Lensing of 21-cm Fluctuations by Primordial Gravitational Waves

TL;DR

This paper discusses how weak gravitational lensing of 21-cm radiation from the dark ages can reveal primordial gravitational waves through a curl component in the lensing pattern, enabling detection of extremely small tensor-to-scalar ratios.

Contribution

It introduces a method to extract the curl component of lensing from 21-cm fluctuations, significantly improving sensitivity to primordial gravitational waves.

Findings

Curl component induced by gravitational waves can be isolated.

Potential to measure tensor-to-scalar ratio as low as 10^{-9}.

21-cm fluctuations provide extensive data across small angular scales.

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.