Doutorado em Astrofísica, Cosmologia e Gravitação

URI Permanente para esta coleção

http://repositorio.ufes.br/handle/10/17489

Nível: Doutorado
Ano de início: 2016
Conceito atual na CAPES: 5
Ato normativo: Parecer 487/2018
Periodicidade de seleção: Semestral
Área(s) de concentração: Astronomia e Física
Url do curso: https://cosmologia.ufes.br/pt-br/pos-graduacao/PPGCosmo/detalhes-do-curso?id=1453

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Agora exibindo 1 - 5 de 15

On the degenerate dynamics of branched hamiltonians
(Universidade Federal do Espírito Santo, 2024-09-21) Ferreira Junior, Alexsandre Leite; Zanelli, Jorge; https://orcid.org/0000-0001-7512-4284; Pinto-Neto, Nelson; https://orcid.org/0000-0001-6713-5290; Fabris, Júlio; Piattella, Oliver Fabio; Bittencourt, Eduardo; Gannouji, Radouane; Helayël-Neto, José
Branched Hamiltonians and the corresponding singularity are present in several inter esting physical systems: Lovelock extension of General Relativity in higher dimensions, classical time crystals, k-essence fields, Horndeski theories, compressible fluids, and nonlinear electrodynamics. The emergent ill defined sympletic structure and tricky dynamical evolution poses challenges to a consistent interpretation. In this thesis, multi-valued Hamiltonians are investigated in the framework of degenerate dynamical system, whose sympletic form does not have a constant rank, allowing novel features and interpretations not present in previous investigations. In particular, it is shown how the multi-valuedness is associated with a dynamical mechanism of dimensional reduction, as some degrees of freedom turn into gauge symmetries when the system degenerates. In the case of classical time crystal, there is no “moving” ground state nor brick wall solution, as described previously. Moreover, the degenerate dynamics of a k–essence model enables it to be responsible for both primordial inflation and the present observed acceleration of the cosmological background geometry, while also admitting a non-singular de Sitter beginning of the Universe (it arises from de Sitter and ends in de Sitter). Furthermore, the model is free of pathologies such as propagating superluminal perturbations, negative energies, and perturbation instabilities. Henceforth, in thesis is demonstrated that the degenerate dynamics offer a consistent interpretation, under which the degeneracy and consequent branching is not a problem but a dynamical feature.
The redshift drift as a new cosmological probe
(Universidade Federal do Espírito Santo, 2024-09-19) Dutra, Pedro Henrique Bessa Rodrigues; Durrer, Ruth; https://orcid.org/0000-0001-9833-2086; Marra, Valerio; https://orcid.org/0000-0002-7773-1579; Marttens, Rodrigo vom; Piattella, Oliver Fabio; Koksbang, Sofie Marie; Martins, Carlos
With the advent of precision cosmology and the next generation of telescopes and sur veys, observations will allow the standard cosmological model to be tested in old and new regimes with unprecedented accuracy, and the data will demand accurate the oretical models of cosmological phenomena in order to be properly interpreted and processed. Theorists are reassessing old and new probes that allow the current cosmo logical paradigm to be tested to its limits. One promising new probe, the measurement which is one of the main objectives of the ELT ANDES spectrograph, is the cosmic redshift drift. One of the theoretical predictions of cosmological models based on Robertson-Walker metrics is the cosmic redshift drift, which measures the real-time rate of change of the spectra of far away sources, and is a generic consequence of cosmological models where the expansion rate is non-constant. Its observation provides a model independent way to measure the time variation of the Universe’s expansion and as such, an independent test of the late-time acceleration phase of the Universe and the existence of a cosmo logical constant. In this thesis we study the redshift drift in realistic cosmological models containing inhomogeneities in the !CDM paradigm. We derive for the first time a gauge-invariant expression for the redshift drift and its power spectrum, and implement the redshift drift f luctuation power spectrum numerically through Einstein-Boltzmann codes and using n-body simulations, providing a solid theoretical foundation for future measurements and observations of the redshift drift in !CDM cosmology and beyond.
Probing cosmology with an eye on Rubin : from strong lensing to the large scale structure of the universe
(Universidade Federal do Espírito Santo, 2024-04-11) Oliveira, Renan Alves de; Ho, Shirley ; https://orcid.org/0000-0002-1068-160X; Makler, Martín; https://orcid.org/0000-0003-2206-2651; http://lattes.cnpq.br/6567844719949395; https://orcid.org/0000-0002-0200-3833; http://lattes.cnpq.br/1895596998416086; Abramo, Luis Raul Weber ; https://orcid.org/0000-0001-8295-7022; http://lattes.cnpq.br/4558796258762790; Bom, Clécio Roque de ; https://orcid.org/0000-0003-4383-2969; http://lattes.cnpq.br/5635352837026339; Velten, Hermano Endlich Schneider ; https://orcid.org/0000-0002-5155-7998; http://lattes.cnpq.br/0282590467459210; Marra, Valerio ; http://orcid.org/0000-0002-7773-1579; http://lattes.cnpq.br/6846011112691877
In 2024, the Vera C. Rubin Observatory will begin observing the Universe for the next ten years. Two key cosmological observables that Rubin will probe are gravitational lensing and the large-scale structure of the Universe. In this thesis, we derive analytical solutions for strongly lensed images that can be useful for generating fast simulations and as a starting point in parameter searches for lens inversion. Then, we obtain an expression in closed form for the magnification cross-section, which can be used to predict the abundance of highly magnified sources. Next, we focus on real data and assemble an extensive compilation of Strong Lensing candidate systems from the literature containing over 30,000 unique objects. We cross-match this sample with the current major photometric and spectroscopic catalogs. As preparation for Rubin, we generate image cutouts for these systems in most current wide-field surveys with subarcsecond seeing, namely DES, HSC, KiDS, CFHTLens, RCSLens, and CS82. This sample dubbed the “Last Stand Before Rubin” (LaStBeRu), has a myriad of applications, from using archival data to selections for follow-up projects and training of machine learning algorithms. As an application, we have performed a test of General Relativity (GR) with these data, combining information from strong lensing and velocity dispersions, which allow one to set constraints on the Post-Newtonian parameter γPPN. From the LaStBeRu database, we were able to provide the first independent test of γPPN from previous results and for the first time only for systems identifiable in ground-based images. We can obtain the most stringent constraint on γPPN by combining these data with the current samples. Moreover, we have obtained new spectroscopic data for systems selected from LaStBeRu, which were used to obtain the first end-to-end determination of γPPN. It is also the first determination derived purely from ground-based data and the first to use self-consistent priors. Our results are consistent with GR at the ∼ 1-σ level and with the previous results from the literature. Finally, in the context of the large structure, we present two neural emulators capable of making fast predictions for the density, displacement, and velocity fields of dark matter particles without necessarily having to run expensive N-body simulations. We compared these emulators with another fast method for the same task, showing that neural emulators provide the best results
An analytic study of lensing by black holes in Kerr-de Sitter spacetimes
(Universidade Federal do Espírito Santo, 2023-08-11) Omwoyo, Eunice Monyenye; Belich Junior, Humberto; https://orcid.org/0000-0002-8795-1735; http://lattes.cnpq.br/3879935393431243; https://orcid.org/0000-0003-4660-8161; http://lattes.cnpq.br/5310812834959626; Fabris, Julio Cesar; https://orcid.org/000000018880107X; http://lattes.cnpq.br/5193649615872035; Falciano, Felipe Tovar; https://orcid.org/0000000322631252; http://lattes.cnpq.br/7214193952056222; Velten, Hermano Endlich Schneider; https://orcid.org/0000000251557998; http://lattes.cnpq.br/0282590467459210; Stuchilk, Zdenek; Dotti, Gustavo
The recent release of the images of M87 and Sagittarius A∗ (SgrA∗ ) black holes by the Event Horizon Telescope (EHT) collaboration has provided unprecedented insights into the emission structure on horizon scales. As technology advances, the aim is to capture even sharper and more detailed images which is among the main aims of the next generation Event Horizon Telescope (EHT). This raises the question of what can be expected and learned from highly resolved black hole images. In-depth studies using general relativistic magnetohydrodynamics simulations reveals that a highly resolved black hole image exhibits a distinct feature called the photon ring. This feature persists in the simulations, regardless of the nature of the astrophysical source profile surrounding the black hole. The photon ring is generated by photons on trajectories that have undergone extreme bending due to the strong gravity of the black hole, causing them to execute multiple orbits. As such, it is intricately connected to the specific properties and spacetime geometry in the vicinity of the black hole and is less sensitive to the astrophysical source profile around the black hole. Besides, the photon ring exhibits a nested sequence of self similar subrings that exponentially converge to the critical curve. The critical curve is purely a theoretical entity whose shape directly follows from General Relativity (GR) but is not in itself observable. However, the photon ring is in principle detectable in the near future observations. Given that this feature is contingent on spacetime geometry and black hole properties, its detection presents the potential for more robust tests of General Relativity (GR) and the Kerr hypothesis. Given the significance thereof, it is vital to conduct an extensive study and make predictions about the explicit nature of the photon ring in various black hole spacetimes. In this thesis, we present the photon ring structure in asymptotically de Sitter spacetimes, with emphasis on the Kerr-de Sitter (KdS) and Kerr-de Sitter Revisited (RKdS) spacetimes. Our analytical approach begins by obtaining solutions to the null geodesic equations in these spacetimes in terms of the Jacobi elliptic functions. These solutions shed light on the overall structure of bound and nearly bound photon orbits, which are the orbits central to this thesis. Subsequently, we delve into the analysis of the critical curve, for which we focus on observers located in the vicinity of the static radius. Moreover, utilizing the solutions we conduct an analytical ray-tracing to explore the properties of direct images, lensed rings, and photon rings. We also consider the special case of zero spin and zero cosmological constant. Our analysis takes into account locally static observers and assumes equatorial disks around the black holes. We compare the various images to the corresponding critical curves. Images arising from photons that have made 2 or more half orbits around the black hole exhibit a remarkable resemblance to the critical curve and are located in close proximity to this curve. Furthermore, these images demonstrate the same universal behavior as the critical curve, such as a more circular shape for small black hole spin and observer inclination angles, as well as a flattened appearance on one side for larger spin and inclination angles. From our study, these images demonstrate a more promising arena for tests of General Relativity (GR) than images arising from photons that have executed one half orbit around the black hole. Besides, we investigate the parameters that govern the subsequent rings’ exponential demagnification, rotation, and detection delay. These parameters are the Lyapunov exponent, the azimuthal angle change, and the time delay.
Enhancing constraints and precision studies for a deeper understanding of the universe using CMB and 21cm maps
(Universidade Federal do Espírito Santo, 2023-08-09) Mokeddem, Rahima; Ricaldi, Wiliam Santiago Hipolito; https://orcid.org/000000021748553X; http://lattes.cnpq.br/0293080746483402; https://orcid.org/0000000283795106; http://lattes.cnpq.br/3993334728622346; Souza, Carlos Alexandre Wuensche de; https://orcid.org/0000-0003-1373-4719; http://lattes.cnpq.br/1448223845901360; Rodrigues, Davi Cabral; https://orcid.org/0000000316835443; http://lattes.cnpq.br/5465449494182034; Leo, Armando Bartolome Bernui; https://orcid.org/0000-0003-3034-0762; http://lattes.cnpq.br/6926651124954505; Marra, Valerio; https://orcid.org/0000000277731579; http://lattes.cnpq.br/6846011112691877
This thesis focuses on enhancing constraints and precision studies to deepen our understanding of the universe. By improving measurements and testing methods, we aim to achieve higher-resolution simulations and more precise results. The thesis specifically addresses the lensing amplitude parameter, investigating its accuracy and quantifying any discrepancies. Additionally, we explore the generation of 21cm mocks in different cosmologies, using the remapping technique to obtain reliable results. The findings of this thesis have the potential to revolutionize our understanding of large-scale structures, the cosmic microwave background, and the early and late stages of the universe by precisely estimating the lensing amplitude AL and by introducing for the first time 21cm mocks in three different cosmologies, that were obtained in a very timely mannered way starting from one dark matter halo catalogue.

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