Přesné prostoročasy v modifikovaných teoriích gravitace

V rešeršní části této diplomové práce shrnujeme poznatky o modifikovaných teori- ích gravitace, především pak takových, které jsou charakterizovány dodatečnými křivostnímy invarianty v hustotě lagrangiánu. Dále uvádíme přehled o netwistu- jících geometriích, speciálně s důrazem na jejich Kundtovu podtřídu. Z principu nejmenší akce poté odvozujeme rovnice gravitačního pole v případě, kdy hustota lagrangiánu odpovídá obecné funkci křivostních invariantů. V rámci původní části této práce explicitně nalézáme jednotlivé složky rovnic pole pro negyratonovou Kundtovu geometrii v obecné kvadratické teorii gravitace v obecné dimenzi. Dis- kutujeme, jak rovnice pole omezují obecnou Kundtovu metriku ve vybraných ge- ometricky privilegovaných případech. Speciální pozornost pak věnujeme případu Gauss-Bonnetovy teorie. 1In the review part of the thesis we summarize various modified theories of gravity, especially those that are characterized by additional curvature invariants in the Lagrangian density. Further, we review non-twisting geometries, especially their Kundt subclass. Finally, from the principle of least action we derive field equations for the case with the Lagrangian density corresponding to an arbitrary function of the curvature invariants. In the original part of the thesis we explicitly express particular components of the field equations for non-gyratonic Kundt geometry in generic quadratic gravity in arbitrary dimension. Then we discuss how this, in general fourth order, field equations restrict the Kundt metric in selected geome- trically privileged situations. We also analyse the special case of Gauss-Bonnet theory. 1Institute of Theoretical PhysicsÚstav teoretické fyzikyMatematicko-fyzikální fakultaFaculty of Mathematics and Physic

Gravitační čočkování substrukturami v halech temné hmoty

Gravitational lensing serves as an invaluable tool for studying the distribution of mat- ter in the universe. This matter is predominantly dark and clumped into centrally concen- trated hierarchically structured halos of galaxies and galaxy clusters. Early comparisons suggested discrepancies between the high population of substructures predicted by cos- mological simulations and the lack of corresponding observational data. More recently, a major discrepancy in the opposite sense was reported from analyses of lensing galaxy clusters: the lensing efficiency of their substructures was found to be much higher than predictions based on cosmological simulations. In this thesis, we examine gravitational lensing by substructures embedded in dark-matter halos with Navarro-Frenk-White (NFW) density profiles. We start by a detailed investigation of a simple model with a single point mass perturbing the halo. Using analytical methods, we study its critical curves, caustics and their transitions. Next, we explore the geometry of lensed images and the weak-lensing characteristics of the same simple model, developing novel ways of their visualization. Finally, we construct a more realistic lens model of a galaxy cluster con- sisting of an ellipsoidal cluster halo combined with a population of truncated ellipsoidal galaxy...Gravitační čočkování je neocenitelným nástrojem ke studiu rozložení hmoty ve ves- míru. Tato hmota je převážně temná a shluknutá do centrálně koncentrovaných hierar- chicky strukturovaných hal galaxií a kup galaxií. Dřívější srovnání poukázala na nesoulad mezi vysokou populací substruktur predikovanou kosmologickými simulacemi a menším množstvím substruktur v pozorováních. Nedávno však byl analýzou čočkujících kup galaxií objeven značný nesoulad opačného typu. Byla pozorována mnohem vyšší účinnost čočkování substrukturami kup, než by odpovídalo kosmologickým simulacím. V této diz- ertační práci zkoumáme gravitační čočkování substrukturami vnořenými v halech temné hmoty s Navarrovým-Frenkovým-Whiteovým (NFW) hustotním profilem. Začali jsme detailním ohledáním jednoduchého modelu s halem perturbovaným jedním hmotným bo- dem. Analytickými metodami jsme studovali kritické křivky, kaustiky a jejich přechody. Dále jsme zkoumali geometrii čočkovaných obrazů a charakteristiky slabého čočkování v témže jednoduchém modelu, k čemuž jsme vyvinuli nové metody vizualizace. Nakonec jsme zkonstruovali realističtější čočkovací model kupy galaxií tvořený hlavním elipsoidál- ním halem kupy kombinovaným s populací oříznutých elipsoidálních hal jednotlivých galaxií. Pro oba typy hal uvádíme analytické vztahy úhlů odklonu....Ústav teoretické fyzikyInstitute of Theoretical PhysicsFaculty of Mathematics and PhysicsMatematicko-fyzikální fakult

Modeling Tool of TRILL Protocol

This paper proposes the first modeling tool of TRILL protocol. The application implements TRILL features, which include the most fundamental concepts of IS-IS protocol. Simplified STP protocol is also implemented. The application can be used to didactically and interactively present the advantages of the TRILL protocol over STP. Alternatively, the tool can be utilized as a decision tool for TRILL deployment in enterprise networks. The functionality of the TRILL protocol is discussed with emphasis on the main differences between STP. The application was tested on five scenarios of TRILL deployment in a typical enterprise network. Data traffic of these different deployments was compared using the application. Finally, results are further discussed

Probing of InAs/AlSb double barrier heterostructures by ballistic electron emission spectroscopy

InAs/AlSb resonant tunneling heterostructures have been studied by ballistic electron emission spectroscopy. Current thresholds attributed to quasibound states in the quantum well and emission over the AlSb barriers are observed. The observed shape of thresholds is consistent with inelastic processes in the InAs layers of the structures, where a high number of electron–hole pairs are generated. A threshold consistent with the generation of electron–hole pairs in quantum well states is observed

Novel Tm3+-doped fluorotellurite glasses with enhanced quantum efficiency

In this paper, new highly Tm3+-doped tellurite glasses with host composition 75TeO2-xZnF2-yGeO2-12PbO-3Nb2O5 [x(5-15), y(0-5) mol%] are presented and compared to the Tm-doped tellurite glasses based on the traditional host composition: 75TeO2-20ZnO-5Na2O mol%. Enhanced quantum efficiency from 3F4 level was observed for the proposed glasses and thermal stability and viscosity values make them suitable for optical fiber drawing. Besides the host composition, substantial influence of Tm3+ concentration on luminescence and lifetime of excited 3F4 and 3H4 states were discusse

Exact spacetimes in modified theories of gravity

In the review part of the thesis we summarize various modified theories of gravity, especially those that are characterized by additional curvature invariants in the Lagrangian density. Further, we review non-twisting geometries, especially their Kundt subclass. Finally, from the principle of least action we derive field equations for the case with the Lagrangian density corresponding to an arbitrary function of the curvature invariants. In the original part of the thesis we explicitly express particular components of the field equations for non-gyratonic Kundt geometry in generic quadratic gravity in arbitrary dimension. Then we discuss how this, in general fourth order, field equations restrict the Kundt metric in selected geome- trically privileged situations. We also analyse the special case of Gauss-Bonnet theory.

Impulsive gravitational waves

In the review part of this bachelor thesis, we summarize various results about solutions to Einstein's gravitational field equations which describe both non-expanding and expanding impulsive gravitation waves in spacetimes of constant curvature. Special attention will be paid to geodesic motion in these spacetimes and to geometrical methods of their construction. In the original part of the thesis, we check compatibility of a direct solution to geodesic equation in (anti-)de Sitter spacetime with non-expanding impulsive wave and refraction formulae derived under the assumption of continuity of geodesics in a specific coordinate system. We also investigate an interaction of test particles with expanding spherical impulsive wave propagating on the Minkowski background which is generated by a pair of perpendicular snapping cosmic strings. Powered by TCPDF (www.tcpdf.org