Cosmic Shear from Scalar-Induced Gravitational Waves

TL;DR

This paper investigates the lensing signatures of primordial and secondary gravitational waves on cosmic shear, finding that their signals are too weak to be detected by upcoming surveys.

Contribution

It derives the curl mode for galaxy and CMB lensing due to both primordial and secondary gravitational waves, highlighting their relative amplitudes.

Findings

Secondary tensor modes dominate over primary gravitational waves in curl mode signals.

Both tensor contributions are below the noise levels of future surveys.

Detection of these tensor-induced curl modes is unlikely with upcoming observational data.

Abstract

Weak gravitational lensing by foreground density perturbations generates a gradient mode in the shear of background images. In contrast, cosmological tensor perturbations induce a non-zero curl mode associated with image rotations. In this note, we study the lensing signatures of both primordial gravitational waves from inflation and second-order gravitational waves generated from the observed spectrum of primordial density fluctuations. We derive the curl mode for galaxy lensing surveys at redshifts of 1 to 3 and for lensing of the cosmic microwave background (CMB) at a redshift of 1100. We find that the curl mode angular power spectrum associated with secondary tensor modes for galaxy lensing surveys dominates over the corresponding signal generated by primary gravitational waves from inflation. However, both tensor contributions to the shear curl mode spectrum are below the projected noise levels of upcoming galaxy and CMB lensing surveys and therefore are unlikely to be detectable.