How Does B Mode Polarization Help Trace Gravitational Radiation?

[wolram]

This seems to crop up a few times now, how does it help to trace gravitational radiation?

 

[Jonathan Scott]

This seems to crop up a few times now, how does it help to trace gravitational radiation?

I don't know, but here's a website which attempts to explain it:

http://cosmology.berkeley.edu/~yuki/CMBpol/CMBpol.htm"

 

[wolram]

That's an excellent web site, thanks Johnathon.

 

[Chalnoth]

Not sure I can really add to that site, because it is excellent, but in case anybody doesn't want to delve that far into and just wants the super-short version, here it is:

When we look at the CMB, the light coming from it is partially polarized. This means that the electric field lines of the incoming radiation tend to be oriented in one particular direction. The specific source of this orientation comes from the gravitational perturbations in the early universe (the above link gives a good description of the physics involved there).

Now, we can decompose any field that picks out a specific direction at every point into two components: an "E mode" and a "B mode". The E mode component, also called the gradient component, is a vector field which only points towards/away from sources. The reason it is called an "E mode" is because this is much like the behavior of the Electric field with unmoving charges.

The B mode component, similarly, behaves like a magnetic field in that it has a curl but no divergence, which means it tends to point in circles around sources.

The important point here is that the physics behaves very differently upon these two different components of the polarization, making them an excellent probe of early-universe physics. For instance, the B-mode polarization is only sourced by tensor perturbations in the early universe, so a positive detection of primordial B-mode signal is as near a direct measurement of inflation as we can get with CMB observations.

 

[Naty1]

The presence of B mode polarization, if detected in the cosmic microwave background radiation, would confirm the inflationary model and discredit the ekpyrotic predictions of a cycling universe...according to Paul Steinhardt and Neil Turok, THE ENDLESS UNIVERSE, page 210.
The WMAP satellite has not yet detected any B mode but is not all that sensitive. A more recent effort, the European Space Agency Planck Satellite, was supposed to report on the polarization...don't know the status of that.

The Planck link at the bottom of the above article under EXPRIMENTS leads nowhere.

According to these physicsts, E mode polarization appears like the pattern of electrostatic field linesof straight radial lines from a point ...(hence the "E" ) ; B mode appears as curved lines emanating from a point, a curved shape, resembling the pattern of magnetic field lines, hence the "B" designation... gravitational waves squeeze space in one direction and stretch it in the perpendicular direction (tidal forces) and this results in both E and B mode polarizations.

 

[Chalnoth]

The WMAP satellite has not yet detected any B mode but is not all that sensitive. A more recent effort, the European Space Agency Planck Satellite, was supposed to report on the polarization...don't know the status of that.

The first-year CMB science release for Planck is supposed to be out about two years from now.

Unfortunately, it remains unlikely that Planck will have much of anything to say about B-mode polarization. The signal is expected to be very small already, and the parameters would have to be just right for the signal to be large enough to be within Planck's range of detectability. There's a chance it will be detectable, but it isn't a very good chance, and it will depend upon the removal of systematic errors being exceedingly accurate (both instrument systematics and foreground sources).

The official Planck page is here:
http://www.sciops.esa.int/index.php?project=PLANCK