Shining a Light into the Dark: Cosmic Voids as Cosmological Probes of Evolving and Interacting Dark Sector Physics

The dark sector of the Universe, which comprises dark matter and dark energy, presents one of the greatest unanswered questions in all of physics. Although it dominates the current mass-energy content of the Universe, little is understood about its true nature. The extent of our understanding is reflected in the current Standard Model of Cosmology, Λ Cold Dark Matter (ΛCDM), which characterises dark matter as a non-relativistic, non-baryonic form of matter, and dark energy as a form of vacuum energy described by the cosmological constant, Λ, in the field equations of General Relativity. Although this characterisation has generally been successful, it does not provide much insight into or theoretical underpinning for the dark sector. In this thesis, I aim to address this problem by searching for the unique signatures of two alternative cosmological models featuring evolving and interacting dark sectors: an uncoupled quintessence model (φCDM) and a coupled dark energy model (CDE). These signatures will provide a way to distinguish between cosmological models featuring different dark sector physics, allowing us to observationally probe and constrain the properties of the dark sector in our Universe. I focus on the signatures of these models within cosmic voids. Using state-of-the-art adiabatic hydrodynamical simulations and a density-based void finder I developed, I study the properties of voids in the two alternative models and compare them to those in ΛCDM, focusing on their population size, volumes, ellipticities, prolateness, and average densities, over the redshift range z = 12 - 0. I discover that void densities are distinctly lower in the two alternative models than in ΛCDM, revealing that the property is sensitive to the presence of an evolving dark sector, and thus can be used as the basis of observational probes of the dark sector in our Universe