Weak Lensing Systematics in the Era of the LSST

In the era of precision cosmology, we will be collecting an unprecedented amount of data in surveys such as the Rubin Observatory’s Legacy Survey of Space and Time, and the High Latitude Survey of the Nancy Grace Roman Space Telescope. This has the potential to vastly increase our understanding of the Universe, and particularly, dark energy and the accelerated expansion of the Universe. Along with this opportunity comes great responsibility, particularly in controlling sources of bias. Therefore, understanding systematic errors has become of the utmost importance in

modern cosmology, especially for probes with faint signals, as is the case with weak gravitational lensing. This thesis describes my work towards understanding some of the systematic errors that could contaminate the weak lensing signal and potentially be the limiting factor in inferring cosmological parameters. In particular, this thesis details my work on (a) optimizing the observing strategy of Rubin Observatory’s survey to mitigate observational weak lensing systematics such as the Point Spread Function, where we found that weak lensing systematics are mitigated with a higher number of well-dithered observations of galaxies; and (b) forecasting the effect of photometric redshift modeling errors on inferences made using the 3x2pt probes of cosmic

shear, clustering, and galaxy-galaxy lensing, where we assess the relative importance of different photometric redshift error parameters on cosmological measurements of

large-scale structure.