Physical Cosmology in the Epoch of Large Surveys

Our understanding of the Universe has advanced tremendously in the past few decades, with General Relativity(GR) laying the ground for a successful model for the Universe, ΛCDM. However, there are still some fundamental open questions that need to be explored. It is therefore the objective of this thesis to highlight some of these questions by exploring them theoretically and observationally. In the first part of the thesis, the Introduction, I present a background review of GR and ΛCDM. The second part is on testing an essential assumption in Cosmology, the Copernican Principle. By distinguishing between line-of-sight and transverse expansion rates in the most general spacetime possible, one can constrain deviations from the Copernican Principle. Observationally, this is done by measuring polarization of Cosmic Microwave Background(CMB) photons that have been inverse-Compton scattered by galaxy clusters. In the third part, the possibility that Dark Matter(DM) is part of Grav- ity is investigated. On Cosmological scales, this hypothesis is tested with a case study model: Mimetic Dark Matter(MDM). By rederiving the model’s equations of motion, extra free functions and parameters appear in need of fine tuning to produce the observationally certified adiabatic initial conditions. To visualize this, I modify the Boltzmann code CLASS to include MDM, and then look at CMB correlation functions and matter power spectra, which show that deviations of at least 10% from adiabatic initial conditions fall beyond cosmic variance limits. On an astrophysical scale, the hypothesis that DM is part of a modified gravity theory(MGT) is tested by examining DM-devoid galaxies. The main argument is that if DM is part of a MGT, then this phenomenon should be found in every gravitational system. The fact that DM-devoid galaxies exist, while other similar ones are DM dominated, constrains severely the above mentioned hypothesis. To quantify this, I derive a generalized Virial theorem for some MGTs, and show that the extra term gives inconsistent results for DM-devoid galaxies. Therefore, unless fine-tuning is used, DM is more likely to be a non-baryonic particle, or a compact object such as primordial black holes, rather than part of a MGT. The fourth part explores the realm of Quantum Field Theory(QFT) in a gravitational background. The goal is to use neutrinos(spinors) as probes for Dark Energy(DE), to distinguish between its different models. After laying down a general formalism, I first investigate three types of interactions between the two fields, in a semi-classical way, and study the consequences on oscillations of neutrinos and their dynamics. This framework is later generalized to a broader class of interactions between neutrinos, as quantum spinors, and DE, either in the form of a Cosmological Constant(CC) or a scalar field. The result is that different DE models have distinct signatures on neutrino oscillation’s probability. This provides a proof of concept for using neutrino oscillations in curved spacetime as a tool to distinguish between models of the late acceleration of the Universe. To put the above in an observational context, I conclude the fourth part by considering the full three-flavor neutrino oscillations within the ΛCDM paradigm. This results in ternary diagrams and flux plots that could be later compared to observations in neutrino observatories. The conclusion is that one can use neutrino oscillations in curved spacetime to distinguish between different values of the present expansion rate of the Universe, H0. This result adds new insight on the Hubble tension and explores Multimessenger astronomy in its full capacity. The fifth and final part summarizes the results and conclusions reached for each work. In addition, future perspectives and further developments are discussed in this section.