The motivation of this thesis is to look for a signature of Lorentz violation, hopefully observable, in the gravitational waves emitted by cosmic strings. Aspects of cosmic strings are reviewed, in particular how focused bursts of gravitational radiation are emitted when a cusp forms on the string. The same phenomenon is then studied in an effective field theory with Lorentz violation called Einstein-aether theory. This is a simple theory with a dynamic preferred frame, but it retains rotational and diffeomorphism invariance. The linearized version of the theory produces five wave modes. We study the usual transverse traceless modes which now have a wave speed that can be lower or greater than the speed of light. This altered speed produces distinctive features in the waves. They depend on two free parameters: roughly the wave speed and the acceleration of the string cusp. The profile of the wave is analyzed in detail for different values of the parameters and explained by close comparison with the string motion. / Science, Faculty of / Physics and Astronomy, Department of / Graduate

The type of feedstock and host rock utilised in ex situ and in situ Carbon Capture and Storage by Mineralisation (CCSM) is an important aspect of these technologies, and detailed appraisal of candidate mineral/rock performance in the presence of CO2 may greatly improve CCSM efficiency. Here, a detailed mineralogical and geochemical investigation of serpentine minerals and ultramafic rocks under laboratory-controlled experiments simulating ex situ and in situ process conditions is presented. Feedstock serpentine minerals were analysed comprehensively, prior to experimental processing. The identification of antigorite was unequivocal using a combination of X-ray diffraction, Fourier Transform Infrared Spectroscopy and/or Thermo-gravimetric Analysis. However, the analysis of chrysotile and lizardite proved to be more challenging, especially when the two polymorphs were finely inter-grown. This study highlighted the structural, textural and chemical complexity of serpentines and showed that great care must be taken when analysing this mineral group for CCSM. Investigation of the acid leaching of serpentine minerals under conditions of 70°C and 1.4M NH4HSO4 provided fundamental insights into the rate and extraction efficiency (EE) of Mg2+ and associated controlling factors, under conditions relevant for ex situ CCSM. It is demonstrated that EE is a function of mineral reactivity and depend on a complex interplay between crystal structure and chemistry. Generally, poorly crystalline and highly disordered phases with low levels of Al3+ were found to be suitable feedstock materials for acid digestion pre-treatment. Chrysotile, lizardite 2H1 and poorly crystalline serpentines showed up to 85% Mg2+ EE after 3 h of acid leaching, and hence are recommended as best feedstock materials for CCSM, whilst antigorite and Al3+-rich serpentines proved to be largely unsuitable, showing low levels of EE of ~ 20%. Examination of dunite, harzburgite and serpentinite under conditions relevant for in situ CCSM, i.e. 70°C and 100 bar CO2 pressure, provided insights into rock reactivity as a function of composition and texture, and the progression of in situ mineral carbonation. The rate of net Mg release and thus the extent of subsequent carbonation were greatest for serpentinite, providing ca. 3% carbonation after six months. However, mineral reactivity within serpentinite was preferential, i.e. significantly enhanced within secondary vein serpentine, being thus, the main source of Mg2+ for magnesite precipitation. Reaction-induced transformation and hence mineral carbonation of dunite and harzburgite were less pronounced over the same time-scale. The reaction of serpentinite with wet supercritical CO2, as opposed to CO2-saturated brine, significantly affected rock integrity, with the exposure of more surface area and promotion of fluid-rock interaction. In particular, it is shown that ferric iron oxidation and the precipitation of goethite impacts upon surface mineral dissolution at exposed surfaces, thereby hindering subsequent carbonation. Overall, this study highlighted the importance of host rock choice for in situ CCSM and the need for detailed petrographical and geochemical investigation of any proposed CCSM repository prior to technological process modelling.

The direct radiative capture reactions D(p,⋎)He³ and O¹⁶(p,⋎)F¹⁷, both of which are of interest in astrophysical processes, have been studied theoretically using a simple two-body direct radiative capture model in order to estimate the cross sections at low energies. In addition, the time inverse of the first reaction, namely the photodisintegration of He³, has been studied for high excitation energies in He³ by applying the reciprocity relations to the direct capture theory. The calculations involve taking matrix elements of the particle-radiation interaction Hamiltonian between bound and continuum states and using first-order perturbation theory to obtain the cross sections. Bound state wave functions are generated in simple potentials involving square-well and Saxon-Woods forms with appropriate Coulomb barriers and with one free parameter which is adjusted to fit the binding energy. The potential parameters for the continuum state wave functions are adjusted to fit available scattering data. For the reaction O¹⁶(p,⋎)F¹⁷ the cross sections for transitions to both the ground and first excited states are in good agreement with the somewhat limited experimental data from 150 KeV to 2.5 MeV and the astrophysical S-factors are shown to be energy dependent even at energies below 100 KeV. The photodisintegration cross section for the reaction He³(⋎,p)D is well fitted in the neighbourhood of the peak at around 11 MeV as well as at lower energies. The D(p,⋎)He³ direct capture cross sections in the energy range around 1 MeV are shown to be sensitive to admixtures of ²S-state of mixed symmetry and of ⁴D-state in the ground state of He³, which is predominantly Symmetric ²S. The same model including the ²S-state of mixed symmetry leads to a capture cross section for thermal neutrons by deuterons in good agreement with the experimental value. / Science, Faculty of / Physics and Astronomy, Department of / Graduate

No description available.

A striking characteristic of Brigham Young's theology was his inclusion of a cosmology in his teachings. In his speeches as President of the Church of Jesus Christ of Latter-day Saints, Young juxtaposed cosmological pronouncements with practical advice. Young regularly opined on such topics as the eternal nature of matter and light and the interaction of gods and humans with these substances. Dovetailed to his cosmic musings was down-to-earth advice on raising children, avoiding the evils of the gold rush, and controlling one's temper. This paper argues that Young's mingling of the abstruse with the mundane functioned as a theodicy for the nineteenth-century Mormons. In order to justify an omnipotent God's allowance of Mormon suffering and persecution, Young framed God and the human experience within a cosmology. He taught that humans exist with the express purpose of accumulating light and truth. This accumulation would continue after death throughout eternity. Young taught that an ineluctable factor in this progression was suffering and for this reason, Young condoned God's allowance of Mormon hardship. By weaving these cosmological teachings with his pragmatic counsel, Young taught the Latter-day Saints to view their daily lives--full of struggles, pain, and fear--within a cosmological framework. Young believed that such a mindset would bolster the faith of the benighted Mormons.

This thesis contains a brief description of the apparatus, the procedure, and the results of an investigation of the absorption of cosmic rays in thin absorbers. The existence of an anomalous maximum at a thickness of 10.5 cm. of lead is reported, which is tenatively interpreted as being caused by the production of a penetrating ionizing radiation by a neutral radiation through some seemingly unknown process. / Thesis / Master of Science (MS)

Environment plays an important role in the evolution of galaxies. In particular, denser environments, such as galaxy clusters and large-scale field filaments of the cosmic web have been found to reduce star formation in galaxies. The intersection of these environments provides an interesting regime of study. We investigate how cosmic filaments impact the quenching of galaxies within one virial radius of 324 simulated clusters. We use hydrodynamic runs from The Three Hundred Project along with the cosmic web extractor DisPerSE to track filaments and the structure finder VELOCIraptor to identify halos hosting galaxies. Limited by the resolution of the simulation, we examine star formation indirectly by way of galaxy colour and cold gas fraction. We find that cluster galaxies residing closer to filaments tend to be star-forming, bluer, and contain more cold gas than their counterparts further away from filaments. This is in stark contrast with galaxies residing outside of clusters, where galaxies close to filaments show clear signs of density related pre-processing. Careful examination of flows around and into cluster galaxies strongly suggests that the colder, dynamically coherent hydrodynamic streams along intra-cluster filaments partially shield galaxies close to them from strangulation by the hot, dense intra-cluster medium. These streams, in addition to the reduced density contrast of intra-cluster filaments with the intra-cluster medium, also limit the ram pressure stripping experienced by cluster galaxies. We further examine stripping in the context of gas disturbances in phase space to create a classification for wet and dry galaxies. / Thesis / Master of Science (MSc)

A surface search was made using a small-area four-layer scintillator detector. Slow lightly ionizing particles created in cosmic ray interactions in the atmosphere which arrived within 13 ms of the shower front were searched for. In a live time of 2.55 x 10⁵ s 394,608 showers of two or more muons and 21,038 showers of four or more muons were recorded. No multiple muon events were recorded in which a trailing particle was identified above the expected random background, with a velocity between 7.4 x 10⁻⁵c and 6.7 x 10⁻³c. The detector operated without a trigger and used interplanar timing information to identify potential events, and operated above a threshold of 1/400 of that of a minimum ionizing muon. / Ph. D.

The existence of a Fisk-type heliospheric magnetic field (HMF) is one of the most debated questions in cosmic-ray modulation. Recently, Burger and Hitge [2004] developed a divergence-free Fisk-Parker hybrid magnetic field model to demonstrate the behaviour of cosmic rays in the heliosphere due to such a field. This approach has been refined and the properties of the consequent field are investigated. It is found that randomly directed magnetic field diffusion in and above the photosphere significantly influences the solar magnetic field both at the solar poles and near the polar coronal hole boundary. The solar cycle dependence of this field is investigated, a study which is of particular importance for studies of the long-term behaviour of cosmic rays, such as those undertaken at the SANAE base in Antarctica. The amplitudes of the 26-day recurrent cosmic-ray variations are modelled as function of both latitudinal gradient and heliolatitude and are found to agree qualitatively and in some cases quantitatively with the observational results reported by Zhang 119971 and Paizis et al. 119991. Although magnetic field data do not clearly indicate the existence of the Fisk field [see, e.g., Fursyth et al., 20021, this study supports the existence of a Fisk-type HMF. / Thesis (M.Sc. (Physics))--North-West University, Potchefstroom Campus, 2006.

Introdução: Os ligamentos denteados são extensões da pia-máter em cada lado da medula espinhal, totalizando cerca de 20 a 21 pares de estruturas fibrosas que se conectam com a dura-máter, ancorando a medula espinhal. Esses ligamentos são importantes referências anatômicas para cirurgias envolvendo estruturas no interior do canal vertebral e, portanto, muito bem conhecidos por cirurgiões que abordam o canal vertebral. No entanto, não encontramos a descrição deste ligamento em estudos de ressonância magnética (RM). Objetivo: Nosso objetivo foi avaliar a detecção dos ligamentos denteados na coluna cervical por meio de imagens de RM em sequências de aquisição volumétrica e com alta resolução espacial, e comparar a frequência de detecção desses ligamentos nos equipamentos de 1,5 Tesla e 3,0 Tesla. Resultados: Foram avaliados 116 exames de ressonância magnética da coluna cervical, sendo observada alta frequência de detecção dos ligamentos denteados na coluna cervical utilizando sequência volumétrica 3D COSMIC, tanto no equipamento de 1,5 Tesla quanto no equipamento de 3,0 Tesla. Não houve diferença estatisticamente significativa quando comparamos a frequência de detecção dos ligamentos entre os equipamentos de RM de 1,5T e 3,0 T. / Introduction: Denticulate ligaments are extensions of the pia mater on each side of the spinal cord, totaling about 20 to 21 pairs of fibrous structures that connect with the dura mater, anchoring the spinal cord. These ligaments are important anatomical references for surgeries involving structures inside the spinal canal and therefore well known to surgeons who approach the spinal canal. To our knowledge, there is no previous study on the feasibility to detect the denticulate ligaments using MRI. Objective: Our aim was to evaluate the detection of denticulate ligaments in the cervical spine using high resolution magnetic resonance imaging (MRI), and to compare the frequency of detection of these ligaments in 1.5 Tesla and 3.0 Tesla equipment. Results: We evaluated 116 MRI scans of the cervical spine and observed high frequency of detection of the denticulate ligaments in the cervical spine using the volumetric sequence 3D COSMIC on both the 1,5 Tesla and 3,0 Tesla MR equipments. There was no statistically significant difference in the frequency of detection of the ligaments between the MRI equipments.