Lensing of 21-cm Fluctuations by Primordial Gravitational Waves
  Weak-gravitational-lensing distortions to the intensity pattern of 21-cm
radiation from the dark ages can be decomposed geometrically into curl and
curl-free components. Lensing by primordial gravitational waves induces a curl
component, while the contribution from lensing by density fluctuations is
strongly suppressed. Angular fluctuations in the 21-cm background extend to
very small angular scales, and measurements at different frequencies probe
different shells in redshift space. There is thus a huge trove of information
with which to reconstruct the curl component of the lensing field, allowing
tensor-to-scalar ratios conceivably as small as r ~ 10^{-9} - far smaller than
those currently accessible - to be probed.
