Untangling the Cosmic Web: Cosmology in the connections between galaxies and the large-scale structure of the Universe

Abstract

We live in a massive web of dark matter which is illuminated by galaxies. Large cosmological surveys map out the positions of galaxies across cosmic time to explore how this structure evolves and the forces which drive it: gravity and dark energy. But galaxies are not indifferent inhabitants of the cosmic web. Large-scale gravitational forces leave detectable imprints on galaxies, affecting their motions, shapes, and orientations. This dissertation untangles these correlations and provides insights into their nature, how they bias cosmological measurements, and original methods for unlocking their potential as a direct cosmological probe. The intrinsic alignment of galaxies (IA) is a subtle effect that is only detectable with tens of thousands of galaxies. Large galaxies surveys like the Dark Energy Spectroscopic Instrument (DESI) have provided the strictest constraints yet on the nature of dark energy, but are also uniquely susceptible to the effects of IA. IA is most commonly studied as a bias for weak lensing measurements: a method of measuring underlying matter through the gravitational distortion it creates in the light of distant galaxies. This dissertation explores how IA can also affect galaxy clustering in DESI, biasing measurements of redshift-space distortions (RSD). We demonstrate that, if unaccounted for, IA will combine with an orientation-dependent selection effect to lower the measured growth rate of structure, particularly at high redshifts. Given the advancements of large cosmological surveys, we also revisit traditional IA methods and propose alternative estimators for measuring them in the presence of RSD. Although IA can trace large-scale structure and the cosmological effects that form it, practical applications are limited by difficulties in directly detecting IA in blue, faint, and distant galaxies. This dissertation documents these difficulties and identifies a novel approach to circumvent them: multiplet alignment. By detecting correlations between the orientations of small galaxy groups and the underlying dark matter, we show that small-scale galaxy clustering preserves an interpretable memory of the cosmic web. It can be used to uniquely explore the fingerprints left on galaxies by dark energy and gravity, probing their fundamental nature. These insights into the nature of IA and optimal measurement methods will advance the power of large and upcoming cosmological surveys.