Campbell, Martine2025-12-222025-12-222025-12-222025-12-21https://hdl.handle.net/10012/22779In this thesis, we present new measurements of the galaxy–galaxy lensing (GGL) signal around Baryon Oscillation Spectroscopic Survey (BOSS) CMASS galaxies using background sources from the Ultraviolet Near-Infrared Optical Northern Survey (UNIONS). With an overlap of approximately 2650 square degrees between CMASS lenses and background source galaxies—the largest to date—we obtain precise large-scale GGL measurements. With these new measurements, we revisit the so-called ‘lensing is low’ problem, wherein galaxy–halo connection models calibrated on clustering data over-predict the GGL signal by 20–40% under cosmic microwave background (CMB)-based cosmologies. We model the galaxy–halo connection using a halo occupation distribution (HOD), and perform joint fits to both GGL and clustering signals across a wide range of scales, as well as a clustering-only fit. Similar to previous work, we find a lensing–is–low effect in the CMASS sample, although our GGL and clustering predictions are less inconsistent with each other. The best joint fits are achieved by lowering the amplitude of the matter power spectrum relative to Planck 2018, driven by the precision of our large-scale GGL measurements. Once a lower matter power spectrum amplitude is adopted, feedback is the only HOD extension that further improves the joint fit. Our feedback model redistributes matter within a halo, modifying the halo–matter cross–power spectrum. Overall, we find that two models describe our observables equally well: one where HOD and cosmological parameters are free, and one where HOD, cosmological, and feedback parameters are free. Importantly, we emphasize the role of large scales in driving the lensing–is–low effect, shifting the narrative away from a purely small-scale issue.encosmologyweak gravitational lensinggalaxy formationMaster Thesis