Establishing the origin of CMB B-mode polarization
  Primordial gravitational waves leave a characteristic imprint on the cosmic
microwave background (CMB) in the form of $B$-mode polarization. Photons are
also deflected by large scale gravitational waves which intervene between the
source screen and our telescopes, resulting in curl-type gravitational lensing.
Gravitational waves present at the epoch of reionization contribute to both
effects, thereby leading to a non-vanishing cross-correlation between $B$-mode
polarization and curl lensing of the CMB. Observing such a cross correlation
would be very strong evidence that an observation of $B$-mode polarization was
due to the presence of large scale gravitational waves, as opposed to
astrophysical foregrounds or experimental systematic effects. We study the
cross-correlation across a wide range of source redshifts and show that a
post-SKA experiment aimed to map out the 21-cm sky between $15 \leq z \leq 30$
could rule out non-zero cross-correlation at high significance for $r \geq
0.01$.
