedly strong performances for peace activists Meretz and the Arab parties, and the ultra-Orthodox foreign policy moderates Shas seem to have soaked up much of the religious vote. It will be trivially easy for Olmert to assemble a large centrist governing coalition; it's a pity that continuing unilateral actions are the best he can do in terms of a plan for dealing with the Palestinians.

The obituaries for Likud are practically writing themselves. Even Netanyahu was quoted as calling it "a broken, shattered movement." Unfortunately, this means that the new voice of fanatical Zionism will be Yisrael Beiteinu's leader Avigdor Lieberman, who is himself a crazy eliminationist settler type even as he heads a party mainly representing practical but poorly-integrated Russian immigrants with little interest in Zionism or even Judaism. Thus it's still premature to count the settler movement out just yet.

Today the news will bear watching, because it's Election Day in Israel. It is universally assumed that the newly-minted center-right Kadima party will win a large plurality in the Knesset, followed by center-left Labor (with whom a ruling coalition will likely be formed) and the religious-nationalist Likud in third. There is a wonderful visualization of the Israeli political field here at Political Arithmetik. However, politicians are afraid that an expected record low voter turnout of 65% or so (!) could mess with the predicted spread. Despite having 31 parties to choose from, the electorate reports frustration that there is nobody representative of their views for whom to vote.

Fig. 1: large-scale polarization structure of the CMB superimposed on the temperature anisotropy map. Click here to enlarge. NASA/WMAP Science Team.

Our story so far:
WMAP first impressions
WMAP and the Axis of Evil
WMAP looks through a galaxy

And now a topic near and dear to my heart, the polarization of the CMB. The WMAP team claims to have detected polarization, although it's a pretty weak and ratty signal and there's massive foreground contamination to deal with. Which is good for me, since there's no shortage of ground left for my mission to cover, although it also emphasizes just how difficult this field is going to be.

First off, let's tackle what I've labeled as Figure 1, a press image that's been widely disseminated. When the 3-year data release appeared, I heard from a number of quarters a sort of "my word, is that their data?" exclamation upon seeing it. Now the underlying temperature map is essentially a linear combination of the temperature data in the five bands that minimizes the foreground contribution (principally from the galaxy), and except perhaps in the plane of the galaxy is highly robust. The big white lines do represent polarization angles, but are far removed from raw data.

Compare to the image I've labeled Figure 2.

According to the Page et al Polarization Analysis paper (available here), the 61 GHz band was the least contaminated with polarized foreground signal, with synchrotron emission rising at lower frequencies and thermal radiation from warm dust kicking in at the highest band. Even so, the polarized part of the CMB amounted to around 0.3 μK of the entire signal, while even in the cleanest band the foreground polarization averaged twice that much. In Figure 2 the color indicates the total polarized intensity. The white lines again indicate the polarization angle measured on the sky, but this time at something like the resolution of the instrument¹. A line is only drawn where the polarization signal-to-noise (S/N) ratio is greater than one, and even after some binning there's not a whole lot of lines that aren't part of the galactic plane.

Increasing the number of samples that go into a single measurement boosts your signal faster than the noise (as the square root of the number of samples, generically). Thus three years of WMAP data are able to reveal details that couldn't be detected in a single year. Or, given a single set of data, it's possible to improve S/N by sacrificing resolution and averaging pixels together, just as is done in Figure 1. As it happens, it's possible to get a handle on reionization using just polarization data at the largest angular scales, so the WMAP team averaged over large enough patches of the sky to leave just the l=2 through l=6 multipole modes (I touched on multipoles earlier in this series). That's what was chosen for display in the nice press release image.

Fig. 2: V-band (61 GHz) polarization intensity and averaged S/N>1 vectors. Click here to enlarge. WMAP Science Team

An l=6 multipole has a characteristic scale of 30° or so, which at the distance of the CMB corresponds to lengths far greater than the horizon size at that epoch. These sorts of large-scale patterns cannot be intrinsic to the CMB; they must arise from physics going on closer to us. To get a feel for what this means, imagine this: shortly after the Big Bang, a magical indestructible transmitter sends out a signal. About 300,000 years later it is received by a moderately less magical station during the epoch of recombination. If you did this experiment today the two receivers would be very nearly 300,000 light-years apart, but thanks to the expansion of the universe that isn't true in any useful sense. However, it turns out that if you observed both of these stations while taking measurements of the CMB, they would appear to be roughly 1° (two full moon's widths) apart on the sky. Anything more widely separated than that hadn't yet had a chance to exchange information, so there can't be any coherent structure in the CMB on larger scales².

In fact, according to the current models, we do expect to see some power in the l=6 and larger polarization multipoles due to scattering during reionization. Post-recombination, the universe evolved in relative darkness for nearly a half-billion years, until it was fully a tenth of its present size. Then the light from the first stars and quasars began to permeate space, creating a patchwork of ionized plasma interspersed with the neutral soon-to-be intergalactic medium. Although few and far between compared to CMB photons, liberated electrons managed to scatter a small portion of the microwave energy, and this led to polarization writ large, if faintly. The amount of scattering is encapsulated in the τ term that I mentioned; now that we know it's value moderately well, it is possible to say that the first generation of stars was born when the universe was approximately 400 million years old.

This is just the beginning, though. There's an abundance of new science to be done with the polarization of the CMB, but it'll take experiments more sensitive than WMAP. The next steps will require raw sensitivity at least ten times better than WMAP, accompanied by clever techniques for removing the foreground noise, and at higher resolution to boot. This generation of experiments is already in design and under construction, so expect results on this front in the next handful of years.

¹ The beam size at V-band is about 30 arcminute. In the sort of high-S/N regime that, say, optical astronomers work in you could do image deconvolution and get down to 10' or better pixel resolution. Here, you instead resort to averaging many beams into a single pixel to push the S/N ratio up. So in Figure 2, the polarization has been smoothed to 2 degree pixels.

² That's probably not true. Blame inflation, which can and does create structures this large and more, right up to waves longer than the observable universe today. However, I'm talking about the polarization of the CMB as seen by WMAP, in which case inflation doesn't play a detectable role. So I'm doing like your high school physics teacher did, and sticking to the simple story.

Tags: CMB, WMAP, astronomy, Cosmology, space

No posting this past weekend primarily because of our youngest roommate's 22nd birthday. Which involved a minor bash, which involved much translocation of household items before, and application of mop and vacuum after. I didn't even make it in to the lab once, although I did get in a few hours of working remotely. Hooray for fast networks. Now if only we were like South Korea and all had optical fiber to our apartments, instead of being like ourselves and having the worst broadband service of the developed world.

At any rate, all is back to normal.

From the WMAP Science Team, images (from here) of the microwave foreground in three bands, plus an explanatory map. Click to enlarge.

Just a tip: I don't have a separate category for these WMAP posts, but thanks to the Technorati tags I've been including at the bottom of these posts they're trivial to find here.

Last time I mentioned some of the uncertainty that can arise on large scales due to the inconvenient fact that there's a big honking galaxy in the way when looking at the background radiation. Unless you're a galactic astronomer, in which case the CMB is just a happy side-effect of WMAP's creation of a map of the Milky Way Galaxy in microwave light. WMAP observes at a total of five frequencies; here I've pulled out maps of the foreground signal for three. At the bottom of the image is a cartoon that identifies the major objects.

The most noticeable feature, of course, is the bright horizontal slash across each map. This is the disk of our galaxy wrapping all the way around the sky. In this coordinate system the center of the image is the center of the galaxy, the top and bottom correspond to looking straight north or south out of the galactic plane, and the sides are 180° from the center, looking directly away from the galactic center. As it's pretty easy to identify, astronomers who survey the entire sky quite often work in these galactic coordinates.

Obviously these are false-color maps, since microwaves are far removed from the wavelengths of light the human eye can perceive. In this case, the colors don't even correspond to different wavelengths, precisely speaking. Here, the colors represent the mechanism by which the radiation was emitted. Red denotes synchrotron radiation, produced by electrons spiraling at relativistic speeds through the tangle of magnetic fields that fills the galaxy. Green is for free-free emission, the result of collisions between electrons and protons in the tenuous 100,000-degree plasma left over from supernovas and massive star winds. Blue signifies the thermal radiation emitted by warm dust, found throughout the galaxy's disk but typically associated with very young or very old stars.

At least that's the theory. The idea is that each source of radiation varies with frequency in a particular way; by combining the five maps it's possible to algebraically separate the signal seen at each pixel into four components (three foreground sources plus the CMB). There's enough data there to do this exactly, but only if the frequency dependance of each source is known exactly. In practice the models are good, but far from perfect. This is a problem, since as I've discussed doing science with the CMB requires being able to pick out tiny variations in the intensity of the background signal. That's why so much suspicion is aroused by the curious alignment between the low-order multipoles and the orientation of the galaxy. It only takes a small error in the foreground models to create a large error in the CMB anisotropy, and in general, you might very well expect such noise to align with the galactic plane.

Thus John's suggestion that a sensible thing to do would be to run a Monte-Carlo simulation of the foreground removal. In principle, this could at least estimate the likelihood of this alignment arising due to contamination by the galactic signal.

Click on the bottom cartoon to find out what other objects are identifiable in these maps. On the far right is the Orion-Eridanus Bubble, which contains the Orion star-forming regions, among other things. it shows up in green on all three maps, outlining the superheated plasma pumped out as stellar winds by the young stars forming in places like the Orion Nebula. This plasma has since expanded to very, very thinly fill a vast cavity, like a bubble blown in the interstellar medium. Another free-free dominated feature is the Large Magellanic Cloud, found in the lower right quadrant of the map.

In the K-band red features are more prominent, especially the diffuse red glow arcing upwards from near the center of the galaxy, known as the North Polar Spur. The Milky Way has several such large-scale features in the synchrotron sky, which also show up in X-ray observations, indicating that they are the site of energetic processes that boost particles to extreme energies. It has been suggested that the Spur is the shockwave from an old nearby supernova, the bubble blown by an ancient starburst in galactic core, or even a jet from the supermassive black hole at the center of the galaxy. The truth is that there is no widely accepted explanation.

Tags: CMB, WMAP, astronomy, Cosmology, space

Speaking of snow sculpture, I passed this example in progress a day or so before my own project. Clearly art types posessing more actual skill than myself at these things, they seemed genuinely peeved that the snow wasn't amenable to adding more detail.

This is way more awesome than the gigantic snow phallus the dorms erect each year after the first big blanketing turns malleable.

Art student snow sculpture. Probably never had much of an audience though, since it went up in the middle of spring break, and fell over before the students got back. 2006:03:16 19:29:10

Returning to the Spring Break Blizzard for a moment, it's easy to tell that this storm just swept in from the north. See, taking this photo I was facing nearly south. If I'd been looking north, the trees would still have been black.

Near the 15th Ave entrance to campus, the north-facing sides of, well, everything, have been plastered solid with snow. 2006:03:13 14:52:52

From the Hinshaw et al WMAP paper, fits of the low-order multipoles of the CMB sky.

Here's a figure from the WMAP temperature results paper (available here) that drew some attention from the theorists in recent discussions. In part because of the provocative-sounding term attached to it: the Cosmic Axis ... of Evil!

First, what's going on in this figure? The top left figure is the familiar temperature map of the microwave background. Now the bread-and-butter of CMB work is breaking this map up into multipoles, or simple functions that each encode structure on a particular scale, and which when added together give you the original map. Reading the maps left to right, top to bottom, the first few multipoles are shown that add up to the large-scale structure of the CMB sky. In the real map, notice that there's a dark blue (cold) patch just right of the center. This sort of large-scale structure is reflected in the multipole plots; several of these low-order maps similarly have a cold peak at about this point.

When this was first done a few years back, ears pricked up because, if you squint, it looks like the l=2 and l=3 (and mayle l=5) multipoles have the same alignment. Almost like they're lined up along a cosmic axis, which you wouldn't expect if the multipoles are randomly aligned. But it's theoretically very naughty to give the Universe any kind of special direction; hence the axis of evil bit. In particular, it's hard to have a preferred cosmic axis, or vector anisotropy, without messing up the electromagnetic force in really obvious ways.

But back then it was pointed out that the supposed Cosmic Axis also lines up with the axis of the galactic coordinate system, and that would be quadruply unlikely. So it was dismissed as an artifact of not being able to perfectly subtract contamination from the galaxy -- for instance, maybe the cold patch I mentioned above isn't real. Except that now we have the 3-year WMAP data release, and it makes a strong case that this is real. So either we have a curious coincidence on our hands (just how curious is being hotly debated, but any sort of curious coincidence always makes theoretical physicists jumpy), or there's something genuinely odd about the very geometry of our universe. So far they think it's not quite curious enough that we need to seriously consider the second possibility, but be sure that they're thinking about it.

Tags: CMB, WMAP, astronomy, Cosmology, space

About once a year, I embark upon a moderately overambitious act of snow sculpture. This year conditions were nearly ideal, having a good depth of reasonably sculptable snow land during spring break. Could have been better, to be sure -- it was a tad warm, and the snow a mite too old by the time it got adequately sticky, so fine detail was a non-starter. So my attempt at a dragon came out looking like a dinosaur-tailed Sphinx with a sheep's head. I rather like the carved-from-granite blockiness of it, though.

My snow dragon-sheep-Sphinx thing guards the Knoll. The snow was so dense when I built it, the thing'll probably stand for weeks. 2006:03:19 14:37:59

The Knoll, a nice tree-ish bit of park near the 15th Ave entrance to campus. 2006:03:13 14:53:24

Having previously declared winter dead, Old Man Winter opted to spend the past week upbraiding me with all the subtlety of a blizzard. Yea, with exactly that much subtlety. On NPR, they were cracking jokes about how Minnesota looks like ANWR now, suggesting maybe we should try drilling for oil. Punxsutawney Phil, I am not.

Not that I wanted to go to campus at all, since I had a cold and was all for calling it a sick/snow day. But work called, literally, and if I'm going to sit on the phone with my advisor, I might as well do it from my desk where I can easily refer to things.

But at least I got some quite decent pictures out of the deal. Wandering into campus, I was reminded of nothing so much as a scene right out of Narnia, pre-Christmas. If only we had more rustic lampposts, the image would be complete.

After a well-earned two-day St. Patrick's Day-weekend and a long Sunday of catching up, I'm back.

Minnesota crew, take note: it's perfect snowball-fight snow out there today. Get outside. Take advantage. This is why the lab looks to me for intra-workday time-wasting field trips.

Rather than respond to the question he actually posed, Connor's post yesterday for some reason inspired me to dig up a photo from last summer's western roadtrip. Maybe next time I have a slow week I should do a series of recap photoblogging from that trip.

A prarie dog at home in one of the villages in Wind Cave NP, in the Black Hills. I don't know about Denver's urban dogs, but out here, you have to move exceeding slowly to get this close to one. 2005:08:19 10:49:18

The WMAP three-year data is out! There was some kvetching that it didn't show up exactly at noon EST, but then the site lit up at a quarter past or so and around the world, cosmologists decided to call it a day and start reading.

The headline numbers don't change much, of course -- the Universe is still flat, still about 13.8 billion years old. But there are some significant surprises nonetheless, that are quite interesting.

One concerns τ, the optical depth to reionization. Now it's about 0.09, which is a factor of two smaller than everyone thought previously. Which makes reionization more sensible, since now the Universe ionizes at about z=10 or so, which is reasonably consistent with where people actually see the Gunn-Peterson trough in quasar spectra. Back when τ~.17, you had reionization happening around z=20, which is very early, and made it hard to explain the much later presence of at least regions of neutral gas.

That, incidentally, will suppress B-mode polarization at large angular scales. I think. Which would be bad for some of our competitor experiments. Not to gloat or anything.

The other surprise is n, the slope of the perturbation power law spectral index; it isn't equal to 1. More like 0.95, with pretty small error bars. That's just weird, and will give the theorists something to chew on for a bit.

Tags: CMB, WMAP, cosmology, astronomy

Enjoy keeping questionable company? Don't mind being disliked by the neighbors? Ever wanted to run an outskirts motel or rental complex in Iowa?

Iowa's new residency restrictions on sex offenders may have created a tasty business opportunity for you! Can't you see the flyer? "Freshly remodeled suburban brownstone appts for rent -- no longer have to lie about your address -- guaranteed 2,000+ ft from nearest school or daycare!"

You'd imagine folks would pay a small premium if the alternative is sleeping at the truck stop.

Still here, but light posting will continue through the end of the week. Between being a bit under the weather and submitting a grant proposal, my free time has been rather constrained. I have photos from this week's snowfall to post (because this season is just transparently mocking me), and will try to provide some coverage of tomorrow's WMAP data release. Otherwise I shall be reading, drinking tea, baking, and otherwise relaxing when not in the lab. It technically is spring break, after all.

Big news afoot. First the backstory...

WMAP, for the non-experts in the room, is the satellite currently making precision measurements of fluctuations in the cosmic microwave background. The first year's worth of data made all kinds of waves, including proving that the Universe is geometrically flat, and giving a pretty good estimate of its age. About two years ago, one started to hear grumbling at cosmology conferences about the whereabouts of the WMAP second year results. At this point it's practically devolved into a running joke, although with a serious undercurrent. Rumors have been swirling for ages about how the data release was held up due to some difficulty with polarization measurements, which is exactly what the current CMB mission race (of which my group is a part) is all about.

Of late the rumors have really been churning, since lots of WMAP scientists have been giving talks recently, and some have hinted that the data could surface sometime soon. Now it's official. There's an email going around from the WMAP team at Goddard.

The WMAP second data release will occur at noon EST, Thursday March 16!

First impressions: One, they mention that there will be no televised media accompanying the release. This suggests that we can stop sweating the possibility that they were going to trump us all and get a strong polarization detection. Two, I wonder if the release date was ever the 15th, and got pushed back because someone at GSFC remembers Caesar. Three, it's no longer called the second-year release, just the second release; sounds like there will be more recent data thrown in as well.

Watch this space.

Tags: CMB, cosmology, WMAP