Large-Scale Structure with Gravitational Waves II: Shear

TL;DR

This paper analyzes how gravitational waves influence galaxy shear measurements, finding intrinsic alignments dominate lensing effects and impact the potential to detect tensor modes with galaxy surveys.

Contribution

It derives full-sky tensor mode contributions to shear from lensing and intrinsic alignments, highlighting the significance of intrinsic alignments in detecting gravitational waves.

Findings

Intrinsic alignments exceed lensing contributions by an order of magnitude.

Tensor mode effects extend to higher multipoles.

Detection prospects are less pessimistic but remain challenging.

Abstract

The B-(curl-)mode of the correlation of galaxy ellipticities (shear) can be used to detect a stochastic gravitational wave background, such as that predicted by inflation. In this paper, we derive the tensor mode contributions to shear from both gravitational lensing and intrinsic alignments, using the gauge-invariant, full-sky results of arXiv:1204.3625. We find that the intrinsic alignment contribution, calculated using the linear alignment model, is larger than the lensing contribution by an order of magnitude or more, if the alignment strength for tensor modes is of the same order as for scalar modes. This contribution also extends to higher multipoles. These results make the prospects for probing tensor modes using galaxy surveys less pessimistic than previously thought, though still very challenging.