The main point is that vector components of defects' contribution to CMB polarization anisotropies peak at scales smaller than those from inflation.

On the other hand, the ordinary

E- and

B-modes depend non-locally on the Stokes parameters, so they cannot be used to put constraints on causal sources like defects using the angular correlation function of

E- and

B-modes on small scales. That is the reason why Baumann and Zaldarriaga [

0901.0958] suggested using instead the local modes. Those are the true causal modes, written in terms of derivatives of the Stokes parameters.

These local

B-modes then have power spectra that are much bluer than the non-local ones, and hence enhance the small scale (high-

l) end of the spectrum. It is by looking at the angular correlation functions at small separations (tens of arcmin) that one has a chance to measure the defect's contribution to the local

B-modes, and distinguish it from the one of inflation.

Of course, the usual white noise power spectrum for polarization will also be modified by this [tex]\ell^4[/tex] factor, but by a suitable gaussian smoothing of the data (following Baumann&Zaldarriaga), we can indeed obtain large signal to noise ratios for binned data at small angular scales.

Baumann&Zaldarriaga looked at the model-independent signature of inflation at angles [tex]\theta>2[/tex] degrees. What we have realiazed is that, although model-dependent, the signal at angles [tex]\theta < 1[/tex] degrees can be much more significant. In fact, the feature at small angles is rather universal. The differences between defect models (and we considered four different ones) is just in the height and width of the first and second oscillations in the angular correlation functions (related to the heigth and position of the angular power spectrum). Therefore, with sufficient angular resolution one could not only detect defects (if they are there) but also differentiate between different models.