Scalar fields in cosmology and their applications beyond the standard cosmological model

Abstract

This dissertation investigates key challenges in cosmology and astrophysics through the development and analysis of models for early dark energy, dynamical dark energy, and CP-violating axion-like particles (ALPs). First, we introduce a novel early dark energy (EDE) model, @EDE, aimed at resolving the Hubble tension by injecting energy before recombination to reduce the sound horizon size and increase the inferred value of H0, thereby addressing the tension between early- and late-time cosmological measurements. We analyze the model’s impact on the Universe’s expansion history and test its viability against cosmological data. Next, we examine two dynamical dark energy models—the single-exponential quintessence model and the pixelated dark energy model—in light of recent DESI BAO observations, which show a growing preference for time-varying dark energy over a cosmological constant. While the w0–wa parameterization provides a better fit to the DESI data than ΛCDM, the quintessence model fails to replicate the rapid low-redshift transition in the equation-of-state implied by the data. In contrast, the pixelated model, in its simplest form with a constant pixel growth rate, is marginally preferred over ΛCDM. However, extending it to allow for time-dependent growth significantly enhances its ability to match the observed equation-of-state behavior. Lastly, we explore the cosmological and astrophysical effects of CP-violating axion-like particles, deriving constraints on their properties and evaluating their influence on neutron star structure and the mass-radius relationship. This work provides fresh theoretical perspectives and practical tools to address unresolved questions in modern cosmology and astrophysics.