Here's more on that result.
The Cosmic Microwave Background is the result of the Universe cooling enough to allow its hydrogen and helium nuclei to capture its electrons, something that happened when its temperature was around 3000 K. That made it much more transparent, allowing its light to travel freely. It eventually got redshifted into the microwave part of the spectrum, where we observe it today.
There are two main types of primordial fluctuations:
- scalar or density fluctuations
- tensor or gravitational radiation
The fluctuations unambiguously observed so far are the density ones. These fluctuations seeded the formation of galaxies and galaxy clusters by giving the Universe's material something to gravitationally collapse around.
The density ones made recombination time alternately early and late, thus making them alternately a little bit more and less redshifted at the present, observed as alternately a little bit cooler and warmer, about 10^(-5) relative difference.
This earlier and later also made the CMB a little bit polarized, and the polarization occurs in patterns called E-modes. These are (up-down) - (sideways) modes relatives to hills and valleys of some intensity function. The other possible patterns are called B-modes. They are (one diagonal) - (other diagonal) modes. They are called E and B in analogy with electric and magnetic fields.
Gravitational radiation moves the Universe's material around, and it also produces CMB polarization: both E-modes and B-modes.
To summarize:
- Density: intensity, E-modes
- G-waves: E-modes, B-modes
So all one has to do is observe CMB B-modes and one is all set, or so it seems. But the CMB photons have to travel all the way to us, and they can be affected by various effects as they do so. Gravitational lensing and our Galaxy's dust cause mixing of E-modes and B-modes.
Untangling these effects can be difficult, and that's why it's taken so long for the Planck team to release its results.
Last year, the BICEP2 team measured some B-modes that implied that the ratio
r = (G-waves)/(density) of primordial fluctuations
is about 0.2. But there were big questions about foreground effects, like what I'd mentioned. The Planck team found something like r < 0.11, meaning that the BICEP2 team didn't see any noticeable G-wave effects -- all galactic dust and gravitational lensing.
