Beyond Lambda and CDM: interacting scalar fields for dark sector cosmology

Since its modern inception in the early twentieth century, cosmology has developed drastically due to new observational methods. With this, our understanding of the Universe has changed accordingly, requiring new theoretical insights. For the past twenty-five years, Lambda-CDM has been considered the standard model of cosmology. However, with ever-increasing observational precision, new tensions in the data have emerged. There are many hints that this crisis in cosmology should be addressed by considering alternatives to Lambda-CDM, especially the dark energy `Lambda' and the cold dark matter `CDM' components. These two highly studied but scarcely understood energy sources form what we refer to as the dark sector. Dark sector physics often results in novel phenomenology that can alleviate cosmological tensions whilst also providing a robust theoretical framework.

In this thesis, we will first review the basics of modern cosmology, culminating in the description of the Lambda-CDM and cosmological tensions. We then introduce alternative dark sector models from the literature, focusing on scalar fields. The bulk of this work focuses on studying new models of interacting scalar field dark matter (DM) and dark energy (DE). We study a model inspired by hybrid inflation, obtaining a previously unknown form of dark energy-dark matter coupling. By comparing this model to cosmological data we conclude that it can alleviate the H_0 tension. We also propose a framework for DM-DE scalar fields based on effective field theory. This results in the DM component developing a negative equation of state at late times. We look for evidence of this new signature in cosmological observations and find that whilst this model can reduce the S_8 tension, new data will be needed to establish whether this model is fully viable.