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Thesis. 1977. M.S.--Massachusetts Institute of Technology. Dept. of Earth and Planetary Sciences. / Microfiche copy available in Archives and Science. / Bibliography : leaves 72-73. / M.S.

<p dir="ltr"> This article-based dissertation provides an account of two distinct classes of expansive astrophysical outflows and techniques to interpret their observations using numerical modeling. The primary purpose of this dissertation is to provide an extensive description of the research projects I undertook during my tenure as a Graduate Research Assistant, under the guidance of my advisor Prof. Paul Duffell.</p><p dir="ltr">Chapter 1 provides a brief introduction to numerical hydrodynamics and techniques of modeling expanding flows numerically. I also introduce the aforementioned classes of astrophysical outflows, namely relativistic jets from Active Galactic Nuclei (AGN), and supernova remnants (SNRs). I provide a general overview of the theoretical picture, and the general strategy used in this work to model them.</p><p dir="ltr">Chapter 2 describes my investigation on the connection of kiloparsec scale AGN jet properties to their intrinsic parameters and surroundings, based on an article published in The Astrophysical Journal. Using a suite of over 40 relativistic hydrodynamic jet models, we find that the dynamics of relativistic jets can be described in terms of only two parameters, the jet to ambient medium energy density ratio, and the jet opening angle. The former is found to strongly control the Fanaroff-Riley (FR) morphological dichotomy, which was previously thought to be tied to the magnitude of the jet luminosity. We also suggest a purely hydrodynamical origin of bright spots observed in some AGN jets. Our models were tested against and found to be consistent with the observations of the jets in M87 and Cygnus A.</p><p dir="ltr">In chapter 3, I present my moving-mesh hydrodynamics code Sprout, also described in an article published in The Astrophysical Journal Supplements. Sprout solves the equations of ideal hydrodynamics on an expanding Cartesian mesh. The expanding mesh can follow fluid outflows for several orders of magnitude with very little numerical diffusion. This allows Sprout to capture expanding flows with very high dynamic range. Sprout is thus particularly suitable for studying expanding outflows such as supernova remnants and active galactic nuclei. Relative to other moving mesh codes, the simple mesh structure in Sprout is also convenient for implementing additional physics or algorithms. I discuss many code tests that were performed to test the accuracy and performance of the numerical scheme.</p><p dir="ltr">Chapter 4 details my study of hydrodynamic instabilities in supernova remnants (SNRs) as they expand against the circumstellar medium (CSM). This is based on an article published in The Astrophysical Journal. A suite of 3D hydrodynamical SNR models, generated using my hydro code \sprout, was used to study the impact of the stellar ejecta density profile and seed anisotropies in the ejecta and the CSM on formation of turbulent structures in the SNRs. We found that most of the turbulent power in these models resides at a typical angular mode or scale that is determined by the ejecta density structure. It was also found that clumps or anisotropies in either the ejecta or CSM do not imprint upon these turbulence structures unless they are massive and form large-scale coherent structures.</p><p dir="ltr">In chapter 5, I discuss the implementation of a technique to measure anisotropies in observed SNRs just using 2D high-resolution images. This technique is calibrated using 3D hydro SNR models and synthetic images derived from them. As seen in Chapter 4, we find a similar dominant angular scale of turbulent structures dictated by the ejecta density structure. Both the 3D models and the synthetic images yield the same value of this scale, which validates the image analysis technique used in this work. As an example of how this technique can be applied to observations, we analyze observations of a known supernova remnant (Tycho's SNR) and compare with our models. Our technique picks out the angular scale of Tycho's fleece-like structures and also agrees with the small-scale power seen in Tycho.</p><p dir="ltr">PhChapter 6 summarizes the results, conclusions, and future prospects of all the research work described so far. It is followed by a bibliography, my curriculum vita, and a list of publications.</p>

Deivisceris is a four-player role-playing tabletop game that focuses on themes of horror. It looks into ideas from the horror genre as a whole while combining aspects from the body horror and cosmic horror subgenres to create a discomforting horror experience. The game features illustrations and written events with a choice-based narrative that can have multiple outcomes depending on a player's decisions, stats, and items. Deivisceris utilizes randomness in order to create a new experience each time it is played through randomized characters and a randomized game board that is built up as it is played. The game reveals its narrative through clues within the gameplay, illustrations, and written text as characters enter the game's world blindly. Deivisceris is an immersive tabletop horror experience that can be further expanded on in the future with the possibility of a larger production. / Master of Fine Arts / Deivisceris is a four-player tabletop game that looks into the ways horror can be created in a board game format. It examines various ideas from the horror genre as a whole while taking inspiration more directly from two subgenres of horror: body horror and cosmic horror, each of which has very different ways of evoking horror. The game includes a variety of full-color illustrations and written situations that give players a chance to make their own decisions. Deivisceris utilizes randomness in order to create a new experience every time it is played. The game board is built up differently every time it is played and characters' stats, such as strength, intelligence, and endurance, may be different in each game. The game's story is revealed through clues within the gameplay, illustrations, and text. Deivisceris is a tabletop role-playing horror experience that can be further expanded on in the future with the possibility of a larger production.

With the start of NASA’s Artemis program, astronauts will soon leave the protection of Earth’s magnetosphere. Previous studies have established a theme of liver dysregulation associated with the harmful radiation and microgravity experienced in deep space. MicroRNAs (miRNAs) are key regulators of many of the pathologies observed during and after spaceflight, including hepatic fibrosis, making them strong potential biomarkers and therapeutic targets. Therefore, in the present study we investigated the effects of simulated microgravity, galactic cosmic radiation (GCR), and solar particle events (SPE) on miRNAs and mRNAs involved in the TGF-β signaling pathway, a key regulator of fibrosis, senescence, and inflammation in the liver. We also analyzed the ability of an antagomir treatment which targets miR-16-5p, miR-125b-5p, and let-7a-5p to modulate these spaceflight-associated changes in liver health. Our results indicate that the observed miRNA profile widely varied depending on whether GCR or SPE irradiation were administered. Likewise, while the antagomir treatment was successful at downregulating expression of its target mRNAs, the type of radiation received greatly influenced whether the miRNA shifts observed correlated with changes in downstream gene expression; the GCR group was the most effected and the SPE the least in terms of both miRNA and mRNA expression. Overall, the current study highlights the profound effects of cosmic radiation on the hepatic miRNA profile and confirms the potential of antagomir treatment as a therapeutic for spaceflight induced changes in expression.

Silicates analogous to cosmic dust were synthesized, modified, and analyzed utilizing ion-beam techniques with Rutherford backscattering spectrometry (RBS) and x-ray diffraction (XRD). Silicate dust is a common constituent in interstellar space, with an estimated 50% of dust produced in the stellar winds of M class Asymptotic Giant Branch (AGB) stars. Silicate dust acts as a surface upon which other chemicals may form (water ice for example), increasing significance in the cosmochemistry field, as well as laboratory astrophysics. Silicate formation in the stellar winds of AGB stars was simulated in the laboratory environment. Three sequential ion implantations of Fe-, MgH2-, and O- with thermal annealing were used to synthesize a mixture appropriate to silicate dust in the surface layers of a p-type Si substrate. Post implantation He+ irradiation was shown to preferentially induce crystalline formation in the analogue prior to thermal annealing. This effect is believed to originate in the ion-electron interaction in the Si substrate. The effects of ionization and ion energy loss due to electronic stopping forces is believed to precipitate nucleation in the amorphous media. For annealing temperatures of 1273 K, predominant quartz formation was found in the substrate, whereas lower annealing temperatures of 1000 K formed enstatite without post-implantation He+ irradiation, and olivine with He+ irradiation. Post annealed crystalline phase modification was investigated via x-ray diffraction and elemental compositions were investigated utilizing RBS. Finally, the interdiffusion of Fe and Mg at temperatures of 900-1100 K was investigated with RBS, and activation energies for interdiffusion were extracted for the transition from amorphous to crystalline phase in the silicate analogues. Fe had an interdiffusion energy of 1.8 eV and Mg 1.5eV. The produced analogues have similar properties to those inferred from infrared spectroscopy of the stellar winds of M-class AGB stars with an oxygen-rich outflow. This work established a method of silicate production using ion beam modifications, explored He+ irradiation effects in the annealed structures, and derived interdiffusion activation energies for Fe and Mg in the amorphous structure. Grain sizes were <100 nm with the observed formation of quartz, enstatite, and olivine.

O conhecimento sobre a composição de raios cósmicos de altas energias é fundamental na abordagem da maior parte das grandes questões referentes à astrofísica de altas energias. Entretanto, do ponto de vista experimental, determinar o tipo de partícula medida nesse regime de energia ainda é um enorme desafio e essa tarefa tem recebido especial atenção por parte das colaborações responsáveis pelos experimentos em atividade. A principal dificuldade está no fato das medidas serem realizadas indiretamente através das cascatas de partículas formadas a partir da interação do raio cósmico inicial com átomos da atmosfera, os chamados chuveiros atmosféricos. Entre os principais experimentos em funcionamento, o Observatório Pierre Auger se destaca por ter a maior área de detecção (3000 km2) e por utilizar pioneiramente um sistema híbrido de detecção, com detectores de superfície e de fluorescência funcionando simultaneamente. Os telescópios de fluorescência medem o número de partículas do chuveiro em função da profundidade, o que chamamos de perfil longitudinal. Alguns parâmetros extraídos desse perfil são sensíveis à composição das partículas primárias. No presente trabalho, aplicamos métodos estatísticos novos aos dados extraídos dos perfis longitudinais de chuveiros medidos pelo Auger com o objetivo de inferir informações sobre a massa média, ou seja, a composição dos raios cósmicos. A primeira análise apresentada é baseada no parâmetro XMax. A evolução do valor médio de XMax com a energia contém informações sobre a composição inicial. Com o objetivo de eliminar vieses experimentais, corrigindo os efeitos dos detectores, aplicamos métodos de deconvolução às distribuições de XMax. A segunda análise é do tipo multiparamétrica e aplica redes neurais do tipo Multilayer Perceptrons a outros parâmetros extraídos dos perfis longitudinais. A partir desse procedimento é possível obter informações sobre a composição média das partículas e também reconstruir a energia dos eventos. / The knowledge about high energy cosmic rays composition is fundamental to approach most of the big questions regarding high energy astrophysics. However, from the experimental point of view, to determine the kind of the measured particle in this energy range is still a huge challenge and this task has received special attention from the collaborations responsible for running the experiments in activity. The main difficulty is on the fact that the measurements are made indirectly by the secondary particles cascades formed by the interaction of primary particles with atmosphere atoms, which are called air showers. Among the main experiments in operation, Pierre Auger Observatory has the larger collecting area (3000 km2) and uses a pioneer hybrid detection system, with surface detectors and fluorescence telescopes working simultaneously. The fluorescence telescopes measure the number of particles in the shower as a function of atmospheric depth, which we call longitudinal profiles. Some parameters extracted from these profiles are sensitive to primary composition. In this study, we applied new statistical methods to the data from longitudinal profiles measured by the Pierre Auger Observatory aiming to infer information about the mean mass, in other words, the composition of cosmic rays. The first analysis shown (chapter 4) is based on the known parameter called XMax. The evolution of XMax mean value with energy contains information about primary composition. Unfolding methods have been applied to the XMax distribution in order to minimize experimental bias and to correct detector effects. The second analysis shown is of the multi-parametric type and applies neural networks of the Multilayer Perceptrons class to longitudinal profiles parameters. From this procedure, it is possible to obtain information about average composition and to reconstruct the energy of events.

O conhecimento sobre a composição de raios cósmicos de altas energias é fundamental na abordagem da maior parte das grandes questões referentes à astrofísica de altas energias. Entretanto, do ponto de vista experimental, determinar o tipo de partícula medida nesse regime de energia ainda é um enorme desafio e essa tarefa tem recebido especial atenção por parte das colaborações responsáveis pelos experimentos em atividade. A principal dificuldade está no fato das medidas serem realizadas indiretamente através das cascatas de partículas formadas a partir da interação do raio cósmico inicial com átomos da atmosfera, os chamados chuveiros atmosféricos. Entre os principais experimentos em funcionamento, o Observatório Pierre Auger se destaca por ter a maior área de detecção (3000 km2) e por utilizar pioneiramente um sistema híbrido de detecção, com detectores de superfície e de fluorescência funcionando simultaneamente. Os telescópios de fluorescência medem o número de partículas do chuveiro em função da profundidade, o que chamamos de perfil longitudinal. Alguns parâmetros extraídos desse perfil são sensíveis à composição das partículas primárias. No presente trabalho, aplicamos métodos estatísticos novos aos dados extraídos dos perfis longitudinais de chuveiros medidos pelo Auger com o objetivo de inferir informações sobre a massa média, ou seja, a composição dos raios cósmicos. A primeira análise apresentada é baseada no parâmetro XMax. A evolução do valor médio de XMax com a energia contém informações sobre a composição inicial. Com o objetivo de eliminar vieses experimentais, corrigindo os efeitos dos detectores, aplicamos métodos de deconvolução às distribuições de XMax. A segunda análise é do tipo multiparamétrica e aplica redes neurais do tipo Multilayer Perceptrons a outros parâmetros extraídos dos perfis longitudinais. A partir desse procedimento é possível obter informações sobre a composição média das partículas e também reconstruir a energia dos eventos. / The knowledge about high energy cosmic rays composition is fundamental to approach most of the big questions regarding high energy astrophysics. However, from the experimental point of view, to determine the kind of the measured particle in this energy range is still a huge challenge and this task has received special attention from the collaborations responsible for running the experiments in activity. The main difficulty is on the fact that the measurements are made indirectly by the secondary particles cascades formed by the interaction of primary particles with atmosphere atoms, which are called air showers. Among the main experiments in operation, Pierre Auger Observatory has the larger collecting area (3000 km2) and uses a pioneer hybrid detection system, with surface detectors and fluorescence telescopes working simultaneously. The fluorescence telescopes measure the number of particles in the shower as a function of atmospheric depth, which we call longitudinal profiles. Some parameters extracted from these profiles are sensitive to primary composition. In this study, we applied new statistical methods to the data from longitudinal profiles measured by the Pierre Auger Observatory aiming to infer information about the mean mass, in other words, the composition of cosmic rays. The first analysis shown (chapter 4) is based on the known parameter called XMax. The evolution of XMax mean value with energy contains information about primary composition. Unfolding methods have been applied to the XMax distribution in order to minimize experimental bias and to correct detector effects. The second analysis shown is of the multi-parametric type and applies neural networks of the Multilayer Perceptrons class to longitudinal profiles parameters. From this procedure, it is possible to obtain information about average composition and to reconstruct the energy of events.

This thesis consists of three articles investigating the asymptotic behaviour of cosmological spacetimes with symmetries arising in Mathematical General Relativity. In Paper A and B, we consider spacetimes with Bianchi symmetry and where the matter model is that of a perfect fluid. We investigate the behaviour of such spacetimes close to the initial singularity ('Big Bang'). In Paper A, we prove that the Strong Cosmic Censorship conjecture holds in non-exceptional Bianchi class B spacetimes. Using expansion-normalised variables, we further show detailed asymptotic estimates. In Paper B, we prove similar estimates in the case of stiff fluids. In Paper C, we consider T2-symmetric spacetimes satisfying the Einstein equations for a non-linear scalar field. To given initial data, we show global existence and uniqueness of solutions to the corresponding differential equations for all future times. In the special case of a constant potential, a setting which is equivalent to a linear scalar field on a background with a positive cosmological constant, we investigate in detail the asymptotic behaviour towards the future. We prove that the Cosmic No-Hair conjecture holds for solutions satisfying an additional a priori estimate, an estimate which we show to hold in T3-Gowdy symmetry. / Denna avhandling består av tre artiklar som undersöker det asymptotiska beteendet hos kosmologiska rumstider med symmetrier som uppstår i Matematisk Allmän Relativitetsteori. I Artikel A och B studerar vi rumstider med Bianchi symmetri och där materiemodellen är en ideal fluid. Vi undersöker beteendet av sådana rumstider nära ursprungssingulariteten ('Big Bang'). I Artikel A bevisar vi att den Starka Kosmiska Censur-förmodan håller för icke-exceptionella Bianchi klass B-rumstider. Med hjälp av expansions-normaliserade variabler visar vi detaljerade asymptotiska uppskattningar. I Artikel B visar vi liknande uppskattningar för stela fluider. I Artikel C betraktar vi T2-symmetriska rumstider som uppfyller Einsteins ekvationer för ett icke-linjärt skalärfält. För givna begynnelsedata visar vi global existens och entydighet av lösningar till motsvarande differentialekvationer för all framtid. I det speciella fallet med en konstant potential, en situation som motsvarar ett linjärt skalärfält på en bakgrund med en positiv kosmologisk konstant, undersöker vi i detalj det asymptotiska beteendet mot framtiden. Vi visar att den Kosmiska Inget-Hår-förmodan håller för lösningar som uppfyller en ytterligare a priori uppskattning, en uppskattning som vi visar gäller i T3-Gowdy-symmetri. / <p>QC 20171220</p>

L'expansion accélérée de l'univers est devenu un fait établi que personne ne pouvait prévoir il y a encore une vingtaine d'années. Pour expliquer l'accélération cosmique, l'univers doit être composé de $75%$ d'énergie noire, une matière hypothétique à pression négative. Une alternative aussi vertigineuse consiste à modifier la relativité générale d'Einstein à l'échelle cosmique.Mes travaux de thèse portent sur la contrainte des modèles d'énergie noire et de gravité modifiée avec les données observationnelles provenant de la croissance linéaire des structures cosmologiques. Une méthode basée sur une nouvelle paramétrisation de l'index de croissance des perturbations linéaires cosmologiques permet d'analyser un grand nombre de modèles "accélératoires" en même temps. Nous avons évalué et validé cette méthode par une analyse systématique de sa précision et de sa performance. Mes résultats montrent que le modèle standard de la cosmologie (le modèle $Lambda$CDM) reste en accord avec les données actuelles. Dans une étude approfondie, nous simulons les contraintes possibles avec les futures sondes cosmologiques de "précision" comme Euclid. Pour analyser encore plus de modèles en même temps, nous introduisons la théorie effective des champs de l'énergie noire (EFT) dans le formalisme développé auparavant. La EFT est un formalisme prometteur qui permet d'explorer d'une manière complète tous les modèles gravitationnels non-standards résultant de l'addition d'un degré de liberté supplémentaire dans l'équation d'Einstein. Nous proposons une paramétrisation de cette théorie que nous confrontons avec les données actuelles et futures. / The accelerated expansion of the universe has become an established fact that nobody could foresee until twenty years ago. To explain the cosmic acceleration, the universe must be composed by $75%$ of dark energy, a hypothetical form of matter with negative pressure. Alternatively, Einstein's field equation must be modified on cosmic scales. During my thesis I have worked on the constraint of dark energy and modified gravity models with data coming from the observed growth rate of cosmic structures. We have introduced a method based on a new parametrization of the growth index of linear cosmological perturbations. An advantage is the possibility of a concurrent analysis of multiple accelerating models. We have evaluated and validated the method in a systematic precision and performance check. My results show that the standard model of cosmology (the $Lambda$CDM model) remains consistent with current data. In an ongoing study, we have simulated future constraints for upcoming cosmological 'precision' probes like Euclid.In a second step, we introduce the effective field theory of dark energy (EFT) into our formalism. The EFT is a promising framework that allows to explore in a complete way all non-standard gravitational models that result from adding one degree of freedom in Einstein's field equation. Another advantage is its neat split of background and perturbation observables. We propose a parametrization of the EFT that we confront with current and simulated future constraints.