No description available.

No description available.

No description available.

京都大学 / 新制・課程博士 / 博士(理学) / 甲第23705号 / 理博第4795号 / 新制||理||1686(附属図書館) / 京都大学大学院理学研究科物理学・宇宙物理学専攻 / (主査)教授 嶺重 慎, 准教授 前田 啓一, 准教授 野上 大作 / 学位規則第4条第1項該当 / Doctor of Science / Kyoto University / DFAM

Astrophysical phenomena such as exploding primordial black holes (PBHs), gamma-ray bursts (GRBs), compact object mergers, and supernovae, are expected to produce a single pulse of electromagnetic radiation detectable at the low-frequency end of the radio spectrum. Detection of any of these pulses would be significant for the study of the objects themselves, their host environments, and the interstellar/intergalactic medium. Furthermore, a detection of a radio transient from an exploding PBH could be a signature of an extra spatial dimension, which would drastically alter our perception of spacetime. However, even upper limits on the existence of PBHs, from transient searches, would be important to discussions of cosmology. We describe a method to carry out an agnostic single-dispersed-pulse search, and apply it to data collected with the Eight-meter-wavelength Transient Array (ETA). ETA is a radio telescope dedicated to searching for transient pulses. It consists of 12 crossed-dipole antenna stands with Galactic-noise-limited performance from 29-47 MHz. There is a vast amount of data collected from an ETA observation. It is therefore greatly beneficial to use a computer cluster, which works in parallel on different parts of a data set, in order to carry out a single-pulse search quickly and efficiently. Each spectrogram in a data set needs to be analyzed individually, without reference to the rest, in order to utilize a computer cluster's capabilities. The data reduction software has been developed for single-dispersed-pulse searches, and is described in this thesis. The data reduction involves sweeping through the collected data with a dedispersion routine assuming a range of dispersion measures. The resulting time series are searched with multiple matched filters for signals above a signal-to-noise threshold. Applying the single pulse search to ~ 30 hours of ETA data did not yield a compelling detection of an astrophysical signal. However, from ≈ 5 hours of interference-free data we find an observational upper limit to the rate of exploding PBHs of r ≈ 4.8 × 10⁻⁷ pc⁻³ y⁻¹ for a PBH with a fireball Lorentz-factor f = 10^4.3. This limit is applicable to PBHs in the halo of the Galaxy to distances ≲ 2 kpc, and dispersion measures ≲ 80 pc cm⁻³ . We also find a source-agnostic rate limit ≲ 0.25 events y⁻¹ deg⁻² for pulses of duration < 3 s, and having apparent energy densities ≳ 2.6 × 10⁻²³ J m⁻² Hz⁻¹ at 38 MHz. / Ph. D.

<p>String theory is a unified framework for general relativity and quantum mechanics, thus being a theory of quantum gravity. In this thesis we discuss various aspects of quantum gravity for particular systems, having in common the existence of horizons. The main motivation is that one major challenge in theoretical physics today is in trying to understanding how time dependent backgrounds, with its resulting horizons and space-like singularities, should be described in a controlled way. One such system of particular importance is our own universe.</p><p>We begin by discussing the information puzzle in de Sitter space and consequences thereof. A typical time-scale is encountered, which we interpreted as setting the thermalization time for the system. Then the question of closed time-like curves is discussed in the combined setting where we have a rotating black hole in a Gödel-like universe. This gives a unified picture of what previously was considered as independent systems. The last three projects concerns $c=1$ matrix models and their applications. First in relation to the RR-charged two dimensional type 0A black hole. We calculate the ground state energy on both sides of the duality and find a perfect agreement. Finally, we relate the 0A model at self-dual radius to the topological string on the conifold. We find that an intriguing factorization of the theory previously observed for the topological string is also present in the 0A matrix model.</p>

String theory is a unified framework for general relativity and quantum mechanics, thus being a theory of quantum gravity. In this thesis we discuss various aspects of quantum gravity for particular systems, having in common the existence of horizons. The main motivation is that one major challenge in theoretical physics today is in trying to understanding how time dependent backgrounds, with its resulting horizons and space-like singularities, should be described in a controlled way. One such system of particular importance is our own universe. We begin by discussing the information puzzle in de Sitter space and consequences thereof. A typical time-scale is encountered, which we interpreted as setting the thermalization time for the system. Then the question of closed time-like curves is discussed in the combined setting where we have a rotating black hole in a Gödel-like universe. This gives a unified picture of what previously was considered as independent systems. The last three projects concerns $c=1$ matrix models and their applications. First in relation to the RR-charged two dimensional type 0A black hole. We calculate the ground state energy on both sides of the duality and find a perfect agreement. Finally, we relate the 0A model at self-dual radius to the topological string on the conifold. We find that an intriguing factorization of the theory previously observed for the topological string is also present in the 0A matrix model.

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. / Doctorat en Sciences / info:eu-repo/semantics/nonPublished

In a preamble, a quick summary of the line of thought from Noether's theorems to modern views on conserved charges in gauge theories is attempted. Most of the background material needed for the thesis is set out through a small survey of the literature. Emphasis is put on the concepts more than on the formalism, which is relegated to the appendices.<p><p>The treatment of exact conservation laws in Lagrangian gauge theories constitutes the main axis of the first part of the thesis. The formalism is developed as a self-consistent theory but is inspired by earlier works, mainly by cohomological results, covariant phase space methods and by the Hamiltonian formalism.<p>The thermodynamical properties of black holes, especially the first law, are studied in a general geometrical setting and are worked out for several black objects: black holes, strings and rings. Also, the geometrical and thermodynamical properties of a new family of black holes with closed timelike curves in three dimensions are described.<p><p><p>The second part of the thesis is the natural generalization of the first part to asymptotic analyses. We start with a general construction of covariant phase spaces admitting asymptotically conserved charges. The representation of the asymptotic symmetry algebra by a covariant Poisson bracket among the conserved charges is then defined and is shown to admit generically central extensions. The asymptotic structures of three three-dimensional spacetimes are then studied in detail and the consequences for quantum gravity in three dimensions are discussed. / Doctorat en Sciences / info:eu-repo/semantics/nonPublished

Masters thesis of Liam Farrell, under the supervision of Dr. Duncan O'Dell. Successfully defended on August 26, 2021. / Caustics are regions created by the natural focusing of waves. Some examples include rainbows, spherical aberration, and sonic booms. The intensity of a caustic is singular in the classical ray theory, but can be smoothed out by taking into account the interference of waves. Caustics are generic in nature and are universally described by the mathematical theory known as catastrophe theory, which has successfully been applied to physically describe a wide variety of phenomena. Interestingly, caustics can exist in quantum mechanical systems in the form of phase singularities. Since phase is such a central concept in wave theory, this heralds the breakdown of the wave description of quantum mechanics and is in fact an example of a quantum catastrophe. Similarly to classical catastrophes, quantum catastrophes require some previously ignored property or degree of freedom to be taken into account in order to smooth the phase divergence. Different forms of spontaneous pair-production appear to suffer logarithmic phase singularities, specifically Hawking radiation from gravitational black holes. This is known as the trans-Planckian problem. We will investigate Hawking radiation formed in an analogue black hole consisting of a flowing ultra-cold Bose-Einstein condensate. By moving from an approximate hydrodynamical continuum description to a quantum mechanical discrete theory, the phase singularity is cured. We describe this process, and make connections to a new theory of logarithmic catastrophes. We show that our analogue Hawking radiation is mathematically described by a logarithmic Airy catastrophe, which further establishes the plausibility of pair-production being a quantum catastrophe / Thesis / Master of Science (MSc)