Monday, January 7, 2013

Polarized Microwaves from the begining of the Universe

I work in the Martin A. Pomerantz Observatory, aka MAPO.  Here's a short video tour I shot of the place:

I neglect to mention that the giant wooden cones around the telescopes are ground shields which prevent our cameras from seeing stray light bouncing off the snow.  When I'm up in the mount, I also mistakenly point at a tray holding pulse-tube lines while describing it as an elevation gear.  At that time, I was paying more attention to not getting my hand caught in the drive than what I was filming.

This observatory has housed two very successful CMB polarization experiments over the past decade, and several of our senior group members have worked on both.  This rest of this post describes what they saw and I'll try to make another post about how our experiment will advance that further (things get nerdy from here on out!).

I previously described how the Big Bang "echoed" in the early hot universe, and how the hot and cold regions of those sound waves left behind hot and cold spots in the microwave sky.  Those spots are also polarized.

You can think of light as similar to shaking waves on a rope- shaking the rope up-down is distinct from shaking left-right- and the same is true for light waves.  We call this property polarization.  But even though our eyes can see different colors or brightness, we cannot see polarization on our own.  Often, both polarizations of light are present, but more of one type than the other.  We can build filters that screen away one of the two polarizations, and this video uses such a filter to show that the afternoon sky is polarized:

That polarizing filter looks dark in a specific position because it is filtering the brighter of the polarizations, passing only the dimmer one.  The sky itself looks polarized because the afternoon sun illuminates air in our atmosphere from only one side, like this:

Light scatters off atoms in the sky, adapted from Wayne Hu
In this cartoon, the light from the sun has both polarizations shown in the light blue crossed lines; that light grabs and shakes electrons in the gas in our atmosphere.  We cannot "see" electron motion towards and away from us, only transverse.  So we see mostly vertical polarization scattered at us.

Those sound waves in the early universe created a similar situation: electrons in the cold low pressure planes (red stripes) will be hit by weaker light from above and below and brighter light from the warmer high pressure planes (blue stripes) coming in from the left and right.  So the scattered light is brighter in the vertical polarization.  The opposite argument holds for electrons in the hotter blue strips, scattering brighter light in the horizontal polarization.
Early universe sound wave scattering polarized radiation, adapted from Wayne Hu
Many different sound-waves will produce a series of polarized spots on the sky.  Here's a map of polarization difference from our sister experiment BICEP-2:

Polarized CMB spots
All of our cameras contain hardware that is similar to the polarizing grid shown in the youtube video above so we can measure the power in both polarizations.

In 2002, the DASI experiment (in MAPO) demonstrated that the CMB is indeed polarized as everyone expected.  In 2006-8, the next experiment in MAPO- QUAD- took a detailed enough map to show that this polarization has similar structure as the temperature maps I showed in this previous post.
from the QUAD experiment
Similar to before, this plot shows how much brighter one polarization is than the other for spots of specific sizes, and it plots difference against spot size.  You can see  a similar peaked structure as before, which is not a surprise since these come the same early-universe sound-waves that produce the temperature spots.  For technical reasons, this sort of measurement can measure many of the features of our universe better (i.e. smaller uncertainty) than the temperature maps.  But no one is good enough at these measurements yet to beat the temperature measurements.


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