Weak lensing: applications in gravitational wave and galaxy cluster cosmology

Abstract

Radiation travelling through the Universe is weakly lensed by Large Scale Structure. Measurements of this effect are a valuable probe of the content and dynamics of the Universe. In this thesis I will explore novel applications for the weak lensing of both gravitational and electromagnetic radiation.

Future gravitational wave (GW) detectors will observe tens of thousands of merging black hole or neutron star binaries through their gravitational radiation. Weak lensing will be the dominant source of uncertainty when using these sources as standard sirens: measuring their distance−redshift relation to probe the geometry of the Universe. I performed a detailed study into the impact of weak lensing on standard siren analyses, finding it to be significant, and demonstrating that current mitigation tactics require improvement. I have also developed new approaches to reducing the impact of weak lensing. Besides being a source of uncertainty, the weak lensing of gravitational waves also contains cosmological information. Using a combination of future GW detectors, the vast number of observations will be sufficient for a statisticalweak lensing analysis— using the weak lensing effect on GWs over the sky to probe structure formation. I have demonstrated the strength of this method, finding several novel applications such as constraining the sum of neutrino masses.

The distortion of light from background galaxies around foreground galaxy clusters allows inference of the cluster density profile, the shape of which is sensitive to properties of structure formation. Comparing cluster profiles in the so-called infall region with predictions from a suite of cosmological simulations allows constraints on cosmological parameters. These constraints exhibit an orthogonal degeneracy to those from other standard tests, such as cluster abundances or cosmic shear, making the infall region a promising complementary probe. I have pioneered this novel test, and performed a first-of-its-kind analysis using weak lensing data from the Ultraviolet Near Infrared Optical Northern Survey, combined with a publicly available galaxy cluster catalogue, obtaining constraints of Ωm = 0.29 ± 0.05 and 𝜎�8 = 0.80 ± 0.04.