At the largest scales, two ingredients dictate the distribution of matter in the Universe. The first is dark matter, acting as an invisible scaffolding held together by gravitational forces. The... Show moreAt the largest scales, two ingredients dictate the distribution of matter in the Universe. The first is dark matter, acting as an invisible scaffolding held together by gravitational forces. The second is dark energy, an enigmatic component responsible for the accelerated expansion of the Universe. Under these two forces, matter in the Universe organizes itself in the so-called cosmic web. The nodes of this network are large dark matter haloes, and this thesis explores how their boundaries provide information about the nature of dark energy and cosmology. Chapters 3 and 4 present robust theoretical predictions for this titular edge and discuss its simple physical interpretation. Chapters 2 and 5 corroborate these results by presenting measurements of this feature in weak-lensing data. The last scientific chapter of this thesis is a collection of studies in gravitational-wave physics. This chapter explores how these spacetime ripples observed from across the cosmos can be used to detect alternative theories of gravity. Show less
The splashback radius rsp has been identified in cosmological N-body simulations as an important scale associated with gravitational collapse and the phase-space distribution of recently accreted... Show moreThe splashback radius rsp has been identified in cosmological N-body simulations as an important scale associated with gravitational collapse and the phase-space distribution of recently accreted material. We employ a semianalytical approach to study the spherical collapse of dark matter halos in symmetron gravity and provide, for the first time, insights into how the phenomenology of splashback is affected by modified gravity. The symmetron is a scalar-tensor theory which exhibits a screening mechanism whereby higher-density regions are screened from the effects of a fifth force. In this model, we find that, as overdensities grow over cosmic time, the inner region becomes heavily screened. In particular, we identify a sector of the parameter space for which material currently sitting at rsp has followed, during the collapse, the formation of this screened region. As a result, we find that for this part of the parameter space the splashback radius is maximally affected by the symmetron force, and we predict changes in rsp up to around 10% compared to its General Relativity value. Because this margin is within the precision of present splashback experiments, we expect this feature to soon provide constraints for symmetron gravity on previously unexplored scales. Show less
