Tesi sul tema "Cosmology"

Els cartografiats de galàxies fotomètrics actuals i futurs observaran un gran volum de l’univers que ens permetrà acotar amb precisió el model cosmològic. Tanmateix, la capacitat de delimitar el model que tenen aquests cartografiats a través de les sondes cosmològiques depèn de la precisió i certesa amb les que es determina el redshift de les galàxies. Per tant, la determinació del redshift fotomètric i els seus efectes en les anàlisis cosmològiques han de ser tractats i estudiats curosament. En la primera part de la tesi, transformem la fotometria de simulacions que ja existeixen per imitar les mesures fotomètriques del Dark Energy Survey (DES). D’aquesta manera, esperem recuperar la distribució real del redshift fotomètric en simulacions, i així crear una base encara més realista per comprovar els resultats de les anàlisis cosmològiques de DES que fan us del redshift fotomètric. Per transformar les simulacions utilitzem diversos mètodes que transfereixen les propietats estadístiques de la fotometria d’observacions real a les simulacions. A la segona part de la tesi, utilitzem la tècnica del Self-Organizing Map per seleccionar galàxies per ser observades amb espectroscòpia, contribuint així al projecte C3R2 que vol establir un mapa correlacional entre l’espai de colors i redshift i omplir-ho amb informació espectroscòpica. En aquesta part també explorem l’espai de colors definit per la fotometria del Physics of the Accelerating Universe Survey (PAUS) per tal d’estudiar la cobertura del redshift espectroscòpic del seu espai de colors. Volem determinar la quantitat d’espai de color sense cobertura espectroscòpica perquè la falta de representació d’espectroscòpia pot ser una font de biaix quan la precisió dels redshifts fotomètrics és avaluada comparant-los amb redshifts espectroscòpics o quan els redshifts espectroscòpics s’utilitzen com a mostra d’entrenament per determinar els redshifts fotomètrics amb algoritmes d’entrenament. Finalment, explorem com la variació en la profunditat de les observacions des de terra combinades amb les d’Euclid afecta la precisió dels redshifts fotomètrics i, per tant, la capacitat de determinar els paràmetres cosmològics d’Euclid sobretot quan utilitza galaxy clustering i galaxy-galaxy lensing com a sondes cosmològiques. També estudiem com la densitat de les mostres de galàxies afecta la capacitat de delimitar els paràmetres cosmològics i quina és la configuració de bins tomogràfics de redshift que permet extreure la màxima informació per delimitar els paràmetres cosmològics. Per tal de dur a terme aquesta anàlisi, creem diverses distribucions realistes de redshift fotomètric basades en la simulació Flagship d’Euclid i utilitzem el formalisme de Fisher per fer una estimació de la capacitat d’acotament dels paràmetres cosmològics de les diferents configuracions de les mostres de galàxies.
Los cartografiados de galaxias fotométricos actuales y futuros observarán un gran volumen del universo que nos permitirá acotar con precisión el modelo cosmológico. Aun así, la capacidad de los cartografiados para delimitar el modelo a través de las sondas cosmológicas depende de la precisión y certeza con las que se determina el redshift de las galaxias. Por lo tanto, la determinación del redshift fotométrico y sus efectos en los análisis cosmológicos deben ser tratados y estudiados cuidadosamente. En la primera parte de la tesis, transformamos la fotometría de simulaciones que ya existen para imitar las mediciones fotométricas del Dark Energy Survey (DES). De esta forma, esperamos recuperar la distribución real del redshift fotométrico en simulaciones, y así crear una base aún más realista para comprobar los resultados de los análisis cosmológicos de DES que usan redshifts fotométricos. Para transformar las simulaciones utilizamos diversos métodos que transfieren las propiedades estadísticas de la fotometría de observaciones reales a las simulaciones. En la segunda parte de la tesis, utilizamos la técnica del Self-Organizing Map para seleccionar galaxias para ser observadas con espectroscopia, contribuyendo así al proyecto C3R2 que quiere establecer un mapa correlacional entre el espacio de colores y redshift y llenarlo con información espectroscópica. En esta parte también exploramos el espacio de colores definido por la fotometría del Physics of the Accelerating Universe Survey (PAUS) con tal de estudiar la cobertura del redshift espectroscópico de su espacio de colores. Queremos determinar la cantidad del espacio de color sin cobertura espectroscópica porque la falta de representación espectroscópica puede originar un sesgo cuando la precisión del redshift fotométrico se evalúa comparándolo con el redshift espectroscópico o cuando el redshift espectroscópico se utiliza como muestra de entrenamiento para determinar el redshift fotométrico con algoritmos de entrenamiento. Finalmente, exploramos como la variación en la profundidad de las observaciones desde tierra combinadas con las de Euclid afecta la precisión de los redshifts fotométricos y, por lo tanto, la capacidad de Euclid para determinar los parámetros cosmológicos sobre todo cuando se utilizan galaxy clustering y galaxy-galaxy lensing como sondas cosmológicas. También estudiamos como la densidad de las muestras de galaxias afecta la capacidad de acotar los parámetros cosmológicos y cuál es la configuración de bines tomográficos de redshift que permiten extraer la máxima información para delimitar los parámetros cosmológicos. Para llevar a cabo este análisis, creamos diversas distribuciones realistas de redshift fotométrico basadas en la simulación Flagship de Euclid y utilizamos el formalismo de Fisher para hacer una estimación de la capacidad de acotar los parámetros cosmológicos de las diferentes configuraciones de las muestras de galaxias.
Current and future photometric surveys will observe a large volume of the universe that will allow us to accurately constrain the cosmological model. However, the constraining power from cosmological probes of photometric surveys highly relies on the accuracy and precision with which we can determine the galaxies redshifts. Therefore, the determination of photometric redshifts (photo-zs) and their effect in cosmological analysis should be treated and studied carefully. In the first part of this thesis, we transform the photometry of existing simulations to mimic the photometric measurements of the Dark Energy Survey (DES). With this exercise, we expect to recover the real photo-z distribution in simulations, thus creating a more realistic environment to crosscheck the performance of DES in cosmological analyses that use photo-z. We transform the simulations using several method to transfer the statistical properties from the real observations photometry to the simulations. In the second part of the thesis, we use the Self-Organizing Map technique to select spectroscopic targets for the C3R2 program aimed at establishing the mapping between color and redshift space. We also explore the color space defined by the photometry of galaxies from the Physics of the Accelerating Universe Survey (PAUS) in order to study the spectroscopic redshift coverage of its color space. We want to quantify the regions of color space without spectroscopic redshifts because the lack of spectroscopic representation can be a source of bias when the accuracy of photo-zs is evaluated by comparing it to spectroscopic redshifts and when the spectroscopic redshifts are used to determine the photo- z with training-based algorithms. Lastly, we explore how the variation of the depth of ground-based observations combined with Euclid observations affects the accuracy and precision of the photo-z and thus the cosmological constraining power of Euclid focusing on photometric galaxy clustering and galaxy-galaxy lensing analyses. We also study how the number density of photometric galaxy samples affects the constraining power and which tomographic redshift binning configuration returns the maximum information to constrain the cosmological parameters. To perform such analyses, we create several realistic photo-z distributions based on the Euclid Flagship simulation and we use the Fisher forecast and the cosmological inference code, CosmoSIS, over the different configurations of the galaxy samples to determine the cosmological constraining power.
Universitat Autònoma de Barcelona. Programa de Doctorat en Física

In this thesis we push forward techniques to learn about the large scale structure formation of the universe. The approach is from theory, observations and simulations. From theory we study the growth of four non standard cosmological models: DGP model, Cardassian model, Chaplygin gas model and Coupled Quintessence model. From observations we look at the ISW effect by means of the cross-correlation between CMB maps and galaxy survey. From a compilation of data we contrain the cosmological parameters and give new evidence of dark energy. We also introduce a new method to compute errors in configuration space and compare it with other methods in the literature. This method takes into account the geometry of the suveys and it is shown to give accurate results at large angles. In this thesis we also use large dark matter simulations to study the clustering and the local halo bias model. KEY WORDS: large scale structure, structure formation, dark energy, halo bias
CATALÀ:

En aquesta tesi estudiem la formació d'estructures a gran escala de l'univers apoximant-nos-hi des de la teoria, les observacions i les simulacions. Respecte la teoria estudiem el creixement d'estructures en quatre models no estàndards: el model DGP, el model Cardassian, el model de gas de Chaplygin i el model de Quintessècia acoplada.
De les observacions estudiem la formació d'estructura a partir de correlacions entre mapes de galàxies amb mapes del CMB, les quals mesuren l'efecte ISW. A partir d'una compilació que d'aquestes mesures restringim els paràmetres cosmològics i mostrem nova evidència de l'energia fosca. També introduim un nou mètode per a calcular els errors de les correlacions en l'espai de configuració. Comparem aquest mètode amb d'altres i mostrem que estima bé els errors a angles grans, doncs tenim en compte l'area del cel observada. Finalment utilitzem simulacions de matéria fosca per estudiar el clustering i el model de bias local.
RESUMEN CASTELLANO:

En esta tesis estudiamos la formación de estructura en el universo en las vertientes teórica, observacional y con simulaciones. Respeto a la teoria estudiamos el crecimiento de estructuras en cuatro modelos no estándares: el modelo DGP, el modelo Cardassian, el modelo de gas de Chaplygin y el modelo de Quintessencia acoplado. Respecto a las observaciones estudiamos la formación de estructura a partir de las correlaciones entre el CMB y mapas de galaxias, las cuales miden el efecto ISW. A partir de una compilación de estas medidas restringimos los parámetros cosmológicos dando nueva evidencia de la energía oscura. También introducimos un nuevo método para el cálculo de errores en espacio de configuración y mostramos, al compararlo con otros, que estima bien los errores a grandes ángulos puesto que tenemos en cuenta la geometria de área observada. Finalmente, utilizando simulaciones de materia oscura, estudiamos el clustering y el modelo de bias local. PALABRAS CLAVE: Estructura a gran escala, energia oscura, bias en halos, formación de estructura.

En aquesta tesi hem tractat alguns aspectes clau de les lents dèbils gravitacionals en el context d’enquestes fotomètriques. En particular, es van utilitzar simulacions i dades preses durant els primers tres anys d’observacions de l’enquesta de l’energia fosca (DES Y3). Les programacions de DES estan publicades a finals d’aquest any amb les principals anàlisis cosmològiques de DES Y3 i aquesta tesi abasta algunes de les anàlisis. A la part II d’aquesta tesi, ens hem centrat en la tècnica de “clustering-redshift” i el seu part en la principal estratègia de calibració de redshift DES Y3. Clustering-redshift és un mètode per obtenir (o calibrar) distribucions redshift que es basa en correlacions creuades amb mostres petites amb redshift segurs. La part III es va dedicar a la prova del catàleg oficial de formes DES Y3, que abasta ~ 4143 dòlars ^ 2 de l’hemisferi sud i que comprèn ~ 100 milions de galàxies, cosa que el converteix efectivament en el catàleg de formes més gran mai creat. A la darrera part de la tesi (capítols 6 i 7), es van presentar els mapes de massa de lents febles amb lents febles DES Y3 i es va discutir una possible aplicació cosmològica dels mapes. En particular, vam introduir al capítol 6 quatre tècniques de reconstrucció massiva de mapes massius diferents, cadascuna d’elles que assumeixen diferents nivells en el camp de convergència recuperat. El capítol 7 presentava una anàlisi de cosmologia simulada utilitzant el segon i el tercer moment dels mapes de massa de lent feble, dirigits a les dades de DES Y3.
En esta tesis hemos estudiado algunos aspectos clave de la lente gravitacional débil en el contexto de los estudios fotométricos. En particular, utilizamos simulaciones y datos tomados durante los primeros tres años de observaciones de la Dark Energy Survey (DES Y3). DES está programado para lanzar su análisis cosmológico principal DES Y3 más adelante este año, y esta tesis cubre algunas partes del análisis. En la Parte II de esta tesis, nos hemos centrado en la técnica de “clustering redshift’’ y su parte en la estrategia principal de calibración del desplazamiento al rojo de DES Y3. El clustering redshift es un método para obtener (o calibrar) distribuciones de desplazamiento al rojo que se basa en correlaciones cruzadas con muestras pequeñas con desplazamiento al rojo seguro. La Parte III se dedicó a la prueba del catálogo oficial de formas de lente gravitacionales de DES Y3, que abarca ~ 4143 $ deg ^ 2 del hemisferio sur y comprende ~ 100 millones de galaxias, lo que lo convierte en el catálogo de formas más grande jamás creado. En la última parte de la tesis (Capítulo 6 y 7), presentamos los mapas oficiales de masa de lentes débiles de DES Y3, y discutimos una posible aplicación cosmológica de los mapas. En particular, introdujimos en el Capítulo 6 cuatro técnicas diferentes de reconstrucción de mapas de masas, cada una de las cuales asumió diferentes antecedentes en el campo de convergencia recuperado. El Capítulo 7 presentó un análisis de cosmología simulada utilizando el segundo y el tercer momento de los mapas de masas de lentes débiles, dirigidos a los datos DES Y3.
In this thesis we have addressed some key aspects of gravitational weak lensing in the context of photometric surveys. In particular, we used simulations and data taken during the first three years of observations of the Dark Energy Survey (DES Y3). DES is scheduled to release their main DES Y3 cosmological analysis later this year, and this thesis covers some parts of the analysis. In Part II of this thesis, we have focused on the “clustering-redshift’’ technique and its role in the main DES Y3 redshift calibration strategy. Clustering-redshift is a method to obtain (or calibrate) redshift distributions which is based on cross-correlations with small samples with secure redshifts. Part III was devoted to the testing of the official DES Y3 shape catalogue, covering ~ 4143$ deg^2 of the southern hemisphere and comprising ~100 million galaxies, which effectively makes it the largest shape catalogue ever created. In the last part of the thesis (Chapter 6 & 7), we presented the official DES Y3 weak lensing mass maps, and discussed a potential cosmological application of the maps. In particular, we introduced in Chapter 6 four different mass map reconstruction techniques, each of those assuming different priors on the recovered convergence field. Chapter 7 presented a simulated cosmology analysis using the second and third moments of the weak lensing mass maps, targeted at the DES Y3 data.

Observations of the Cosmic Microwave Background (CMB) are crucial components of our understanding of cosmology. Modern high resolution, ground-based CMB survey instruments provide important information about the mass and energy content of our present Universe and the high-energy physics of the Big Bang. In this work we present several aspects of our work on the Atacama Cosmology Telescope (ACT), a 6m telescope in Northern Chile that observed the CMB in three millimetre wavelength bands from 2007–2010. We begin with a description of the Multi-Channel Electronics readout system, an important component of the data acquisition systems for ACT and several other CMB observatories. The system provides room-temperature electronics and software for controlling and reading out arrays of Transition Edge Sensor bolometers via a cryogenic time-domain multiplexing system. We next present our measurement of the ACT point spread function, or beam, using observations of Solar System planets. An accurate understanding of the beam and its covariant error is essential for interpretation of astrophysical and cosmological signal in the ACT data. We then use our understanding of the beam and the instrument calibration to measure the brightness temperatures of Uranus and Saturn at millimetre wavelengths. Precise measurements of planetary brightnesses provide convenient calibration sources for other observatories at these wavelengths. Finally we present a sample of galaxy clusters detected in the ACT maps. We develop a new approach for the analysis of Sunyaev-Zeldovich signal that incorporates a model for the typical cluster pressure to better understand selection effects and evaluate cluster masses. Addressing the current level of systematic uncertainty in the overall mass calibration of clusters, we explore the cosmological constraints obtained when calibrating the mass relation based on pressure profile measurements from X-ray data and from models that take different approaches to the cluster physics. Ultimately we use dynamical mass estimates based on optical velocity dispersion measurements to obtain constraints on the amplitude of scalar fluctuations, the matter density, the Dark Energy equation of state parameter, and the sum of the neutrino mass species.

Conselho Nacional de Desenvolvimento Científico e Tecnológico
Formação de estruturas em larga escala é um problema em aberto em Cosmologia. Há um consenso de que a variedade de estruturas observadas tais como galáxias e aglomerados de galáxias tiveram origem a partir de pequenas flutuações do fluido cósmico possivelmente geradas durante a época inflacionária na era dominada pela matéria. Os estágios iniciais da evolução destas flutuações são descritas pela teoria de Jeans resultante da aproximação linear das equações hidrodinâmicas. Basicamente, a instabilidade devido à ação do campo gravitacional induz à existência de dois tipos de modos perturbativos: os modos instáveis que crescem, e os estáveis que oscilam como ondas sonoras. A distinção entre estes modos depende se o comprimento de onda de um determinado modo perturbativo é maior ou menor que um comprimento de onda típico conhecido como o comprimento de Jeans. Eventualmente, o crescimento dos modos instáveis quebra a aproximação e efeitos não-lineares tornam-se cruciais para a formação de estruturas. Neste sentido, nosso objetivo é estudar o problema não-linear de formação de estruturas em um Universo em expansão dominado pela matéria considerando uma extensão consistente da teoria linear de Jeans. Uma aproximação por sistema dinâmico é fornecida pelo método de Galerkin usado para integrar as equações dinâmicas do fluido auto-graviante. Conseqüentemente é exibido o comportamento dos modos perturbativos instáveis e estáveis do fluido cósmico além do regime linear anterior à formação de estruturas. Nós também consideramos consistentemente a influência da viscosidade ao invés de introduzi-la de um modo artificial como no modelo de Adesão.
The formation of large scale structure is an outstanding problem in Cosmology. It is a consensus that the observed variety of structures such as galaxies and clusters of galaxies have originated from small fluctuations of the cosmic fluid possibly generated during the inflationary epoch-in the matter dominated era. The early stages of evolution of these fluctuations are described by the Jeans theory resulting from the linear approximation of the hydrodynamical equations. Basically, the instabilities due the action of the gravitational field induce the existence of two types of perturbativo modes: the unstable modes that grow, and the stable one that oscillate as sound waves. The distinction between these modes depend son whether the wave length of a given perturbativo mode is greater or smaller than atypical wave-length known as the Jeans length. Eventually, the growth of the unstable modes breaks the linear approximation and nonlinear effects turn out to be crucial for the formation of structures. In this vein, our objective here is to study the problem of nonlinear structure formation in a matter dominated expanding universe considering a consistent extension of the linear Jeans theory. A dynamical system approach is provided by the Galerkin method used to integrate the self-gravitating fluid dynamical equations. Therefore, the behavior of the unstable and stable perturbative modes of the cosmic fluid are exhibited beyond the linear regime prior the formation of structures. We have also considered the influence of viscosity consistently rather than introducing it in an artificial way as in the Adhesion Model.

Orientador: Rogério Rosenfeld
Banca: Bruto Max Pimentel Escobar
Banca: Rudnei de Oliveira Ramos
Resumo: O modelo cosmológico inflacionário postula uma rápida expansão do universo primordial, denominada inflação, que pode resolver certas diflculdades da cosmologia padrão. Em modelos inflacionários mais simples a expansão acelerada é causada pela densidade de energia armazenada em um campo escalar denominado inflaton. Após a fase inflacionária, o universo encontra-se em um estado frio e sem partículas. Deve haver um mecanismo responsável pelo reaquecimento do universo. Nesta dissertação, primeiramente fazemos uma revisão simples da cosmologia padrão e inflacionária. Posterior mente, realizamos um estudo detalhado de três diferentes processos de reaquecimento: ressonância paramétrica, instabilidades taquiônicas e reaquecimento perturbativo. Finalmente, fazemos uma análise deste processo para um modelo de inflação quintessencial usando o programa LATTICEEASY
Abstract: The inflationary cosmological model postulates a fast expansion of the eaxly universe, called inflation, which can solve some issues of the standard cosmological model. In simple inflationary models the accelerated expansion is caused by the energy density stored in a scalar field called inflaton. After the inflationary phase, the universe is in a cold state and without particles. There must be a mechanism responsable for its reheating. In this dissertation we flrstly review the standard and inflationary cosmologies. We then perform a detailed study of three different reheating processes: parametric ressonance, tachyonic instabilities and perturbative reheating. Finally, we mahe an analysis of this process for a quintessencial inflation model using the program LATTICEEASY
Mestre

“Computational Cosmology” is the modeling of structure formation in the Universe by means of numerical simulations. These simulations can be considered as the only “experiment” to verify theories of the origin and evolution of the Universe. Over the last 30 years great progress has been made in the development of computer codes that model the evolution of dark matter (as well as gas physics) on cosmic scales and new research discipline has established itself. After a brief summary of cosmology we will introduce the concepts behind such simulations. We further present a novel computer code for numerical simulations of cosmic structure formation that utilizes adaptive grids to efficiently distribute the work and focus the computing power to regions of interests, respectively. In that regards we also investigate various (numerical) effects that influence the credibility of these simulations and elaborate on the procedure of how to setup their initial conditions. And as running a simulation is only the first step to modelling cosmological structure formation we additionally developed an object finder that maps the density field onto galaxies and galaxy clusters and hence provides the link to observations. Despite the generally accepted success of the cold dark matter cosmology the model still inhibits a number of deviations from observations. Moreover, none of the putative dark matter particle candidates have yet been detected. Utilizing both the novel simulation code and the halo finder we perform and analyse various simulations of cosmic structure formation investigating alternative cosmologies. These include warm (rather than cold) dark matter, features in the power spectrum of the primordial density perturbations caused by non-standard inflation theories, and even modified Newtonian dynamics. We compare these alternatives to the currently accepted standard model and highlight the limitations on both sides; while those alternatives may cure some of the woes of the standard model they also inhibit difficulties on their own. During the past decade simulation codes and computer hardware have advanced to such a stage where it became possible to resolve in detail the sub-halo populations of dark matter halos in a cosmological context. These results, coupled with the simultaneous increase in observational data have opened up a whole new window on the concordance cosmogony in the field that is now known as “Near-Field Cosmology”. We will present an in-depth study of the dynamics of subhaloes and the development of debris of tidally disrupted satellite galaxies.1 Here we postulate a new population of subhaloes that once passed close to the centre of their host and now reside in the outer regions of it. We further show that interactions between satellites inside the radius of their hosts may not be negliable. And the recovery of host properties from the distribution and properties of tidally induced debris material is not as straightforward as expected from simulations of individual satellites in (semi-)analytical host potentials.
Die Kosmologie ist heutzutage eines der spannendsten Arbeitsgebiete in der Astronomie und Astrophysik. Das vorherrschende (Urknall-)Modell in Verbindung mit den neuesten und präzisesten Beobachtungsdaten deutet darauf hin, daß wir in einem Universum leben, welches zu knapp 24% aus Dunkler Materie und zu 72% aus Dunkler Energie besteht; die sichtbare Materie macht gerade einmal 4% aus. Und auch wenn uns derzeit eindeutige bzw. direkte Beweise für die Existenz dieser beiden exotischen Bestandteile des Universums fehlen, so ist es uns dennoch möglich, die Entstehung von Galaxien, Galaxienhaufen und der großräumigen Struktur in solch einem Universum zu modellieren. Dabei bedienen sich Wissenschaftler Computersimulationen, welche die Strukturbildung in einem expandierenden Universum mittels Großrechner nachstellen; dieses Arbeitsgebiet wird Numerische Kosmologie bzw. “Computational Cosmology” bezeichnet und ist Inhalt der vorliegenden Habilitationsschrift. Nach einer kurzen Einleitung in das Themengebiet werden die Techniken zur Durchführung solcher numerischen Simulationen vorgestellt. Die Techniken zur Lösung der relevanten (Differential-)Gleichungen zur Modellierung des “Universums im Computer” unterscheiden sich dabei teilweise drastisch voneinander (Teilchen- vs. Gitterverfahren), und es werden die verfahrenstechnischen Unterschiede herausgearbeitet. Und obwohl unterschiedliche Programme auf unterschiedlichen Methoden basieren, so sind die Unterschiede in den Endergebnissen doch (glücklicherweise) vernachlässigbar gering. Wir stellen desweiteren einen komplett neuen Code – basierend auf dem Gitterverfahren – vor, welcher einen Hauptbestandteil der vorliegenden Habilitation darstellt. Im weiteren Verlauf der Arbeit werden diverse kosmologische Simulationen vorgestellt und ausgewertet. Dabei werden zum einen die Entstehung und Entwicklung von Satellitengalaxien – den (kleinen) Begleitern von Galaxien wie unserer Milchstraße und der Andromedagalaxie – als auch Alternativen zum oben eingeführten “Standardmodell” der Kosmologie untersucht. Es stellt sich dabei heraus, daß keine der (hier vorgeschlagenen) Alternativen eine bedrohliche Konkurenz zu dem Standardmodell darstellt. Aber nichtsdestoweniger zeigen die Rechnungen, daß selbst so extreme Abänderungen wie z.B. modifizierte Newton’sche Dynamik (MOND) zu einem Universum führen können, welches dem beobachteten sehr nahe kommt. Die Ergebnisse in Bezug auf die Dynamik der Satellitengalaxien zeigen auf, daß die Untersuchung der Trümmerfelder von durch Gezeitenkräfte zerriebenen Satellitengalaxien Rückschlüsse auf Eigenschaften des ursprünglichen Satelliten zulassen. Diese Tatsache wird bei der Aufschlüsselung der Entstehungsgeschichte unserer eigenen Milchstraße von erheblichem Nutzen sein. Trotzdem deuten die hier vorgestellten Ergebnisse auch darauf hin, daß dieser Zusammenhang nicht so eindeutig ist, wie er zuvor mit Hilfe kontrollierter Einzelsimulationen von Satellitengalaxien in analytischen “Mutterpotentialen” vorhergesagt wurde: Das Zusammenspiel zwischen den Satelliten und der Muttergalaxie sowie die Einbettung der Rechnungen in einen kosmologischen Rahmen sind von entscheidender Bedeutung.

The variability of fundamental physical constants has been a topic of interest both theoretically and experimentally for many years. Although it is interesting to investigate the consequences of such a variation, it is important to realise that only the variation of dimensionless combination of constants can be meaningfully measured and discussed. In this thesis, I try to justify this way of thinking and apply it to two basic cosmological observables, Big Bang Nucleosynthesis and Cosmic Microwave Background anisotropies. I will mention some related studies that are either wrong or not complete because of being dimensionful. Variation of constants could be considered on two different levels. On the first level one assumes that the constants are time invariant but they can assume different values in different Universes or patches of sky. A thought experiment describing a discussion with aliens having a different system of units with different coupling constants could be helpful, this idea will be returned to throughout the thesis. On the next level, the constants can be promoted to being smooth functions of time or space. It is good to have a firm understanding of what happens on the previous level before trying to consider genuinely variable constants. For variable constants we need to consider theories beyond the currently accepted ones, which are capable of consistently describing such a variation. I briefly review the scalar-tensor theory of gravity as a possible way to describe the variation of the gravitational coupling. I give a brief historical review on the subject and consider the theory in the two so called ‘frames’, discussing about the benefits of each frame mathematically and the physical meaning of these frames. Such theories could form the frame work in which further study of variable con- stants could be carried out.

L'histoire séculaire des systèmes dynamiques puise ses originesdansle développement du cadre mathématique en astronomie.L'objet de cette thèse est l'étude de propriétés de la gravitation de ce point de vue de la dynamiqueà différentes échelles cosmologiques.Dans la théorie du potentiel, l'isochronie définit généralement le mouvement d'oscillation harmonique de pendules.En 1959, le mathématicien et astronome Michel Hénon étend cette définition afin de caractériser les oscillationsorbitales d'étoiles, autour du centre du système à symétrie sphérique auquel elles appartiennent.Dans ce cas, la période d'oscillation peut dépendre de l'énergie de l'étoile.Aujourd'hui, son potentiel isochrone est majoritairement utiliséen simulation numérique pour ses propriétés analytiques d'intégrabilité, mais demeure par ailleurs souvent méconnu.Dans cette thèse, nous revisitons la caractérisation géométrique de l'isochronie comme initiée par Michel Hénon etcomplétons ainsi la famille des potentiels isochrones en physique. La classification de cet ensemblesous l'action de divers groupes mathématiques met en évidence une relation privilégiéeentre les isochrones.Nous montrons alors la nature keplérienne intrinsèque aux isochrones, laquelle est au coeur dela nouvelle relativité isochrone que nous présentons.Les conséquences de cette relativité en mécanique céleste, à savoirla généralisation de la troisième loi de Kepler, celle de la transformation de Bohlin ou Levi-Civita,et le théorème de Bertrand, conduisent à l'analyse du résultat d'un effondrement gravitationnel.Une analyse isochrone est développée pour caractériser un état de quasi-équilibredesystèmes auto-gravitants isolés, comme certains amas stellaires ou galaxies dynamiquement jeunes,à partir de propriétés orbitales de leurs étoiles ou contenu physique.A l'échelle cosmologique, la dynamique de l'univers dépendde sa composition énergétique. Elle peut s'exprimer sous forme d'unsystème dynamique conservatif, bien connu en écologie pour décrire la dynamiquede populations variées. Ce modèle dit de Lotka-Volterra est exploité pour décrireun espacetemps globalement homogène et isotrope, dont les composantes peuventêtre en interaction non uniquement gravitationnelle.Dans cet univers jungle, des comportements dynamiques effectifs à grande échelle pourraient conduire à une expansionaccélérée de l'univers sans nécessité d'énergie noire
Dynamical systems have a centuries-long history with roots going back to the mathematical development for astronomy. In the modern formalism, the present thesis investigates dynamical properties of gravitation at different astrophysical or cosmological scales.In potential theory, isochrony often refers to harmonic oscillations of pendulums. In 1959, the mathematician and astronomer Michel Hénon introduced an extended definition of isochrony to characterize orbital oscillations of stars around the center of the system to which they belong. In that case, the period of oscillations can depend on the energy of the star. Today, Michel Hénon’s isochrone potential is mainly used for its integrable property in numerical simulations, but is not widely known. In this thesis, we revisit his geometrical characterization of isochrony and complete the family of isochrone potentials in physics. The classification of this family under different mathematical group actions highlights a particular relation between the isochrones. The actual Keplerian nature of isochrones is pointed out and stands at the heart of the new isochronerelativity, which are presented together.The consequences of this relativity in celestial mechanics — a generalization of Kepler’sThird law, Bohlin or Levi-Civita transformation, Bertrand’s theorem — are applied to analyze the result of a gravitational collapse. By considering dynamical orbital properties, an isochrone analysis is developed to possibly characterize a quasi-stationary state of isolated self-gravitating systems, such as dynamically young stellar clusters or galaxies.At a cosmological scale, the dynamics of the universe depends on its energy content. Its evolution can be expressed as an ecological dynamical system, namely a conservative generalized Lotka-Volterra model. In this framework of a spatially homogeneous and isotropic spacetime, named Jungle Universe, the dynamical impact of a non-gravitational interaction between the energy components is analyzed. As a result, effective dynamical behaviors could account for an accelerated expansion of the universe without dark energy

Orientador: Pedro Cunha de Holanda
Tese (doutorado) - Universidade Estadual de Campinas, Instituto de Física Gleb Wataghin
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Resumo: Cosmologia de neutrinos é a atividade de pesquisa dedicada ao teste de propriedades de neutrinos através de observáveis cosmológicos. Particularmente, as propriedades estudadas e testadas ao longo desta tese de doutorado foram à massa dos neutrinos e o número de estados estéreis. Ambas as propriedades são motivadas pela observação em experimentos terrestres do fenômeno de oscilação entre estados de interação. Diferentemente da existência de estados massivos, os indícios da existência de estados estéreis são tenuemente sugeridos, contudo ambas as propriedades representariam um grande impacto no modelo cosmológico caso sejam observados sinais positivos de suas existências e por esse motivo justificam-se suas análises. A utilização da cosmologia como um laboratório de física de neutrinos é possível graças à imensa abundância de neutrinos remanescentes do universo primordial além da sensibilidade sem precedentes das observações realizadas. Telescópios e detectores em planejamento ou em implementação supostamente alcançarão a sensibilidade equivalente aos valores mínimos das massas determinados pelo fenômeno de oscilação. A perspectiva de tais sensibilidades experimentais deve ser acompanhada pelo aumento equivalente da precisão obtida nas previsões teóricas dos efeitos pelos quais pretende-se detectar a presença de neutrinos massivos ou de estados estéreis. Nesta tese de doutorado, além de ser detalhadamente desenvolvida a teoria e a metodologia convencionais na análise de dados cosmológicos para obtenção de medidas ou limites para estes dois parâmetros, também foram desenvolvidos métodos para aumentar a precisão da previsibilidade teórica. Foram abordados dois desafios teóricos prementes, a imprecisão das previsões no regime não-linear de perturbação para neutrinos massivos e a degenerescência em relação aos parâmetros de modelos cosmológicos estendidos. As melhorias obtidas não compõem soluções definitivas, mas sim metodologias a serem desenvolvidas sistematicamente ao longo da obtenção de novos dados reais e simulados.
Abstract: Neutrino cosmology is the research activity dedicated to test neutrino properties by cosmological observables. Specially, the properties studied and tested along this graduate research were the neutrino mass and the sterile states. Both properties are motivated from observations in terrestrial experiments of the phenomenon of oscillation between interaction states. Differently from the existence of massive states, the signals of sterile states are weakly suggested, however both properties would mean a sizeable impact in the cosmological model in case any positive signal is detected and, therefore, their analysis are justified. The utilization of cosmology as a laboratory of neutrino physics is possible thanks to the high abundance of remaining neutrinos from the primordial universe, besides the unprecedented sensitivity of experimental observations. Telescopes and detectors planned or under construction will supposedly reach the sensitivity equivalent to the minimal neutrinos masses given by the oscillation phenomenum. The perspective of such experimental sensitivities must be followed by the equivalent improvement in the theoretical predictability for the effects with which it is intended to detect a positive signal of massive neutrinos or sterile states. In this Ph.D. Thesis, besides being developed in detail the theory and methodology used in the analysis of cosmological data to measure or constrain these two parameters, we also developed methods and tools to improve the theoretical predictability. Two pressing theoretical challenges were addressed, the imprecision of the predictions in the non-linear regime of perturbations for massive neutrinos and the degeneracy related to parameters of extended cosmological models. The improvements obtained do not make up a definitive solution, but rather methodologies to be systematically developed along with the achivement or the eventuality of new real and simulated data.
Doutorado
Física
Doutor em Ciências

The cosmological phenomenology of branes is tested, concentrating on quantum mechanical models at all levels. In increasing order of energy, we discuss low-energy observable quantum fields on the brane, higher-energy quantum fields which live in the new bulk space provided by brane scenarios, quantum mechanical models of the brane world itself, and simple phenomenological models of quantum mechanical strings. We calculate radiative gravitational corrections to the mass of a class of light scalar fields, which include the inflation and the quintessence field, and discuss constrains which arise from requiring consistency with observation. We calculate a class of corrections to the quintessence potential, and show that the classical tree-level potential is typically not destroyed by radiative effects. A systematic path integral theory of Kaluza-Klein fields is developed and applied to stationary and ‘breathing’ orbifolds, and in the process we calculate first order corrections to the spectrum of tensor modes produced during an epoch of early universe inflation. We go on to discuss aspects of a quantum cosmology of the braneworld, and describe how the on-brane Wheeler-de Witt equation descends from the bulk quantum constraint. In our formalism, a tower of corrections to the four-dimensional quantum cosmology appears, and we show how this framework directly addresses consistency issues in the brane world. Finally, a simple phenomenological model of quantum mechanical strings stretching across the brane world is introduced. We discuss the low-energy phenomenology, and discuss how such effects may influence dynamical models in which the Hot Big Bang is connected with a collision between branes in the early universe.

[Abstract]: This thesis presents a new cosmological model based on a blank-slate reconsideration of the issue of the first cause in cosmology. It is proposed that a pre-Big Bang evolution of nature occurred that removes the need for postulating the existence of matter, energy and time. This new approach leads to an underlyingconceptualisation of nature consistent with quantum mechanics.The problems of first cause and initial conditions in cosmology are reconsidered. It is proposed that the initial conditions were flawed and evolved toproduce the Big Bang as a natural response to these flaws. This contrasts with the traditional approach of postulating a homogeneous initial state requiring perturbation by an additional first cause.In flawed nature cosmology the origin of time occurs as a natural response to the flawed set of initial conditions, and removes the need to postulate time. The development of causality remains an ongoing process rather than being fully determined by the first cause. Ongoing development of causality provides a conceptual understanding of the probabilistic nature of quantum mechanics and itsrelationship with classical physics.Flawed nature cosmology is used to examine pre-Big Bang evolution, in order to justify rather than postulate a set of conditions leading to the Big Bang. Thisexamination of pre-Big Bang evolution also introduces a structured method to start addressing the question of the origin of matter and the forces of nature.Flawed nature cosmology reconsiders the issues that introduced the manyuniverses concept into physics such as spontaneous first causes, the many-worlds interpretation of quantum mechanics, brane cosmology’s use of the extra dimensions in string theory, and parallel universes to solve the fine tuning problem. The manyuniversesconcept has found favour, as much of the puzzling behaviour of the universe can be avoided by simply stating that if there are many universes, one couldmatch our experience. In contrast, flawed nature cosmology demonstrates that the universe we experience is the unique product of its evolutionary history.

The substantial recent advances in early universe observational cosmology have confronted theoretical physics with a number of deep conceptual questions. The standard model of particle physics and its natural extensions have proven to be insufficient for explaining the recent observations. String theory as the main candidate for a complete theory of quantum gravity provides us with the possibility of resolving this discrepancy. In this dissertation, I begin by an overview of thermodynamics in string theory. The issue of stability and the existence of the thermodynamic limit have been addressed. I will describe the string gas cosmology in detail, as the leading candidate of early universe cosmology. Successes and shortcomings of the most recent model of string gas cosmology in matching with observations are addressed. I conclude with comments on interesting directions for future study and a possible link with black hole physics.
A la lumiere des plus recentes observations cosmologiques, la physique theorique se trouve confrontee a de nombreuses questions conceptuelles. Le Modele Standard de physique des particules et ses plus simples extensions se sont averes insuffisants pour expliquer les recentes observations. La theorie des cordes, qui est presentement la principale candidate pour une theorie complete de la gravite quantique, nous offre la possibilite de resoudre ces lacunes du Modele Standard. Nous debutons cette these par un apercu de la thermodynamique dans la theorie des cordes. Nous nous penchons en particulier sur la stabilite et sur l'existence d'une limite thermodynamique en theorie des cordes. Nous presentons par la suite les details de la cosmologie des gaz de cordes et discutons des succes et des problèmes de cette approche pour decrire l'evolution cosmologique du debut de l'univers. Nous concluons en commentant sur les possibles travaux futurs et sur un lien entre ce sujet et la physique des trous noirs.

Recently, the ideas of extra dimensions and brane world scenarios have captured considerable interest. The potential ability of these brane-based models to explain the gauge hierarchy problem and the cosmological constant problem has been examined in numerous recent papers (on these subjects). In the first part of this thesis we examine many aspects of warped brane world scenarios, focusing on the Randall-Sundrum model. We begin with the stability issue of the Randall-Sundrum model and its cosmological implications. We will specifically verify that the Randall-Sundrum scenario; once stabilized, will reproduce the late-time conventional cosmology. In further studies, the possibility of providing a very small cosmological constant, in accordance with recent observations, is examined in the context of the warped brane world scenario. We also show that the "self-tuning" mechanism of the cosmological constant problem does not improve in the brane world models and the unnatural fine-tuning mystery of the cosmological constant problem remains unexplained. In the second part of this thesis some cosmological implications of the tachyon are examined. In particular, a mechanism is presented to take advantage of a time-varying tachyonic background to convert the energy of the tachyon to radiation at the end of inflation.

The standard ΛCDM cosmological model has been successful in accommodating most of the cosmological observations to date. However, there are still many theoretical and observational issues that this model needs to address before it can constitute a reliable framework in this era of 'precision cosmology'. In this thesis, we focus on some of these issues and shortcomings of the ΛCDM model that stem from inhomogeneities in the universe. We begin by focusing on the generation of inhomogeneities by suggesting a generic approach to building suitable models for the origin of structure within the inflationary paradigm. This is in contrast with the study of specific models that has been commonly practised so far. Within the inflationary paradigm the universe in the earliest stages undergoes exponential expansion that stretches quantum fluctuations to astronomical scales, thus providing seeds of future structure. However, this inflationary phase can be generated by many different theoretical 'toy' models of inflation. To alleviate this arbitrariness, we argue that certain potentials for a scalar field are preferred from the viewpoint of the renormalisation group. We arrive at these potentials by decomposing generic UV complete theories for a scalar field in a way that isolates the part that dominates at energies lower than the initial UV scale of the theory. In our calculations, we adopt approximations usually made within the inflationary paradigm. We obtain predictions for the main inflationary observables consistent with the current measurements. We continue by studying the way how the inhomogeneous distribution of matter can be characterised. The structure in the universe at late times exhibits complicated fractal-like behaviour. We propose new methods built on the ideas of anomalous diffusion and random walks to characterise this behaviour. Our methods complement the wellestablished 'count-in-spheres' approach. Moreover, we draw attention to and explicitly demonstrate biases that have existed in the previous applications of these methods, thus improving on them. Finally, we determine the distance scale above which the fractal behaviour disappears and the distribution of matter can be considered homogeneous. The inhomogeneities can also be analysed in terms of the velocity field. We study the most extreme example of it observed to date, namely the 'Bullet Cluster', a system of two massive clusters of galaxies which have collided at a high relative speed. This collision poses a potential issue for the standard cosmological model since such mergers are expected to be extremely rare. We calculate the observationally relevant quantity - the expected number of such mergers on the sky in a survey complete up to some redshift. We find that, up to the redshift of the Bullet Cluster, we only expect about 0.1 such collisions. Many more similar merging clusters have been found recently, thereby increasing the tension with the ΛCDM model. We provide a formula for the expected number of mergers within the ΛCDM model given their collisional parameters and their redshift, which can be used to aid the future analyses and directly confront the model. Finally, we focus on the role of the inhomogeneous velocity field in determining the background rate of expansion of space. The Hubble parameter, when measured in the frame of the Local Group of galaxies, has less variation about the background value than when it is measured in the Cosmic Microwave Background frame. This appears to be at odds with the standard cosmological model in which the CMB frame is the one where quantities are expected to be most uniform. We quantify this apparent discrepancy and show that the boost to the frame that makes the Hubble flow most uniform for our position is in fact consistent with the standard model. We show that such boosts to the frame of most uniform flow are typical if observers are located in underdensities and participate in local bulk flows, which is indeed the case for our position.

The following thesis project explores the design of a nature center and planetarium within Rock Creek Park, in Washington, DC. The project evolved from a desire to re-imagine a relationship with nature in a way that allows conversation, reflective thinking, and allows oneâ s sense of wonder to enter a place which is between science and myth. The design of the nature center and planetarium was developed by investigating the dualities which exist within and around the site, such as earth and air, day and night,and winter and summer. The goal was to create a building which is fully embedded in the site around it, to create a unique environment for exploration and conversation, and a place to contemplate oneâ s relationship with nature.
Master of Architecture

Bibliography: leaves 147-153.
This thesis contains two distinct parts: the first part introduces and explains the relevant theory and background necessary for the analytic work done on magnetized cosmological perturbations at the end of the first part. The second part discusses some issues related to observational cosmology. After an introductory chapter including an overview of the thesis, PART I starts with a discussion of the covariant approach to cosmology, introducing notation needed in the thesis. The covariant approach to perturbations is then discussed, and the basic inhomogeneity variables describing energy density, pressure and expansion pertubations are introduced. Their exact evolution equations are presented before being linearized about an FRW background.

Kyoto University (京都大学)
0048
新制・課程博士
博士(人間・環境学)
甲第9664号
人博第148号
13||133(吉田南総合図書館)
新制||人||35(附属図書館)
UT51-2002-G422
京都大学大学院人間・環境学研究科人間・環境学専攻
(主査)教授 松田 哲, 助教授 阪上 雅昭, 助教授 早田 次郎
学位規則第4条第1項該当

Thesis (Ph. D.)--University of North Carolina at Chapel Hill, 2009.
Title from electronic title page (viewed Jun. 26, 2009). "... in partial fulfillment of the requirements for the degree of Doctor of Philosophy in the Department of Physics and Astronomy." Discipline: Physics and Astronomy; Department/School: Physics and Astronomy.

Kyoto University (京都大学)
0048
新制・課程博士
博士(理学)
甲第12085号
理博第2979号
新制||理||1444(附属図書館)
23921
UT51-2006-J80
京都大学大学院理学研究科物理学・宇宙物理学専攻
(主査)助教授 早田 次郎, 教授 中村 卓史, 教授 畑 浩之
学位規則第4条第1項該当

Tese (doutorado) - Universidade Federal de Santa Catarina, Centro de Comunicação e Expressão, Programa de Pós-Graduação em Inglês: Estudos Linguísticos e Literários, Florianópolis, 2014.
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Abstract : Based on cosmologies and constellations within Legitimation Code Theory (LCT) interfaced with appraisal framework within Systemic Functional Linguistics (SFL), this research aims at analyzing sustainability knowledge management by Kraft Foods Company in their Annual Sustainability Report based on the meanings that their constellations convey. As conceptualized by Maton (in press) constellations refer to clusters of stances condensing social actors? practices, ideas and beliefs related to a given field of science. These clusters are the mechanism through which cosmologies are built. Through the lens of SFL, the hierarchy Individuation, situated in between the hierarchies of Instantiation and Realization, provides an open space for the activation of the LCT model, since in individuation, as Martin (2007, 2009) postulates, the central focus is on the language users rather than the language use. As regards the methodological framework, because of its interdisciplinary perspective, this research is split into two different stages: 1) the handling of the internal relations; 2) the handling of the external relations. These relations make up the overall procedures that enable the development of works on cosmologies. In stage (I), the handling of the former consists of clustering the company?s practices, beliefs and values in the form of stances through which Kraft?s constellations are identifiable. Thanks to the appraisal resources by Martin and White (2005), the inscriptions of the attitudinal subsystem and the grading resources, in particular, that identification is possible. Stage (II) discusses the constellations analysis regarding the external relations allowing for gauging the epistemological and axiological underpinnings in the investigated discourse, the means by which the type of knower is known. In this phase, the theoretical accounts of Maton?s (in press, 2013b, 2010a, 2010b, 2007, 2006, 2004) knowledge-knower structure interfaced with Bernstein?s concepts of the code theory are brought into play. The results have shown that by means of appreciation and graduation interplay within appraisal theory the majority of the stances in Kraft?scosmology are acknowledgeable within the reservoir of meanings of sustainability. By the same token, appreciation-graduation interplay has revealed that four of the stances identified in Kraft?s cosmology do not comply with the reservoir of meanings of sustainability, indicating that the ?grammar? power in Kraft?s cosmology is, in some sense, downplayed. Nevertheless, this is not to argue that the epistemic relations in Kraft?s knowledge structure become weak. In truth, they remain untouchable, since the only hierarchy individuated by Kraft?s company is the one of sustainability whereby the presence of the majority of the stances is justifiable. Therefore, the social actors in Kraft?s cosmology are the type of knowers who emphasize the specialist knowledge over the knowers? attributes and gaze from the view that in Kraft?s knowledge structure epistemic relations are strongly classified and social relations are weakly framed. Technically considered, this means that the company investigated is described under the knowledge code +ER (+C, + F); - SR (-C, -F), strong epistemic relations and weak social relations, thus indicating the company?s inclination towards the specialist knowledge.

No presente estudo, o viés interdisciplinar justifica-se pela conexão estabelecida entre o aspecto sociológico por intermédio do modelo 'Legitimation Code Theory' (LCT), especificamente referente às dimenensões das cosmologias e constelações, e da perspectiva linguística sistêmico-funcional através da teoria da avaliatividade. Esta tese tem como objetivo investigar de que forma a Multinacional ?Kraft Foods? lida com o conhecimento da sustentabilidade com base nos significados que suas constelações constroem. Maton (in press) define constelações os grupos das ideias, crenças, e valores que refletem as práticas de atores sociais em um específico tempo e espaço em uma dada cosmologia. 'Cosmologia', por conseguinte, consiste em uma configuração de ideáis que caracteriza as práticas socioculturais desses atores atrelados a um dado campo da ciência. No que concerne à perspectiva da linguística sistêmico-funcional, a hierarquia Individuação, localizada entre as dimensões Instanciação e Realização, possibilita a ativação dos recursos da LCT, haja vista ser a individução âmibito de proliferção das ideologias em que se privilegia o repertório do falante. No tocante aos pressupostos teórico-metodológicos, a presente pesquisa alicerça-se em duas fases distintas:1) manuseio das relações internas; 2) manuseio das relações externas. Tais relações compreendem os procedimentos sobre os quais trabalhos desenvolvidos sob a ótica ?cosmologia? são viavéis. No estágio (I), acontence o manuseio das relações internas, o que significa que os recursos linguísticos contidos nos subsistemas de Atitude e Gradação dentro da teoria da avaliatividade (Martin & White, 2005) são ativados a fim de identificar as unidades de significados constituídas a partir da configuração de ideias, práticas, valores e crenças dos atores sociais referente ao reservatório de significados da sustentabilidade. A fase (II) compreende a análise das constelações, ocasião em que as relacões externas são tratadas de modo que se mensure o sustentáculo epistemológico e axiológico existente na cosmologia investigada. Nesta fase, os postulados de Maton ( in press, 2013b, 2010a, 2010b, 2007, 2006, 2004 ) sobre a estrutura do conhecimento e conhecedor, emanada das concepões de Bernstein acerca da estruturação e transmissão do conhecimento, são explorados a fim de caracterizar o tipo de conhecedor exitente na cosmologa em questão. Dos resultados obtidos, evidenciou-se que o intercâmbio entre Apreciação, uma das três dimensões de Atitude, e Gradação propiciou o reconhecimento da maioria das unidades significativas pertencentes ao reservatório designificados da sustentabilidade. Outrossim, percebeu-se que os mesmos recursos linguísticos corroboraram a descorbeta de que há quatro unidades significativas desvínculadas do campo da sustentabilidade, o que sugere uma diminuição do poder da ?gramática? na cosmologia sob análise.É importante salientar que, no cenário desta investigação, a diminuição do poder da gramática não implica no enfraquecimento da hierarquia (episteme), uma vez que os referentes epistêmicos no disurso analisado são integralmente explorados pela multinacional em questão. Ademais, a hierárquia da sustentabilidade é a única base científica evidenciada no discurso sobre o qual a existência da maioria dos referentes e descrições empíricas (unidades significativas) é justificável. Por fim, os referentes epistêmicos são fortemente 'classificados' enquanto que as relações sociais- os atributos dos atores sociais em relaçao ao campo do conhecimento proposto - são fracamente ?enquadrados?, fato que caracteriza os atores socias sob os códigos +ER (+C, +F) e - SR (-C, -F), fortes relaçoes epistêmicas e fracas relações sociais. Tecnicamente, esses códigos indicam que na cosmologia investigada os atores sociais são os tipos de conhecedores que sobrepõem os pressupostos teóricos às suas relaçoes sociais com o objeto invesitgado.

Nesta dissertação, uma revisão dos modelos cosmológicos newtonianos e neo-newtonianos baseados na formulação da hidrodinâmica clássica é apresentada, com especial ênfase para os resultados básicos e as limitações mais importantes dessas abordagens. Em seguida, mostramos que a descrição Lagrangiana clássica proposta por Lima, Moreira e Santos (1998) para fluidos simples, pode ser generalizada para incluir modelos com misturas de fluidos, e portanto, cosmologias mais realísticas contendo bárions, matéria escura e energia escura, bem como qualquer forma de interação entre essas componentes. Neste trabalho propomos uma descrição lagrangiana clássica para modelos relativísticos do tipo FRW. Nesta descrição, o comportamento dinâmico do fator de escala a(t), como previsto pelas cosmologias relativísticas, é substituído pelo movimento unidimensional de uma partícula teste de massa m sob a ação de um potencial clássico, V(x), onde x é a coordenada unidimensional da partícula. O tratamento pode ser aplicado para os mais diversos cenários de energia escura. Para exemplificar, discutimos com detalhe os seguintes modelos contendo matéria escura e energia escura: XCDM, X(z)CDM, Lambda CDM, Lambda(t) e gás de Chaplygin. Por completeza, modelos multidimensionais do tipo FRW também são considerados. Em todos esses modelos, o parâmetro de curvatura k das seções espaciais das cosmologias determina a energia total da partícula teste pela relação, E=-mk/2, tal como ocorre nos modelos de fluidos simples. As propriedades dinâmicas associadas com o presente estágio de aceleração do universo são univocamente descritas em termos da função potencial do sistema. Finalmente, utilizando os dados da distância de luminosidade provenientes das supernovas do tipo Ia, discutimos como o potencial unidimensional pode ser reconstruído a partir das observações.
In this dissertation, a review of the Newtonian and neo-Newtonian cosmological models based on the classical hydrodynamics formulation is presented with special emphasis to the basic results and the main limitations of such approaches. Next, we show that the classical Lagrangian description as proposed by Lima, Moreira & Santos (1998) for simple fluids, can be generalized to include fluid mixtures, and, therefore, more realistic cosmologies containing baryons, dark matter and dark energy, as well as, any kind of interaction among such components. In the lagrangian description, the dynamic behavior of the scale factor a(t), as predicted by the relativistic cosmologies, is replaced by the unidimensional motion of a test particle with mass m under the action of a classical potential, V(x), where x(t) is the coordinate of the particle. The treatment can be applied for many different scenarios of dark energy. In order to exemplify, we discuss with detail the following models containing dark matter and dark energy: XCDM, X(z)CDM, Lambda(t)CDM and the Chaplygin gas. For completeness, FRW type multidimensional models are also considered. For all these models, the curvature parameter k of the spatial sections in the relativistic cosmologies determines the total energy by the relation, E=-mk/2, as occurs in the simple fluid models. The dynamic property associated with the present accelerating stage of the Universe are univocally described in terms of the potential function of the system. Finally, by using the data from luminosity distance of supernovae type Ia, we discuss how the unidimensional potential can be reconstructed from the observations.

Ma thèse porte sur deux questions importantes de la Cosmologie:(i) L'origine des champs magnétiques cosmologiques:L'Univers semble magnétisé à absolument toutes ses échelles (spatiales et temporelles), y compris le milieu intergalactique. Mais leur origine est encore inconnue à l'heure actuelle, malgré les nombreux efforts pour essayer de répondre à cette question. On pense qu'ils ont d'abord été générés avec de très faibles amplitudes, puis qu'ils ont été amplifiés au cours de la formation des structures. La turbulence dans les galaxies et les amas de galaxies modifie totalement l'organisation initiale de ces champs, ce qui fait que les champs observés actuellement dans les structures ne nous renseignent pas sur leur origine. Il convient donc de s'intéresser aux champs intergalactiques. J'ai dévelopé analytiquement un modèle de magnétogénèse basé sur la photoionisation du milieu intergalactique par les premières étoiles et les premières galaxies apparues dans l'Univers, il y a environ 13 milliards d'années. Puis, en collaboration avec H. Tashiro et N. Sugiyama (Japon), j'ai calculé de façon analytique la densité d'énergie moyenne injectée par ce processus dans le contexte cosmologique, et en parallèle, en collaboration avec D.Aubert (France), j'ai étudié les propriétés statistiques du champs généré à travers des simulations numériques. Nos prédictions sont compatibles avec les observations actuelles. Ce mécanisme a donc dû participer à la magnétisation de l'Univers à ses plus grandes échelles.(ii) Fragmentation gravitationnelle de la toile cosmique:Les simulations numériques suggèrent que la matière dans l'Univers est répartie de façon filamentaire, les noeuds de ce réseau étant les amas de galaxies. La matière s'écoule le long de ces filaments. L'accrétion dans les noeuds est donc anisotrope, et il s'avère qu'elle est aussi en partie intermittente. Cela indique que la matière ne se structure pas uniquement dans les amas, mais aussi dans les filaments, voire les nappes ou les vides cosmiques. Je me suis donc intéressé à l'instabilité gravitationnelle dans les milieux stratifiés. J'ai proposé une nouvelle approche, dans le cadre de la théorie spectrale, en m'inspirant de la littérature plasma
My thesis deals with two important topics of Cosmology:(i) Origin of cosmological magnetic fields:Magnetic fields seem ubiquitous in the Universe, present at all scales and all times, probably even in the entire intergalactic medium. Their origin is still unclear, especially on the largest scales. The current paradigm is that they were first generated with extremely weak strengths, and later amplified during structure formation. Because of turbulence, the fields we observe in galaxies and galaxy clusters lost their initial characteristics. However, in less dense regions such as cosmological filaments, sheets or voids, magnetic fields have evolved more mildly. Therefore, intergalactic magnetic fields may still possess a memory of the processes that generated them and hold the key to their origin. I developed analytically a detailed physical model of a natural astrophysical mechanism that generates intergalactic magnetic fields during the first billion year, namely at the time when first stars and galaxies were born. Then, in collaboration with H. Tashiro and N. Sugiyama (Japan), I computed analytically the mean energy density injected in the entire Universe through this mechanism. Independently, in collaboration with D. Aubert (France), I derived the topological and statistical properties of the magnetic field thus generated, using cosmological numerical simulations. This way I demonstrated that this simple, natural photoionization-based magnetogenesis must have created magnetic seed fields with properties a priori perfectly compatible with present day observations.(ii) Gravitational fragmentation of the cosmic web:Cosmological numerical simulations suggest that the Universe has a web-like structure, the nodes of which are galaxy clusters. These nodes are supplied with matter flowing along the filaments interconnecting them. Part of this accretion occurs intermittently, which indicates that clumps of matter form not only inside clusters themselves, but also either in cosmic voids, walls and/or filaments. I studied gravitational instability in stratified media in the frame of spectral theory, in planar and cylindrical geometries, relevant for cosmic walls and filaments, for isothermal, polytropic, and with and without an external gravitational background (e.g. Dark Matter). I have recasted the problem as an eigenvalue problem in the force operator formalism, and derived the wave equation governing the growth of perturbations. I also studied it in matrix form, which gives complementary information

Per constrènyer models cosmològics mitjançant el creixement de les fluctuacions a gran escala de la matèria és cabdal entendre com les galàxies que observem tracen el camp de densitat de tot el conjunt de matèria. La relació entre el camp de densitat de matèria i el de galàxies s'acostuma a aproximar amb una expansió de segon ordre de la funció anomenada bias. La llibertat en els paràmetres d'aquesta funció redueix la informació cosmològica que es pot extreure de les observacions. En aquesta tesi estudiem dos mètodes per determinar els paràmetres del bias independentment del creixement. L'anàlisi es basa en la distribució de matèria de la gran simulació MICE Grand Challenge. Als halos, identificats en aquesta simulació, se'ls associen galàxies. El primer mètode consisteix en mesurar directament els paràmetres del bias d'estadístiques de tercer ordre de les distribucions d'halos i de matèria. El segon en predir-los a partir de l'abundància d'halos en funció de la seva massa (concepte al qual ens referirem com a funció de massa). Les nostres estimacions del bias amb estadístiques de tercer ordre es basen en les autocorrelacions i correlacions creuades de tres punts dels camps de densitat d'halos i de matèria, en l'espai de configuració tridimensional. Usant les autocorrelacion de tres punts i un model local i quadràtic del bias trobem una sobreestimació del $\sim20\%$ en el paràmetre lineal del bias respecte a la referència provinent de correlacions de dos punts. Aquesta desviació es pot deure a ignorar contribucions no locals i d'ordre superior a la funció bias, així com sistemàtics en les mesures. L'efecte d'aquestes inexactituds en les estimacions del bias en les mesures del creixement són comparables amb els errors en les nostres mesures, procedents de la variància de la mostra i del soroll. També presentem un nou mètode per mesurar el creixement que no requereix un model per a la correlació de tres punts de la matèria fosca. Els resultats d'ambdós enfocaments estan en acord amb les prediccions. Combinant les autocorrelacions i les correlacions creuades de tres punts, per una banda podem mesurar el bias lineal sense ser afectats per termes quadràtics (locals o no locals) en les funcions del bias, i de l'altra podem aïllar aquests termes i comparar-los amb les prediccions. Les nostres mesures de bias lineal a partir d'aquestes combinacions són molt consistents amb el bias lineal de referència. La comparació de les contribucions no lineals amb les prediccions revelen una forta dependència de les mesures amb desviacions significatives de les prediccions, inclús a escales molt grans.  El nostre segon enfoc per obtenir els paràmetres de bias són prediccions derivades de la funció de massa a través de l'aproximació de "peak-background !split". Trobem desviacions significatives del 5-10% entre aquestes prediccions i la referència a partir de les estadístiques de dos punts. Aquestes desviacions poden ser explicades només en part a partir dels sistemàtics que afecten les prediccions de bias, provinent del "binning" de la funció de massa d'halos, l'estimació de l'error de la funció de massa i la parametrització de la funció de massa a partir de la qual se'n deriven les prediccions de bias.  Estudiant la funció de massa trobem relacions entre diferents parametritzacions de la funció de massa. A més, trobem que el mètode estàndard de Jack-Knife sobreestima la covariança d'error de la funció de massa en el rang de baixa massa. Expliquem aquestes desviacions i presentem un nou i estimador de covariança millorat.
For constraining cosmological models via the growth of large-scale matter fluctuations it is important to understand how the observed galaxies trace the full matter density field. The relation between the density fields of matter and galaxies is often approximated by a second- order expansion of a so-called bias function. The freedom of the parameters in the bias function weakens cosmological constraints from observations. In this thesis we study two methods for determining the bias parameters independently from the growth. Our analysis is based on the matter field from the large MICE Grand Challenge simulation. Haloes, identified in this simulation, are associated with galaxies. The first method is to measure the bias parameters directly from third-order statistics of the halo and matter distributions. The second method is to predict them from the abundance of haloes as a function of halo mass (hereafter referred to as mass function). Our bias estimations from third-order statistics are based on three-point auto- and cross- correlations of halo and matter density fields in three dimensional configuration space. Using three-point auto-correlations and a local quadratic bias model we find a ∼ 20% overestimation of the linear bias parameter with respect to the reference from two-point correlations. This deviation can originate from ignoring non-local and higher-order contributions to the bias function, as well as from systematics in the measurements. The effect of such inaccuracies in the bias estimations on growth measurements are comparable with errors in our measurements, coming from sampling variance and noise. We also present a new method for measuring the growth which does not require a model for the dark matter three-point correlation. Results from both approaches are in good agreement with predictions. By combining three-point auto- and cross-correlations one can either measure the linear bias without being affected by quadratic (local or non-local) terms in the bias functions or one can isolate such terms and compare them to predictions. Our linear bias measurements from such combinations are in very good agreement with the reference linear bias. The comparison of the non-local contributions with predictions reveals a strong scale dependence of the measurements with significant deviations from the predictions, even at very large scales. Our second approach for obtaining the bias parameters are predictions derived from the mass function via the peak-background split approach. We find significant 5−10% deviations between these predictions and the reference from two-point clustering. These deviations can only partly be explained with systematics affecting the bias predictions, coming from the halo mass function binning, the mass function error estimation and the mass function parameterisation from which the bias predictions are derived. Studying the mass function we find unifying relations between different mass function parameterisation. Furthermore, we find that the standard Jack-Knife method overestimates the mass function error covariance in the low mass range. We explain these deviations and present a new improved covariance estimator.

Thesis (Ph. D.)--University of California, San Diego, 2005.
Title from first page of PDF file (viewed March 7, 2006). Available via ProQuest Digital Dissertations. Vita. Includes bibliographical references (p. 344-361).

State of the inhomogeneous universe and its geometry throughout cosmic history can be studied by measuring the clustering of galaxies and the gravitational lensing of distant faint galaxies. Lensing and clustering measurements from large datasets provided by modern galaxy surveys will forever shape our understanding of the how the universe expands and how the structures grow. Interpretation of these rich datasets requires careful characterization of uncertainties at different stages of data analysis: estimation of the signal, estimation of the signal uncertainties, model predictions, and connecting the model to the signal through probabilistic means. In this thesis, we attempt to address some aspects of these challenges.

The first step in cosmological weak lensing analyses is accurate estimation of the distortion of the light profiles of galaxies by large scale structure. These small distortions, known as the cosmic shear signal, are dominated by extra distortions due to telescope optics and atmosphere (in the case of ground-based imaging). This effect is captured by a kernel known as the Point Spread Function (PSF) that needs to be fully estimated and corrected for. We address two challenges a head of accurate PSF modeling for weak lensing studies. The first challenge is finding the centers of point sources that are used for empirical estimation of the PSF. We show that the approximate methods for centroiding stars in wide surveys are able to optimally saturate the information content that is retrievable from astronomical images in the presence of noise.

The fist step in weak lensing studies is estimating the shear signal by accurately measuring the shapes of galaxies. Galaxy shape measurement involves modeling the light profile of galaxies convolved with the light profile of the PSF. Detectors of many space-based telescopes such as the Hubble Space Telescope (HST) sample the PSF with low resolution. Reliable weak lensing analysis of galaxies observed by the HST camera requires knowledge of the PSF at a resolution higher than the pixel resolution of HST. This PSF is called the super-resolution PSF. In particular, we present a forward model of the point sources imaged through filters of the HST WFC3 IR channel. We show that this forward model can accurately estimate the super-resolution PSF. We also introduce a noise model that permits us to robustly analyze the HST WFC3 IR observations of the crowded fields.

Then we try to address one of the theoretical uncertainties in modeling of galaxy clustering on small scales. Study of small scale clustering requires assuming a halo model. Clustering of halos has been shown to depend on halo properties beyond mass such as halo concentration, a phenomenon referred to as assembly bias. Standard large-scale structure studies with halo occupation distribution (HOD) assume that halo mass alone is sufficient to characterize the connection between galaxies and halos. However, assembly bias could cause the modeling of galaxy clustering to face systematic effects if the expected number of galaxies in halos is correlated with other halo properties. Using high resolution N-body simulations and the clustering measurements of Sloan Digital Sky Survey (SDSS) DR7 main galaxy sample, we show that modeling of galaxy clustering can slightly improve if we allow the HOD model to depend on halo properties beyond mass.

One of the key ingredients in precise parameter inference using galaxy clustering is accurate estimation of the error covariance matrix of clustering measurements. This requires generation of many independent galaxy mock catalogs that accurately describe the statistical distribution of galaxies in a wide range of physical scales. We present a fast and accurate method based on low-resolution N-body simulations and an empirical bias model for generating mock catalogs. We use fast particle mesh gravity solvers for generation of dark matter density field and we use Markov Chain Monti Carlo (MCMC) to estimate the bias model that connects dark matter to galaxies. We show that this approach enables the fast generation of mock catalogs that recover clustering at a percent-level accuracy down to quasi-nonlinear scales.

Cosmological datasets are interpreted by specifying likelihood functions that are often assumed to be multivariate Gaussian. Likelihood free approaches such as Approximate Bayesian Computation (ABC) can bypass this assumption by introducing a generative forward model of the data and a distance metric for quantifying the closeness of the data and the model. We present the first application of ABC in large scale structure for constraining the connections between galaxies and dark matter halos. We present an implementation of ABC equipped with Population Monte Carlo and a generative forward model of the data that incorporates sample variance and systematic uncertainties. (Abstract shortened by ProQuest.)

Les missions modernes permettent d’accéder à des mesures de qualité pour les grandes structures, en échantillonnant la distribution de galaxies en détail jusque dans les régions les moins denses, les vides. Toutefois, nous observons les vides dans l’espace des redshift, ce qui limite notre connaissance de ces structures. Afin d’utiliser les vides en tant qu’outils cosmologiques de précision, il est fondamental d’obtenir leur forme dans l’espace réel. Dans cette thèse nous présentons un algorithme non-paramétrique permettant de reconstruire les profils de densité sphérique des vides empilés dans l’espace réel, sans assomption pour les distorsions en redshift. Nous obtenons donc les premiers profils de densité des vides empilés dans l’espace réel, à travers lesquels nous étudions la compensation de masse et calculons les profils de vitesses particulières des vides, se basant sur la théorie linéaire et le modèle cosmologique. Nous discutons l’utilisation des profils pour contraindre indépendamment les vitesses. Avec des catalogues simulés de galaxies, nous analysons l’effet des vitesses particulières sur les propriétés physiques des vides. Enfin nous calculons une prédiction du nombre de vides que fournira la future mission Euclid et des contraintes que ce nombre de vides donnera sur les paramètres cosmologiques (grâce au formalisme de Fisher). Les profils de densité de vides dans l’espace réel peuvent être utilisés pour tester les modèles cosmologiques (à travers l’étude de l’effet des vitesses particulières et l’amélioration du test de Alcock-Paczynski); l’étude des vides promet donc d’apporter des informations indépendantes pour éclaircir le mystère de l’énergie obscure
Modern surveys allow us to access to high quality measurements, by sampling the galaxy distribution in detail also in the emptier regions, voids. When we observe cosmic voids, however, we observe them in redshift-space: their real shape remains inaccessible to us, thus limiting our knowledge about such structures. To employ voids as a precision tool for Cosmology, it is fundamental to obtain their real-space shape. This thesis presents a model-independent non-parametric algorithm to reconstruct the spherical density profiles of stacked voids in real space, without assumptions about redshift distortions. With this algorithm, we obtain the first ever real-space density profiles of stacked voids. With the profiles we study the mass compensation and obtain a theoretical prediction for the velocity profiles of voids based on linear theory and assuming cosmological parameters. In parallel, we discuss the use of the real-space profiles to obtain model-independent information about the peculiar velocity profiles of voids. Also, using mock catalogues, we analyse the effect of peculiar velocities on void properties and discuss it in the framework of current and future surveys. Finally we calculate a forecast for void abundances with the future Euclid mission and obtain, using the Fisher matrix formalism, a prediction for the constraints that void abundances will set on cosmological parameters. The real-space profiles of voids can be used to test cosmological models (through the understanding of peculiar velocities effects and the improvement of the Alcock-Paczynski test); and void abundances promise to bring independent information and to shed light on the mystery of dark energy

What is essential is invisible to the eye. Antoine de Saint-Exupery
Of course, Saint-Exupery didn't have extra dimensions in mind when he wrote this famous line. Nevertheless, the recent realisation that standard model degrees of freedom can naturally be restricted to a submanifold embedded in a higher dimensional Universe means that an ingredient essential to our description of nature might quite literally be "invisible to the eye".
Exploring the consequences of such braneworld scenarios has occupied a large part of the theoretical physics community over the last seven years, and this thesis is a collection of contributions to this endeavour.
After reviewing the motivations for and early successes of braneworld scenarios, we examine rho2 corrections to the Hubble rate in the stabilized Randall-Sundrum I model, where the hierarchy problem is solved in a natural way, in order to ascertain whether such corrections might be of help in addressing some issues with inflation and baryogenesis. The three following chapters are concerned with six-dimensional models that have been advertised as possibly leading to a self-tuning solution to the cosmological constant problem. We examine this claim thoroughly, through the study of thick codimension-two braneworlds. This allows us to provide a generalization of the relationship between the deficit angle and the brane matter content. We also present the first derivation of the Friedmann equations on a codimension-two brane containing matter with an arbitrary equation of state, first in the context of Einstein-Hilbert gravity and then in six dimensional supergravity.

This thesis is concerned with the interface of cosmology and condensed matter. Although at either end of the scale spectrum, the two disciplines have more in common than one might think. Condensed matter theorists and high-energy field theorists study, usually independently, phenomena embedded in the structure of a quantum field theory. It would appear at first glance that these phenomena are disjoint, and this has often led to the two fields developing their own procedures and strategies, and adopting their own nomenclature. We will look at some concepts that have helped bridge the gap between the two subjects, enabling progress in both, before incorporating condensed matter techniques to our own cosmological model. By considering ideas from cosmological high-energy field theory, we then critically examine other models of astrophysical condensed matter phenomena. In Chapter 1, we introduce the current cosmological paradigm, and present a somewhat historical overview of the interplay between cosmology and condensed matter. Many concepts are introduced here that later chapters will follow up on, and we give some examples in which condensed matter physics has had a very real effect on informing cosmology. We also reflect on the most recent incarnations of the condensed matter / cosmology interplay, and the future of these developments. Chapter 2 presents the Einstein-Klein-Gordon system of equations and their non-relativistic and nonlinear counterparts, the Schrodinger-Poisson, and nonlinear Schrodinger (Gross Pitaevskii)-Poisson systems. We give a more technical overview of the various applications of these systems of equations, as well as discussing the role and interpretation of condensates in the field of cosmology. In Chapter 3 we discuss more qualitatively the fluid-mechanical methods used in a wave-mechanical approach to structure formation, and in formulations of condensed matter models. Taking a lead from the condensed matter side, we look at some of the details of the Gross-Pitaevskii equation, particularly with regard to quantum vortices, and then put this quantum-mechanical system into a cosmological environment by coupling it to the Poisson equation, in an effort to pin down some of the parameters that may be consistent with the existence of vortices in a cosmological Bose-Einstein condensate. In Chapter 4 we turn to high-energy field theory and elucidate further some of the relationships with condensed matter physics that are present. We also critically examine a Bose-Einstein dark matter model in light of these considerations. Chapter 5 rounds off with a discussion and suggestions for further work based upon models we have discussed, as well as some ideas for models that have not yet been mentioned. An appendix discusses techniques for moving from the relativistic Einstein-Klein-Gordon equations to the Schrodinger-Poisson system.

The motivation of this study is the eventual application to calculating braneworld cosmological perturbations in order to determine observational signatures for the existence of extra dimensions. First, we construct five-dimensional bulk spacetime solutions, empty except for a negative cosmological constant, in the absence of any cosmological perturbations. Taking a higher-dimensional variant of Birkoff’s theorem, we find anti-de Sitter bulk solutions to the five-dimensional Einstein equations exhibiting three-dimensional spatial homogeneity and isotropy. We introduce branes as spacetime trajectories endowed with the same three-dimensional spatial symmetry as the bulk, hence capable of reproducing the observed homogeneity and isotropy of the universe. Second, we analyse the viability of a mechanism for the generation of a braneworld immersed in five-dimensional anti-de Sitter spacetime. An initial phase of de Sitter spacetime, which rapidly tends to an attractor vacuum state, is postulated. Through quantum tunnelling, bubbles of anti-de Sitter nucleate in the background de Sitter spacetime. From the collision of two such bubbles results a four-dimensional surface of constant negative spatial curvature where our four-dimensional universe will be localized. This mechanism has the advantage of explaining the bulk smoothness and horizon problems, of reproducing three-dimensional spatial homogeneity and isotropy, and also of giving rise to a warped geometry with the potential to explain the hierarchy problem. Third, we explore the nature of the brane-bulk interaction with a view towards computing the linearized cosmological perturbations for the models previously constructed. In particular, we focus on the study of the production and propagation of gravitational waves, since these are the only bulk degrees of freedom. We present the study of the quantum field theory of modes of a spring connected to a semi-infinite string in situations of increasing complexity, which exhibit all the qualitative features of the brane-bulk system except for gravitational gauge invariance. The production of gravitons is described by the motion of the spring, with the waves thus produced propagating through the string. A perturbative expansion of the brane propagator in five dimensions is contructed. Fourth, we consider the problem of gravitational gauge fixing for a brane moving on the boundary of the bulk spacetime solutions, its trajectory being determined by the evolution of its stress-energy content. The formulation of boundary conditions is essential for the complete definition of the problem of computing cosmological perturbations on a moving boundary. We derive the boundary conditions for linearized perturbations in a moving brane background spacetime and study the compatibility with the induced Lorentz gauge conditions. Fifth, we work out the effects on the graviton propagator due to the presence of simple forms of matter on the brane, namely a scalar field and a perfect fluid, which reproduce and inflationary and radiation/matter-dominated regimes from the four-dimensional perspective. We derive the boundary conditions modified by the presence of matter on the brane and expand perturbatively the brane and the bulk propagators.

In this thesis we investigate a number of roles that scalar fields can play in cosmology. In particular it is generally believed that the early Universe underwent a period of very rapid expansion. We call this epoch inflation. Initially we investigate the evolution of two slow-rolling scalar fields with potentials of the form V = V_oφ^-a exp(-bφ^m). By considering different values of the parameters a, b and m, we drive several new inflationary solutions in which one field just evolves in the background and is not important for the inflationary dynamics. In addition, we find new solutions where both fields are dynamically important during inflation. Moreover, we discuss the evolution of perturbations in both the scalar fields and the spacetime metric, concentrating on the production of entropy perturbations. We find that for a large region in parameter space and initial conditions, tensor modes are negligible, and that adiabatic and isocurvature perturbations are essentially uncorrelated. We move on to consider perturbations in the Randall-Sundrum braneworld scenario. At energies higher than the brane tension, the dynamics of a scalar field rolling down a potential are modified compared with the predictions of General Relativity. These modifications imply, among other things, that steeper potentials can be used to drive an epoch of slow-roll inflation. The evolution of entropy and adiabatic modes during inflation driven by two scalar fields confined to the brane is studied. We show that the amount of entropy perturbation produced during inflation is suppressed compared to the predictions made by General Relativity. As a consequence, the initial conditions do not matter in multiple field inflation in braneworlds if inflation is driven at energies much higher than the brane tension. Following this, we study the evolution of slow-roll inflation in a five-dimensional braneworld model with two boundary branes and a bulk scalar field. Assuming that the inflationary scale is below the brane tension, we can employ the moduli space approximation to study the dynamics of the system. Detuning the brane tension results in a potential for the moduli fields which we show will not support a period of slow-roll inflation. We then study an inflation field confined to the positive tension brane, to which the moduli fields are non-minimally coupled. We discuss in detail the two cases of V (χ) = ½m² χ ² and V (χ) = l χ ⁴ and demonstrate that increasing the coupling results in spectra which are further away from scale-invariance. Furthermore, there is an increase in the tensor mode production, while entropy perturbations are subdominant. Finally, we point out that the five-dimensional spacetime is unstable during inflation because the negative tension brane collapses.

This thesis is concerned with the amount and distribution of dark matter in the universe and makes use of Bayesian techniques to extract maximal information. I have been using two independent approaches. I have (i) compared and combined cosmological parameter estimates from various cosmological probes and (ii) I have developed a method for estimating the mass distribution in clusters of galaxies using gravitational lensing. The first approach tests cosmological theories and estimates the cosmological parameters, including the total amount of matter. The second produces maps of the dark matter in the largest gravitationally bound structures in the universe. Chapter 3 is also published as 'Cosmological Parameters from Cluster Abundances, CMB and IRAS' by Bridle et al. 1999 (MNRAS, 310, 565). Chapter 4 is also available as 'Cosmological Parameters from Velocities, CMB and Supernovae' by Bridle et al. 2000 (astro-ph/0006170, MNRAS accepted). Chapter 5 contains the work presented in Lahav, Bridle, Hobson, Lasenby and Sodré Jr. 2000 (MNRAS, 315, L45) entitled 'Bayesian 'Hyper-Parameters' Approach to Joint Estimation: the Hubble Constant from CMB Measurements' and further demonstrates the properties of this approach by applying it to toy models. Part II concerns maximum-entropy reconstruction of mass distributions from weak gravitational lensing data and consists of two chapters. Chapter 6 sets out the basic method, also published in 'A maximum-entropy method for reconstructing the projected mass distribution of gravitational lenses' by Bridle et al. 1998 (MNRAS, 299, 895). Chapter 7 details extensions to this work, also shortly to become available in Bridle et al. 2000, entitled 'Maximum-Entropy Reconstruction of Gravitational Lenses using Shear and/or Magnification Data'. Future directions are suggested in the conclusions to each part and in the Concluding Remarks.

We outline the application of a new method of evidence calculation called nested sampling Skilling (2004). We use a clustered ellipsoidal bound to restrict the parameter space sampled, that is generic enough to be used for even complex multimodal posteriors. We demonstrate our algorithms, COSMOCLUST makes important savings in computational time when compared with previous methods. The study of the primordial power spectrum, which seeded the structure formation observed in both the CMB and large scale structure, is crucial in unravelling early universe physics.  In this thesis we analyse a number of spectral parameterisations based on both physical and observational grounds. Using the evidence we determine the most appropriate model in both WMAP 1 year and WMAP 3 year data (including additionally a selection of high resolution CMB and large scale structure data). We conclude that currently the evidence does suggest the need for a tilt in the spectrum, however the presence of running of the spectral index is dependent on the inclusion of, specifically Ly-α data. Bayesian analysis in cosmology is computationally demanding. We have succeeding in improving the efficiency of inference problems for a wide variety of cosmological applications by training neural networks to ‘learn’ how observables such as the CMB spectrum change with input cosmological parameters. We demonstrate that improvements in speed of several orders of magnitude are possible using our algorithm COSMONET.

This thesis is devoted to studying two important aspects of braneworld physics: their cosmology and their holography. We examine the Einstein equations induced on a general (n - 2)-brane of arbitrary tension, embedded in some n-dimensional bulk. The brane energy-momentum tensor enters these equations both linearly and quadratically. From the point of view of a homogeneous and isotropic brane we see quadratic deviations from the FRW equations of the standard cosmology. There is also a contribution from a bulk Weyl tensor. We study this in detail when the bulk is AdS-Schwarzschild or Reissner-Nordstrom AdS. This contribution can be understood holographically. For the AdS-Schwarzschild case, we show that the geometry on a brane near the AdS boundary is just that of a radiation dominated FRW universe. The radiation comes from a field theory that is dual to the AdS bulk. We also develop a new approach which allows us to consider branes that are not near the AdS boundary. This time the dual field theory contributes quadratic energy density/pressure terms to the FRW equations. Remarkably, these take exactly the same form as for additional matter placed on the brane by hand, with no bulk Weyl tensor. We also derive the general equations of motion for a braneworld containing a domain wall. For the critical brane, the induced geometry is identical to that of a vacuum domain wall in (n-l)-dimensional Einstein gravity. We develop the tools to construct a nested Randall-Sundrum scenario whereby we have a "critical" domain wall living on an anti-de Sitter brane. We also show how to construct instantons on the brane, and calculate the probability of false vacuum decay.