Philosophiae Doctor - PhD (Physics)

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High-dimensional bayesian modelling approaches for 21cm cosmology
(University of the Western Cape, 2025) Murphy, Geoff Grant
The 21cm hyperfine transition of neutral hydrogen, HI, is an invaluable source of cosmological information, with its detection offering the opportunity to constrain models of the Universe, and thereby improving our understanding of its evolution. Numerous, and ever more sensitive, experiments target this signal across a range of frequencies, and, correspondingly, a range of redshifts, from Cosmic Dawn, to the Dark Ages, the Epoch of Reionisation, and the present-day Universe. Common among these experiments, however, are numerous performance-limiting effects, be it signal chain systematics, foreground contamination, etc. This work focuses on the mitigation of these examples in two different 21 cm experiments — signal chain systematics in simulated Hydrogen Epoch of Reionisation Array (HERA) data, and the separation of foregrounds from the 21 cm signal in the MeerKAT Large Area Synoptic Survey (MeerKLASS), both in a statistically rigorous Bayesian framework. The first chapter statistically models signal chain systematics in simulated HERA visibilities, namely cable reflections and antenna cross-coupling. We employ a Hamiltonian Monte Carlo (HMC) sampler to constrain our forward-model of the systematics-corrupted autocorrelation and cross-correlation visibilities as observed in HERA, and use our results to remove the systematics from mock data. This method allows us to not only recover the underlying power spectra, but to also provide rigorously propagated statistical uncertainties, a property which is important in the reporting of upper limits on the Epoch of Reionisation (EoR) signal. In the presence of high noise, we are able to effectively mitigate the systematics down to the noise level. Incoherent averaging of the recovered visibilities further reduces the noise, suggesting there is minimal residual systematics present. In cases of negligible noise, the majority of the systematics can be mitigated, but a fair amount of residual systematics remain, primarily due to the density of the systematics, i.e. confusion between systematics. However, this work performs similarly in terms of recovery in comparison to the currently implemented filtering and f itting methods in the HERA observational pipeline, but with the addition of statistical uncertainty estimates being placed on the recovered visibilities and power spectra. There is no significant signal loss in any of our results, an important consideration in the recovery of the weak Hi signal. The second chapter separates the 21 cm signal from the orders-of-magnitude more powerful foregrounds in simulated single-dish intensity maps, with future applications to MeerKLASS (MeerKAT Large Area Synoptic Survey) data being the intended outcome of this work.
Enabling scientific discovery in astronomical data with machine learning
(University of the Western Cape, 2025) Etsebeth, Verlon
The history of astronomy is defined by observational milestones that have profoundly transformed the understanding of the universe. Galileo’s pioneering use of the telescope allowed for detailed observations of celestial objects, such as the moons of Jupiter, providing empirical support for the then controversial heliocentric model (Galilei, 1610). Later advancements, including the development of space-based observatories like the Hubble Space Telescope (HST, Neal, 1990) and Karl Jansky’s groundbreaking discovery of radio waves from the Milky Way (Jansky, 1933), laid the foundation for modern astronomy. Astronomy has always been a data-driven science, but recent technological advances have brought about unprecedented volumes of data. Large-scale surveys and modern telescopes, such as the Sloan Digital Sky Survey (SDSS, York et al., 2000) and the Dark Energy Camera Legacy Survey (DECaLS, Dey et al., 2019) already produced terabytes of data. Future facilities will produce an even greater flow of data. The Vera C. Rubin Observatory’s Legacy Survey of Space and Time (LSST, Ivezi´c et al., 2019) is projected to capture approximately 20 TB per night. Similarly, the Square Kilometre Array (SKA, Dewdney et al., 2009) is expected to generate up to 700 PB of data annually. In addition, Euclid (Scaramella et al., 2022) has begun delivering data and is expected to contribute significantly to the wealth of astronomical data and aims to map the geometry of the dark Universe with unprecedented accuracy (Tutusaus et al., 2023).
Aspects relevant for nucleosynthesis in novae and 136Xe neutrinoless double beta decay, probed via 32S(d; t) and 137Ba(3He; ) reactions
(University of the Western Cape, 2025) Kamil, Mohamed
This work uses two independent neutron removal reactions to investigate nuclear states relevant for two contexts: the 30P(p; )31S resonance reaction, which plays a crucial role in nucleosynthesis pathways in novae, and the neutrinoless double beta decay (0 ) of 136Xe ! 136Ba. To achieve these objectives, high-resolution measurements using the 32S(d; t) and the 137Ba(3He; ) reactions were employed to probe excitation energies of interest in 31S and 136Ba, respectively. Both experiments were performed using the Q3D spectrograph at the Maier-Leibnitz Laboratory (MLL) in Garching, Germany. A rigorous statistical analysis of the 32S(d; t) data was used to resolve a discrepancy concerning the existence of an astrophysically relevant state in 31S at around 6.4 MeV. The analysis of the second dataset provided valuable spectroscopic information to test calculated matrix elements for 136Xe 0 .
A study of satellite contamination for single dish Hi intensity mapping with MeerKAT
(University of the Western Cape, 2025) Engelbrecht, Brandon Nicholas
HI Intensity Mapping is a novel technique that involves detecting the 21 cm emission lines from a large sky area without resolving individual sources, which proves to be a powerful probe for large scale structure cosmology. However, due to the weak nature of the 21 cm line, the majority of the received signal is dominated by astrophysical foregrounds and instrumental systematics that are several orders of magnitude stronger than the HI signal. Therefore, it becomes essential to mitigate these foregrounds and study their properties with respect to parameters such as frequency, scale, and redshift. The data used for this work has been obtained from the MeerKAT pilot survey which was collected from 62 hours of observation using the MeerKAT telescope’s 64 dish array in single dish mode. In this work, we attempt to use the higher L-Band frequency data (1300 - 1500 MHz) and understand the foreground behavior at higher frequencies. We use a foreground removal pipeline used for previous works with the lower L-band frequencies and make additions to it which allow us to identify and remove lowlevel RFI from the data and also compare the noise level estimates with theoretical values. We present the final results in the form of 1D power spectra and residual maps that indicate a decrease in amplitude due to the removal of foregrounds. We also conclude that the noise levels for most of the frequencies are higher than their theoretical estimates, which can be attributed to the presence of low-level contamination still be present in the dataset. Moreover, we conclude that the combined band results show noise levels an order of magnitude higher than individual bands, possibly due to cross-contamination between the foregrounds leading to a more complex structure.
Vapour transport deposition of multi-dimensional tin-lead perovskite thin films
(University of the Western Cape, 2024) Magubane Siphesihle Siphamandla
Thin film solar technology offers a promising solution to address the escalating global demand for clean, affordable, and renewable energy. Despite significant advancements in traditional solar cell (SC) materials, the emergence of perovskite materials has sparked renewed innovation in the field. However, challenges related to lead (Pb) toxicity and the instability of Pb-based perovskites impede the widespread adoption of perovskite solar cells (PSCs). This study aims to overcome these challenges by integrating tin (Sn) atoms into Pb-based perovskite structures to mitigate Pb toxicity issues and environmental concerns. Employing a comprehensive approach centered on a multi-step chemical vapour deposition (MS-CVD) technique, the research endeavors to synthesize and characterize Pb-Sn perovskite thin films. Structural and electronic properties are meticulously analyzed utilizing state-of-the-art techniques such as Rutherford Backscattering Spectrometry (RBS), X-ray diffraction (XRD), and optical property measurements. Incorporating Sn into Pb-based perovskite films significantly alters the material energy bandgap (Eg) and the crystal structure. The utilization of a three-step sequential deposition method yields superior quality Pb-Sn perovskite films compared to the two-step approach, resulting in enhanced structural stability and reduced defect density. Synthesized Pb-Sn 3-dimensional (3D) perovskite films exhibit a tetragonal crystal structure (Space Group, SG: I4cm), with a reduced unit cell volume attributed to the smaller atomic radius of Sn compared to the Pb atom. The effect of Sn concentration on Pb-Sn perovskites was manifested by the decrease in the Eg with increasing Sn concentration, while the Urbach energy (EU) escalated, signifying an augmented defect density in the films. Investigation into the effect of conversion substrate temperature on the structural and electronic properties of Pb-Sn 2D perovskite films reveals an optimal conversion temperature of 100°C. This led to the synthesis of high-quality Pb-Sn 2D perovskite films with enhanced structural stability from -273.15°C to 87°C as confirmed by the temperature-dependent photoluminescence (PL) analysis.
Probing large-scale structure with MeerKAT, the SKA and other surveys
(University of the Western Cape, 2024) Kopana Mponeng
The fluctuations produced during cosmic inflation may exhibit non-Gaussian characteristics that are imprinted in the large-scale structure of the Universe. This non-Gaussian imprint is an ultra-large scale signal that can be detected using the power spectrum. We focus on the local-type non-Gaussianity fNL and employ a multi-tracer analysis that combines different probes in order to mitigate cosmic variance and maximize the non-Gaussian signal. In this work we combine the 21cm intensity mapping survey optimised for interferometer mode with spectroscopic galaxy surveys. Combining the same 21 cm experiments, including also single-dish surveys, with photometric galaxy surveys. The 21 cm single-dish surveys are based on MeerKAT and SKAO and the interferometric surveys are alike to HIRAX and PUMA. We implement foreground-avoidance filters and utilize models for the 21 cm thermal noise associated with single-dish and interferometer modes. The photometric galaxy surveys are similar to the DES and LSST while spectroscopic galaxy surveys are similar to the Euclid and MeggaMapper. Our multi-tracer Fisher forecasts show that combining spectroscopic galaxy surveys with 21 cm intensity mapping surveys in interferometer mode could lead to a ∼ 20-30% improvement in the precision on this non-Gaussian signal. Furthermore, we investigate the effects on constraints of varying the parameter of non-Gaussian galaxy assembly bias and of varying the parameters of the intensity mapping foreground filters. We find that the non-Gaussian galaxy assembly bias parameter causes a greater change in the constraints on local primordial non-Gaussianity than the foreground filter parameters. Again our multi-tracer Fisher forecasts show a better precision for the combination of the photometric galaxy surveys and 21 cm interferometric surveys than with the 21 cm single-dish surveys - leading to at most an improvement of 23% in the former case and 16% in the latter case. Furthermore, we examine the impact of varying the foreground filter parameter, redshift range and sky area on the derived constraint. We find that the fNL constraint is highly sensitive to both the redshift range and sky area. The foreground filter parameter shows negligible effect.
Cosmological constraints using 2- and 3-point correlations with meerkat, the ska and other surveys
(Universty of the Western Cape, 2024) Randrianjanahary, Liantsoa Finaritra; Maartens, Roy
We explore the information from HI power spectra (tree-level and one-loop) in redshift space and bispectrum tree-level models. 21-cm emission from neutral hydrogen is a promising tool for probing the matter distribution in the universe post-reionization. The HI signals contain signatures of the primordial universe and the growth of large-scale structure in the universe. These signatures are typically analyzed via the 2-point correlation function or power spectrum. However, adding the information from the 3-point correlation function or bispectrum will be crucial to exploiting next-generation intensity mapping experiments. Upcoming surveys by SKAO and HIRAX will undertake intensive line-intensity observations, complementing galaxy surveys. This study provides new forecasts on cosmological constraints derived from the combined analysis of the 21cm power spectrum and bispectrum. We use Fisher predictions to examine how useful these surveys might be for constraining cosmological parameters, BAO distance functions, growth function, and what they mean for dynamical dark energy and modified gravity. We account for telescope beam effects, instrumental noise, foreground avoidance, the Alcock-Paczynski effect, and theoretical modeling errors in the correlators. The investigation also includes assessments of 21-cm clustering bias up to the second order.
Using skao and other surveys to study the large-scale structure of the universe
(Universty of the Western Cape, 2024) Dlamini, Simthembile; Maartens, Roy
We are currently in a time of precision cosmology, marked by an extensive accumulation of data for comprehending our Universe. Nonetheless, our current grasp of the subject remains incomplete. Our most successful cosmological model relies on components that have yet to be directly detected and do not harmonize with our broader physics model. This implies that there is still much to probe and a high demand for novel techniques to explore the cosmos. One method gaining traction over the last decade is mapping the intensity of the 21cm emission from neutral hydrogen (HI) within unresolved galaxies. Understanding the large-scale structure of the Universe, characterized by galaxies, clusters, and superclusters, is a cornerstone of modern astrophysics. This research presents an investigation into the Universe’s large-scale structure, employing the Square Kilometre Array Observatory (SKAO) in conjunction with other surveys. The study leverages the analytical tool of Fisher forecast analysis to estimate the constraining power of these surveys. The SKAO offers a unique opportunity to probe the Universe’s large-scale structure via the 21 cm emission line of HI. By incorporating complementary surveys, this research develops a multi-tracer view of cosmic structure, encompassing various redshift ranges and investigating the constraints on key cosmological parameters.
Exploring poly (2, 5) benzimidazole enhanced with carbon nanotubes for space applications
(University of the Western Cape, 2023) Fourie, Lionel Fabian; Square, Lynndle
This work explores using polymeric materials for space radiation shielding in low-earth orbit. Shielding against radiation is essential on any space mission. Low atomic number materials, such as hydrogen, have shown to be effective in shielding ionising radiation. However, compared to metallic alloys, these materials suer from relatively low mechanical and thermal properties, which limit their application. Aluminium (Al) enjoyed wide use in space applications as a structural and radiation shielding material. However, weight and secondary radiation generation issues have made its use as a shielding material less viable on modern space missions where cost and safety play a crucial role in planning these missions. On modern space missions, conventional shielding materials include Al alloys, high-density polyethylene, and water. The disadvantages include low thermal properties, high atomic numbers, and complex maintenance systems. This lead to exploring other materials that can mitigate some of these drawbacks. A proposed approach to replacing high atomic number metals is deploying hydrogen-rich polymers enhanced with nanoﬁller materials to form polymer nanocomposites. Poly-mers enhanced with nanoﬁllers can achieve improved physical properties while pro-viding adequate radiation shielding functions at a lower weight with less secondary radiation generation.
Chemical pressure-induced transition of the magnetic ground state from ferromagnetic to antiferromagnetic order in CeCuGe and DyCuIn alloys
(University of the Western Cape, 2023) Altayeb, Anas Alamin Hassan; Tchoula Tchokont´e, Moise
Rare-earth intermetallic compounds continue to attract considerable attention, due to their fundamental importance in understanding physical properties and potential applications based on a variety of phenomena. The ternary intermetallic compounds of the RTX series (R = rare-earth element, T = 3d / 5d transition element, X = p-block element) in particular, were studied extensively for the past two decades. A number of interesting magnetic and electrical properties of practical and fundamental importance, were found in different compounds of the RTX series, including giant and large magnetocaloric effects and magnetoresistivity, antiferromagnetic (AFM) to ferromagnetic (FM) order transitions and rich magnetic transition phase diagrams. Most of these properties are related to the interaction between the R-4f (localized) electronic states and other (itinerant) electronic states in the electronic system. Several experiments have provided evidence for magnetic ground state switching between FM and AFM ordering, driven by pressure or chemical substitution in d and f electron compounds. The methods used to investigate the magnetic phase transitions could be divided into macroscopic and microscopic ones. The macroscopic method relied on the temperature dependences of magnetization, magnetic susceptibility and specific heat measurements while the microscopic method relied on neutron diffraction and Mössbauer effect.
Physics graduate preparedness for work and society
(University of the Western Cape, 2023) Audu, Bako Nyikun; Marshall, Delia
Both internationally and in South Africa, there have been renewed debates about the purposes of higher education and the attributes that should be developed in graduates. In the field of physics, many studies have reported that the preparation of physics graduates in the 21st century needs to be broader, to prepare graduates for a range of careers and roles in society. This case-study aims to understand the formation of graduate preparedness and the development of graduate attributes in a Physics Department at a South African university. A framework of physics graduate attributes was developed, drawing on skills and attributes benchmarked by several international physics bodies, as well as the South African Institute of Physics; the national SAQA critical cross-field outcomes and the UWC Charter of Graduate Attributes were also drawn on. The study adopted a range of research methods: document analysis of physics module descriptors was used to examine the embeddedness of graduate attributes in the physics curriculum; a questionnaire was developed to gauge students’ perceptions of their preparedness for work and society; focus group discussions with students, and interviews with graduates, were used to understand the structural and institutional arrangements that enabled or hindered the development of physics graduate attributes. The theoretical framework for this study is the capability approach, as developed by Amartya Sen and Martha Nussbaum. It offers an enlarged view of the purposes of higher education beyond the development of human capital; it also enabled an analysis of the structural challenges that students faced, while at the same time foregrounding student agency, and the strengths and resources that students bring to higher education. The findings revealed that the physics graduate attributes were unevenly and mostly scantily embedded as learning outcomes in the physics module descriptors; attributes least embedded in the curriculum were social and environmental awareness, ethical behaviour, and teamwork. Analysis of questionnaire and focus group discussion data showed that students felt that more could be done in the Physics programme to explicitly develop a wider set of physics graduate attributes to enhance their sense of graduate preparedness. In particular, the development of ICT skills, communication skills, practical skills and social awareness was found to be lacking. Concepts from the capability approach, including conversion factors, well-being and agency, were used to understand the ways in which the development of students’ graduate preparedness and their attainment of academic success and well-being were enabled or hindered by various structural and institutional factors.
A case study of university students' experiences of introductory physics drawn from their approaches to problem solving
(University of the Western Cape, 2001) Alant, Busisiwe Precious; Linder, Cendric; Marshall, Delia
This thesis explores the experience of leaming physics through a particular medium: problem-solving, which is seen by many educators as the primary medium in which physics is learnt at university. Situating itself within two theoretical perspectives: phenomenography and actor-network theory, the dissertation explores the variation in the ways of experiencing introductory physics leaming through problem-solving. phenomenography, which is the main theoretical framework, places emphasis on the variation of experience of phenomenon at a supra-individual level. Leaming is regarded as relational, which means that the act of leaming is apprehended (in terms of how the learning is done as well as what is leant) in the relation between the leamer and the phenomenon. Rather than regard the content of physics learning as the phenomenon, the study proposes the process of learning physics through problem, solving as the phenomenon under investigation. The thesis draws on insights from actor-network theory, particularly with regard to the spatiality of leaming. Learning is seen as a function of enrolment. Fifteen students were interviewed on introductory physics problems encountered in four end-of-module tests. The data were analyzed on the basis of strategy - conceived as "moments,' of problem-solving, as well as the factors (intentional and contextual) that could be seen to influence the strategy adopted. Two qualitatively distinct problem-solving strategies were identified, derived from the relative presence of reflective awareness. Further, factors influencing the strategies were identified and found to be indicative of two qualitatively distinct ways in which the students focused on the problems - either on problem content (the physics concepts) or on problem requirement (the formal requirements of the task within the test setting). These findings are seen to constitute the structural aspect of the students' experience of physics learning through problem solving. With regard to the referential aspect of the experience, the study derives two overall meanings that the students attached to their experience of physics learning through problem-solving, namely physics leaming as "reconstituting understanding', and physics learning as, confirming convention". It is argued that the variations identified in the strategies employed by the students, in the ways they focus on problems, in their perception of the problem-solving settings, in the meanings they attach to physics learning through problem-solving - call for a framework of learning that takes account of spatiotemporal intricacy. The notion of conceptual understanding in the learning of physics should be informed by the specific demands of the medium of problem-solving through which physics is learnt at undergraduate level
Synthesis of bimetallic immiscible alloy nanoparticles through green and gamma radiolysis approaches for environmental remediation applications
(University of the Western Cape, 2022) Noukelag, Sandrine Kamdoum; Arendse, Christopher
The synthesis of bimetallic immiscible alloy nanoparticles (NPs) using versatile routes, is a major concern since physio-chemical methods are not environmentally benign. Breaking down the immiscibility would generate NPs with remarkable properties and consequently more applications. As a result, it urges the development of one-step, eco-friendly, efficient, and reliable methods for getting more metastable bimetallic alloys from immiscible metals. To that aim, unconventional approaches such as green and gamma radiolysis were considered as the paths forward in this thesis. The wide immiscibility gaps of iron-silver (Fe-Ag), and iron-zinc (Fe-Zn) led to their selection.
HI intensity mapping: Impact of primary beam effects
(University of the Western Cape, 2023) Matshawule, Siyambonga Donald; Santos, Mario G.
Neutral hydrogen (HI) intensity mapping surveys with upcoming and future radio telescopes such as the MeerKAT, a precursor to the Square Kilometre Array Observatory (SKAO) MID telescope, have great potential for constraining cosmology, particularly in the post-reionization Universe provided that e ective cleaning methods are available to separate the strong foregrounds from the cosmological signal. The application of cleaning methods is usually conducted under the assumption of simplistic primary beam models. In this thesis, I simulate a single-dish wide-area survey with MeerKAT characteristics, and test foreground subtraction with a realistic model for the primary beam that contains a non-trivial frequency dependence. I also probe the impact of strong point sources on the cleaning. To conduct this evaluation, point source maps from a much more realistic full-sky point source catalogue are included as part of the foregrounds present in the sky model.
Measurements and applications of radon in South African aquifer and river waters
(University of the Western Cape, 2009) Abdalla, Siddig Abdalla Talha; Lindsay, Robbie; de Meijer, Rob J.; Newman, Richard T
In the natural decay series of 238Uan inert radioactive gas, 222Rn(radon) is formed in the decay of 226Ra.Because radon is relatively soluble in water, it migrates from places of its generation in rocks and soils to other places either by soil air, or travels with underground water. Therefore, there is a growing interest among hydrogeologists in using radon as a natural tracer for investigating and managing fresh water reservoirs. This work is aimed at investigating and developing radon-in-water measuring techniques applicable to aquifers and rivers. A gamma-ray spectrometry method using a hyper-pure germanium (HPGe) detector, based at iThemba LABS, Cape Town and Marinelli beakers, has been optimized to measure radon in borehole water via the y-rays associated with the decay of radon daughters 214Pband 214Bi(in secular equilibrium with their parent). An accuracy better than 5% was achieved. Moreover, long-term measurements of radon in water from an iThemba LABS borehole have been carried out to investigate the role of radon for characterizing aquifers. These investigations led to the development of a simplified physical model that reproduces the time-evolution of radon concentration with borehole pumping and may be used to estimate the time for representative sampling of the aquifer. A novel method is also proposed in this thesis to measure radon-in-water in the field after grab sampling - a so-called quasi in-situ method. The quasi in-situ method involves inserting a y-ray detector in a container of large volume filled with water of interest. The y-ray spectra are analyzed using an approach involving energy intervals on the high-energy part of the spectrum (1.3 - 3.0 MeV). Each energy interval corresponds to contributions from one of the major y-ray sources: 40K and the decay series of 238Uand 23~h, and cosmic rays. It is assumed that the U interval will be dominated by y-rays emitted from the radon daughters e14Pband 214Bi).Minor contributions to an interval with major radionuclide are corrected using an MCNPX simulated standard spectra. The two methods in this thesis make a significant contribution to measuring and modelling of radon in aquifers and surface waters. It forms a basis for further development in an interactive mode with hydrological applications.
New Collective structures in the Z=76 stable odd neutron nucleus, 187Os
(University of the Western Cape, 2021) Sithole, Makuhane Abel; Lawrie, E A
Low- and medium-spin bands of 187Os have been studied using the AFRODITE array, following the 186W(4He,3n)187Os reaction at a beam energy of 37 MeV. The measurements of γ − γ coincidences, angular distribution ratios (RAD), polarization and γ-intensities were performed using eleven High Purity Germanium (HPGe) clover detectors. In the current work, all the previously known bands have been significantly extended and five new bands have been added to the level scheme. The observed bands are interpreted within the cranked shell model (CSM), cranked Nilsson-StrutinskyBogoliubov (CNSB) formalism and Quasiparticle-plus-Triaxial-Rotor (QTR) model. Systematic comparison of bands with the neighbouring isotopes has also been made. Comparison of the models with experimental data shows good agreement. The configurations of some of the previously observed bands have been modified. Most importantly, the coupling of 2+ γ band to the 11/2+[615] neutron configuration is observed for the first time.
New collective structures in the Z=76 stable odd neutron nucleus, 187Os
(University of Western Cape, 2021) Sithole, Makuhane Abel; Lawrie Elena
Low- and medium-spin bands of 187Os have been studied using the AFRODITE array, following the 186W(4He,3n)187Os reaction at a beam energy of 37 MeV. The measurements of 􀀀 coincidences, angular distribution ratios (RAD), polarization and -intensities were performed using eleven High Purity Germanium (HPGe) clover detectors. In the current work, all the previously known bands have been signi cantly extended and ve new bands have been added to the level scheme. The observed bands are interpreted within the cranked shell model (CSM), cranked Nilsson-Strutinsky- Bogoliubov (CNSB) formalism and Quasiparticle-plus-Triaxial-Rotor (QTR) model. Systematic comparison of bands with the neighbouring isotopes has also been made. Comparison of the models with experimental data shows good agreement.
Probing large-scale structure with the SKAO and other cosmological surveys
(University of the Western Cape, 2022) Viljoen, Jan-Albert; Maartens, Roy
In recent history there have been several advances in cosmology, which has significantly shaped our understanding of the Universe. The current leading theory is called ΛCDM, which can successfully model the expansion of the Universe from a primordial state and describe the dynamics of its contents, thereby resulting in the large-scale structure present today. The model is based on general relativity, that describes gravitational interaction as the curvature of a four-dimensional manifold called space-time. However, despite the many successes of ΛCDM, there are a number of things that need further investigation. The Cosmic Microwave Background (CMB) is the oldest observable radiation in the Universe, and this cosmological relic contains a detectable structure. The process leading up to the CMB determines the initial conditions of ΛCDM, but is still poorly understood. It is widely accepted that inflation was responsible for the rapid expansion after the Big Bang, although this is yet to be verified experimentally. The distribution of the primordial potential is imprinted on ultra-large scales of the matter distribution, which offers an important insight into uncovering this mystery. In addition to the primordial Universe, there are other concepts that still puzzle us in ΛCDM itself. The fact that we have been unable to directly detect and explain these dark components (that make up around 96% of the Universe) has prompted several theorists to consider alternative cosmological models. Therefore, testing general relativity and ΛCDM is still an essential part of cosmological research. A key observational discriminant between general relativity and modified theories of gravity is the rate at which the large-scale structure grows from small perturbations. The relativistic effects (or light-cone effects) expected in general relativity also offer an independent test of the gravitational model.
A multi-wavelength study of powerful high redshift radio galaxies
(2021) Marubini, Takalani; Jarvis, J
We present a new sample of distant powerful radio galaxies, in order to study their host-galaxy properties and provide targets for future observations of Hi absorption with new radio telescopes. We cross-match the Sydney University Molonglo Sky Survey radio catalogue at 843 MHz with the VISTA Hemisphere Survey near-infrared catalogue using the Likelihood Ratio technique, producing contour plots as a way to inspect by eye a subset of bright sources to validate the automated technique. We then use the Dark Energy Survey optical and near-infrared wavelength data to obtain photometric redshifts of the radio sources. We find a total of 249 radio sources with photometric redshifts over a 148 square degree region. By fitting the optical and near-infrared photometry with spectral synthesis models, we determine the stellar masses and star-formation rates of the radio sources. We find typical stellar masses of 1011−1012 M for the powerful high-redshift radio galaxies. We also find a population of low-mass blue galaxies. We then report results from the first search for associated Hi 21 cm line absorption with the new MeerKAT radio telescope (shared-risk early science programme). We used a 16-antenna sub-array of MeerKAT to carry out a survey for Hi absorption in the host galaxies of nine powerful (L1.4 GHz > 1026 W Hz−1 ) radio galaxies at cosmological distances (z = 0.29 to 0.54). We found no evidence of absorption with 5σ optical depth detection limits. We only obtain a tentative absorption towards a radio source 3C 262 at z = 0.44 with significant ongoing star formation at a rate of 10.5 M yr−1 . The source consists of two radio lobes separated by 28.5 kpc with no evidence of a compact core. If the absorption arises from neutral gas from an extended disc, the line is redshifted by 79(21) km s−1 with respect to the nucleus and has an average Hi column density across the source of NHI ∼ 7 × 1019−20 cm−2 , which is consistent with the rate of star formation. But after further tests, we find that the Hi detection towards 3C 262 is likely to be an artefact. We conclude that the new correlator with 32 k channel resolution will be needed before searching for its associated absorption in MIGHTEE data.
Nial and steel as matrices and tic and oxynitrides as reinforcements in metal-matrix composite fabrication
(University of Western Cape, 2021) Camagu, Sigqibo Templeton; Bolokang, A. S.
Metal matrix composites harness the superior attributes of their individual constituents to form high performance materials that would rather be impossible from monolithic substances. Owing to many possible combinations, a myriad of metal matrix composite systems can be fabricated with a metal (or a metal alloy) as a matrix (continuous) phase and a ceramic as a reinforcement (discontinuous) phase. The current study focuses on two matrices, namely Nickel Aluminide and Austenitic Steel as well as two reinforcements namely, Titanium Carbide and Oxynitrides. NiAl alloys are candidates for high temperature structural materials due to their high melting temperature, low density, good thermal conductivity, and excellent oxidation resistance.

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