pleasehelpmeno said:
Can anyone explain:
why COBE measured IR and microwave as opposed to visible light, when surely we are measuring photons that have decoupled?
why Planck is measuring light polarisation?
While the CMB originally decoupled at around 3000K, which would have appeared rather yellow to our eyes. But our universe has expanded by a little more than a factor of 1000, bringing the temperature of that radiation down from around 3000K down to 3K, which is far, far below the range of visible light. Rather, it's in the millimeter-microwave range.
As for polarization, there are two different effects at work. First, the simple fact that the CMB was emitted from a plasma that was slightly warmer in some areas and slightly cooler in others tends to polarize the light in a particular way: it sets up what is known as E-mode polarization that is tightly correlated with the differences in temperature from place to place on the sky. So the E-mode polarization acts, for the most part, as an independent check on our understanding of the physics that emitted the CMB, and is particularly useful in nailing down just how much of the light from the CMB was absorbed by intervening matter between us and the CMB (answer: about 8-10%).
There is also a B-mode polarization which the plasma physics described above don't produce at all, but can be created by gravitational waves in the early universe. It is possible for inflation to produce such gravitational waves, and a detection of this form of polarization could tell us what the energy scale of inflation was. The difficult is that the B-mode signal is much smaller than the E-mode signal, and gravitational lensing from matter between us and the CMB mixes the much larger E-mode polarization with B-mode polarization.
Our best bet at measuring the B-mode polarization signal in the next few years is the EBEX balloon experiment which just recently launched, and should hopefully be publishing its results in the coming months. Planck will also be releasing its results soon, but it just doesn't have the sensitivity to definitively detect the B-mode polarization signal, barring an exceptionally-high B-mode signal.
You're probably asking by now what the E-mode and B-mode polarization are. Well, consider that when we look at the sky, each place on the sky can be seen as having a direction of polarization. See here, for example:
http://physicsworld.com/cws/article/news/32769/1/cmb
The little white lines in that image are the direction (and magnitude) of polarization of the various places on the sky. Most of what you see is polarization from dust in our own galaxy (that can be cleaned up with multi-frequency analysis). Anyway, an E-mode signal is one that only consists of lines entering or exiting some sources with no circular behavior, while a B-mode signal is one that is only composed of lines swirling in circles, with no radial behavior. In technical terms, the E-mode signal is has no curl, while the B-mode signal has no divergence.
