I raised a question a while ago regarding weak gravitational lensing of galaxies and the CMB. With all the fuzz with the BICEP2 data, I think it is time to raise even more questions about this amazing subject.

One of the assumptions being applied is that if we remove all the gravitational lensing happening to the CMBR, then we are only left with the "non-lensing" CMBR, and any B-mode polarization left over must be primordial gravitational waves

What is not clear is why can't be assumed that all the the remaining B-mode polarization is due to weak gravitational lensing of unseen dark matter. Note: Remember that, given the question I referenced at the beginning, we use gravitational lensing already to 'measure' dark matter (well, at least extra-galactic dark matter distributions)


From the paper itself:

Gravitational lensing of the CMB’s light by large scale structure at relatively late times produces small deflections of the primordial pattern, converting a small portion of E-mode power into B-modes. The lensing B-mode spectrum is similar to a smoothed version of the E-mode spectrum but a factor ~100 lower in power, and hence also rises toward sub-degree scales and peaks around ~1000. The inflationary gravitational wave (IGW) B-mode, however, is predicted to peak at multipole ~80 and this creates an opportunity to search for it around this scale where it is quite distinct from the lensing effect.

The mass of these large structures has been defined/found including the mass of dark matter. In this sense dark matter can be considered as matter and does contribute to the gravitational lensing whether it emits light or not, it is already included. The additional constraints in the paragraph above allow for a clear signal.


Dark matter is generally not associated with a total lack of visible matter. From formation simulations we predict that luminous matter is really only clumped together because dark matter was there already (gross simplification, but sorta descriptive). In addition, although the distribution of luminous matter and dark matter don't exactly match (ie bullet cluster), when lensing sources are removed they are generally removing in regions which would include any dark matter associated with it.

Combine that with the fact that the observed B-modes are seen in regions around 2 degrees in size, much larger than any known dark matter source, and I think it's pretty easy to be convinced that removing luminous lensing sources removes dark lensing sources as well.


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