Open Collections

Abstract

Although the Standard Model of particle physics has been a phenomenal success in modelling known particles and predicting new, theoretically founded particles, it is known to be incomplete. And while the Standard Model of cosmology has been a phenomenal success in modelling the evolution of the Universe, it too has open questions that remain unresolved. In this thesis, we aim to address properties of new physics models that are being developed that aim to answer these questions. In particular, we wish to focus on and examine in detail the connection between the dark sector of the Universe and the visible sector. In examining this connection, we may use cosmological observables to place strict limits on new theories that go beyond the Standard Model. In the first part of this thesis we will address the flow of energy from the visible sector to the hidden via a phenomenon known as freeze-in. Here, we explore the effects that early-time, ultraviolet energy transfer may have on the infrared, late-time evolution of a dark matter candidate. We use a simplified hidden-sector model to highlight the notion that operators that are typically considered early may have relevant late-time effects. Following this, we consider the reverse energy flow, and consider how dark-sector energy injection via decays of electromagnetic radiation may affect the products of Big Bang Nucleosynthesis. In this section, we focus on arbitrary light particle ($