Cosmology with the line-of-sight shear of strong gravitational lenses

TL;DR

This paper introduces a novel cosmological probe using line-of-sight shear perturbations of strong gravitational lenses, expanding the standard correlation analysis to enhance the extraction of cosmological information from upcoming large surveys.

Contribution

It proposes the use of LOS shear of strong lenses as a new observable, developing estimators and covariance analysis for a 6x2pt correlation scheme, demonstrating high potential for cosmological insights.

Findings

High signal-to-noise detection prospects for LOS shear in stage-IV surveys.

The new correlation functions significantly improve cosmological parameter constraints.

Strong lenses can serve as powerful complementary probes in large-scale structure studies.

Abstract

Stage-IV photometric galaxy surveys are designed to measure the position and shapes of billions of galaxies. Their aim is to characterise the large-scale distribution of matter in the Universe using galaxy clustering and weak gravitational lensing. As a byproduct, stage-IV surveys are expected to detect more than a hundred thousand strong gravitational lenses. In this article, we propose the use of weak-lensing perturbations to strong lenses, specifically their line-of-sight (LOS) shear, as a cosmological probe. This new observable allows us to define three new correlation functions: the LOS shear with itself, with galaxy positions, and with galaxy shapes, thereby promoting the standard $3\times 2$pt correlation method to a $6\times 2$pt scheme. We design estimators for these new correlation functions and determine their expectation values as a function of the matter power spectrum. We then derive the analytical expression for the full covariance matrix of the $6\times 2$pt correlation scheme. Considering various scenarios for the stage-IV strong-lensing samples, we demonstrate that the cosmological information carried by the LOS shear of strong lenses will be detectable with a very high signal-to-noise ratio, even in the most pessimistic of cases. Strong lenses are thus extremely promising cosmological probes, whose synergy with galaxy positions and shapes should also contribute to mitigating systematics in stage-IV surveys.