https://www.um.edu.mt/library/oar/handle/123456789/63159 2026-04-04T15:53:11Z 2026-04-04T15:53:11Z https://www.um.edu.mt/library/oar/handle/123456789/70206 2021-03-02T13:42:22Z 2020-01-01T00:00:00Z

Title: Modified gravity : cosmological tests Abstract: The formulation of General Relativity revolutionised the principles to understand the mechanics of gravitation. Under this viewpoint, gravity was no longer a force as postulated by Newton but a manifestation of curvature of the unification of space and time, spacetime. Despite its success, General Relativity started to be incompatible with observations as it required the introduction of dark matter and dark energy. Furthermore, General Relativity is a classical theory and is not renormalisable. Modifications to General Relativity have therefore been proposed. In this work, an alternative proposal, known as Teleparallel gravity, is investigated. This formulation reintroduces gravity as a force by replacing curvature of spacetime through torsion. Furthermore, the theory can reproduce the field equations of General Relativity, referred to as Teleparallel Equivalent of General Relativity, and hence can be deemed as an alternative description of gravitation. Throughout this work, various teleparallel gravity models are studied under different topics: (i) stability of the Friedmann-Lemaître-Robertson-Walker geometry through homogeneous and isotropic perturbations, (ii) reconstruction of the gravitational Lagrangian, (iii) gravitational waves, and (iv) large scale structure. It is shown that this formulation is capable of accounting for the accelerating universe without invoking dark energy, while also being stable under homogeneous perturbations, hosting various expansion histories, agreeing with the observed predictions of gravitational waves, while also being able to correctly generate the observed large scale structure. Description: PHD.SPACE SCIENCES&ASTRONOMY

2020-01-01T00:00:00Z https://www.um.edu.mt/library/oar/handle/123456789/63800 2020-11-16T05:41:21Z 2020-01-01T00:00:00Z

Title: Detection of orbital debris in low Earth orbit Abstract: The ever increasing satellite population in near-Earth orbit has made the monitoring and tracking of active satellites, and orbital debris objects ever more critical. As the in-orbit population grows, so does the risk of a collision. In recent years, the European Space Agency (ESA)’s Space Situational Awareness (SSA) programme has been assisting national institutions in the upgrading of their space detection and monitoring capabilities. One of the latest such systems within this programme is the BIstatic RAdar for LEo Survey(BIRALES) space surveillance system consisting of a radio transmitter in Caligari, Sardinia and the BEST-2 phased array in Medicina, near Bologna, Italy. This research lays out the foundation for a new space debris detection system for this novel sensor. First, this work introduces a new software backend that makes use of data processing pipelines to process the incoming data from the 32-antenna radio telescope in real-time. The detection pipeline channelises and beamforms the incoming antenna signals, creating a multi-pixel of beams covering the Field of View (FoV) of the instrument. The detection algorithm uses a series of filters to pre-process the incoming data from any interference. In this study, two novel track detection algorithms are presented. These algorithms identify the unique doppler echo tracks emanating from resident space objects crossing the FoV of this bi-static radar. Candidates are identified by these algorithms are validated to reject false positives. The trajectory of the detected objects is determined by considering the illumination sequence of the multi-pixel. Initial experimental results from observation campaigns of known objects show that the radar can reliably detect in-orbit objects down to a few centimetres in size in Low Earth Orbit (LEO). These encouraging results represent the latest scientific contribution from Europe’s emerging space debris monitoring radar within its growing network of European Space Surveillance and Tracking (SST) systems. Description: PHD.SPACE SCIENCES&ASTRONOMY

2020-01-01T00:00:00Z