Astrofísica, Cosmologia e Gravitação

URI Permanente desta comunidade

http://repositorio.ufes.br/handle/10/17488
Programa de Pós-Graduação em Astrofísica, Cosmologia e Gravitação

Centro: CCE
Telefone: (27) 4009 2488
URL do programa: https://cosmologia.ufes.br/pt-br/pos-graduacao/PPGCosmo

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Navegando Astrofísica, Cosmologia e Gravitação por Assunto "Cosmologia"

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    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.
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    Modified gravity and large scale structure cosmology: a linear and non-linear treatment
    (Universidade Federal do Espírito Santo, 2022-06-10) Oliveira, Guilherme Brando de; Falciano, Felipe Tovar; https://orcid.org/0000000322631252; http://lattes.cnpq.br/7214193952056222; https://orcid.org/0000000308051905; Fabris, Julio Cesar; https://orcid.org/000000018880107X; http://lattes.cnpq.br/5193649615872035; Bragança, Vinicius Miranda; Quartin, Miguel Boavista; https://orcid.org/0000000158536164; http://lattes.cnpq.br/3080181268936724; Abramo, Luis Raul Weber; Wands, David Graham
    This thesis consists of a comprehensive study of beyond-ΛCDM cosmologies, in particular I investigate possible consequences of scalar-tensor theories of gravity on the Large Scale Structure of the Universe. Within the Standard Model of Cosmology, General Relativity is assumed to be the theory that describes gravity in all scales and this is supported by the highly accurate Astrophysical and Solar System tests. Notwithstanding, at cosmological scales, we still lack gravity tests with the same constraining power. Therefore, in addition to the motivation from the well-known conceptual problems of the Cosmological Constant, it is reasonable to investigate if General Relativity is the correct gravity theory at the largest scales of the Universe. In order to increase the accuracy of our cosmological tests of gravity, I develop numerical tools based on the linear and nonlinear regimes of cosmological perturbation theories, as well as a non-perturbative approach using quasi N-body simulations. I also present different ways of testing the large freedom introduced by modified theories of gravity in the parameter space. Indeed, modified gravity models cannot avoid introducing extra parameters besides the usual six cosmological parameters of the ΛCDM model. The main results of the thesis have been published in four papers cited along the text and I have tried to condensate them mainly in Chapters 3, 4 and 6. In chapter 3 I discuss the impact of modified gravity on cosmological observables such as the modifications Horndeski theories introduce in the growth and light propagation equations of motion. In particular, I perform a detailed analysis of the No Slip Gravity at the linear regime of structure formation. Then, I discuss how early modified gravity theories change the matter power spectrum at large and small scales. In Chapter 4, I start by analyzing the matter power spectrum at linear scales, namely how it is defined within ΛCDM and how massive neutrinos introduce a scale dependent on the growth function. Then, I introduce the formulation of the N-Body gauge, a specific coordinate system that facilitates the interpretation of Newtonian simulations within a relativistic framework, by consistently introducing the effects coming from photons, neutrinos and dark energy. As stage-IV LSS surveys will probe the Universe at increasingly large scales; it is imperative to include these species in our analysis inasmuch at large scales their imprint can be above the 1% threshold. I also present new cosmological tests of gravity by combining this framework with relativistic N-Body simulations. At the end I show how to correctly combine modified gravity effects and Newtonian simulations. In Chapter 5, I outline all the nonlinear mathematical tools have I have studied and developed during this project and on Chapter 6 I present the results of how we can construct computationally fast new numerical tools using all the new developments I have done in modified gravity, from linear to nonlinear scales. Chapter 7 ends the thesis with some conclusions and three future avenues I plan to pursue in the next few years.
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    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
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    Pushing the boundaries of modern cosmology: physics beyond the Copernican principle
    (Universidade Federal do Espírito Santo, 2022-08-16) Torres, David Francisco Camarena; Marra, Valerio; https://orcid.org/0000000277731579; http://lattes.cnpq.br/6846011112691877; https://orcid.org/0000000171650439; http://lattes.cnpq.br/2093261235489631; Piattella, Oliver Fabio; https://orcid.org/0000000345580574; http://lattes.cnpq.br/5707156831919279; Amendola, Luca; https://orcid.org/0000-0002-0835-233X; http://lattes.cnpq.br/7512327723238183; Sapone, Domenico; https://orcid.org/; http://lattes.cnpq.br/; Clarkson, Chris
    The standard paradigm of modern cosmology relies on a set of fundamental assumptions that simplify and make possible the modeling of the Universe. Among these critical hypotheses, there is the presumption that we do not occupy a special place in the Uni- verse, the so-called Copernican principle. The assumption of this principle constrains the degrees of freedom allowed by the theory, and, in particular, within the framework of the General Theory of Relativity, leads to a spatially homogeneous and isotropic space-time. Here, we present a program to observationally test the Copernican princi- ple and study the cosmological applications of inhomogeneous cosmologies. Under the assumption of a spherically inhomogeneous extension of the standard model and using the latest cosmological data, we test the Copernican principle by placing constraints on radial deviations of the spatially homogeneous and isotropic space-time. We also forecast the precision with which future surveys, such as DES, Euclid and LSST, will be able to test the Copernican principle and test their ability to detect any possible violations. Furthermore, we investigate if a local void could explain away the 5σ discrepancy between the early and late times determinations of the Hubble constant. Our goal is to take the first steps to extend the boundary of the standard paradigm of modern cosmology, and, in particular, to develop a suitable framework for the development of physics beyond the Copernican principle.
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    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.