title: Lensing of 21-cm fluctuations by primordial gravitational waves.
abstract: 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.
authors.affiliation: California Institute of Technology, Pasadena, California 91125, USA.
authors.forename: Laura
authors.initials: L
authors.lastname: Book
authors.forename: Marc
authors.initials: M
authors.lastname: Kamionkowski
authors.forename: Fabian
authors.initials: F
authors.lastname: Schmidt