Thesis (Ph. D.)--University of Texas at Austin, 2006. / Vita. Includes bibliographical references.

Thesis (Ph. D.)--University of Texas at Austin, 2008. / Vita. Includes bibliographical references.

abstract: The Cosmic Microwave Background (CMB) has provided precise information on the evolution of the Universe and the current cosmological paradigm. The CMB has not yet provided definitive information on the origin and strength of any primordial magnetic fields or how they affect the presence of magnetic fields observed throughout the cosmos. This work outlines an alternative method to investigating and identifying the presence of cosmic magnetic fields. This method searches for Faraday Rotation (FR) and specifically uses polarized CMB photons as back-light. I find that current generation CMB experiments may be not sensitive enough to detect FR but next generation experiments should be able to make highly significant detections. Identifying FR with the CMB will provide information on the component of magnetic fields along the line of sight of observation. The 21cm emission from the hyperfine splitting of neutral Hydrogen in the early universe is predicted to provide precise information about the formation and evolution of cosmic structure, complementing the wealth of knowledge gained from the CMB. 21cm cosmology is a relatively new field, and precise measurements of the Epoch of Reionization (EoR) have not yet been achieved. In this work I present 2σ upper limits on the power spectrum of 21cm fluctuations (Δ²(k)) probed at the cosmological wave number k from the Donald C. Backer Precision Array for Probing the Epoch of Reionization (PAPER) 64 element deployment. I find upper limits on Δ²(k) in the range 0.3 < k < 0.6 h/Mpc to be (650 mK)², (450 mK)², (390 mK)², (250 mK)², (280mK)², (250 mK)² at redshifts z = 10.87, 9.93, 8.91, 8.37, 8.13 and 7.48 respectively Building on the power spectrum analysis, I identify a major limiting factor in detecting the 21cm power spectrum. This work is concluded by outlining a metric to evaluate the predisposition of redshifted 21cm interferometers to foreground contamination in power spectrum estimation. This will help inform the construction of future arrays and enable high fidelity imaging and cross-correlation analysis with other high redshift cosmic probes like the CMB and other upcoming all sky surveys. I find future arrays with uniform (u,v) coverage and small spectral evolution of their response in the (u,v,f) cube can minimize foreground leakage while pursuing 21cm imaging. / Dissertation/Thesis / Doctoral Dissertation Physics 2018

Neste trabalho realizamos cálculos teóricos das razões de povoamento dos níveis de estrutura fina do C0, C+ e Si+ considerando em detalhes os efeitos dos diversos mecanismos de excitação: colisões, fluorescência e a radiação cósmica de fundo. Empregamos dados atômicos recentes coletados na literatura na resolução das equações de equilíbrio estatístico pertinentes. A confrontação das razões de povoamento calculadas com as razões de densidades de coluna observadas disponíveis na literatura nos permite obter informações acerca das condições físicas (densidades volumétricas, intensidade de um campo de radiação UV presente, temperatura da radiação cósmica de fundo) e propriedades (dimensão característica e massa) dos sistemas damped Lyman a e Lyman Limit vistos em absorção no espectro de QSOs. Como um aparte, e por sua relação com o tema do trabalho, também investigamos a lei de temperatura da radiação cósmica de fundo. / In this work we perform theoretical calculations of the population ratios of fine structure levels of C0, C+ and Si+ considering in detail the effect of the various excitation mechanisms: collisions, fluorescence and the cosmic background radiation. We employ recent atomic data, gathered in the literature, to solve the related statistical equilibrium equations. The comparison of the calculated population ratios with the observed column densities ratios available in the literature allows us to obtain informations regarding the physical conditions (volume densities, intensity of a UV radiation field present, temperature of the cosmic background radiation) and properties (characteristic size and mass) of damped Lyman a and Lyman Limit systems seen in absorption in spectra of QSOs. We also investigate the temperature law of the cosmic background radiation, which bears a tight relationship with this work.

The C-Band All-Sky Survey (C-BASS) is a project to produce an all-sky map in intensity and polarization at a central frequency of 5 GHz with 1 GHz bandwidth and approximately 1 degree resolution. The central frequency is low enough for the map to be dominated by synchrotron and free-free emission but high enough so that Faraday rotation and depolarization are small across most of the sky. The C-BASS map will enable a more accurate removal of contaminating foregrounds from measurements of the cosmic microwave background, particularly in polarization where the B-mode signal from inflation is likely to be orders of magnitude weaker than the diffuse Galactic foreground emission. To produce an all-sky map from the ground requires two telescopes, one in the northern and one in the southern hemisphere. This thesis focuses on analysis of C-BASS North data. The noise properties of time-ordered data are characterised by fitting a noise model to periodograms. Using simulations, the errors introduced into the C-BASS maps by a destriping mapmaker are quantified and we reduce the signal error by masking the brightest pixels during baseline offset estimation. Jackknife tests are used to test the C-BASS data for systematics and to test the accuracy of the sensitivity maps. In total intensity, the spectral index of diffuse Galactic emission between 5 GHz and 408 MHz is measured using an extended T-T plot method and the results are compared to simulations. The spectral index of polarized diffuse Galactic emission between 5 GHz and 30 GHz is estimated in 55 arcminute pixels, modelling the polarized intensity as a Rician random variable.

Le rayonnement fossile, ou CMB, est un sujet d’étude clé pour la cosmologie car il indique l’état de l’univers à une époque primordiale. Le CMB est observable dans le ciel dans la bande de fréquences des micro-ondes. Cependant, il existe des processus astrophysiques, les avant-plans, qui émettent dans les micro-ondes, et rendent indispensable le traitement des données avec des méthodes de séparation de composantes. J'utilisé la méthode aveugle SMICA pour obtenir une estimation directe du spectre de puissance angulaire du CMB. La détermination des petites échelles de ce spectre est limité par les avant-plans comme les galaxies lointaines, et par le biais du bruit. Dans cette analyse, ces deux limitations sont abordées. En ajoutant des hypothèses sur la physique des galaxies lointaines, il est possible de guider l’algorithme pour estimer leur loi d'émission. Un spectre de puissance angulaire obtenu d'une carte du ciel a un biais dû au bruit à petites échelles. Toutefois, les spectres obtenus en croisant différentes cartes n'ont pas ce biais. J'ai donc adapté la méthode SMICA pour qu'elle n'utilise que ces derniers, diminuant l'incertitude due au bruit dans l'estimation du CMB. Cette méthode a été étudiée sur des nombreuses simulations et sur les données Planck 2015, afin d'estimer des paramètres cosmologiques. Les résultats montrent que la contamination résiduelle des avant-plans présente dans le spectre CMB, même si fortement réduite, peut introduire des biais dans l'estimation des paramètres si la forme des résiduels n'est pas bien connue. Dans cette thèse, je montre les résultats obtenus en étudiant un modèle de gravité modifiée. / The Planck satellite observed the whole sky at various frequencies in the microwave range. These data are of high value to cosmology, since they help understanding the primordial universe through the observation of the cosmic microwave background (CMB) signal. To extract the CMB information, astrophysical foreground emissions need to be removed via component separation techniques. In this work I use the blind component separation method SMICA to estimate the CMB angular power spectrum with the aim of using it for the estimation of cosmological parameters. In order to do so, small scales limitations as the residual contamination of unresolved point sources and the noise need to be addressed. In particular, the point sources are modelled as two independent populations with a flat angular power spectrum: by adding this information, the SMICA method is able to recover the joint emission law of point sources. Auto-spectra deriving from one sky map have a noise bias at small scales, while cross-spectra show no such bias. This is particularly true in the case of cross-spectra between data-splits, corresponding to sky maps with the same astrophysical content but different noise properties. I thus adapt SMICA to use data-split cross-spectra only. The obtained CMB spectra from simulations and Planck 2015 data are used to estimate cosmological parameters. Results show that this estimation can be biased if the shape of the (weak) foreground residuals in the angular power spectrum is not well known. In the end, I also present results of the study of a Modified Gravity model called Induced Gravity.

Thesis (M.Med. (Diagnostic Radiology)) -- University of Limpopo, 2022 / BACKGROUND: Ionizing radiation is detrimental to growing cells. The potential risk of any dose of radiation in growing cells can lead to permanent damage of basic cellular structure resulting in a high risk of developing cancer in children. Therefore, paediatric doctors need sufficient knowledge to protect their patients from late effects of radiation resulting from medical use. Most studies report poor knowledge of radiation safety measures among doctors. PURPOSE: The purpose of this study is to assess the knowledge of radiation safety and radiation doses among paediatric doctors in Pietersburg and Mankweng Hospitals, Limpopo province, South Africa. OBJECTIVE: To assess the paediatric doctors’ knowledge of radiation doses used in radiological diagnostic imaging examinations, as well as their knowledge of radiation safety measures. METHODOLOGY: This is a survey of paediatric doctors in the paediatrics general ward, paediatric oncology, intensive care and neonatal units, and paediatric surgery department, at Pietersburg and Mankweng Hospitals. A self-administered questionnaire with 23 items on knowledge of radiation safety and radiation doses emitted during normal radiological examinations is used to collect the data in May 2021. Data are analysed using Statistical Package for Social Sciences (SSPS) version 26.0 software. Chi-squared test is used to analyse the relationship between variables. Analysis of variance (ANOVA) is used to analyse the differences between variables. A p value of <0.05 is considered statistically significant. A total score of 50% in each section is used to denote adequate knowledge. The result are presented in charts and tables. RESULTS: Out of 52 paediatric doctors, 47 completed the questionnaire achieving a 90.4% response rate. Majority of participants were females (n = 31; 66%). Overall, only 10 (21.2%) of the participants scored 50% and above, including three consultants, two registrars, two medical officers and one medical officer intern. Twenty-five (53.1%) participants scored 50% and above on knowledge of radiation safety. Nearly half (44.7%) of the doctors were familiar with the concept of ALARA principle in radiation dose optimisation. Three participants scored more than 50% in the knowledge of radiation doses section. Only five (10.6%) and 17 (36%) participants correctly identified MRI and ultrasound, respectively, as the radiological diagnostic modalities that do not utilize ionizing radiation. CONCUSION: The level of knowledge of radiation safety and radiation doses among the paediatric doctors are poor. A very small number of the paediatric doctors were able to identify non-ionizing radiation modalities that can be used as an alternative to reduce radiation exposure to paediatrics during radiological investigations. Paediatric doctors will benefit from courses on radiation safety and radiation doses to improve their knowledge and/or eliminate unwarranted exposure of their patients to ionizing radiation.

The Cosmic Microwave Background (CMB) serves as a powerful backlight, illuminating thestructures throughout the universe. As the CMB photons travel to our telescopes from the surface of last scattering, their interactions with matter imprint detectable signatures in the CMB spectrum, known as the CMB secondary anisotropies. Among these late-time phenomena, the Sunyaev–Zel’dovich (SZ) effect—caused by scattering of the CMB photons off free electrons—is one of the most powerful, providing a unique window into the pressure and density of the electron gas. As the ionized gas and its feedback on the underlying dark matter distribution via high-energy processes present a significant obstacle to obtain precise cosmological constraints from the matter power spectrum, the SZ effect serves as an invaluable tool to address these challenges. This thesis uses the measurements of the CMB secondary anisotropies, particularly the SZ effect, from the state-of-the-art experiments, the Planck satellite and the ground-based Atacama Cosmology Telescope (ACT), in combination with Large-Scale Structure data to probe the ionized gas in distant galaxies. Chapter 2 presents the second measurement of the kinetic SZ effect in the unWISE galaxies with Planck using the projected-fields estimator. This work concludes that the ionized gas abundance in these galaxies matches the primordial-CMB predictions. Chapter 3 describes the work done to model the galaxy-halo connection of the unWISE catalog with Planck CMB lensing data using the halo model framework. It constrained the halo masses of these samples to ≈ 2 ×10¹³ _⊙/ℎ, and found that they are dominated by central galaxies, rather than satellites. These constraints can be directly used in other cross-correlations of unWISE with, e.g., the tSZ or the kSZ effect in the halo model with the upcoming CMB experiments. Chapter 5 discusses the ongoing work of cross-correlating the Dark Energy Survey Maglim galaxies with the thermal SZ maps from ACT. It measures very extended pressure profiles around Maglim, which suggest strong feedback activity in low mass objects, pushing the ionized gas far outside of the halo. This thesis also presents novel techniques to tackle the key systematics in cosmological cross-correlations. The analysis of Maglim galaxies employs the new Cosmic Infrared Background (CIB) cleaning technique, the moment-deprojection method, which ensures that the measurement is robust to this foreground. Chapter 4 discusses three new methods to remove the CIB and tSZ contamination, using the external Large-Scale Structure data which show a large correlation with both fields (e.g., the unWISE catalog). With the new methods presented, it is possible to remove those contaminants to enhance the measurements of the blackbody component of a CMB map. The results presented in this thesis offer a unique window into the baryons residing in distant galaxies through the SZ effect, confirming there is no missing baryons, and indicating that the feedback is stronger than predicted in simulations. These analyses lay the groundwork for cross-correlations of the upcoming high-resolution, low-noise CMB experiments such as the Simons Observatory, and high density galaxy surveys, including DESI, Euclid, or LSST. The upcoming measurements will yield precise constraints on gas physics, transforming our understanding of galaxy formation, and enabling cosmological constraints from the matter power spectrum, where baryons currently represent the primary uncertainty.

Observations from the 1930s until the present have established the existence of dark matter with an abundance that is much larger than that of luminous matter. Because none of the known particles of nature have the correct properties to be identified as the dark matter, various exotic candidates have been proposed. The neutralino of supersymmetric theories is the most promising example. Such cold dark matter candidates, however, lead to a conflict between the standard simulations of the evolution of cosmic structure and observations. Simulations predict excessive structure formation on small scales, including density cusps at the centers of galaxies, that is not observed. This conflict still persists in early 2007, and it has not yet been convincingly resolved by attempted explanations that invoke astrophysical phenomena, which would destroy or broaden all small scale structure. We have investigated another candidate that is perhaps more exotic: Lorentz-violating dark matter, which was originally motivated by an unconventional fundamental theory, but which in this dissertation is defined as matter which has a nonzero minimum velocity. Furthermore, the present investigation evolved into the broader goal of exploring the properties of Lorentz-violating matter and the astrophysical consequences – a subject which to our knowledge has not been previously studied. Our preliminary investigations indicated that this form of matter might have less tendency to form small-scale structure. These preliminary calculations certainly established that Lorentz-violating matter which always moves at an appreciable fraction of the speed of light will bind less strongly. However, the much more thorough set of studies reported here lead to the conclusion that, although the binding energy is reduced, the small-scale structure problem is not solved by Lorentz-violating dark matter. On the other hand, when we compare the predictions of Lorentz-violating dynamics with those of classical special relativity and general relativity, we find that differences might be observable in the orbital motions of galaxies in a cluster. For example, galaxies – which are composed almost entirely of dark matter – observed to have enlarged orbits about the cluster center of mass may be an indication of Lorentz violation.

We calculate the angular power spectrum of the cosmic microwave background temperature fluctuations induced by the kinetic Sunyaev-Zel'dovich (kSZ) effect from the epoch of reionization (EOR). We use detailed N-body+radiative-transfer simulations to follow inhomogeneous reionization of the intergalactic medium. For the first time, we take into account the "self-regulation" of reionization: star formation in low-mass dwarf galaxies or minihalos is suppressed if these halos form in the regions that were already ionized or Lyman-Werner dissociated. Some previous work suggested that the amplitude of the kSZ power spectrum from the EOR can be described by a two-parameter family: the epoch of half-ionization and the duration of reionization. However, we argue that this picture applies only to simple forms of the reionization history which are roughly symmetric about the half-ionization epoch. In self-regulated reionization, the universe begins to be ionized early, maintains a low level of ionization for an extended period, and then finishes reionization as soon as high-mass atomically cooling halos dominate. While inclusion of self-regulation affects the amplitude of the kSZ power spectrum only modestly (~10%), it can change the duration of reionization by a factor of more than two. We conclude that the simple two-parameter family does not capture the effect of a physical, yet complex, reionization history caused by self-regulation. When added to the post-reionization kSZ contribution, our prediction for the total kSZ power spectrum is below the current upper bound from the South Pole Telescope. Therefore, the current upper bound on the kSZ effect from the EOR is consistent with our understanding of the physics of reionization. / text