Polarization, Calibration and Chocolate Cream Pie

I've been spending the last week or so (along with the rest of the calibration team)  working on setting up and making measurements with the SPT polarization calibration source.   The source is located a couple kilometers away from the station and telescope, so on a day with bad visibility you become very isolated.    We've had a couple of those, where all you can see is the snowy ground around you and then nothing in the background (just a wall of snowy wind).  When riding back in, eventually SPT and then the station start to come into focus, like looming giants in the distance.  The last time we went out, the weather was fairly nice, and we were able to see SPT in the distance (it's the dark circle right above the fence in the picture).   The temperature has steadily increased since we first arrived.  The average temperature is about -35 degrees Celsius, but the wind has also increased.  We were getting gusts of about 20 knots the other day, which takes the windchill back down to -50 C.  It's amazing how quickly being outside in the cold can sap your energy.  It's very easy when you're out at the calibration source to imagine how isolated Scott and Admunsen must have felt when trekking to and from the South Pole.  Personally, I'm happy to hop on the snowmobile and drive to a heated station for a good meal and clean socks.

The calibration measurements we're taking will be used to measure the polarization of the Cosmic Microwave Background.   Light has a total intensity as well as a directionality (called the polarization).  Polarization occurs when the electromagnetic (EM) waves from a source are aligned in a given direction.   One way to visualize it is with two people holding the ends of a long piece of string.  If one person starts moving their hand up and down, there will be a vertical wave in the string.  But if the same person instead moves their hand from side to side, the wave will be horizontal.  The polarization of light waves is similar, the waves can be vertically or horizontally polarized (or any angle in between).  This is referred to as 'linear polarization'.   Light can also be unpolarized (each EM wave is polarized at an angle between 0-360 degrees so when you sum them up you can't tell the difference), or circularly polarized (but that's a whole other story).  We expect the CMB to have a slight linear polarization, but from two very different sources.  

In my previous post, I said that after neutral hydrogen forms, the light from the CMB can travel freely through space.  As time continues this neutral hydrogen begins to form structures and eventually evolves in to the people, planets, stars, galaxies and galaxies clusters we see today (this is a bit simplified, but I'm trying to summarize billions of years into a couple sentences).   The CMB photons interact with these structures through gravitational lensing.  The gravitational force of the matter distorts the path through space that the light travels (just like a glass lens would, hence the name gravitational lensing).   The gravitational lensing of the CMB polarizes the light on small scales (size of galaxy clusters).  

The second source is far more exotic.  The current theory of cosmology, where the universe started with a Big Bang and rapidly expanded is not the full story.   In order to explain current observations, cosmologists and created the theory of inflation.  The idea is for a short period of time after the Big Bang, the universe was expanding exponentiall.  After it was over, the infant universe would be 10^26 times bigger in size (that's 100000000000000000000000000 times larger)! 

Inflation is still just a theory, but there is a measurement that could prove it!  Inflation is predicted to leave a very specific pattern polarization in the CMB light.  There are no other ways (that anyone has theorized) that would create this pattern, so a measurement of it would point directly towards inflation.  It's an extremely challenging measurement, because the signal is so faint.   Which brings me back to the calibration measurements we're taking right now.   In order to measure such a faint signal, we have to understand how much and what angle of polarization each of our detectors measures.  The calibration has to be extremely precise, or the faint CMB polarization signal will be washed out by contaminating signals from the instrument itself. 

Overall, this past week living in the station has been a bit surreal.  One minute I'm working on a piece of analysis software in our communal lab office and it's basically just like being at home (with different faces and furniture).  Then I walk down the hallway to dinner and look out the window onto the Antarctic snow, complete with telescopes, the summer camp housing,  giant pallets of cargo waiting to be loaded on a plane, and snow mobiles zooming around.  It's a jolt in consciousness to remember where I am are.  The little details of life are different from home as well, but walking up and down the same corridor everyday and traveling out the telescope quickly became very normal.  It's very easy to be comfortable here, as everyone is friendly (there is a real sense of community) and the food is excellent.   Who doesn't love a meal of steak, potatoes and chocolate cream pie after a long day of work?