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Fields of current loops around black holes
Vlasáková, Zuzana ; Semerák, Oldřich (advisor) ; Svítek, Otakar (referee)
The magnetic field of a test circular current loop placed symmetrically around a Schwarzschild black hole has been determined several times in the literature and solutions has been expressed by different formulas. We compare these formulas analytically as well as numerically, and show, in particular, how they behave on the symmetry axis, in the equatorial plane and on the horizon. The problem is relevant for modelling accretion discs around black holes.
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Covariant Loop Quantum Gravity
Irinkov, Pavel ; Hinterleitner, Franz (advisor) ; Svítek, Otakar (referee)
In this thesis we offer a broad introduction into loop quantum gravity against the backdrop of the quantum gravity research as a whole. We focus on both the canonical and covariant version of the theory. In the latter version we investigate the dynamics of some simple configurations in the simplified setting of Ponzano-Regge model. We ascertain that the naïve approach to define a consistent dynamics, where the path integral's partition function is computed as a sum of amplitudes corresponding to all boundary and bulk states, fails in this case, on account of an appearance of divergences. This opens up space for the utilization of some more sophisticated methods.
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Gravitational waves in cosmology
Kadlecová, Alžběta ; Svítek, Otakar (advisor) ; Žofka, Martin (referee)
In this work, we study the backreaction of high-frequency gravitational waves on cosmological backgrounds. To describe the wave, we use the Isa- acson formalism, specifically the WKB approximation, which allows us to express the backreaction through an effective stress-energy tensor of the gra- vitational wave. First, we consider the inhomogeneous cosmological model of Charach and Malin, which contains gravitational waves and a massless scalar field mini- mally coupled to gravity. We show that although this is a spatially compacti- fied solution, it is possible to add a high-frequency perturbation and solve Einstein's equations with the effective stress-energy tensor in a consistent way. The bacreaction is of the same order as the influence of the scalar field. Second, we add multiple incoherent high-frequency waves to the homoge- neous Kasner background, and discuss the relation to the late-time limit of the Gowdy (vacuum Charach and Malin) model. 1
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Hamilton's function in classical and quantum mechanics
Černý, Jiří ; Svítek, Otakar (advisor) ; Žofka, Martin (referee)
In this work we will examine an on-shell action, its basic properties and its relation to transition amplitude. Derivatives of on-shell action with respect to position and time are equal to momentum and energy. On-shell action of a system is sufficient for determining the trajectory describing time evo- lution of the system. On-shell action can be computed as (off-shell) action of specific physical trajectory connecting initial position in initial time with final position in final time but it can also be found from solutions to two Hamilton-Jacobi equations, one in initial time and position variables and the other in final time and position variables. In quantum mechanics a transition amplitude is directly proportional to the complex exponential of the on-shell action. Work with on-shell action is demonstrated on examples such as free particle, harmonic oscillator and partly also on central force problem.
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Shell sources and interpretation of extremally charged spacetimes
Veselý, Jiří ; Žofka, Martin (advisor) ; Svítek, Otakar (referee)
The subject of study of this thesis is the so-called ECS spacetime. It originated as an extension to the Majumdar-Papapetrou solution for an infinite extremally-charged string (hence ECS). In the first part of the thesis, some general properties of the spacetime are examined. However, the main method of research is Israel formalism, which is used to find an alternate and more physically-elegant source of the spacetime in question. Nine different model scenarios are thoroughly investigated. In the end, we succeed in finding a single source that is not singular, does not necessitate the presence of exotic matter and has acceptable properties even in the Newtonian limit of weak gravitation: two infinite cylinders filled with Minkowski spacetime. Powered by TCPDF (www.tcpdf.org)
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Quasilocal horizons
Klozová, Eliška ; Svítek, Otakar (advisor) ; Scholtz, Martin (referee)
In this thesis we discuss drawbacks of the event horizon which is defined glo- bally in spacetime and we introduce a quasilocal definition of black hole boundary foliated by marginally trapped surfaces on which the expansion of the outer null normal congruence becomes zero. List of different types of quasilocal horizons follows, i.e. apparent horizon, trapping horizon and isolated and dynamical hori- zon. Subsequently we calculate and analyse quasilocal horizons in two dynamical spacetimes which are used as inhomogeneous cosmological models. We discover future and past horizon in spherically symmetric Lemaître spacetime and we come to conclusion that both are null and have locally the same geometry as the ho- rizons in the LTB spacetime. Then we study Szekeres-Szafron spacetime with no symmetries, particularly its subfamily with β,z ̸= 0, and we derive the equation of the horizon. However, because of the lack of symmetries the spacetime is not adapted to double-null foliation, therefore we were unsuccessful in our attempts to estimate the equation's solution. Only in a special case when the function Φ does not depend on the coordinate z we found a condition on the existence of the horizon, that is Φ,t Φ > 0. 1
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Geodetic structure of multi-black-hole spacetimes
Ryzner, Jiří ; Žofka, Martin (advisor) ; Svítek, Otakar (referee)
V klasické fyzice m·že být ustavena statická rovnováha v soustavě nabitých hmotných bod·, jsou-li poměry náboje a hmotnosti každého hmotného bodu stejné. Udivujícím faktem je, že tato situace m·že nastat i pro černé díry v relativistické fyzice. Obecný případ takovéhoto systému poprvé popsali Majumdar a Papapetrou nezávisle na sobě v roce 1947. Tato práce se zabývá jeho speciálním případem obsahujícím dvě nabité černé díry, zkoumá elektrogeodetiky v tomto prostoročasu a srovnává je se situací v klasické fyzice. Dále též shrnujeme situaci v případě nestatického vesmíru, kterou popsali Kastor a Traschenová v roce 1992, a tuto geometrii srovnáváme se statickou verzí. 1
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Inhomogeneous cosmology and averaging methods
Kašpar, Petr ; Svítek, Otakar (advisor) ; Balek, Vladimír (referee) ; Carloni, Sante (referee)
In this work we have examined different methods of averaging in general relativity and cosmology. We developed the method based on Cartan scalars. We computed the backreaction term for a flat LTB model with a special ansatz for the radial function. We found out that it behaves as a positive cosmological constant. In the next part of this thesis we were interested in averaging inside LRS class II dust model. For this family we averaged all the Einstein equations and the resulting system generalizes the Buchert equations. We numerically worked out two concrete examples where deceleration parameter changes its sign from positive to negative. Powered by TCPDF (www.tcpdf.org)
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Inhomogeneous cosmological models
Vrba, David ; Svítek, Otakar (advisor) ; Pravda, Vojtěch (referee) ; Žofka, Martin (referee)
In this work we study inhomogeneous cosmological models. After a brief review of applications of inhomogeneous solutions to Einstein equations in cosmology, we give a short description of the most widely used inhomogeneous cosmological models. In the second chapter we study in detail geometrical prop- erties of the Szekeres spacetime and we are concerned with the interpretation of the metric functions in different types of geometries. In the last chapter we model inhomogeneity in Szekeres spacetime. We derive formula for the density contrast and investigate its behaviour. We also derive conditions for the density extremes that are necessary for avoiding the shell crossing singularity in Szekeres spacetime. 1
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Dynamics of causal sets
Káninský, Jakub ; Svítek, Otakar (advisor) ; Žofka, Martin (referee)
The causal set theory represents a possible approach to quantum gravity. It is based upon two leading concepts, causality and discreteness. The causal set, embodying the fundamental structure, forms the discrete substratum for spacetime. Relations to the continuous spacetime are then studied by kinematics while the dynamics' task is to provide a set of rules that would conduct the process of formation. Within this thesis we give a coherent introduction to the causal set theory, focusing on the classical dynamics, in particular its general form called the classical sequential growth. In the sequel, we explore some of the applications, taking advantage of growth simulations.
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