Thesis (Ph. D.)--University of Oregon, 2006. / Typescript. Includes vita and abstract. Includes bibliographical references (leaves 75-80). Also available for download via the World Wide Web; free to University of Oregon users.

Made available in DSpace on 2016-05-17T16:51:00Z (GMT). No. of bitstreams: 0 Previous issue date: 2007-12-19. Added 1 bitstream(s) on 2016-05-17T16:54:21Z : No. of bitstreams: 1 000855561.pdf: 728928 bytes, checksum: d9c47745ba9f0ff381d16d64c98da423 (MD5) / Investigamos a formação, em larga escala, de estruturas no Universo, na presença da energia escura. Sua influência sobre o crescimento de perturbações cosmológicas é exercida tanto através do efeito sobre a taxa de expansão do fundo cósmico homogêneo, quanto de suas próprias flutuações de densidade de energia. Para calcularmos a taxa de formação de aglomerados de galáxias, empregamos uma generalização do formalismo de colapso esférico para a inclusão de fluidos com pressão. Um importante efeito de flutuações de energia escura associados a halos de matéria escura é a indução de halos de energia escura, que reprimem o crescimento de estruturas quando temos equações de estado não phantom; por outro lado, quando temos equações de estado phantom, são gerados vazios de energia escura, aumentando o crescimento de estruturas de matéria. Outro importante efeito ocorre quando consideramos a possibilidade da energia escura mudar sua equação de estado quando há grandes variações de sua densidade no interior dos halos em relação ao fundo homogêneo. O grande número de parametrizações da energia escura que foram obtidos com dados, de supernovas Ia são sensíveis apenas até desvios para o vermelho de ordem um. Mostramos que as parametrizações produzem assinaturas distintas na formação de aglomerados com o uso do formalismo de Press-Schechter. Portanto, futuras observações de aglomerados galácticos podem prover vínculos importantes no comportamento da energia escura durante a evolução do Universo / We investigate large scale structure formation in universe with dark energy presence. The dark energy influence on cosmological perturbation growth is exerted both through its effect on the expansion rate of background, and through its own density fluctuation as well. To compute the rate of formation of massive objects we employed the spherical collapse formalism, which was generalized to include fluids with pressure. An important effect caused by fluctuations in dark energy associated with dark matter halos is the induction of dark energy halos damping the growth of structures when the equations of state are non-phantom; on the other hand, phantom models generate dark energy voids, enhancing the growth of matter halos. Other important effect occurs when we consider the possibility of dark energy changing its equation of state when there are large differences between densities in the background and in the halos. The large number of dark energy parametrizations obtained with supernova Ia data are only sensitive to redshifts up to order one. We show these parametrizations produce distinguishable signatures in cluster formation using the Press-Schechter formalism. Therefore, future observations of galaxy clusters can provide important constraints on the behavior of dark energy in the course of universe evolution

Orientador: Rogério Rosenfeld / Banca: Ruben Aldrovandi / Banca: Luís Raul Weber Abramo / Resumo: Nos últimos anos diversas evidências acumularam-se indicando que o universo é plano e dominado por alguma forma de energia escura, cuja pressão negativa está atualmente fazendo com que ele esteja em expansão acelerada. Vários modelos têm sido propostos para a energia escura, entre os quais destacam-se os modelos de quintessência, nos quais essa energia é modelada por um campo escalar. Neste trabalho analisamos alguns vínculos observacionais nos modelos de quintessência com potenciais exponenciais, e obtivemos limites para o espaço de parâmetros desses modelos no caso em que a quintessência está desacoplada dos demais componentes do universo e no caso em que ela está acoplada à partícula de matéria escura. No caso desacoplado, estudamos as soluções do tipo scaling, e mostramos que nesse regime, o único em que a quintessência exponencial desacoplada apresenta soluções cosmologicamente realísticas, esses modelos não podem ser considerados menos naturais que os demais potenciais de quintessência. Obtivemos ainda que o caso acoplado, assim como o desacoplado, também não resolve o problema da coincidência cósmica, e que a idade do universo nestes modelos é consideravelmente maior que no caso desacoplado, de modo que os limites na idade podem ser úteis para distinguir observacionalmente entre as quintessências acoplada e desacoplada / Abstract: During the last years many evidences are indicating that the universe is flat and dominated by some form of dark energy, whose negative pressure is currently driving its accelerated expansion. A plenty of models have been proposed, with special attention to the quintessence models, in which the dark energy is modelled by a scalar field. In this work we have analysed some observational constraints in the quintessence models with exponential potentials, and we have put limits on the parameter space in both coupled and uncoupled cases. In the uncoupled case, we have studied the scaling Solutions, and we have showed that in this regime, that is the only one in which the exponential uncoupled quintessence presents realistic Solutions, such models can not be considered less natural than others quintessence potentials. We have also obtained that in the case in which the quintessence is coupled to dark matter the cosmic coincidence problem can not be solved, and that the age for coupled models is considerably higher than the age for non-coupled models, in such a way that limits on the age can be useful in distinguishing between coupled and non-coupled models. / Mestre

Orientador: Rogério Rosenfeld / Banca: Reuven Opher / Banca: Ioav Waga / Banca: Ruben Aldrovandi / Banca: José Ademir Sales de Lima / Resumo: Investigamos a formação, em larga escala, de estruturas no Universo, na presença da energia escura. Sua influência sobre o crescimento de perturbações cosmológicas é exercida tanto através do efeito sobre a taxa de expansão do fundo cósmico homogêneo, quanto de suas próprias flutuações de densidade de energia. Para calcularmos a taxa de formação de aglomerados de galáxias, empregamos uma generalização do formalismo de colapso esférico para a inclusão de fluidos com pressão. Um importante efeito de flutuações de energia escura associados a halos de matéria escura é a indução de halos de energia escura, que reprimem o crescimento de estruturas quando temos equações de estado não phantom; por outro lado, quando temos equações de estado phantom, são gerados vazios de energia escura, aumentando o crescimento de estruturas de matéria. Outro importante efeito ocorre quando consideramos a possibilidade da energia escura mudar sua equação de estado quando há grandes variações de sua densidade no interior dos halos em relação ao fundo homogêneo. O grande número de parametrizações da energia escura que foram obtidos com dados, de supernovas Ia são sensíveis apenas até desvios para o vermelho de ordem um. Mostramos que as parametrizações produzem assinaturas distintas na formação de aglomerados com o uso do formalismo de Press-Schechter. Portanto, futuras observações de aglomerados galácticos podem prover vínculos importantes no comportamento da energia escura durante a evolução do Universo / Abstract: We investigate large scale structure formation in universe with dark energy presence. The dark energy influence on cosmological perturbation growth is exerted both through its effect on the expansion rate of background, and through its own density fluctuation as well. To compute the rate of formation of massive objects we employed the spherical collapse formalism, which was generalized to include fluids with pressure. An important effect caused by fluctuations in dark energy associated with dark matter halos is the induction of dark energy halos damping the growth of structures when the equations of state are non-phantom; on the other hand, phantom models generate dark energy voids, enhancing the growth of matter halos. Other important effect occurs when we consider the possibility of dark energy changing its equation of state when there are large differences between densities in the background and in the halos. The large number of dark energy parametrizations obtained with supernova Ia data are only sensitive to redshifts up to order one. We show these parametrizations produce distinguishable signatures in cluster formation using the Press-Schechter formalism. Therefore, future observations of galaxy clusters can provide important constraints on the behavior of dark energy in the course of universe evolution / Doutor

Philosophiae Doctor - PhD / The matter power spectrum is key to understanding the growth of large-scale structure in the Universe. Upcoming surveys of galaxies in the optical and HI will probe increasingly large scales, approaching and even exceeding the Hubble scale at the survey redshifts. On these cosmological scales, surveys can in principle provide the best constraints on dark energy (DE) and modified gravity models and will be able to test general relativity itself. However, in order to realise the potential of these surveys, we need to ensure that we are using a correct analysis, i.e. a general relativistic analysis, on cosmological scales. There are two fundamental issues underlying the general relativistic (GR) analysis. Firstly, we need to correctly identify the galaxy overdensity that is observed on the past light cone. Secondly, we need to account for all the distortions arising from observing on the past light cone, including redshift distortions (with all general relativistic effects included) and volume distortions. These general elativistic effects appear in the angular power spectra of matter in redshift space. We compute these quantities, taking into account all general relativistic large-scale effects, and including the important contributions from redshift space distortions and lensing convergence. This is done for self-consistent models of DE, known as ‘quintessence’, which have only been very recently treated in the GR approach. Particularly, we focus mainly on computing the predictions (i.e. the power spectra) that need to be confronted with future data. Hence we compute the GR angular power spectra, correcting the 3D Newtonian calculation for several quintessence models. We also compute the observed 3D power spectra for interacting DE (which until now have not previously been studied in the GR approach) – in which dark matter and DE exchange energy and momentum. Interaction in the dark sector can lead to large-scale deviations in the power spectrum, similar to GR effects or modified gravity. For the quintessence case, we found that the DE perturbations make only a small contribution on the largest scales, and a negligible contribution on smaller scales. Ironically, the DE perturbations remove the false boost of large-scale power that arises if we impose the (unphysical) assumption that the DE perturbations vanish. However, for the interacting DE (IDE) case, we found that if relativistic effects are ignored, i.e. if they are not subtracted in order to isolate the IDE effects, the imprint of IDE will be incorrectly identified – which could lead to a bias in constraints on IDE, on horizon scales. Moreover, we found that on super-Hubble scales, GR corrections in the observed galaxy power spectrum are able to distinguish a homogeneous DE (being one whose density perturbation in comoving gauge vanishes) from the concordance model (and from a clustering DE) – at low redshifts and for high magnification bias. Whereas the matter power spectrum is incapable of distinguishing a homogeneous DE from the concordance model. We also found that GR effects become enhanced with decreasing magnification bias, and with increasing redshift.

No description available.

Philosophiae Doctor - PhD / We present a new sample of distant powerful radio galaxies, in order to study their host-galaxy properties and provide targets for future observations of Hi absorption with new radio telescopes. We cross-match the Sydney University Molonglo Sky Survey radio catalogue at 843 MHz with the VISTA Hemisphere Survey near-infrared catalogue using the Likelihood Ratio technique, producing contour plots as a way to inspect by eye a subset of bright sources to validate the automated technique. We then use the Dark Energy Survey optical and near-infrared wavelength data to obtain photometric redshifts of the radio sources. We find a total of 249 radio sources with photometric redshifts over a 148 square degree region. By fitting the optical and near-infrared photometry with spectral synthesis models, we determine the stellar masses and star-formation rates of the radio sources. We find typical stellar masses of 1011−1012 M for the powerful high-redshift radio galaxies. We also find a population of low-mass blue galaxies. We then report results from the first search for associated Hi 21 cm line absorption with the new MeerKAT radio telescope (shared-risk early science programme). We used a 16-antenna sub-array of MeerKAT to carry out a survey for Hi absorption in the host galaxies of nine powerful (L1.4 GHz > 1026 W Hz−1 ) radio galaxies at cosmological distances (z = 0.29 to 0.54). We found no evidence of absorption with 5σ optical depth detection limits. We only obtain a tentative absorption towards a radio source 3C 262 at z = 0.44 with significant ongoing star formation at a rate of 10.5 M yr−1 . The source consists of two radio lobes separated by 28.5 kpc with no evidence of a compact core. If the absorption arises from neutral gas from an extended disc, the line is redshifted by 79(21) km s−1 with respect to the nucleus and has an average Hi column density across the source of NHI ∼ 7 × 1019−20 cm−2 , which is consistent with the rate of star formation. But after further tests, we find that the Hi detection towards 3C 262 is likely to be an artefact. We conclude that the new correlator with 32 k channel resolution will be needed before searching for its associated absorption in MIGHTEE data.

No description available.

Philosophiae Doctor - PhD / The standard model of cosmology (the ɅCDM model) has been very successful and is compatible with all observational data up to now. However, it remains an important task to develop and apply null tests of this model. These tests are based on observables that probe cosmic distances and cosmic evolution history. Supernovae observations use the so-called `standard candle' property of SNIa to probe cosmic distances D(z). The evolution of the expansion rate H(z) is probed by the baryon acoustic oscillation (BAO) feature in the galaxy distribution, which serves as an effective `standard ruler'. The observables D(z) and H(z) are used in various consistency tests of ɅCDM that have been developed. We review the consistency tests, also looking for possible new tests. Then the tests are applied, first using existing data, and then using mock data from future planned experiments. In particular we use data from the recently commissioned Dark Energy Survey (DES) for SNIa. Gaussian Processes, and possibly other non-parametric methods, used to reconstruct the derivatives of D (z) and H (z) that are needed to apply the null tests of the standard cosmological model. This allows us to estimate the current and future power of observations to probe the ɅCDM model, which is the foundation of modern cosmology. In addition, we present an improved model of the HI galaxy number counts and bias from semi-analytic simulations, and we use it to calculate the expected yield of HI galaxies from surveys with a variety of phase 1 and 2 SKA configurations. We illustrate the relative performance of the different surveys by forecasting errors on the radial and transverse scales of the BAO feature. We use the Fisher matrix method to estimate the error bars on the cosmological parameters from future SKA HI galaxy surveys. We find that the SKA phase 1 galaxy surveys will not contend with surveys such as the Baryon Oscillation Spectroscopic Survey (BOSS) whereas the full "billion galaxy survey" with SKA phase 2 will deliver the largest dark energy Figure of Merit of any current or future large-scale structure survey. / South African Square Kilometre Array Project (SKA) and German Academic Exchange Service (DAAD)

Doctor of Philosophy / Department of Physics / Bharat Ratra / High precision cosmological observations in last decade suggest that about 70% of our universe's energy density is in so called "Dark Energy" (DE). Observations show that DE has negative effective pressure and therefore unlike conventional energy sources accelerates the cosmic expansion instead of decelerating it. DE is highly uniform and has become a dominant component only recently. The simplest candidate for DE is the time-independent cosmological constant. Although successful in fitting available data, the cosmological constant model has a number of theoretical shortcomings and because of that alternative models of DE are considered. In one such scenario a cosmological scalar field that slowly rolls down its potential acts like a time-dependent cosmological constant. I have used different independent cosmological data sets to constrain the time dependence of DE's energy density in the framework of the slowly-rolling cosmological scalar field model. Present data favors a time-independent cosmological constant, but the time-dependent DE can not be ruled out at high confidence level. Ongoing and planned cosmological probes and surveys will provide more and better quality data over the next decade. When the new data sets are available we will be able to either detect the time dependence of DE or constrain it to a very small physically uninteresting value.