https://www.um.edu.mt/library/oar/handle/123456789/132032 Sat, 25 Apr 2026 10:13:22 GMT 2026-04-25T10:13:22Z https://www.um.edu.mt/library/oar/handle/123456789/134917 Title: Secular evolution in the Milky Way Abstract: This work and its abstract are both under embargo until the restriction is lifted. Description: Ph.D.(Melit.) Mon, 01 Jan 2024 00:00:00 GMT https://www.um.edu.mt/library/oar/handle/123456789/134917 2024-01-01T00:00:00Z https://www.um.edu.mt/library/oar/handle/123456789/132261 Title: Cosmological predictions of scalar-tensor theories in teleparallel gravity Abstract: The cosmological tensions following observational analysis as well as the late-time cosmic accelerated expansion provide a solid motivation for the adequacy of General Relativity as the primary theory for describing gravity. Under that novel viewpoint, scalar-tensor theories are commonly utilized as the typical method for investigating potential deviations from the ΛCDM model. Scalar-tensor theories are among the most extensively examined topics of modified gravity since their dynamical analysis reveals quite interesting behaviour associated with the various eras of cosmic evolution. The capacity of scalar-tensor theories to elucidate different cosmological epochs validates the increasing interest of the scientific community in the Horndeski theory of gravity, which is considered the most general scalar-tensor theory resulting in second-order field equations. The revival of the Horndeski theory in its contemporary form has generated profound research beyond the standard model of cosmology involving the broader framework of scalartensor theories. Following the detection of the GW170817 event, the constraints imposed on the terms of the Horndeski Lagrangian served as a starting point for its incorporation into Teleparallel Gravity. This results in the Teleparallel Analogue of Horndeski theory, known as the BDLS theory, which is discussed in the current thesis, followed by the classification of its models using the Noether Symmetry Approach. The investigation of the BDLS cosmological perturbations is also included along with their potential for further research on whether the No-go argument could be circumvented in a healthy manner. Furthermore, the Einstein Gauss-Bonnet model is examined by utilizing a dynamical system approach. This scalar-tensor theory contains a highly diverse phase space due to the inclusion of the fourth-order Gauss-Bonnet invariant combined with the second-order scalar field contribution. The critical points that emerge have the potential to enhance our comprehension of cosmic evolution as they differ noticeably compared to those already present in the literature. Description: Ph.D.(Melit.) Mon, 01 Jan 2024 00:00:00 GMT https://www.um.edu.mt/library/oar/handle/123456789/132261 2024-01-01T00:00:00Z https://www.um.edu.mt/library/oar/handle/123456789/132260 Title: Machine learning applications in exoplanet host star recommendation and determination Abstract: In the past three decades, parallel to the detection and characterisation of extrasolar planets, the link between a planet and its host star has been explored extensively. The presence of planetary companions has been linked to the star’s chemical, physical and galactic properties, such that particular configurations may potentially lead to a greater likelihood of formation. If these correlations are truly astrophysical, these characteristics can be leveraged to recommend potential host stars based solely on their stellar properties. This work aims to use the power of ML to develop, train and test recommendation engines capable of discriminating between the stellar host and comparison star samples. The project explores two approaches with different applications. The first is a spectral classification model, which provides a score for the likelihood of a star hosting a giant planet based on its spectrum. An exploratory phase extensively validates the use of machine vision and the architecture used for this application. This leads to the development of two upgraded designs to enhance performance and overall stability. The project establishes this method as a credible tool which can be implemented in spectrograph pipelines to recommend follow-up observations. The second application is to develop a tool to aid in analysing incomplete stellar abundance data in differentiating between hosts and comparison stars by leveraging supervised and unsupervised learning, as well as MOO, to allow for reliable imputation. The complete dataset could then be applied to comparison tests and host star recommendation strategies. This project presents the first implementation of the MOO algorithm, constraining its usage for binary classification of giant-planet hosts and multi-label classification for giant, low-mass and multiple-planet hosts by evaluating its performance on a stellar abundance catalogue. Performance in both approaches is then analysed to direct future work. Description: Ph.D.(Melit.) Mon, 01 Jan 2024 00:00:00 GMT https://www.um.edu.mt/library/oar/handle/123456789/132260 2024-01-01T00:00:00Z https://www.um.edu.mt/library/oar/handle/123456789/132259 Title: Cosmological constraints on physics beyond general relativity Abstract: The standard cosmological model, ΛCDM, has long been successful in explaining the Universe’s evolution. However, as observational precision has advanced, significant tensions, such as the Hubble tension and the S8,0 tension, have emerged, challenging the model’s validity and hinting at the potential need for theories beyond General Relativity. Among these alternatives, Teleparallel Gravity, where gravity is described through torsion rather than curvature, offers a promising avenue. In particular, the Teleparallel Equivalent of General Relativity (TEGR) is considered, a formulation that replicates General Relativity’s field equations but uses the torsion scalar T as the primary contributor to the gravitational Lagrangian. To explore the potential of this framework in addressing these tensions, generalised functions such as f(T) and f(T, B) gravity models are investigated. Consequently, in this work a comprehensive suite of cosmological datasets are utilised. The Pantheon compilation of Type Ia Supernovae (SNe Ia) provides precise luminosity distance measurements, constraining the expansion history. Cosmic Chronometers (CC) data offer model-independent estimates of the Hubble parameter based on differential age dating of galaxies. Baryon Acoustic Oscillations (BAO) measurements probe the imprint of sound waves in the early Universe, refining constraints on the late-time expansion. Redshift Space Distortions (RSD) trace the growth of cosmic structures, offering insights into large-scale structure formation. Additionally, the BICEP/Keck BB-spectrum enables exploration of primordial gravitational waves, shedding light on early Universe physics. Alongside these datasets advanced techniques like Gaussian Processes (GPs) for reconstructing the arbitrary function, and Markov Chain Monte Carlo (MCMC) for constraining the parameters of these models are employed. The results establish f(T) gravity as a promising alternative to ΛCDM, capable of addressing persistent tensions such as H0 and S8,0. The models provide observationally consistent solutions for late-time cosmic acceleration, the growth of structures, and primordial gravitational waves. These findings present the most comprehensive constraints on f(T) gravity to date, underscoring its potential as a viable framework and enhance our understanding of the Universe’s evolution, offering valuable insights into the late- and early-time Universe. As a result, this work contributes to the growing body of research exploring alternatives to ΛCDM and highlighting the potential of modified gravity models as viable solutions. Description: Ph.D.(Melit.) Mon, 01 Jan 2024 00:00:00 GMT https://www.um.edu.mt/library/oar/handle/123456789/132259 2024-01-01T00:00:00Z