Thursday, November 24, 2011

... 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!

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