... The Rest of the Story Part II: Polarization

In "The Rest of the Story Part I," I discussed how measuring tiny temperature fluctuations in the CMB provides us with a multitude of data about the early Universe. At the end of that post, I mentioned this wasn't the end of the line for CMB measurements as we can also learn a ton from CMB polarization anisotropies. That's the topic of this post.

There's really only three kinds of information we can get from measuring light: we can learn how bright the light from an object is at a given set of colors or frequencies (the spectrum of the object), we can learn how bright the object is as a function of where we're pointing our telescope (an image of the object), and we can study the orientation of the light coming from the object as a function of where we're pointing our telescope (the polarization of the object). Remember that polarization is the direction that the E-field of the light is pointing, on average.  First, the COBE satellite measured the spectrum of the CMB in the early '90s and showed that it matched that of a theoretical blackbody.  Then, over the next twenty years we've imaged the CMB with higher and higher resolution and sensitivity enabling us to do statistical analyses and tests on the variations in intensity/temperature we found (the TT power spectrum).  Now, the bleeding edge of CMB observations is trying to characterize and map out its polarization properties.

First, COBE measured the spectrum of the CMB.

Since then, the WMAP satellite (which collected the data that make up this map) and many ground-based and high altitude balloon-born experiments have imaged and mapped the CMB temperature anisotropies.

We now strive to map out and understand polarization of the CMB. The white lines in this image are actually polarization directions in our Milky Way galaxy, measured by WMAP, created in large part by dust grains aligning themselves along our galaxy's magnetic field lines.

You're probably more familiar with polarization than you think.  Take sunlight, for example.  Light coming from the Sun starts out unpolarized. While some light from point A on the Sun might be oriented "up-down" and some light from point B oriented "left-right," on average there is no preferred direction of the light when it reaches you. This changes when light interacts with matter. For example, sunlight that scatters off smooth surfaces like a relatively calm lake or the roof of a car (glare) is polarized in the "left-right" direction. When you use polarized sunglasses, the direction of polarization of the lenses is oriented 90 degrees off from the glare's direction ("up-down"), which blocks the glare. The closer to 90 degrees the sunglasses are to the direction of the polarized glare, the more the glare will be blocked. (So turning your head left and right keeps the lenses oriented 90 degrees with the glare, but rocking your head from shoulder to shoulder reduces that angle and more glare is let through the sunglasses).

[As a really cool aside, this is how LCD computer monitors and televisions work. These monitors have two polarized screens, with polarizations oriented 90 degrees apart. This means that the light coming from the backlight lamp would get polarized by the first screen and then totally blocked out by the second screen, leaving all the pixels dark. Between the two polarized screens, each pixel has liquid crystals in it. By putting a different voltage across the crystals, you can change the crystals' shapes. Changing their shapes alters the path that light takes through the crystals and therefore changes the polarization of the light passing through it from the first screen. That means when it reaches the SECOND polarized screen the polarization is no longer 90 degrees out of sync and some light gets to pass through, making that pixel brighter. So, the brightness of each pixel is controlled by changing the polarizing properties of the liquid crystals. You can see this for yourself! Take a pair of polarized sunglasses or 3D glasses from the theater and rotate them in front of an LCD screen. Watch as some (or all) of the colors start bright, get dim or get totally blocked out, then get bright again as you rotate the polarization angle of the glasses and change the amount of (polarized) light that passes through. How cool is that?!?]

     Since polarization is an averaged effect, exactly how much the light is polarized depends on how much of it is oriented "up-down" compared to "left-right."  This means measuring polarization is a differential measurement.  You can't know how polarized the light is on average if you're only sensitive to "up-down" polarization - you also need to measure the brightness of "left-right" polarized light and then take the difference of the two.  What's left is the average polarization.  So, each polarization-sensitive pixel, which I'll call a polarimeter, needs two detectors which are sensitive to polarization directions offset by 90 degrees.  One detector measures "up-down" light and the other detector measures "left-right" light, and subtracting the two measurements gives you the average polarization.  Below is an image of a single polarimeter.  The triangle leads in the center act in pairs.  The top and bottom ones pick up "up-down" polarization, and the left and right ones pick up "left-right" polarization.  The signals are then transfered by the electrical lines in the pixel and are detected by the island-like structures on the top and right of the pixel.  These are the actual detectors.  There's a third detector in this pixel, but it's not connected to the triangle leads, so we call it a "dark" detector - it shouldn't be seeing any light being funneled to the pixel and so it can be used to diagnose the background signal each detector sees regardless of what the pixel is looking at.

A single prototype polarimeter pixel designed and tested by the TRUCE collaboration, another group I'm involved in.  The whole pixel is 5 mm in diameter.

     Just as sunlight can be polarized by scattering off of surfaces on Earth, scattering CMB off of matter in the early universe also polarizes it.  The intensity of the polarization, and the direction it's pointing in tells scientists and awful lot about the make up of the Universe and what was going on in the earliest moments of the Universe's past.  Just like with temperature anisotropies, one can measure the power spectrum of CMB polarization anisotropies.  There was only one temperature power spectrum (the TT spectrum), but you can actually break up the polarization into two related but fundamentally different types (called E-mode or EE and B-mode or BB power spectra).  The trouble with polarization is that the CMB is only very weakly polarized, which makes it a very small signal.  It took 30 years for scientists to detect temperature anisotropies, and polarization anisotropies [at their strongest] are more than 100 times fainter.

EE and BB (upper limits) polarization power spectra as published in Chiang et al., 2010. The sister experiments QUAD and BICEP currently have the lowest upper limit constraints on the level of B-mode polarization. The dashed lines on the lower left of the BB plot are the expected gravitational wave B-modes for a particularly nice inflationary model, while the dotted curve on the bottom right of the same plot is the expected lensed B-mode spectrum. (See below). QUAD was also the first experiment to confidently show bumps and wiggles in the E-mode power spectrum, which match very well to expectations arising from fitting the temperature anisotropy power spectrum.

     My PhD adviser's thesis project, the DASI experiment, was the first to measure E-mode polarization in the CMB.  A decade later, a whole slew of experiments have measured E-mode polarization with higher and higher precision, mapping out the bumps and wiggles of the EE power spectrum.  But the real prize is still B-mode polarization, which is 10-100 times fainter still.  It has yet to be observed.  Two different physical phenomena could produce B-mode polarization.  One is gravitational lensing.  As light passes through really massive objects, gravity changes the direction of the light, making the massive object act like an optical lens.  If E-mode polarized light is gravitationally lensed by a massive object like a galaxy cluster, then B-mode polarized light is produced.  We've seen countless examples of gravitational lensing, predicted by General Relativity, so we know it takes place, and we've measured E-mode polarization, so we know it exists.  That makes us all but completely certain B-modes from gravitational lensing exist.  It's just a matter of making a camera sensitive enough to detect it (and that's exactly what SPTpol should do).

An example of gravitational lensing. The yellow blobs are all galaxies within a galaxy cluster - a massive collection of galaxies, hot gas, and dark matter all bound together into a single entity by gravity. Much farther away, and behind the yellow cluster, is a single young and blue galaxy. As the light from the blue galaxy travels through the cluster, the gravity of the cluster bends the direction the light is traveling, producing arcs and multiple images of the same blue background galaxy. Gravitational lensing of E-mode polarization produces B-mode polarization, which has yet to be detected.

     The other source of B-modes is even more exciting.  We expect that there was a brief period of time, a fraction of a fraction of a second, right after the Big Bang when the Universe expanded much faster than the speed of light, a period of time we call the epoch of inflation.  (In case you're wondering, nothing in GR says space can't move faster than light, only matter and energy in space-time).  Inflation is a really nice theory because it solves a number of problems that crop up when you stop to consider certain properties of the CMB, and it solves them in a relatively simple and elegant way.  The problem with inflation is that there's never been any direct evidence that it actually took place.  The problem is compounded by the fact that you can't actually see inflation happen by looking farther away (and further back in time).  The Universe was a dense fog before the CMB was emitted, and just like on a foggy day, there's no way to see through the fog using light.

     .... But General Relativity comes to the rescue!  It predicts a phenomenon (so far undetected) called gravitational waves.  A gravitational wave (GW) is a distortion of space-time that passes by.  Say you have a circle of particles hanging out in space.  As a GW passed through space itself would be distorted, and the circle would be distorted into an oval, oscillating back and forth between being oriented "up-down" and "left-right."  Well, you might have guessed it, gravitational waves polarize light, and in particular produce B-mode polarization.  This is some of the real juicy stuff...  if we detect B-mode polarization in the CMB at the angular scales expected for gravitational waves from inflation, we prove inflation took place.  That would be HUGE!  It would also mark the first time in the history of science of an (albeit indirect) measurement of a whole new spectrum of radiation to measure and study and to use to diagnose the universe.  It wouldn't be regular light from the electromagnetic spectrum... it'd be gravitational radiation from the gravitational wave spectrum.  Each time in history when we measured a new part of the electromagnetic spectrum our view of the Universe completely changed.  Imagine how much will change if we can measure a completely different spectrum entirely!

What would happen to a perfect circle of particles as a GW passed by.  The particles don't change where they are in space (they stay at the same coordinates).  Instead, the distance between two set points of space changes.  GIF taken from the GW Wikipedia entry.

     After we install the brand new polarization-sensitive camera we've all been working so hard on, SPTpol will try to measure both types of B-mode polarization (and E-modes with better precision than previous experiments as well).  We're confident we'll see the lensed B-modes, but we're also trying to detect the gravitational wave B-modes (or inflationary B-modes).  Either way, we'll be doing broundbreaking new science and helping to usher in a new era of CMB measurements and cosmology.

...

     That's why I'm going down to the South Pole. :)


     I know that was a lot... so thanks for braving through it.  I just wanted to put my trip in context.  We're going down there for serious work and serious science and this series of posts hopefully gives people some idea of what the science is and why it's exciting.  The next post will likely have neat hardware pictures, which will be tons more fun.  Promise!

It's Starting to Get Real

     Hi everyone!  Sorry for the long delay between posts.  Things have been super busy in the lab while we get ready to deploy.  It's been an eventful couple of weeks.  The camera receiver shipped to the Pole earlier today and that means we're really close to deploying this experiment!  The receiver is the cryostat that will house the camera and much of the passive readout electronics, all of which has to be cooled to 4 K or below (-452.5 F) to operate.  The detectors actually sit much colder than this, at only 280 mK (0.28 K, or -459.3 F).  That's really cold. 0 K is absolute zero, the point at which all motion stops, so we're only 1.3 F away from as cold as you can possibly get... period.  With some rare exceptions, (like when the CMB gets absorbed by clouds of, say, formaldehyde), the coldest any place in the Universe gets is 2.7 K because the CMB is 2.7 K.  In general, if some gas cloud or whatever were to get colder CMB photons would just re-thermalize it to 2.7 K, so the Universe is pretty much locked to whatever the CMB temperature is.

     All of this means that our receiver (when in operation) is literally and figuratively one of the coolest places in the Universe.  If the current cosmological model holds true, it will take another 36.2 billion years of expansion for the Universe to cool to our operating temperature.  I mean... the Universe is only 13.76 billion years old.  It's a mere teenager right now, and it will be 50 [billion] years old when it has finally cooled down below our receiver's temperature.  (For those interested, the lowest temperature ever reached in a laboratory was 100 pK, or 1x10^-10 K.  That's 3 billion times colder than what our camera will be in operation).  A couple pictures of the camera and the camera installed in the receiver cryostat when it was still in Chicago are below.

A backside view of the SPTpol camera (receiver). The back end of one of the 150 GHz camera modules I designed can be seen installed (the tower with the red circuit board). The green circuit boards will have 192 individual 90 GHz pixels soldered on from the front side. Most of the holes in the circuit boards are for alignment pins to hold the pixels in exactly the right locations.

A view of the receiver installed in the receiver cryostat. While the camera itself is really cold, it can't just float in space - it has to eventually be connected to the rest of the world. This means to stay cold on the camera end all the readout wiring has to be clamped and thermally sunk at several temperatures on the way from 280 mK to 300 K (room temperature). Those awesome star-shaped hex rings provide clamping area to do just that.

     But the REALLY exciting news is that I have my plane ticket information for my trip to Antarctica!  The receiver team (that's me, a grad student from Berkeley named Liz, a grad student from Chicago named Abby, and a post-doc from Chicago named Brad: we get to actually install the camera) has been waiting for confirmation that the receiver worked and could cool and operate detectors properly.  That milestone was reached last week at Chicago, and with the receiver shipping out to meet us at the Pole we got the okay from the higher ups of SPTpol to allow Raytheon to ticket us last Friday.  I got my flight information yesterday and...  I officially leave for the South Pole next Thursday, December 1.  It's going to be quite the adventure....

     I start out leaving Denver at 4:40 PM Mountain time (Thursday Dec 1) to arrive in Los Angeles at 6:20 PM Pacific time.  I'll be hand-carrying four of seven camera modules I've built as carry-on.  They're way too precious to pack as checked baggage.  Liz will have the other three since they'll be in Berkeley being tested until we leave next week.  I then have a 5 hour layover at LAX.  I think at that point I'll be meeting Liz, Abby, and Brad since we're all on the same flights from there on out.  The next leg is the longest - a 13.5 hour flight from Los Angeles to Auckland, New Zealand (the north island).  We take off at 11:25 PM Thursday, Dec 1 and arrive at 9:50 AM Saturday Dec 3.  The flight might be only 13.5 hours, but New Zealand is 20 time zones ahead of me in Colorado (and 21 ahead of LA).  I'll lose nearly an entire day going over the International Dateline.  How messed up is that?  It's a crazy world.

     From there it's pretty easy going.  I take off from Auckland that same day at 12:10 PM and arrive at 1:35 PM in Christchurch, NZ (the south island).  Christchurch was in the news earlier this year when it was hit by a pretty major earthquake.  I think it was the second-largest natural disaster in New Zealand's history - really sad.  A lot of people were killed and a lot of old/iconic buildings were damaged or destroyed.  Thankfully it wasn't any worse.

     I'm in Christchurch for a couple days, and while there I'll get all of my ECW (Extreme Cold Weather) gear for life at the South Pole.  Then I just wait for a phone call confirming my flight to McMurdo station on the coast of Antarctica, which will be the morning of Dec 5 (Dec 4 back in the US).  I'm at McMurdo for a day and then I have my fifth and final flight on Dec 6 when I will finally reach the South Pole, weather permitting.

     This is going to be crazy exciting and I'll be taking pictures and telling everyone about the trip very soon.  In the next week I hope to post a couple more entries on the science, instrumentation, and technology to finish up all the background material and give everyone a decent sense of what we're doing, how we're doing it, and why.  I'll be busy with final lab preparations and packing, though, so no promises.  In any event, this blog will FINALLY have an entry about my actual experiences at the Pole in just a couple weeks.

Mail at the South Pole

     Some people have asked if it's possible to send and receive mail at the South Pole.  The answer to both questions is yes!  If you want to send me anything you can reach me at:

Jason Henning, A-379-S

South Pole Station

PSC 468 Box 400

APO AP 96598

This includes packages as well, but I understand that flat mail is given priority, and science equipment and supplies highest priority of all, so it could take a while to get to me.  If you're sending a package, it can take up to two months depending on the size and weight, which means don't send it past the first week of December or I can't guarantee I'll still be at the Pole when it arrives!  Also, please don't use packing peanuts or other packing materials that are generally thrown away.  The USAP recommends using recyclable materials and items that can be re-used or packed with me for re-deployment.


     I'm happy to send post cards too.  If you're interested, just shoot me an email with the address you want me to send it to and I'll make sure you get one.  I'll mail post cards from the Pole, but I can't guarantee you'll get it before I return to the States...

... The Rest of the Story Part I: Temperature

     This blog will actually contain posts about going to the Pole, I promise.  But I want to take the opportunity now when I have a little time to keep explaining the background science and why I'm actually going to the Pole.  I think it always helps to have a big picture in mind.  So thanks for your patience.

     Let's see... I left off with describing what the CMB is and why it's important.  I mentioned it has an incredibly pure blackbody spectrum, meaning the radiation has the same temperature no matter where you look out into space.  (That's a bit incorrect - a photon doesn't have a temperature, but you can infer a temperature of the matter that emitted it by looking at how the intensity of the light changes with color, i.e., its spectrum).  This fact proves a Big Bang took place.  But as with everything in science, look a little deeper and the answers aren't so black and white.  The temperature of the CMB, now a meager 2.7 K, isn't exactly 2.7 K everywhere you look.  From spot to spot on the sky the temperature changes ever so slightly. One spot, say in the direction of the Big Dipper, might be 2.7 K plus just a tiny bit, and another spot a couple full moon diameters away might be 2.7 K minus a little bit.  We call these changes temperature anisotropies.  When something is isotropic it means it looks the same from every direction.  When something is anisotropic, it looks different depending on where you're looking.

     CMB temperature anisotropies are really tiny, so tiny in fact it took us 30 years to first observe them.  The CMB was observed (accidentally) for the first time by a couple of scientists (Penzias and Wilson) at AT&T Bell Labs in 1965.  (They won Nobel prizes for this... I'm telling you CMB science is super important!  I should also point out that several theorists had been predicting the existence of the CMB for decades, so while it was accidentally discovered, the discovery didn't surprise anyone, except for maybe Fred Hoyle.)  No matter where they looked in the sky they saw this extra microwave light and it looked to be the same temperature everywhere.  For 30 years scientists tried to find deviations from this constant temperature, and only with NASA's COBE satellite in the early '90s did we finally see them.  30 years!  How small are these fluctuations?  Well, let's say the Earth's surface is as smooth as the temperature of the CMB (the Earth is still the same size, I'm just re-scaling surface topography).  If that were the case, Mount Everest (20320 feet above sea level) would be a pathetic 1.5 feet tall.  This corresponds to deviations at a few parts in 10⁵.  That's tiny.  You've got this whomping signal at 2.7 K, and scientists are trying to measure tiny temperature fluctuations at the level of 10^-6 K (10 μK, or micro-kelvin) and smaller.  That's really hard.

Penzias and Wilson in front of the Bell Labs Horn Antenna with which they observed the CMB for the first time.

     But, who cares?  Why does it matter that the temperature of the CMB isn't the same in every direction?  Nothing is perfect, right?  Well, precisely! And we owe our existence to that fact, (and gravity).  The CMB was created a long time ago, when the Universe was only 380,000 years old.  Since light can't travel faster than light speed (duh), it takes a long time to get here... 13.3 billion years.  So, when we look at the CMB, we're looking at what the Universe looked like 13.3 billion years ago - the CMB is a baby snapshot of the Universe.  This was before stars and galaxies and planets.... there was just a bunch of hot hydrogen and helium gas.  That's it.  None of the heavier elements (except for a bit of lithium) even existed.  They would be made in the cores of stars tens of millions of years later.

     Anyway, remember that the temperature of blackbody radiation tracks the temperature of the matter that emitted it.  That means a hot spot in the CMB is a region of space where matter was just a bit denser and hotter than the average.  A cold spot is a region where matter was just a bit less dense (rarefied) and cooler than the average.  Over time, gravity amplified these tiny differences.  Hot/dense spots would become even denser, and cool/rarefied spots even less dense.  Eventually, as gravity did its thing the hot spots would become galaxies and clusters of galaxies (e.g., the Coma Cluster), while the cool spots would become giant cosmic voids where next to nothing exists (like the Boötes Void).  So, it turns out that CMB temperature anisotropies are the seeds of structure in the Universe.  When we look at the CMB, we're looking at the blueprint for what the Universe looks like today.  That's powerful, and amazing.

     The power of these temperature anisotropies can be turned into quantitative predictions about the contents and evolution of the early Universe, when it was a relatively simple place before gravity mucked it up and made stars and galaxies and planets and us.  You can study how large the anisotropies are as a function of the size of a patch of the sky you're looking at.  So, let's say you observe a chunk of the sky 10 degrees by 10 degrees square.  First, you measure the deviation from the average CMB temperature for all patches that are, say, 5 degrees square within that 10x10 area.  If you were to plot these temperature up, you'd see a nice bell curve (a Gaussian) with a mean at the CMB average of 2.7 K, with some standard deviation (how wide the bell curve is).  The wider the bell curve, the larger the deviations from the mean of 2.7 K.  If you keep repeating this, and sample the 10x10 patch of sky at smaller and smaller intervals -  a degree, a tenth of a degree, hundredth, and so on -  you can plot up the size of the deviations (the width of the bell curves) as a function of the size of the patches of sky you broke the original 10x10 patch into.  We call this a power spectrum.  For the case of the CMB, you have temperature squared (the power or amplitude of the signal), on the y-axis and the angular size of  sub-patches on the x-axis.  Fitting models to the bumps and wiggles in the CMB temperature anisotropy (TT) power spectrum tells you all sorts of information about the Universe, like how much matter there is compared to dark matter, the geometric shape of the Universe, how much helium there was compared to hydrogen right after the Big Bang, and a whole lot more.  (If you're curious to see how changing parameters like this affect the shape of the CMB TT power spectrum, I highly recommend this site by NASA: the Build a Universe tool.)

     Take a look at the two images below.  The one on the top is a map of CMB temperature anisotropies.  Blue/black spots are cooler than the average, yellow/red warmer, and green are spots that show very little deviation from 2.7 K.  The image on the bottom is the TT power spectrum of this map, and shows how big the temperature fluctuations are as a function of angular size of patch on the sky.  You'll note that the power spectrum has three large bumps, and then a tail at smaller angular scales of bumps that slowly die away.  The largest bump happens at about the degree scale.  Now look back at the map on top.  Notice how your eye can see tiny clumps and patches?  The angular size of the patches your eyes are picking out is about a degree.  You're eyes are actually picking out the first peak in the CMB TT power spectrum!  (This is a bit different than what I just described, but it turns out power spectra are related to Fourier transforms, and your eyes actually take the Fourier transform of light as it passes through your lens, so your eyes are really good at taking and interpreting power spectra).

WMAP 7-year map of CMB Temperature Anisotropies.

CMB TT Power spectrum. Power (temperature [μK] squared) on the y-axis, angular size of patches on the sky on the x-axis.

     This is fantastic!  If we measure the power spectrum of temperature deviations in the CMB, we can learn about the contents and evolution of the Universe in a quantitative way.  No longer must we just say "the Universe is expanding" or "the Universe is 10-20 billion years old."  We can pin these down more accurately.  Using the CMB and a few observations of other objects and phenomena, and by fitting a model to the power spectrum bumps and wiggles, we can say the Universe is 13.76 +/- 0.11 billion years old, the Universe is spatially flat to 1% (as opposed to positively curved like a beach ball or negatively curved like a pringle), regular matter makes up 4.5%, dark matter 22.6%, and dark energy 72.9% of everything in the Universe, and lots of other more technical parameters.

     For the past 20 years scientists have been measuring temperature anisotropies with greater and greater precision, and to smaller and smaller angular scales.  But does information in the CMB stop at temperature deviations?  No way!  Turns out, we expect light from the CMB to be weakly polarized.  Polarization is the orientation of light.  Remember that light is made up of alternating electric and magnetic fields.  Polarization is defined as the direction that the electric (E) field of the light is pointing.  Mapping out how the light's polarization changes with position on the sky tells us even more information.  That's what I've been working on - a camera that's sensitive to CMB polarization to replace the temperature sensitive one on the South Pole Telescope (SPT).  But let's leave that for the topic of a future post.

Luggage Tags and Penguins

     I got my luggage tags in the mail the other day for my flights.  I received three of these bad boys, along with customs paperwork for the equipment I'll be hand-carrying down and stuff like that.  Two for my personal bags, and one for the briefcase-like box I'll have as a carry-on that will contain in it six or seven of the modules I'm building.  I've seen these tags before and I always think the penguin on them is the Linux penguin at first glance, but it isn't.  USAP stands for United States Antarctic Program.

My USAP luggage tag.

     And speaking of penguins, a lot of people ask (in jest, of course) if I would bring them back a penguin.  Truth of the matter is, I may not even see any penguins.  There aren't any at the South Pole so my chance to see them is when I'm passing through McMurdo Station on the coast of Antarctica.  On the departing (deploying) and returning (redeploying) trip I could be sitting at McMurdo for as little as a day and as long as a week or more, depending on the weather.  Those will be my chances to see penguins.  Some times they're around, sometimes they aren't.  But if they ARE there, you can count on me taking pictures and posting them.