https://www.um.edu.mt/library/oar/handle/123456789/82070 2026-04-15T04:17:41Z https://www.um.edu.mt/library/oar/handle/123456789/133787 Title: Confronting hubble tension using scalar-tensor theories Abstract: The Hubble tension, an ongoing debate in astrophysics and cosmology, pertains to different measures of the universe’s current rate of expansion, the Hubble constant (H0). This tension stems from discrepancies in the results acquired using various observational techniques and datasets. To better understand and possibly resolve these differences, it is necessary to investigate different cosmological theories. An interesting path is to modify the basic character of gravity by investigating theories such as scalar-tensor models, which include a scalar field as an extra degree of freedom. These changes seek to correlate theoretical predictions with empirical evidence, perhaps providing fresh insights into the universe’s fundamental features. In this project, cosmological models are analysed using the MCMC technique and the emcee Python toolkit, with an emphasis on the exponential and Higgs scalar field models. The MCMC approach allows for a thorough statistical study of parameter spaces by utilising current Hubble parameter measurements and other observational data, such as CC and Sn Ia. These models are then compared to the ΛCDM model, which provides the benchmark, evaluating their effectiveness in resolving the Hubble tension and establishing tighter restrictions on cosmological parameters. In comparison to the ΛCDM model, the scalar field models displayed a slightly lower value of H0 and hence a slightly larger value of Ωm,0. The inclusion of priors provided larger values of H0 for all models. Description: B.Sc. (Hons)(Melit.) 2024-01-01T00:00:00Z https://www.um.edu.mt/library/oar/handle/123456789/133782 Title: Comparing quantum computing algorithms for physics simulations Abstract: We find the Boltzmann probability distribution function (PDF) of the quantum Ising model through a variational quantum algorithm (VQA) using state-vector, shots, and noisy simulations and compare their respective fidelity with the exact PDF at different temperatures and magnetic field strengths. The Rudolph and Grover quantum circuit was used alongside the classical optimizers, Powell for ‘state-vector’ and COBYLA for ‘shots’ and ‘noisy’ modes. The state vector result of a quantum circuit represents the optimal theoretical quantum state of the system. A real quantum computer can never output the optimal state vector result and instead utilises ‘shots’. ’Shots’ rely on repeatedly running a quantum circuit to obtain statistical results as a measure. Given enough shots it was shown that the results approximate state-vector results for small qubit numbers. Because real quantum computers are noisy, a noisy simulation was created using one of the most common types of noise in quantum computers, depolarizing noise. The results were then compared across the three types of simulations. One of the major findings was that even a small degree of depolarizing noise randomly implemented on the gates of a quantum circuit reduced the fidelity for different temperatures and magnetic field strengths tested. Future research analysing the fidelity of the Boltzmann distribution in noisy intermediate scale quantum (NISQ) computers would benefit from more noise-resilient qubits or more efficient noise reduction schemes. Description: B.Sc. (Hons)(Melit.) 2024-01-01T00:00:00Z https://www.um.edu.mt/library/oar/handle/123456789/133781 Title: Compact stars in a cosmological background Abstract: Compact stars, such as neutron stars, are dense celestial objects whose gravitational fields are so intense that they warp the spacetime around them, showcasing the cosmos’ profound connection between mass and gravity. The effect of cosmic expansion on local physics continues to be an open problem in astrophysics. Compact stars offer an ideal lab in which to test strong field physics. The underlying gravitational field equations governing compact stars can also be applied to cosmology. The growing tension in the standard model of cosmology in the cosmological context makes the need for more tests of these models all the more important. In this project, the local effects of cosmological models, such as the cosmological constant in standard model physics, will be probed using local physics such as the TolmanOppenhiemer-Volkoff equations where higher-order modified theories of gravity are employed. Through this work, a better understanding of the effect of cosmological evolution on astrophysical phenomena will be explored. Two different models of the higher modified theory of gravity - f(R) will be under observation, namely the Starobinsky model and the Hu-Sawicki model. Description: B.Sc. (Hons)(Melit.) 2024-01-01T00:00:00Z https://www.um.edu.mt/library/oar/handle/123456789/133780 Title: Dynamical systems in scalar-tensor cosmology Abstract: The value of the Hubble constant, H0, as predicted using Cosmic Microwave Background (CMB) data under the ΛCDM model stands at a 5σ tension with that obtained from measurements of Cepheid variables. Efforts to find a solution to this Hubble tension focus significantly on modifications of ΛCDM prior to the emission of the CMB. However, it has been claimed that only a combination of new physics in the early and late Universe, and new local physics can ultimately resolve the tension. The aim of this work is to examine whether these modifications to early- and late-time physics still approach the evolution observed in ΛCDM. Scalar fields, with dynamics governed by their potential energy, were introduced within the concordance model, and the expansion of the Universe was analysed using the framework of dynamical systems and phase portraits. Three free, self-interacting potentials were examined, with the dynamics obtained being similar throughout. While these models reproduced the expected ΛCDM evolution with a new epoch occurring before recombination in which the early-time modifications dominate the expansion, significant fine-tuning of the boundary conditions of the system was required to achieve this. In an attempt to mitigate this issue, an additional model was analysed, in which the potential energy was dampened by a tempering function, localizing the respective field to reach a maximum at a specific point in the history of the universe. While this also reproduced the previous evolution, the tempering did not resolve the fine-tuning issues of the model. Despite these problems, such modifications can reproduce the evolution of the Universe as predicted by ΛCDM, making further research in this area, possibly focusing on resolving the fine-tuning problem, a promising avenue to pursue. Description: B.Sc. (Hons)(Melit.) 2024-01-01T00:00:00Z