Tesi sul tema "Black holes"

Over recent years there has been an increase of the number of secure supermassive black hole (SMBH) detections. These SMBH measurements have lead astronomers to establish well defined empirical relationships between the SMBH mass and some of the properties of the host galaxy. The number of galaxies with SMBH mass measurements is currently limited to about 100. One approach of expanding the study of the SMBH is to use the empirical relations for estimating M[subscript(bh)] for larger samples of galaxies. The investigation of the SMBH population (or SMBH mass function) for large sample of galaxies in the nearby universe has helped to constrain the SMBH and the galaxy evolution. Previous estimates of the SMBH mass function at low redshift were produced mainly by combining the measurements of the galaxy luminosity or velocity function with one of the SMBH scaling relations. In the first part of the thesis I will present an independent construction of the nearby supermassive black hole mass function by applying the optical M[subscript(bh)]–L relation onto the Millennium Galaxy Catalogue (MGC). Additionally, in the second part I will provide photometric analysis of all UKIDSS galaxies for which SMBH masses have been measured. I will derive composite profiles of brightness, ellipticity and position angles of each galaxy. I will show that the Sérsic function fits the brightness profile of the majority of the elliptical galaxies and the bulge of disk galaxies and I will provide alternative multi-component fits when necessary. Then these photometric parameters will be used for constructing the M[subscript(bh)]–L relation in the near-IR and to investigate the M[subscript(bh)]–n relation. In the third part I will construct the near-IR SMBH mass function for the Galaxy and Mass Assembly (GAMA) survey. For this purpose I will apply the newly derived M[subscript(bh)]–L relation onto an elliptical subsample of K-band images. The advantage of this SMBH mass function is that during the M[subscript(bh)]–L construction I used the same quality images and techniques used on the GAMA survey. Apart from the M[subscript(bh)]–L relation, the M[subscript(bh)]–sigma relation was used as an alternative approach for a subsample of galaxies for which the velocity dispersions were available. Furthermore, I employed both local SMBH mass functions (MGC & GAMA) for estimating the SMBH mass density at redshift zero and accounted for the dependence of the total SMBH density on the look-back time by comparing with semi-analytic SMBH mass functions. Finally, from the SMBH mass density I estimated the baryon fraction that is locked into SMBHs.

We study charged rotating black hole solutions of various supergravity theories, both ungauged and gauged, focusing on spacetime dimensions <i>D</i> = 4,5,6,7. These solutions may carry several independent angular momenta and U(1) charges, the precise number depending on the theory. We find asymptotically anti-de Sitter black holes in six-and seven-dimensional gauged supergravity. The six-dimensional solution has two independent angular momenta and one U(1) charge from an SU(2) gauge group. The seven-dimensional solution has three independent angular momenta and two equal U(1) charges from an SO(5) gauge group. We study the thermodynamics of the solutions. These solutions include supersymmetric solutions, amongst which are supersymmetric black holes without naked closed timelike curves. We study the hidden symmetries of the metrics involved. To further illustrate the context of these symmetries, we construct a higher-dimensional charged Kerr solution with NUT parameters. In all cases, we find Killing-Stäckel tensors for a conformally related Jordan-frame metric. These satisfy the conditions for a separability structure and induce conformal Killing-Stäckel tensors for the Einstein-frame metric. We directly verify the separation of the null Hamilton-Jacobi and massless Klein-Gordon equations for some of these solutions.  Simple choices of vielbeins are seen to correspond to eigenforms of privileged Killing-Stäckel tensors of the conformally related metric.

The braneworld paradigm provides an interesting framework within which to explore the possibility that our Universe lives in a fundamentally higher dimensional space- time. In this thesis we investigate black holes in the Randall-Sundrum braneworld scenario. We begin with an overview of extra-dimensional physics, from the original proposal of Kaluza and Klein up to the modern braneworld picture of extra dimensions. A detailed description of braneworld gravity is given, with particular emphasis on its compatibility with experimental tests of gravity. We then move on to a discussion of static, spherically symmetric braneworld black hole solutions. Assuming an equation of state for the "Weyl term", which encodes the effects of the extra dimension, we are able to classify the general behaviour of these solutions. We then use the strong field limit approach to investigate the gravitational lensing properties of some candidate braneworld black hole solutions. It is found that braneworld black holes could have significantly different observational signatures to the Schwarzschild black hole of standard general relativity. Rotating braneworld black hole solutions are also discussed, and we attempt to generate rotating solutions from known static solutions using the Newman-Janis complexification "trick".

This thesis involves the study of strong and weak gravity phenomenology within the braneworld paradigm. We begin with a general overview of the hypothesised concept of extra spatial dimensions and explain why they are so interesting. Turning next to the topic of classical four-dimensional black holes, we discuss their formation via gravitational collapse and indicate some of the strong observational evidence of their existence. We then merge the two independent theories of extra dimensions and black holes together to form braneworld black holes. Focusing our attention on two distinct braneworld scenarios, we examine the effects produced from either strong or weak gravity. The prospect of obtaining experimental verification of the existence of additional spacelike dimensions in the upcoming ground-based accelerators, makes the theoretical research of braneworld gravity within this thesis even more enticing. We start with a non-perturbative approach to look for exact, spherically symmetric star or black hole solutions on a Randall-Sundrum brane from the perspective of the five-dimensional spacetime. By fixing the background, we explore the permissible braneworld trajectories within it that correspond to a braneworld observer, the solutions of the brane Tolmann-Oppenheimer-Volkoff equations. A variety of static gravitating matter sources on the brane are obtained in a range of different backgrounds. Our final aim is a consistent brane embedding in a Schwarzschild- Anti de Sitter spacetime as these solutions are potential candidates for brane stars or black holes. The weak and dominant energy conditions determine the physically sensible solutions which have the interpretation of braneworld stars. We then study time-dependent trajectories as a possible description of time-dependent braneworld black holes. This work is then generalised by relaxing the simplifying assumption of Z(_2)-symmetry, previously imposed around the brane. Non-Z(_2) symmetric spacetimes are applicable in processes which concern only one side of the brane, for example black hole recoil or the emission of Hawking radiation. We determine that a subset of the allowed brane trajectories in an asymmetric background are exactly the same as the Z(_2)-symmetric case. Next, we explore perturbative gravity in the Hofava-Witten model of heterotic M-theory. The study of scalar and gravitational fluctuations determines that the radion mode is coupled to the bulk scalar field, indicating only one single degree of freedom. Our analysis also determines the instability of a black string. We then compute the complete mass spectrum of the graviton mode. Using the five-dimensional gravitational physics, we determine what the gravitational interaction an observer on the braneworld would perceive. This analysis involves the computation of the Newtonian potential between two test masses on the visible brane, together with the four-dimensional tensor structure of the massless graviton propagator. Finally, as an application to the earlier work, we comment on work which is in progress: the study of possible brane black hole solutions in low energy heterotic M-theory.

An understanding of radiation is paramount for connecting observations of accretion disks with the theory of black holes. In this thesis, we explore via radiative transfer postprocessing calculations the observational signatures of black holes. We investigate disk spectra by analyzing general relativistic magnetohydrodynamic (GRMHD) simulations of accretion disks. For the most part there are no surprises -- the resulting GRMHD spectrum is very close to the analytic Novikov & Thorne (1973) prediction from decades past, except for a small modification in the case of spinning black holes, which exhibit a high-energy power-law tail that is sourced by hot Comptonized gas from within the plunging region of the accretion flow. These conclusions are borne out by both 1D and 3D radiative transfer calculations of the disk. Significant effort was spent in developing from scratch the 3D radiative code that we used for the analysis. The code is named HERO (Hybrid Evaluator for Radiative Objects) and it is a new general purpose grid-based 3D general relativistic radiative solver.<br>Astronomy

We perform magnetohydrodynamic simulations in full general relativity of disk accretion onto nonspinning black hole binaries with mass ratio q = 29/36. We survey different disk models which differ in their scale height, total size and magnetic field to quantify the robustness of previous simulations on the initial disk model. Scaling our simulations to LIGO GW150914 we find that such systems could explain possible gravitational wave and electromagnetic counterparts such as the Fermi GBM hard x-ray signal reported 0.4 s after GW150915 ended. Scaling our simulations to supermassive binary black holes, we find that observable flow properties such as accretion rate periodicities, the emergence of jets throughout inspiral, merger and postmerger, disk temperatures, thermal frequencies, and the time delay between merger and the boost in jet outflows that we reported in earlier studies display only modest dependence on the initial disk model we consider here.

Supermassive black holes are a fundamental component of the universe in general and of galaxies in particular. Almost every massive galaxy harbours a supermassive black hole (SMBH) in its center. Furthermore, there is a close connection between the growth of the SMBH and the evolution of its host galaxy, manifested in the relationship between the mass of the black hole and various properties of the galaxy's spheroid component, like its stellar velocity dispersion, luminosity or mass. Understanding this relationship and the growth of SMBHs is essential for our picture of galaxy formation and evolution. In this thesis, I make several contributions to improve our knowledge on the census of SMBHs and on the coevolution of black holes and galaxies. The first route I follow on this road is to obtain a complete census of the black hole population and its properties. Here, I focus particularly on active black holes, observable as Active Galactic Nuclei (AGN) or quasars. These are found in large surveys of the sky. In this thesis, I use one of these surveys, the Hamburg/ESO survey (HES), to study the AGN population in the local volume (z~0). The demographics of AGN are traditionally represented by the AGN luminosity function, the distribution function of AGN at a given luminosity. I determined the local (z<0.3) optical luminosity function of so-called type 1 AGN, based on the broad band B_J magnitudes and AGN broad Halpha emission line luminosities, free of contamination from the host galaxy. I combined this result with fainter data from the Sloan Digital Sky Survey (SDSS) and constructed the best current optical AGN luminosity function at z~0. The comparison of the luminosity function with higher redshifts supports the current notion of 'AGN downsizing', i.e. the space density of the most luminous AGN peaks at higher redshifts and the space density of less luminous AGN peaks at lower redshifts. However, the AGN luminosity function does not reveal the full picture of active black hole demographics. This requires knowledge of the physical quantities, foremost the black hole mass and the accretion rate of the black hole, and the respective distribution functions, the active black hole mass function and the Eddington ratio distribution function. I developed a method for an unbiased estimate of these two distribution functions, employing a maximum likelihood technique and fully account for the selection function. I used this method to determine the active black hole mass function and the Eddington ratio distribution function for the local universe from the HES. I found a wide intrinsic distribution of black hole accretion rates and black hole masses. The comparison of the local active black hole mass function with the local total black hole mass function reveals evidence for 'AGN downsizing', in the sense that in the local universe the most massive black holes are in a less active stage then lower mass black holes. The second route I follow is a study of redshift evolution in the black hole-galaxy relations. While theoretical models can in general explain the existence of these relations, their redshift evolution puts strong constraints on these models. Observational studies on the black hole-galaxy relations naturally suffer from selection effects. These can potentially bias the conclusions inferred from the observations, if they are not taken into account. I investigated the issue of selection effects on type 1 AGN samples in detail and discuss various sources of bias, e.g. an AGN luminosity bias, an active fraction bias and an AGN evolution bias. If the selection function of the observational sample and the underlying distribution functions are known, it is possible to correct for this bias. I present a fitting method to obtain an unbiased estimate of the intrinsic black hole-galaxy relations from samples that are affected by selection effects. Third, I try to improve our census of dormant black holes and the determination of their masses. One of the most important techniques to determine the black hole mass in quiescent galaxies is via stellar dynamical modeling. This method employs photometric and kinematic observations of the galaxy and infers the gravitational potential from the stellar orbits. This method can reveal the presence of the black hole and give its mass, if the sphere of the black hole's gravitational influence is spatially resolved. However, usually the presence of a dark matter halo is ignored in the dynamical modeling, potentially causing a bias on the determined black hole mass. I ran dynamical models for a sample of 12 galaxies, including a dark matter halo. For galaxies for which the black hole's sphere of influence is not well resolved, I found that the black hole mass is systematically underestimated when the dark matter halo is ignored, while there is almost no effect for galaxies with well resolved sphere of influence.<br>Supermassereiche Schwarze Löcher sind ein fundamentaler Bestandteil unseres Universims im Allgemeinen, und von Galaxien im Besonderen. Fast jede massereiche Galaxie beherbergt ein supermassereiches Schwarzes Loch in seinem Zentrum. Außerdem existiert eine enge Beziehung zwischen dem Wachstum des Schwarzen Loches und der Entwicklung seiner umgebenden Galaxie. Diese zeigt sich besonders in der engen Beziehung zwischen der Masse eines Schwarzen Loches und den Eigenschaften der sphäroidalen Komponente der Galaxie, beispielsweise seiner stellaren Geschwindigkeitsdispersion, seiner Leuchtkraft und seiner Masse. Diese Beziehung erklären zu können, sowie das Wachstum von Schwarzen Löchern zu verstehen, liefert einen wichtigen Beitrag zu unserem Bild der Entstehung und Entwicklung von Galaxien. In dieser Arbeit steuere ich verschiedene Beiträge dazu bei unser Verständnis des Vorkommens Schwarzer Löcher und der Beziehung zu ihren Galaxien zu verbessern. Zunächst versuche ich ein vollständiges Bild der Anzahl und Eigenschaften Schwarzer Löcher zu erhalten. Dazu beschränke ich mich auf aktive Schwarze Löcher, wie man sie im Universum als Aktive Galaxienkerne (AGN) in großen Himmelsdurchmusterungen finden kann. Ich benutze eine solche Durchmusterung, das Hamburg/ESO Survey (HES), um die AGN Population im lokalen Universum zu studieren. Dazu habe ich die optische Leuchtkraftfunktion von AGN bestimmt. Diese habe ich mit anderen Ergebnissen leuchtschwächerer AGN kombiniert um die bisher beste AGN Leuchtkraftfunktion im lokalen Universum zu erhalten. Der Vergleich mit Ergebnissen bei höherer kosmischer Rotverschiebung bestätigt unser Bild des sogenannten "AGN downsizing". Dies sagt aus, dass leuchtkräftige AGN bei hoher Rotverschiebung am häufigsten vorkommen, während leuchtschwache AGN bei niedriger Rotverschiebung am häufigsten sind. Allerdings verrät uns die AGN Leuchtkraftfunktion allein noch nicht das ganze Bild der Demographie Schwarzer Löcher. Vielmehr sind wir an den zugrunde liegenden Eigenschaften, vor allem der Masse und der Akkretionsrate der Schwarzen Löcher, sowie deren statistischen Verteilungsfunktionen, interessiert. Ich habe eine Methode entwickelt um diese beiden Verteilungsfunktionen zu bestimmen, basierend auf der Maximum-Likelihood-Methode. Ich habe diese Methode benutzt um die aktive Massenfunktion Schwarzer Löcher, sowie die Verteilungsfunktion ihrer Akkretionsraten für das lokale Universum aus dem HES zu bestimmen. Sowohl die Akkretionsraten, als auch die Massen der Schwarzen Löcher zeigen intrinsisch eine breite Verteilung, im Gegensatz zur schmaleren beobachtbaren Verteilung. Der Vergleich der aktiven Massenfunktion mit der gesamten Massenfunktion Schwarzer Löcher zeigt ebenfalls Hinweise auf "AGN downsizing". Als nächstes habe ich mich mit Untersuchungen zur zeitlichen Entwicklung in den Beziehungen zwischen Schwarzem Loch und Galaxie beschäftigt. Diese kann helfen unser theoretisches Veständnis der physikalischen Vorgänge zu verbessern. Beobachtungen sind immer auch Auswahleffekten unterworfen. Diese können die Schlussfolgerungen aus den Beobachtungen zur Entwicklung in den Beziehungen beeinflussen, wenn sie nicht entsprechend berücksichtigt werden. Ich habe den Einfluss von Auswahleffekten auf AGN Stichproben im Detail untersucht, und verschiedende möchgliche Einflussquellen identifiziert, die die Beziehung verfälschen können. Wenn die Auswahlkriterien der Stichprobe, sowie die zugrunde liegenden Verteilungen bekannt sind, so ist es möglich für die Auswahleffekte zu korrigieren. Ich habe eine Methode entwickelt, mit der man die intrinsische Beziehung zwischem Schwarzem Loch und Galaxie aus den Beobachtungen rekonstruieren kann. Schließlich habe ich mich auch inaktiven Schwarzen Löchern und der Bestimmung ihrer Massen gewidmet. Eine der wichtigsten Methoden die Masse Schwarzer Löcher in normalen Galaxien zu bestimmen ist stellardynamische Modellierung. Diese Methode benutzt photometrische und kinematische Beobachtungen, und rekonstruiert daraus das Gravitationspotenzial aus der Analyse stellarer Orbits. Bisher wurde in diesen Modellen allerdings der Einfluss des Halos aus Dunkler Materie vernachlässigt. Dieser kann aber die Bestimmung der Masse des Schwarzen Loches beeinflussen. Ich habe 12 Galaxien mit Hilfe stellardynamischer Modellierung untersucht und dabei auch den Einfluss des Halos aus Dunkler Materie berücksichtigt. Für Galaxien bei denen der Einflussbereich des Schwarzen Loches nicht sehr gut räumlich aufgelöst war, wird die Masse des Schwarzen Loches systematisch unterschätzt, wenn der Dunkle Materie Halo nicht berücksichtigt wird. Auf der anderen Seite ist der Einfluss gering, wenn die Beobachtungen diesen Einflussbereich gut auflösen können.

This thesis concerns the use of observations of gravitational waves as tools for astronomy and fundamental physics. Gravitational waves are small ripples in spacetime produced by rapidly accelerating masses; their existence has been predicted for almost 100 years, but the first direct evidence of their existence came only very recently with the announcement in February 2016 of the detection by the LIGO and VIRGO collaborations. Part I of this thesis presents an introduction to gravitational wave astronomy, including a detailed discussion of a wide range of gravitational wave sources, their signal morphologies, and the experimental detectors used to observe them. Part II of this thesis concerns a particular data analysis problem which often arises when trying to infer the source properties from a gravitational wave observation. The use of an inaccurate signal model can cause significant systematic errors in the inferred source parameters. The work in this section concerns a proposed technique, called the Gaussian process marginalised likelihood, for overcoming this problem. Part III of this thesis concerns the possibility of testing if the gravitational field around an astrophysical black hole conforms to the predictions of general relativity and the cosmic censorship hypothesis. It is expected that the gravitational field should be well described by the famous Kerr solution. Two approaches for testing this hypothesis are considered; one using X-ray observations and one using gravitational waves. The results from these two approaches are compared and contrasted. Finally, the conclusions and a discussion of future prospects are presented in part IV of this thesis.

The description of a black hole in the limit of very large number of spacetime dimensions D simplifies considerably. When D approaches infinity, the gravitational field lines of force are infinitely dispersed among the infite number of spacetime directions. This implies that outside the near-horizon region of the black hole the background spacetime will be flat, while the gravitational field will be strongly localized near the horizon. Thus, we can attempt to replace the black hole by a sphere cut off at the horizon in an empty background. In this project we attempt to obtain the physical conditions that the sphere has to meet in order to be able to reproduce the dynamics of the black hole when embedded in the empty background. This is described in the effective equation that we derive starting from the Einstein equations. Finally, we apply our results to take a look at ’black droplets’, black holes localized at the boundary of AdS and extending a finite distance into the bulk.

Des trous noirs supermassifs (TNs) de plusieurs millions de masses solaires occupent le centre de la plupart des galaxies proches. La découverte du TN Sagittarius A* au centre de notre galaxie, La Voie lactée, l'a confirmé. Pour autant, certaines galaxies semblent dépourvues de TNs (par exemple NGC205, M33), ou alors ne posséder un TN que de quelques milliers de masses solaires. D'autre part, des TNs dans leur forme la plus lumineuse, appelés quasars, dont la luminosité est plus importante que des centaines de fois celle d'une galaxie toute entière, ont été observés à très grand décalage spectral, lorsque l'Univers n'était alors âgé que d'un milliard d'années. Les modèles de formation des TNs doivent expliquer à la fois l'existence des TNs de faibles masses observés aujourd'hui dans les galaxies de faibles masses, mais aussi leur prodigieux homologues quasars dans l'Univers jeune. La formation des TNs pose encore de nos jours de nombreuses questions: comment se forment les TNs au début de l'histoire de l'Univers? Quelle est leur masse initiale? Quelle est la masse minimale d'une galaxie pour posséder un TN? Pour répondre à ces questions et pour étudier la formation des TNs dans le contexte de l'évolution des galaxies, nous avons utilisé des simulations hydrodynamiques cosmologiques, qui offrent l'avantage de suivre l'évolution temporelle de nombreux processus comme la formation stellaire, l'enrichissement en métaux, les mécanismes de rétroactions des TNs et des supernovae. J'ai particulièrement dirigé mes recherches sur les trois principaux modèles de formation des TNs à partir du reliquat des premières étoiles, d'amas d'étoiles, ou encore par effondrement direct<br>Supermassive black holes (BHs) harboured in the center of galaxies have been confirmed with the discovery of Sagittarius A* in the center of our galaxy, the Milky Way. Recent surveys indicate that BHs of millions of solar masses are common in most local galaxies, but also that some local galaxies could be lacking BHs (e.g. NGC205, M33), or at least hosting low-mass BHs of few thousands solar masses. Conversely, massive BHs under their most luminous form are called quasars, and their luminosity can be up to hundred times the luminosity of an entire galaxy. We observe these quasars in the very early Universe, less than a billion years after the Big Bang. BH formation models therefore need to explain both the low-mass BHs that are observed in low-mass galaxies today, but also the prodigious quasars we see in the early Universe.BH formation is still puzzling today, and many questions need to be addressed: How are BHs created in the early Universe? What is their initial mass? How many BHs grow efficiently? What is the occurrence of BH formation in high redshift galaxies? What is the minimum galaxy mass to host a BH? We have used cosmological hydrodynamical simulations to capture BH formation in the context of galaxy formation and evolution. Simulations offer the advantage of following in time the evolution of galaxies, and the processes related to them, such as star formation, metal enrichment, feedback of supernovae and BHs. We have particularly focused our studies on the three main BH formation models: Pop III remnant, stellar cluster, and direct collapse models

The study of geodesics is of intrinsic significance in the study of the geometry of space-time. In this thesis null, space-like and time-like geodesics are studied in the case of the space-times of Schwarzschild, Reissner-Nordstrouml;m and Kerr black holes. These space-times have been investigated with varying degrees of thoroughness in many articles and some books. However, there are some significant gaps in these treatments and the central aim of this thesis is to fill these gaps where necessary. Moreover, the following topics are covered for the first time. 1. In Chapter 4 a thorough treatment of the space-like geodesics of the Schwarzschild solutions has been given. These geodesics are the trajectories of Tachyons (faster than light particles) and are treated in a complete manner. This has been done by obtaining exact solutions and solving them numerically. 2. In Part II all solutions for geodesics for a Reissner-Nordstrouml;m black hole have been given in complete detail, i.e. time-like, null and space-like geodesics and orbit of a charged particle. 3. In Chapter 14 all solutions for geodesics in the equatorial plane of a Kerr black hole have been given in complete detail, i.e. time-like, null and space-like geodesics. 4. The study of special types of non-equatorial geodesics for a Kerr black hole have been given in complete detail, i.e. time-like (Chapter 17), null (Chapter 15) and space-like (Chapter 16). This has been done in order to distinguish the qualitatively different types of solutions. Calculation of the explicit formulas, which describe these geodesics, as well as numerically computed diagrams representing the geodesics have been incorporated in these studies. The following subjects have been also treated: 5. Solutions for the geodesics in Reissner-Nordstrouml;m black holes with |Q_*| gt;= M, which are black holes with one (|Q_*| = M) or no horizon (|Q_*|gt; M) (Chapter 8). 6. Solutions of geodesics in extreme and fast Kerr black holes, i.e. black holes with a = M (extreme) and a gt; M (fast). As in the case of |Q_*| gt; M, fast black holes have naked singularities (Chapter 14). 7. Some general observations about orbit types of the Kerr black holes regarding relationships between parameters such as angular momentum, energy, Carter constant and mass and angular momentum of black holes (Chapter 13). 8. Some corrections to errors found in the literature. While it has not been possible to cover all different cases which occur for possible relations amongst the parameters specifying a general black hole, interesting geodesics have, however, been studied and a more thorough presentation of the properties of geodesics has now been given.

In this thesis, we study toy models of two-dimensional gravity. We first review two known models: the classical and quantum corrected CGHS models and the quantum corrected model of RST. These two models have black holes solutions with curvature singularities, similar to the Schwartzschild black hole. This singularity becomes naked in the RST model at a certain event during the evaporation. In the third chapter, we build a more general version with new quantum corrections beyond those presented in the RST model, which enable us to find a model without curvature singularities. We will also see that these new quantum corrections can affect the rate of Hawking radiation flowing from the black hole.

In this thesis we study string theory on orbifolds of AdS3. Non-extremal BTZ black holes have been shown to offer a good opportunity to study closed string tachyon condensation, as there are tachyons in the winding sector even in superstring theory. We study extremal BTZ black holes, both M = O and M = J from a world sheet perspective. The string spectrum is calculated within bosonic string theory and tachyons are identied within the spectrum. The at space limit of the M = 0 black hole is considered and an extension to superstring theory is discussed. In the second half of the thesis we discuss the self dual orbifold. The self dual orbifold is a simple example of a geometry which contains an AdS2 factor. AdS2 factors also appear in the near horizon limit of extremal Kerr and Reissner-Nordström black holes. Using the AdS/CFT correspondence we conjecture that the self dual orbifold is dual to a CFT on two distinct boundary regions and nd evidence to support this statement. We consider asymptotically self dual orbifold spacetimes, one of which is dual to a single copy of the groundstate of the CFT.

The work of this thesis falls into two parts: a discussion of decoherence in quantum Kaluza-Klein theories and a study of some of the properties of general black hole metrics in de Sitter spacetime. Kaluza-Klein theories permit a variety of compactifications and arbitrary scales for the internal space. There must be no quantum interference between these different possibilities. In chapter one it is demonstrated that in the Salam-Sezgin compactification interference between differently scaled interenal spaces is suppressed. In chapter two new gravitational instantons are presented which are related to charged, rotating black holes with a cosmological constant A. These instantons correspond to black holes in de Sitter space with identical Hawking temperatures. Their action contributes terms of order A-1/2 to path integrals with quantum wormholes. The metrics of these general de Sitter black holes show that the spacetimes have wormholes connecting different asymptotic regions. In chapter three the theory of black hole perturbations is extended to these metrics. It appears that the holes are stable even at the Cauchy horizon. This implies that cosmic censorship is violated. The stability of the spacetimes also implies the existence of a cosmological no hair theorem.

We study the ultraviolet properties of quantum gravity and its consequences for black hole physics using the functional renormalisation group (RG). In particular we concentrate on the asymptotic safety scenario for quantum gravity put forward by S. Weinberg. This approach relies on the existence of an ultraviolet fixed point in the renormalisation group flow. In chapter 2 we review the functional renormalisation group formalism that is used in order to search for the existence of a fixed point with the properties required for asymptotic safety. Following this introduction, in chapter 3 we use these methods to find ultraviolet fixed points in four-dimensional quantum gravity to high order in a polynomial approximation in the Ricci scalar. In the following three chapters we concentrate on the implications of the renormalisation group for black hole physics. In chapter 4 we study quantum gravitational corrections to black holes in four and higher dimensions using a renormalisation group improvement of the metric. The quantum effects are worked out in detail for asymptotically safe gravity, where the short distance physics is characterised by a weakening of gravity due to the nontrivial fixed point. Furthermore, mini-black hole production in particle collisions, such as those at the Large Hadron Collider (LHC), is analysed within low-scale quantum gravity models. In chapter 5 we investigate the thermodynamical properties of the RG improved metrics in detail and study their evaporation process. In chapter 6 we study renormalisation group improved black hole thermodynamics in a metric free approach. Conditions are formulated under which the thermodynamic properties of four dimensional Kerr-Newman type black holes persist under the RG evolution of couplings. We show that the RG scale must be set by the horizon area of the black hole which acts as a diffeomorphism invariant cut-off for the underlying Wilsonian action.

We address a long-standing problem of describing the thermodynamics of an accelerating black hole. We derive a standard first law of black hole thermo- dynamics, with the usual identification of entropy proportional to the area of the event horizon — even though the event horizon contains a conical singularity. We show how to generalise this result, formulating thermodynamics for black holes with varying conical deficits. We derive a new potential for the varying tension defects: the thermodynamic length, both for accelerating and static black holes. We discuss possible physical processes in which the tension of a string ending on a black hole might vary, and also map out the thermodynamic phase space of accelerating black holes and explore their critical phenomena. We then revisit the critical limit in which asymptotically-AdS black holes develop maximal conical deficits, first for a stationary rotating black hole, and then for an accelerated black hole, by taking various upper bounds for the parameters in the spacetimes presented. We explore the thermodynamics of these geometries and evaluate the reverse isoperimetric inequal- ity, and argue that the ultra-spinning black hole only violates this condition when it is nonaccelerating. Finally, we return to some of our earlier findings and adjust them in light of new results; a new expression for the mass is obtained by computing the dual stress-energy tensor for the spacetime and finding that it corresponds to a relativistic fluid with a nontrivial viscous shear tensor. We compare the holographic computation with the method of conformal completion showing it yields the same result for the mass.

Doctor of Philosophy<br>Department of Mathematics<br>Louis Crane<br>We exhibit a static, cylindrically symmetric, exact solution to the Euler-Heisenberg field equations (EHFE) and prove that its effective geometry contains (optical) black holes. It is conjectured that there are also soliton solutions to the EHFE which contain black hole geometries.

Thesis (Ph. D.) -- University of Maryland, College Park, 2008.<br>Thesis research directed by: Dept. of Astronomy. Title from t.p. of PDF. Includes bibliographical references. Published by UMI Dissertation Services, Ann Arbor, Mich. Also available in paper.

This thesis presents results from general relativistic numerical computations of primordial black-hole formation during the radiation-dominated era of the universe. Growing-mode perturbations are specified within the linear regime and their subsequent evolution is followed as they become nonlinear. We use a spherically symmetric Lagrangian code and study both super-critical perturbations, which go on to produce black holes, and sub-critical perturbations, for which the overdensity eventually disperses into the background medium. For super-critical perturbations, we revisit the results of previous work concerning scaling-laws, noting that the threshold amplitude for a perturbation to lead to black-hole formation is substantially reduced when the initial conditions are taken to represent purely growing modes. For sub-critical cases, where an initial collapse is followed by a subsequent re-expansion, strong compressions and rarefactions are seen for perturbation amplitudes near to the threshold. We have also investigated the effect of including a significant component of vacuum energy and have calculated the resulting changes in the threshold and in the slope of the scaling law. The specification of the growing-mode perturbations in the above work is approximate and in the later part of the thesis, we introduce a more sophisticated and elegant formulation in terms of curvature perturbations. This allows a direct connection to be made with the spectrum of perturbations coming from inflation and also, using this, we find that there is no longer evidence of shock production in connection with primordial black hole formation. Introducing adaptive mesh refinement into our code, we are able to follow black hole formation nearer to the critical limit and find evidence suggesting that scaling laws may continue down to very small n1asses, in contrast with previous suggestions in the literature.

Thesis: Ph. D., Massachusetts Institute of Technology, Department of Physics, 2015.<br>Cataloged from PDF version of thesis.<br>Includes bibliographical references.<br>In this thesis, I discuss the application and development of black hole perturbation theory both from an observational standpoint via gravitational waves and also tidal distortions of black hole horizons. The promise of gravitational wave astronomy depends on our ability to accurately model gravitational wave signals from astrophysical sources. This requires large numbers of accurate theoretical template waveforms spanning large regions of parameter space to be cross-correlated against the output of gravitational-wave detectors. Numerical simulations of binary black-hole evolution are now possible but remain CPU costly. They also have problems with small mass ratios where perturbative analyses are efficient. This high computational cost has motivated the development of the effective-one-body (EOB) formalism, a framework which models the three phases of binary black hole coalescence - inspiral, plunge/merger, and ring down - by combining information from a variety of modeling techniques. In this thesis, we combine EOB with black hole perturbation theory to study the transition from inspiral to plunge-merger and ringdown. This allows us to tune and improve the accuracy of EOB. In Newtonian gravity, tidal coupling between members of a binary system has an influence on that binary's dynamics. There are also well-understood connections between the geometry of the binary's distorted members and the impact of tides on the orbit's evolution. In this thesis we develop tools for investigating the tidal distortion of black holes for tides arising from a body in a bound orbit. We also develop tools to visualize the horizon's distortion for black hole spin a/M < V/3/2. In analyzing how a Kerr black hole is distorted by a small body for a circular equatorial orbit, we find that Newtonian intuition is not applicable. We also apply these techniques to generic Kerr black hole orbits, which enables us to look at time-dependent phenomena on the horizon. In particular, we find significant offsets between the applied tide and the horizon's response, as well as small amplitude coherent wiggles in the horizon's shear response to the applied tide. These appear to arise from the teleological nature of the horizon's response to tides.<br>by Stephen Gerard O'Sullivan.<br>Ph. D.

This work investigates how black holes can be described in terms of different definitions of horizons. Global definitions in terms of event horizons and Killing horizons are contrasted with local definitions in terms of trapping horizons and dynamical horizons. The discussion is framed in the context of the laws of black hole thermodynamics.

We study the coevolution of black holes (BHs) and their host galaxies through cosmic time. The calculation is embedded in the GALFORM semi-analytic model which simulates the formation and evolution of galaxies in a cold dark matter (CDM) universe. The BH and galaxy formation models are coupled: during the evolution of the host galaxy, hot and cold gas are added to the BH by flows triggered by halo gas cooling, disc instabilities and galaxy mergers. This builds up the mass and spin of the BH, and the resulting accretion power regulates gas cooling and subsequent star formation. Using the Blandford–Znajek mechanism for jet production to calculate the jet power, our model reproduces the radio loudness of radio galaxies, LINERS and Seyferts, suggesting that the jet properties of active galactic nuclei (AGN) are a natural consequence of both the accretion rate onto and the spin of the central BH. The model also reproduces the observed luminosity functions (LF) of AGN (optical, soft and hard X-ray, and bolometric) for a wide range of redshifts (0 < z < 6). We find downsizing in the AGN population, in terms of the differential growth with redshift of the space density of faint and bright AGN. This arises naturally from the interplay between the different accretion channels that drive the growth of BHs. The predictions of our model are extended to 6 < z < 20 to study the early growth of BHs. Our model predicts that the first 10^{8} M⊙ BHs appear at z = 14, along with the first luminous quasars. Finally, we explore the dependence of AGN activity and luminosity on environment. We find that quasars inhabit haloes with masses 10^{12} − 10^{13} M⊙. Quasar activity in more massive haloes in suppressed due to AGN feedback. In contrast, radio galaxies occupy the centres of the most massive haloes. Our model represents the first consistent demonstration that the phenomenology and evolution of AGN can be naturally explained by the coeval evolution of galaxies and BHs, coupled by AGN feedback, in a CDM universe.

L’approccio innovativo alla fisica dei buchi neri quantistici proposto da Dvali e Gomez, permette di trovare nuove soluzioni ad alcuni dei principali problemi dell’unione tra relatività generale e meccanica quantistica. In particolare, porta a un’interpretazione originale della geometria classica, che diventa un concetto emergente dalla descrizione della gravità attraverso uno stato quantistico di gravitoni con numero d’occupazione elevato. Queste idee sono la principale motivazione del nostro lavoro, che chiarisce alcuni aspetti delicati stabilendo un netto collegamento tra il modello quantistico corpuscolare di cui abbiamo appena parlato e la teoria della relatività di Einstein, in approssimazione post-Newtoniana. Questo studio si basa in particolare sulla ricerca di una descrizione quantistica effettiva del potenziale gravitazionale statico, per sistemi a simmetria sferica, al primo ordine non lineare nel limite di campo debole e velocità non relativistiche. Verifichiamo esplicitamente che il nostro modello recuperi i risultati classici per due diverse distribuzioni di materia (omogenea e gaussiana). Infine, procediamo alla quantizzazione del sistema e troviamo uno stato quantistico di gravitoni (virtuali) con le correzioni necessarie per riprodurre il potenziale post-Newtoniano.

Studiando le equazioni di Einstein-Maxwell si ricava la soluzione di Kerr-Newman che descrive il comportamento generale di un buco nero rotante e carico. Questa soluzione dipende da tre parametri m, a e Q che definiscono rispettivamente la massa, il momento angolare e la carica del buco nero. Per a=Q= 0 si ottiene la soluzione di Schwarzschild che presenta un orizzonte deglieventi in r= 2m. Per a= 0 si ottiene la soluzione di Reissner-Nordstrøm per buchi neri carichi e per Q= 0 la soluzione di Kerr per buchi neri rotanti. La termodinamica dei buchi neri è l’area di studio che cerca di estendere ai buchi neri le leggi e i principali risultati della termodinamica classica e di farli riconciliare conl’esistenza degli orizzonti degli eventi. Ciò è possibile solo con l’inclusione della meccanica quantistica. Nonostante il collasso gravitazionale conduca, apparentemente, a uno stato di entropia illimitata, l’inclusionedi questi effetti quantistici elimina questa divergenza, assegnando a un buco nero una entropia definita. Jacob Bekenstein, nel 1972, congetturò che l’entropia del buco nerofosse proporzionale all’area del suo orizzonte degli eventi A. Hawking, nel 1974, mostrò che i buchi neri emettono radiazione termica corrispondente a una certa temperatura (temperatura di Hawking). Questo permette di di fissare il coefficiente di proporzionalità tra S e A. Sulla base di questi risultati si dimostra che i buchi neri sono soggetti almeno alle primedue leggi della termodinamica, mentre le condizioni di Nernst per la terza legge dellatermodinamica non sono soddisfatte completamente: non c’è una chiara ragione termodinamica per cui un buco nero non possa essere raffreddato sotto lo zero assoluto e convertito in una singolarità nuda. Tra i risultati di questa teoria si ha che tutta l’informazione riguardo allo stato termodinamico del buco nero è contenuta nella relazione di Smarr, che lega M, J e Q.

In this thesis we consider the production of primordial black holes (PBH) in the context of single field inflation with the aim of describing a significant fraction of dark matter. In the models we consider, the inflaton is a string modulus and its potential is typical of type IIB fibre inflation. The potential presents a plateau at CMB scales and an extremely flat region on smaller scales. The background is analysed by solving the Friedmann's and the Klein-Gordon equations for the system. Perturbations are introduced through the usual Mukhanov-Sasaki equation for the gauge invariant curvature perturbations, whose solution allows us to find the primordial power spectrum which is then compared to observations. In the class of models considered there is an often occurring tension between the tilt of the scalar power spectrum and observations. We study this tension and propose mechanisms to minimise it. We modify the form of the fibre inflationary potential, modifying therefore the slope of the ultra slow-roll plateau. We find that a better agreement with the experimentally measured value of the spectral index can be reached. Therefore that tension between the value of the spectral index on CMB scale and the power spectrum enhancement on PBH scales can be explained as a consequence of the class of potential taken into account. This tension can be avoided in models that provide a different plateau form.

The main objective of this thesis is to understand from a microscopic point of view some of the characteristic phenomena of rotating black holes. The inclusion of rotation gives rise to physics that allows a more precise and detailed understanding of the microscopic string theory of black holes. In this thesis we focus on two models of particular interest: one is based on the D0-D6 system and the other on the D1-D5-P system. The former is interesting because, through its connection to M-theory, it yields a statistical-mechanics description of neutral black holes. The latter allows to have better control over the microscopic conformal field theory and yields a cleaner picture of the origin of superradiance. We extend the microscopic analysis of extremal dyonic Kaluza-Klein (D0-D6) black holes to cover the regime of fast rotation in addition to slow rotation. Fastly rotating black holes, incontrast to slow ones, have non-zero angular velocity and possess ergospheres, so they are more similar to the Kerr black hole. The D-brane model reproduces their entropy exactly, but the mass gets renormalized from weak to strong coupling, in agreement with recent macroscopic analyses of rotating attractors. We discuss how the microscopic model accounts for the fact that fastly rotating extremal KK black holes possess an ergosphere and exhibit superradiance while slow ones don't. In addition, we show in full generality how Myers-Perry black holes are obtained as a límit of Kaluza-Klein black holes, and discuss the slow and fast rotation regimes and superradiance in this context. A, perhaps surprising, consequence of our analysis is that both slowly and fastly-rotating KK black holes provide microscopic accounts of the entropy formula of MP black holes, even if they correspond to rather different microscopic states. As we discuss, this does not pose any problem, since the microscopic theory always retains a memory of how the 5D black hole is embedded within Taub-NUT. For a more detailed and quantitative study of black hole superradiance from the stringy microscopic side, we consider the D1-D5-P system. In order to disentangle superradiance from finite-temperature effects, we consider an extremal, rotating D1-D5-P black hole that has an ergosphere and is not supersymmetric. We explain how the microscopic dual accounts for the superradiant ergosphere of this black hole. The bound 0 < ω < mΩH on superradiant mode frequencies is argued to be a consequence of Fermi-Dirac statistics for the spin-carrying degrees of freedom in the dual CFT. We also compute the superradiant emission rates from both sides of the correspondence, and show their agreement. This is an extension of previous analyses of radiation from the D1-D5-P black holes. We generalize those results to include momentum for the bulk scalar. It would be interesting to extend our picture for superradiance to the smooth SUGRA solitons with D1-D5-P charges which correspond to CFT states such that both sectors are in pure states. Another issue to be investigated would be the absence of fermionic superradiance emission by the previously considered systems with ergoregion.<br>L'objectiu principal d'aquesta tesi és comprendre des d'un punt de vista microscòpic alguns dels fenòmens característics dels forats negres amb rotació. Ens centrem en forats negres amb rotació, extremals, però no supersimètrics. Aquesta mena de solucions són les més adequades per al nostre propòsit. A diferència del cas supersimètric, presenten una ergosfera i, per tant, s'hi pot produir el fenomen de la superradiància. Però, en tenir temperatura nul·la aquest fenomen no apareix “barrejat” amb la radiació de Hawking d'origen purament tèrmic. Alhora, aquestes geometries extremals també preserven el nombre de microestats en passar d'acoblament feble a acoblament fort. La primera part de la tesi, se centra en l'estudi microscòpic de l'entropia dels forats negres de Kaluza-Klein amb rotació. Aquesta família de solucions, presenta dos límits extremals: el de “rotació lenta” i el de “rotació ràpida”. Treballs anteriors ja havien aconseguit reproduir l'entropia del primer cas. Aquí, estenem aquest càlcul al límit extremal amb rotació ràpida. Alhora, mostrem que el fenomen de la superradiància es pot explicar de manera anàloga a la radiació de Hawking, en termes de dues excitacions que xoquen i provoquen l'emissió d'una corda tancada. En aquest cas, aquest mode emès té necessàriament un moment angular diferent de zero. A la segona part, ens centrem en aquesta interpretació microscòpica de la superradiància amb un tractament més quantitatiu. En aquest cas, considerem les solucions extremals no supersimètriques del sistema D1-D5-P amb rotació. A partir de la seva descripció microscòpica, aconseguim reproduir la condició de superradiància ω < mΩH i mostrem que es pot entendre com a conseqüència de l'estadística de Fermi-Dirac. També avaluem els ritmes d'emissió de superradiància des del punt de vista macroscòpic i microscòpic i analitzem si concorden.

The scalar tensor theory of gravitation is constructed in D dimensions in all possible geometries of spacetime. In Riemannian geometry, theory of gravitation involves a spacetime metric g with a torsion-free, metric compatible connection structure. If the geometry is non-Riemannian, then the gauge theory of gravitation can be constructed with a spacetime metric g and a connection structure with torsion. In non-Riemannian theory, connections may be metric compatible or non-metric compatible. It is shown that theory of gravitation which involves non-metric compatible connection and torsion, can be rewritten in terms of torsion-free theory. It is also shown that scalar tensor theory can be reformulated in Einstein frame by applying a conformal transformation. By adding an antisymmetric axion field, the axi-dilaton theory is studied in Riemannian and non-Riemannian geometries. Motion of massive test particles is examined in all these geometries. The static, spherically symmetric and stationary, Kerr-type axially symmetric solutions of the scalar tensor and axi-dilaton theories are presented. As an application, the geodesic elliptical orbits based on a torsion-free connection and the autoparallel orbits based on a connection with a torsion, are examined in Kerr Brans-Dicke geometry. Perihelion shift of the elliptical orbit is calculated in both cases and the results are compared.

This thesis is divided in two parts: the first part is dedicated to the study of black hole solutions in a theory of modified gravity, called massive gravity, that may be able to explain the actual stage of accelerated expansion of the Universe, while in the second part we focus on constraining primordial black holes as dark matter candidates.<p><p>In particular, during the first part we study the thermodynamical properties of specific black hole solutions in massive gravity. We conclude that such black hole solutions do not follow the second and third of law of thermodynamics, which may signal a problem in the model. For instance, a naked singularity may be created as a result of the evolution of a singularity-free state.<p><p>In the second part, we constrain primordial black holes as dark matter candidates. To do that, we consider the effect of primordial black holes when they interact with compact objects, such as neutron stars and white dwarfs. The idea is as follows: if a primordial black hole is captured by a compact object, then the accretion of the neutron star or white dwarf’s material into the hole is so fast that the black hole destroys the star in a very short time. Therefore, observations of long-lived compact objects impose constraints on the fraction of primordial black holes. Considering both direct capture and capture through star formation of primordial black holes by compact objects, we are able to rule out primordial black holes as the main component of dark matter under certain assumptions that are discussed.<p><p>To better understand the relevance of these subjects in modern cosmology, we begin the thesis by introducing the standard model of cosmology and its problems. We give particular emphasis to modifications of gravity, such as massive gravity, and black holes in our discussion of the dark sector of the Universe./<p>Cette thèse est divisée en deux parties :la première partie est consacrée à l’étude de certaines solutions de trous noirs dans une théorie modifiée de la gravité, appelée la gravité massive, qui peut être en mesure d’expliquer l’expansion accélérée de l’Univers; tandis que dans la seconde partie, nous nous concentrons sur des contraintes sur les trous noirs primordiaux comme candidats de matière noire.<p><p>En particulier, au cours de la première partie, nous étudions les propriétés thermodynamiques de solutions spécifiques de trous noirs en gravité massive. Nous en concluons que ces solutions de trous noirs ne suivent ni la deuxième, ni la troisième loi de la thermodynamique, ce qui semble indiquer une inconsistance dans le modèle. Par exemple, une singularité nue peut être créée à la suite de l’évolution d’un état sans aucune singularité.<p><p>Dans la deuxième partie, nous mettons des contraintes sur les trous noirs primordiaux en tant que candidats de matière noire. Pour ce faire, nous considérons l’effet des trous noirs primordiaux lorsqu’ils interagissent avec des objets compacts, tels que les étoiles à neutrons et les naines blanches. L’idée est comme suit :si un trou noir primordial est capturé par un objet compact, alors l’accrétion du matériel constituant l’étoile à neutrons ou la naine blanche est si rapide que le trou noir détruit l’étoile en un temps très court. Par conséquent, les observations d’objets compacts imposent des contraintes sur la fraction de trous noirs primordiaux. Considérant à la fois la capture directe des trous noirs primordiaux par les objets compacts et la capture au travers de la formation stellaire, nous sommes en mesure d’exclure les trous noirs primordiaux comme la composante principale de matière noire sous certaines hypothèses qui sont discutées.<p><p>Pour mieux comprendre la pertinence de ces sujets dans la cosmologie moderne, nous commençons la thèse par l’introduction du modèle standard de la cosmologie et de ses problèmes. Nous donnons une importance particulière aux modifications de la gravité, telles que la gravité massive, et aux trous noirs dans notre discussion sur le secteur sombre de l’Univers.<br>Doctorat en Sciences<br>info:eu-repo/semantics/nonPublished

The focus of this work is the attempt to detect gravitational waves emitted by compact binary coalescences (CBCs) using gravitational wave interferometers. We begin by reviewing the basic theory of gravitational waves and the methods for their detection, focusing on CBCs. We also briefly describe the laser interferometers that are being used to attempt to detect gravitational radiation. We describe in detail the search pipeline that has been used to search for gravitational waves emitted from CBCs in data taken by the LIGO and Virgo detectors. We present the latest results of the all-sky, all-time search and electromagnetically triggered searches. We introduce a fully coherent, multi-detector analysis that can be used to search for CBC signals in coincidence with electromagnetically observed events. Using a number of signal consistency tests, including a coherent extension of the often used x2 test, we demonstrate that the coherent search offers an improvement in sensitivity when compared to the previous search method. Additionally we describe an extension of the coherent search that can be used to search for CBC signals where one of the components has spin. This method is well suited to searches for neutron star, black hole binaries. We introduce a "stochastic" algorithm that can be used to create template banks in arbitrary parameter spaces of arbitrary dimension. We demonstrate this method in a search for super-massive CBCs in the mock LISA data challenge. Finally, we present the black hole hunter game, which has been widely used in outreach projects.

I present work on the accretion onto stellar mass black holes in several scenarios. Due to dynamical friction stellar mass black holes are expected to form high density cusps in the inner parsec of our Galaxy. These compact remnants may be accreting cold dense gas present there, and give rise to potentially observable X-ray emission. I build a simple but detailed time-dependent model of such emission. Future observations of the distribution and orbits of the gas in the inner parsec of Sgr A* will put tighter constraints on the cusp of compact remnants. GRS 1915+105 is an LMXB, whose large orbital period implies a very large accretion disc and explains the extraordinary duration of its current outburst. I present smoothed particle hydrodynamic simulations of the accretion disc. The models includes the thermo-viscous instability, irradiation from the central object and wind loss. I find that the outburst of GRS 1915+105 should last a minimum of 20 years and up to ∼ 100 years if the irradiation is playing a significant role in this system. The predicted recurrence times are of the order of 10^4 years, making the duty cycle of GRS 1915+105 to be a few 0.1%. I present a simple analytical method to describe the observable behaviour of long period black hole LMXBs, similar to GRS 1915+105. Constructing two simple models for the surface density in the disc, outburst and quiescence times are calculated as a function of orbital period. LMXBs are an important constituent of the X-ray light function (XLF) of giant elliptical galaxies. I find that the duty cycle can vary considerably with orbital period, with implications for modelling the XLF.

In Part I we investigated the phenomenon that photons propagating in curved space-time may, depending on their direction and polarisation, travel at speeds greater than the speed of light <I>c</I>. The explicit cases calculated for, were the Reissner Nordstrom and Kerr metrics. This led to a postulated Horizon Sum Rule and Polarisation Theorem for photons propagating in general black hole spacetimes. The effects of a purely electromagnetic background were also calculated on photon propagation, with an intriguing possible link with the conformal anomaly appearing. We argue that the 'faster than light effect' does not violate causality, but rather implies the breakdown of the Strong Equivalence Principle for interacting Quantum Field Theories in curved space-time. In Part II we calculated the tree level and one loop quantum corrections, to the entropies of Rindler space and a Schwarzschild black hole, in two dimensions, due to a minimally coupled, massive scalar field, via two differing approaches. The first, the conical singularity method, relied on shifting the Hawking temperature away from its equilibrium value, inducing temperature dependent corrections to the entropy. For the Schwarzschild black hole case, the effective action was found to possess the property of invariance under temperature duality. The second, 'Brick Wall' method, involved counting the number of states that a scalar field could occupy in the vicinity of an event horizon. The entropies consequently obtained for Rindler space and a Schwarzchild black hole, in two dimensions, were found to be divergent as the horizon was approached. We discussed how both investigations might shed some light on the nature and origin of black hole entropy.

Many galaxies in the Universe show evidence of a supermassive black hole at their dynamical centre. About 10-20% of these galaxies also contain an extremely bright, point-like continuum source in their nuclear region. These luminous objects are called active galactic nuclei (AGN), and are thought to be powered by material accreting onto the central black hole. The infalling gas loses its energy and angular momentum by passing through an accretion disc. The disc then radiates away this energy, and it is this radiation that is observed as an AGN. When observed in the X-ray waveband AGN are found to be very bright and rapidly variable (on timescales as small as 1000 s), implying that the emission must originate from the innermost regions of the central engine. X-ray spectra of AGN often exhibit distinct features that are attributed to radiation reflecting off of the accretion disc. Therefore, it is possible to use sensitive X-ray spectroscopic observations of AGN to directly probe the physics of accretion flows only a short distance away from the black hole. Comparing the results from AGN with different properties, such as luminosity or radio power, may allow general conclusions on the AGN phenomenon to be drawn. This thesis applies computations of X-ray reflection spectra to observations of different AGN in order to determine various properties of their accretion flows. The calculations take into account the ionization effects of the incident radiation and consider different density structures for the surface of the accretion disc. It was found that the model spectra were a good description of the observed X-ray data for many AGN between 1 and 10 keV. In particular, five narrow-line Seyfert 1 galaxies were well fit with reflection spectra from a highly ionized accretion disc, consistent with the idea that they contain rapidly accreting black holes. A similar result was found when fitting the data of 3C 120, a broad-line radio galaxy, which argues against the claim that radio-loud AGN have truncated accretion discs. On the other hand, a weakly ionized reflector proved to be a better fit to the Seyfert 1 galaxy MCG-6-30-15.

This thesis investigates black holes in string theory through string amplitudes and through gauge-gravity duality. The research presented in this thesis supports the claim that string theory is capable of a consistent quantum-mechanical description of black holes and develops techniques which may prove useful in testing this claim in new scenarios. The thesis comprises two parts. Part I describes novel disk amplitudes which derive the supergravity elds sourced by a D-brane with a travelling wave, and Part II describes free particle structures arising in a matrix model which is related through gauge-gravity duality to asymptotically anti-de Sitter black holes. The disk amplitudes calculated in Part I provide a direct connection between the microscopic worldsheet description of a D-brane with a travelling wave and its macroscopic supergravity description. A D-brane carrying a travelling wave can be mapped via string dualities to the two-charge D1-D5 black hole and this research opens up the possibility to use these techniques to study the three-charge D1-D5-P black hole. Part II of the thesis identi es free particle descriptions of non-holomorphic operators in a complex matrix model derived from dimensional reduction of N = 4 Super-Yang-Mills theory. This research generalizes the free particle description in the half-BPS sector of this theory which was realized in supergravity and enabled studies of the microscopics of singular geometries. The free particle descriptions have been derived at zero gauge coupling; if these or similar structures are also present at strong coupling this research could be used to study the microscopics of non-extremal asymptotically anti-de Sitter black holes.

Thesis (Ph. D.)--Massachusetts Institute of Technology, Dept. of Physics, 2005.<br>This electronic version was submitted by the student author. The certified thesis is available in the Institute Archives and Special Collections.<br>Includes bibliographical references (p. 201-212).<br>This Thesis describes the basic framework of a relativistic ray-tracing code for analyzing accretion processes around Kerr black holes. We begin in Chapter 1 with a brief historical summary of the major advances in black hole astrophysics over the past few decades. In Chapter 2 we present a detailed description of the ray-tracing code, which can be used to calculate the transfer function between the plane of the accretion disk and the detector plane, an important tool for modeling relativistically broadened emission lines. Observations from the Rossi X-Ray Timing Explorer have shown the existence of high frequency quasi-periodic oscillations (HFQPOs) in a number of black hole binary systems. In Chapter 3, we employ a simple \hot spot" model to explain the position and amplitude of these HFQPO peaks. The power spectrum of the periodic X-ray light curve consists of multiple peaks located at integral combinations of the black hole coordinate frequencies, with the relative amplitude of each peak determined by the orbital inclination, eccentricity, and hot spot arc length. In Chapter 4, we introduce additional features to the model to explain the broadening of the QPO peaks as well as the damping of higher frequency harmonics in the power spectrum. The complete model is used to fit the power spectra observed in XTE J1550-564, giving confidence limits on each of the model parameters. In Chapter 5 we present a description of the structure of a relativistic alpha-disk around a Kerr black hole. Given the surface temperature of the disk, the observed spectrum is calculated using the transfer function mentioned above.<br>(cont.) The features of this modified thermal spectrum may be used to infer the physical properties of the accretion disk and the central black hole. In Chapter 6 we develop a Monte Carlo code to calculate the detailed propagation of photons from a hot spot emitter scattering through a corona surrounding the black hole. The coronal scattering has two major observable effects: the inverse-Compton process alters the photon spectrum by adding a high energy power-law tail, and the random scattering of each photon effectively damps out the highest frequency modulations in the X-ray light curve.<br>by Jeremy David Schnittman.<br>Ph.D.