Separation of Gravitational-Wave and Cosmic-Shear Contributions to   Cosmic Microwave Background Polarization
  Inflationary gravitational waves (GW) contribute to the curl component in the polarization of the CMB. Cosmic shear--gravitational lensing of the CMB-- converts a fraction of the dominant gradient polarization to the curl component. Higher-order correlations can be used to map the cosmic shear and subtract this contribution to the curl. Arcminute resolution will be required to pursue GW amplitudes smaller than those accessible by Planck. The finite cutoff in CMB power at small scales leads to a minimum detectable GW amplitude corresponding to an inflation energy near 10^15 GeV. 
Michael Kesden, Asantha Cooray, Marc Kamionkowski (Caltech)