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Sökning: hsv:(NATURVETENSKAP) hsv:(Fysik) hsv:(Astronomi astrofysik och kosmologi) > Doktorsavhandling

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1.
  • Persson, Daniel, 1978 (författare)
  • Arithmetic and Hyperbolic Structures in String Theory
  • 2009
  • Doktorsavhandling (övrigt vetenskapligt/konstnärligt)abstract
    • This thesis consists of an introductory text followed by two separate parts which may be read independently of each other. In Part I we analyze certain hyperbolic structures arising when studying gravity in the vicinity of spacelike singularities (the BKL-limit). In this limit, spatial points decouple and the dynamics exhibits ultralocal behaviour which may be mapped to an auxiliary problem given in terms of a (possibly chaotic) hyperbolic billiard. In all supergravities arising as low-energy limits of string theory or M-theory, the billiard dynamics takes place within the fundamental Weyl chambers of certain hyperbolic Kac-Moody algebras, suggesting that these algebras generate hidden infinite-dimensional symmetries of gravity. We investigate the modification of the billiard dynamics when the original gravitational theory is formulated on a compact spatial manifold of arbitrary topology, revealing fascinating mathematical structures known as galleries. We further use the conjectured hyperbolic symmetry E10 to generate and classify certain cosmological (S-brane) solutions in eleven-dimensional supergravity. Finally, we show in detail that eleven-dimensional supergravity and massive type IIA supergravity are dynamically unified within the framework of a geodesic sigma model for a particle moving on the infinite-dimensional coset space E10/K(E10). Part II of the thesis is devoted to a study of how (U-)dualities in string theory provide powerful constraints on perturbative and non-perturbative quantum corrections. These dualities are typically given by certain arithmetic groups G(Z) which are conjectured to be preserved in the effective action. The exact couplings are given by moduli-dependent functions which are manifestly invariant under G(Z), known as automorphic forms. We discuss in detail various methods of constructing automorphic forms, with particular emphasis on a special class of functions known as (non-holomorphic) Eisenstein series. We provide detailed examples for the physically relevant cases of SL(2,Z) and SL(3,Z), for which we construct their respective Eisenstein series and compute their (non-abelian) Fourier expansions. We also discuss the possibility that certain generalized Eisenstein series, which are covariant under the maximal compact subgroup K(G), could play a role in determining the exact effective action for toroidally compactified higher derivative corrections. Finally, we propose that in the case of rigid Calabi-Yau compactifications in type IIA string theory, the exact universal hypermultiplet moduli space exhibits a quantum duality group given by the Picard modular group SU(2,1;Z[i]). To verify this proposal we construct an SU(2,1;Z[i])-invariant Eisenstein series, and we present preliminary results for its Fourier expansion which reveals the expected contributions from D2-brane and NS5-brane instantons.
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2.
  • Scott, Pat, 1982- (författare)
  • Searches for Particle Dark Matter : Dark stars, dark galaxies, dark halos and global supersymmetric fits
  • 2010
  • Doktorsavhandling (övrigt vetenskapligt/konstnärligt)abstract
    • The identity of dark matter is one of the key outstanding problems in both particle and astrophysics. In this thesis, I describe a number of complementary searches for particle dark matter. I discuss how the impact of dark matter on stars can constrain its interaction with nuclei, focussing on main sequence stars close to the Galactic Centre, and on the first stars as seen through the upcoming James Webb Space Telescope. The mass and annihilation cross-section of dark matter particles can be probed with searches for gamma rays produced in astronomical targets. Dwarf galaxies and ultracompact, primordially-produced dark matter minihalos turn out to be especially promising in this respect. I illustrate how the results of these searches can be combined with constraints from accelerators and cosmology to produce a single global fit to all available data. Global fits in supersymmetry turn out to be quite technically demanding, even with the simplest predictive models and the addition of complementary data from a bevy of astronomical and terrestrial experiments; I show how genetic algorithms can help in overcoming these challenges.
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3.
  • Dall` Olio, Daria, 1981 (författare)
  • Magnetic fields around massive protostars as traced by masers and dust emission
  • 2020
  • Doktorsavhandling (övrigt vetenskapligt/konstnärligt)abstract
    • It is not fully clear how the magnetic field acts during the first stages of star formation. A possible way to clarify its role is to observe the polarized light coming from masers and thermal dust emission. By measuring linear polarization angles and Zeeman splitting of different maser species it is possible to study the magnetic field morphology and strength in different parts of the protostar. Polarized emission of thermal dust has also been used extensively to probe the magnetic field at the onset of star formation. In this thesis we study the magnetic field properties of two well-known sources: the massive protostar IRAS18089-1732, showing a hot core chem- istry and a disc-outflow system, and the high-mass star forming complex G9.62+0.19, presenting several cores at different evolutionary stages. We also investigate the polarization properties of selected methanol masers, con- sidering newly-calculated methanol g-factors and hyperfine components. We compare our results with previous maser observations and we evaluate the contribution of preferred hyperfine pumping and non-Zeeman effects. We make use of MERLIN and ALMA observations and we analyse the polarized emission by 6.7 GHz methanol masers and thermal dust. Simulations were run using the radiative transfer code CHAMP for different magnetic field values, hyperfine components and pumping efficiencies. We observe that the large scale field probed by dust continuum emission is consistent with the small scale magnetic field probed by masers. Moreover, in the G9.62+0.19 complex we resolved several cores showing polarized emission. We propose an evolutionary sequence of magnetic field in this complex, where the less evolved stellar embryo exhibits a magnetic field stronger than the more evolved one. From our simulations, we find that preferred hyperfine pumping can explain some high levels of linear and circular polarization. We also notice that non-Zeeman effects need to be considered in magnetic field studies. In conclusion, our work indicates that there is a link between the magnetic field at different scales. More masers observations will help in evaluating the relevance of non-Zeeman effects and obtain good estimates of magnetic fields close to the protostar. Future multi-wavelength and multi-scale observations, aimed at detecting polarized light from masers, thermal dust and thermal molecular lines, will help to constrain magnetic field properties around massive protostars.
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4.
  • Urdshals, Einar, 1995 (författare)
  • Dark matter electron interactions in detector materials
  • 2024
  • Doktorsavhandling (övrigt vetenskapligt/konstnärligt)abstract
    • Dark Matter (DM) makes up 85% of the matter content of the universe, and its gravitational effects are seen on scales ranging from that of cosmology to that of galactic astrophysics. The nature of DM is, however, unknown. Study- ing DM in the lab with a class of experiments called direct detection (DD) experiments is key to understanding its properties. For decades, experiments have been attempting to do this through searches for DM induced nuclear recoils. These have not been found, and a possible reason for this is that the hypothetical DM particle is too light to induce nuclear recoils. Therefore, in the last decade experiments have been built to study DM through electron recoils instead. As the electron is 4 orders of magnitude lighter than the nu- cleus, electron recoils can be induced by DM down to 4 orders of magnitude lighter than the lightest DM particle probeable with nuclear recoils. In order to understand current and upcoming results from experiments searching for DM induced electron recoils, a theoretical understanding of DM electron scatterings in detector materials is needed. When modelling such electron interactions, one need input both from DM and material physics. This thesis improves the theoretical understanding by both improving the material description using density functional theory (DFT), and by extending the DM description using non-relativistic effective theory (NR-EFT) tools. The improvement gives not only a more accurate description of the DM- electron interactions that the experiments are expected to see; it also vastly extends the forms of DM that can be studied in direct detection experiments. Before this extension, one typically focused on a benchmark case of DM, the so called dark photon model. With this extension, one can cover all forms of gravitationally bound DM with spins of 0, 1/2 or 1. In the included works, advances are made in the description of DM-electron interactions in common detector materials such as liquid xenon, silicon and germanium, as well as to materials in the research and development phase, such as graphene and carbon nanotubes (CNTs).
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5.
  • Zema, Vanessa, 1991 (författare)
  • Unveiling the nature of dark matter with direct detection experiments
  • 2020
  • Doktorsavhandling (övrigt vetenskapligt/konstnärligt)abstract
    • The desire of discovery is an anthropic need which characterises and connects the human being over the eras. In particular, observing the sky is an instinctive drive exerted by the curiosity of the mysteries which it retains. At the present time, the tremendous advances in the exploration of space have opened even more challenges than back in the days. One of the most urgent question is unveiling the nature of dark matter (DM). As stated by Neta A. Bahcall (Professor at Princeton University), "Cosmology has revealed an amazing universe, filled with a "dark sector" that composes 95% of the energy density of our cosmos [...]" ( Dark matter universe , PNAS, 2015). About one-third of this dark sector is associated to an invisible and still undetected form of matter, the so-called dark matter, whose gravitational effect manifests at all cosmological scales. Both theoretical and experimental observations based on ordinary gravity reinforced the evidences for the existence of DM, since its first appearance in the pioneering calculations of F. Zwicky (1933). This PhD project explores the hypothesis that DM is made of new particles beyond the standard model. More specifically, it focuses on those DM particles which are trapped into the galactic gravitational field and populate the galactic halo. If DM interacts with ordinary particles, extremely sensitive detectors operating in very low-background environments, are expected to detect galactic DM particles scattering off their target material. This widely employed experimental technique is known as DM direct detection and it is the focus of my studies, where I consider the further hypothesis that DM interacts with atomic nuclei. The research I conducted during my PhD program consists of two main parts: the first part focused on purely phenomenology aspects of the DM direct detection (namely on the DM annual modulation treated using a non-relativistic effective theory and on the scattering of spin-1 DM particles off polarised nuclei) and the second one is more closely connected to experimental applications. The latter has been strongly stimulated by my collaboration with the two DM direct detection experiments CRESST and COSINUS.  For CRESST, I compute the DM-nucleus cross-section for the conventional spin-dependent interactions, used to analyse the data collected with a prototype Li-based detector module, and I derive some prospects for a time dependent analysis of CRESST-III data, using a statistical frequentist approach based on Monte Carlo simulations. For COSINUS, I provide a significant extension of the pulse shape model currently used by CRESST and COSINUS in order to explain experimental observations related to the COSINUS detector response. Finally, I contribute to ongoing studies on the phonon propagation in NaI crystals based on solid state physics. This PhD thesis has been oriented to fill the gap between theoretical and experimental efforts in the DM field. This approach has facilitated the exchange of expertise, has driven the trend of my research and has stimulated the development of the ideas and methods described in this PhD thesis.
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6.
  • Forsberg, Mats, 1978- (författare)
  • Gravitational perturbations in plasmas and cosmology
  • 2010
  • Doktorsavhandling (övrigt vetenskapligt/konstnärligt)abstract
    • Gravitational perturbations can be in the form of scalars, vectors or tensors. This thesis focuses on the evolution of scalar perturbations in cosmology, and interactions between tensor perturbations, in the form of gravitational waves, and plasma waves. The gravitational waves studied in this thesis are assumed to have small amplitudes and wavelengths much shorter than the background length scale, allowing for the assumption of a flat background metric. Interactions between gravitational waves and plasmas are described by the Einstein-Maxwell-Vlasov, or the Einstein-Maxwell-fluid equations, depending on the level of detail required. Using such models, linear wave excitation of various waves by gravitational waves in astrophysical plasmas are studied, with a focus on resonance effects. Furthermore, the influence of strong magnetic field quantum electrodynamics, leading to detuning of the gravitational wave-electromagnetic wave resonances, is considered. Various nonlinear phenomena, including parametric excitation and wave steepening are also studied in different astrophysical settings. In cosmology the evolution of gravitational perturbations are of interest in processes such as structure formation and generation of large scale magnetic fields. Here, the growth of density perturbations in Kantowski-Sachs cosmologies with positive cosmological constant is studied.
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7.
  • Georgieva, Iskra, 1987 (författare)
  • A Cosmic Quest for New Worlds. Characterising Exoplanet Signals via Radial Velocity and Transit Photometry.
  • 2023
  • Doktorsavhandling (övrigt vetenskapligt/konstnärligt)abstract
    • Since the first unambiguous detection of a planet around a Sun-like star, the interest in the new and exciting field of exoplanets has grown immensely. New and exciting developments are seen at a pace unparalleled for most subfields of astronomy. In this thesis, I describe the two most successful techniques for exoplanet detection and characterisation – transits and radial velocities – and the challenges commonly encountered in extracting the planets from the data. Transit photometry allows us to measure the planet radius, while radial velocity measurements give us the planet’s minimum mass. These methods’ true strength, however, manifests in their combination as it allows us to estimate the true mass, which, together with the radius, gives us the planet’s bulk density. This is a powerful quantity, which allows us to construct models and make predictions about the structure and composition of a planet’s interior, as well as its atmosphere. Zeroing in on the latter two is currently one of the biggest challenges for exoplanet characterisation. I describe the process of detecting a planet in a stellar light curve, and how transits and radial velocities are modelled together in order to determine the planet parameters. This is then followed by the ideal theoretical approach, which can be used to study a system in practice. However, the current challenges in exoplanet characterisation surpass the ideal case, leading us to explore more complex models. I then discuss the biggest nemesis to planet discovery, particularly in radial velocity timeseries – stellar activity, and the problem of its often stochastic manifestation. A special focus is given to one method for its mitigation – modelling the radial velocities alongside activity indicators. This is the core concept of multi-dimensional Gaussian process regression, particularly with the quasi-periodic covariance function, which is used in a large part of this work. Finally, the last part if the thesis shows that while the ideal planet case can sometimes be applicable for quiet stars, as is the case of the TOI-2196 system, extending to non-parametric models, such as Gaussian processes, can help us to detect planets in complicated datasets, as demonstrated by the cases of the TOI-1260, TOI-733, TOI-776 and TOI-1416 systems.
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8.
  • Kobyakov, Dmitry, 1985- (författare)
  • Hydrodynamics of Binary Bose-Einstein Condensates and Hydro-elasticity of the Inner Crust of Neutron Stars
  • 2014
  • Doktorsavhandling (övrigt vetenskapligt/konstnärligt)abstract
    • In the present thesis, “Hydrodynamics of Binary Bose-Einstein Condensates and Hydro-elasticity of the Inner Crust of Neutron Stars”, the hydrodynamic effects, instabilities and superfluid turbulence in binary immiscible ultracold gases, and hydro-elastic macroscopic coupled modes and microscopic structure of the inner layers of the crust of neutron stars, are studied. The ultracold gas dynamics can be realized in the laboratory. The excitation modes of the inner crust determine a number of observable properties such as elasticity, thermal properties and mass transport properties. Here we focus on expanding the details, rather than repeating the results presented in the published articles.In the part of the thesis related to atomic ultracold gases, we utilize the physical parameters in the experimentally realizable parameter region. We numerically simulate the coupled non-linear Schrödinger equations, and calculate observable quantities, such as phase and modulus of the order parameter, conditions needed for observation of the Rayleigh-Taylor instability and for turbulence generation. The numerical calculations are accompanied by analytical description of the processes. The dispersion relation for capillary-gravitational waves at the interface between two ultracold gases, is derived straightforwardly from the superfluid Lagrangian. The equations of motion for centre-of-mass of the superfluids are derived, and then used in our model of the quantum swapping of immiscible superfluids pressed by a strong external force. By numerical simulation, we find that the Kelvin-Helmholtz instability which occurs at the non-linear stage of the Rayleigh-Taylor instability, can generate quantum turbulence with peculiar properties. We find that two-dimensional superfluid systems with weak inter-component repulsion are different from previously studied strongly repulsive binary superfluids, because the quantum Kelvin-Helmholtz instability in weakly repulsive superfluids rolls up the whole interface forming a vortex bundle, similarly to dynamics of the shear fluid layers in the classical hydrodynamics. Production of vortex bundles favours the Kolmogorov spectrum of turbulence, and we find that the Kolmogorov scaling indeed is present in a freely decaying turbulence.In the part of the thesis related to neutron stars, we study the inner crust of neutron stars, where the fully ionized atomic nuclei coexist with a superfluid of neutrons. The interaction between superfluid neutrons and the crystallized Coulomb plasma is due to the interaction between density perturbations (interaction of the scalar type), and between the current - the non-dissipative entrainment effect (interaction of the vector type). We calculate velocities of the collective modes of the crystal coupled to superfluid neutrons. As an input we use the results of microscopic nuclear calculations in the framework of the compressible liquid drop model (the Lattimer and Swesty equation of state), and more recent effective Thomas-Fermi calculations with shell corrections (N. Chamel, and the Brussels theoretical nuclear physics group). Knowledge of velocities as functions of the matter density in the inner crust is important for calculation of a number of dynamic and transport properties. The heat transport properties of the inner crust are directly observable in accreting binary systems (low-mass x-ray binaries). The mass transport properties of the inner crust are directly linked to the rotational evolution, being a key physical ingredient of the pulsar glitch phenomenon. The elastic properties are related to the vibrational modes of the star, and to the breaking stress of the crust.In the second part of our work on neutron stars we investigate the microscopic structure of the inner crust treating the structure as an anisotropic crystal coupled to s-wave superfluid neutron liquid. As we analyse dynamics of the elementary excitations at higher wavenumbers (smaller scales), we reach the edge of the first Brillouin zone. The Lattimer-Swesty data is applicable for wavenumbers much smaller than the edge of the first Brillouin zone. We extrapolate the data through the whole first Brillouin zone to calculate the fastest growth rate of the unstable modes. The crucial step is to calculate the mode velocities in anisotropic crystal incorporating both the induced neutron-proton interactions, and the electron screening properties. We find that the combined influence of these two effects leads to softening of the longitudinal phonon of the lattice above about the Thomas-Fermi screening wavenumber of the electrons. The critical wavenumber when the frequency becomes purely imaginary is about  1/5 - 2/3  of the reciprocal lattice vector, thus validating our assumption. The imaginary mode frequency implies instability at finite wavenumbers. Our calculations suggest that the mode at the first Brillouin zone edge is the most unstable, and thus the structure experiences a displacive phase transition when the central ion of a unit cell of the body-cubic-centred lattice, is displaced to the cube face. Thus, the electronic structure of matter at densities above the neutron drip [1], is richer than previously appreciated, and new microscopic calculations of nuclear structure are necessary which take into account the high-wavenumber physics. Such calculations will provide crucial input to models interpreting the quasi-periodic oscillations in Soft Gamma Repeaters as magnetar x-ray flares, and to the theory of glitches of neutron stars.[1] The neutron drip density is ~3×1011 g cm-3.
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9.
  • Akrami, Yashar, 1980- (författare)
  • Supersymmetry vis-à-vis Observation : Dark Matter Constraints, Global Fits and Statistical Issues
  • 2011
  • Doktorsavhandling (övrigt vetenskapligt/konstnärligt)abstract
    • Weak-scale supersymmetry is one of the most favoured theories beyond the Standard Model of particle physics that elegantly solves various theoretical and observational problems in both particle physics and cosmology. In this thesis, I describe the theoretical foundations of supersymmetry, issues that it can address and concrete supersymmetric models that are widely used in phenomenological studies. I discuss how the predictions of supersymmetric models may be compared with observational data from both colliders and cosmology. I show why constraints on supersymmetric parameters by direct and indirect searches of particle dark matter are of particular interest in this respect. Gamma-ray observations of astrophysical sources, in particular dwarf spheroidal galaxies, by the Fermi satellite, and recording nuclear recoil events and energies by future ton-scale direct detection experiments are shown to provide powerful tools in searches for supersymmetric dark matter and estimating supersymmetric parameters. I discuss some major statistical issues in supersymmetric global fits to experimental data. In particular, I further demonstrate that existing advanced scanning techniques may fail in correctly mapping the statistical properties of the parameter spaces even for the simplest supersymmetric models. Complementary scanning methods based on Genetic Algorithms are proposed.
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10.
  • Burgman, Alexander, 1991- (författare)
  • Bright Needles in a Haystack : A Search for Magnetic Monopoles Using the IceCube Neutrino Observatory
  • 2020
  • Doktorsavhandling (övrigt vetenskapligt/konstnärligt)abstract
    • The IceCube Neutrino Observatory at the geographic South Pole is designed to detect the light produced by the daughter-particles of in-ice neutrino-nucleon interactions, using one cubic kilometer of ice instrumented with more than 5000 optical sensors.Magnetic monopoles are hypothetical particles with non-zero magnetic charge, predicted to exist in many extensions of the Standard Model of particle physics. The monopole mass is allowed within a wide range, depending on the production mechanism. A cosmic flux of magnetic monopoles would be accelerated by extraterrestrial magnetic fields to a broad final velocity distribution that depends on the monopole mass.The analysis presented in this thesis constitutes a search for magnetic monopoles with a speed in the range [0.750;0.995] in units of the speed of light. A monopole within this speed range would produce Cherenkov light when traversing the IceCube detector, with a smooth and elongated light signature, and a high brightness.This analysis is divided into two main steps. Step I is based on a previous IceCube analysis, developed for a cosmogenic neutrino search, with similar signal event characteristics as in this analysis. The Step I event selection reduces the acceptance of atmospheric events to lower than 0.1 events per analysis livetime. Step II is developed to reject the neutrino events that Step I inherently accepts, and employs a boosted decision tree for event classification. The (astrophysical) neutrino rate is reduced to 0.265 events per analysis livetime, corresponding to a 97.4 % rejection efficiency for events with a primary energy above 1E+5 GeV.No events were observed at final analysis level over eight years of experimental data. The resulting upper limit on the magnetic monopole flux was determined to 2.54E–19 per square centimeter per second per steradian, averaged over the covered speed region. This constitutes an improvement of around one order of magnitude over previous results.
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