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
For centuries astronomers studied the Universe by collecting light. Nowadays, we are living in times of great technological advancements, which allow us to explore our Universe in a new way -... Show moreFor centuries astronomers studied the Universe by collecting light. Nowadays, we are living in times of great technological advancements, which allow us to explore our Universe in a new way - though gravitational wave radiation. There are many gravitational wave sources in our own Galaxy, the Milky Way. For example, white dwarf stars in tight binary systems spinning around each other in less than 1 hour. LISA is a future ESA mission, that will detect a large variety of gravitational wave sources including Galactic double white dwarfs. Although quite faint, double white dwarfs can also be seen by optical telescopes. Thus, astronomers call them “multi-messenger” sources. This means that we can collect information from them using more than one messenger: electromagnetic waves, messengers of the electromagnetic field, and gravitational waves, messengers of the gravitational field. This thesis proposes to use gravitational wave signals from Galactic double white dwarfs to study the Milky Way and its neighbourhood. In particular, it explores how by collecting many electromagnetic and gravitational wave signals from thousands to millions of binary double white dwarfs spread all across our Galaxy, we can perform multi-messenger Galactic Astronomy and learn about the structure and history of the Milky Way. Show less