### Browsing by Author "Weltman, Amanda"

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- ItemOpen AccessA review of the Hubble tension(2022) Houliston, Rebecca; Larena, Julie; Weltman, AmandaThe Hubble constant H0 is the rate of expansion of the universe today. The discrepancy between the early universe H0 value, inferred using ΛCDM from Planck observations of the CMB, and the late universe H0 value, obtained using luminosity and distance measurements from a Type Ia Supernova (SNIa) distance ladder, has now reached 4.2σ. Despite improvements in precision, this tension has increased. This thesis studies these two measurements, as well as various other H0 determinations, which are independent of both the CMB and the SNIa distance ladder and corroborate the Hubble tension (with the caveat that many have large uncertainties), with a particular focus on lensing and gravitational waves. Some of the very many solutions proposed to resolve the Hubble tension are also explored, with an emphasis on late universe solutions and Early Dark Energy. The improvement in precision, the growing discrepancy, and the supporting independent measurements of the Planck and SNIa distance ladder H0 values are strong evidence that a significant tension between the early and late universe exists. This indicates that some modification to or expansion of ΛCDM is required. A great deal of models and solutions have been proposed to do so, however none have managed to fully resolve the tension yet.
- ItemOpen AccessA study of circuit Complexity for Coherent States(2022) Tladi, Mpho; Haque, Shajid; Murugan, Jeffrey; Weltman, AmandaComputational complexity is a popular quantity in quantum information theory. It has made huge strides in recent years in the study of black hole dynamics. A brief definition of complexity is the measure of how difficult it is to implement a task. For a quantum system, complexity evaluates the difficulty of preparing a quantum state from a given reference state by unitary transformations. However, in the dual gravity theory complexity has a geometric meaning. In some black hole context, Leonard Susskind and collaborators proposed two holographic conjectures. The Complexity=Volume (CV) states that complexity of the boundary field theory is dual to the volume of a co dimension one maximal surface that extends to the boundary of the Ads space. Complexity=Action (CA) posits that complexity of the boundary is the same as the action evaluated as an action on patch in the bulk defined as the Wheeler De Witt patch. In recent years, these two conjectures have initiated an extensive study of complexity. This thesis is also motivated by these conjectures and will investigate complexity in the field theory side of the story. Specifically, we will explore the complexity for coherent states. We will start with a review of different methods of computing complexity. Finally, we then investigate the complexity for coherent states by using the methods of circuit complexity and operator complexity
- ItemOpen AccessA study of vortex lattices and pulsar glitches(2019) Nkomozake, Thando; Weltman, Amanda; Murugan, JeffIn this project we study the three fundamental theories that explain the phenomenon of superconductivity: The London theory, the Ginzburg-Landau theory and the BCS theory. We review works by several authors who utilized these theories as the basis for their investigation. In our literature review we study the behavior of single and multivortex states in mesoscopic thin superconducting discs whose dimensions are comparable to the penetration depth λ and the coherence length ξ of a superconductor. We learn about the types of phase transitions that the vortex configurations undergo and the stability of the resulting states. Our aim is to investigate how vortex configurations reorganize after phase transitions and whether their reorganization releases any energy into the system of vortices in the disc. If so, then what is the precise mechanism through which the released energy is transferred into the disc? We aim to answer this question and generalize the results to neutron star interiors in order to explain and predict the behavior of pulsar glitches.
- ItemOpen AccessAnomalous coupling of scalars to gauge fields(2011) Brax, Philippe; Burrage, Clare; Davis, Anne-Christine; Seery, David; Weltman, AmandaWe study the transformation properties of a scalar tensor theory coupled to fermions under the Weyl rescaling associated with a transition from the Jordan to the Einstein frame. We give a simple derivation of the corresponding modification to the gauge couplings. After changing frame this gives rise to a direct coupling between the scalar and the gauge fields.
- ItemOpen AccessArtificial Neural Networks as a Probe of Many-Body Localization in Novel Topologies(2022) Beetar, Cameron; Murugan, Jeffrey; Rosa, Dario; Weltman, AmandaWe attempt to show that artificial neural networks may be used as a tool for universal probing of many-body localization in quantum graphs. We produce an artificial neural network, training it on the entanglement spectra of the nearest-neighbour Heisenberg spin1/2 chain in the presence of extremal (definitely ergodic/localizing) disorder values and show that this artificial neural network successfully qualitatively classifies the entanglement spectra at both extremal and intermediate disorder values as being in either the ergodic regime or in the many-body-localizing regime, based on known results. To this network, we then present the entanglement spectra of systems having different topological structures for classification. The entanglement spectra of next-to-nearest-neighbour (J1 − J2, and, in particular, Majumdar-Ghosh) models, star models, and bicycle wheel models - without any further training of the artificial neural network - are classified. We find that the results of these classifications - in particular how the mobility edge is affected - are in agreement with heuristic expectations. This we use as a proof of concept that neural networks and, more generally, machine learning algorithms, endow physicists with powerful tools for the study of many-body localization and potentially other many-body physics problems.
- ItemOpen AccessComputational analysis techniques using fast radio bursts to probe astrophysics(2021) Platts, Emma; Weltman, Amanda; Shock, JonathanThis thesis focuses on Fast Radio Bursts (FRBs) and presents computational techniques that can be used to understand these enigmatic events and the Universe around them. Chapter 1 provides a theoretical overview of FRBs; providing a foundation for the chapters that follow. Chapter 2 details current understandings by providing a review of FRB properties and progenitor theories. In Chapter 3, we implement non-parametric techniques to measure the elusive baryonic halo of the Milky Way. We show that even with a limited data set, FRBs and an appropriate set of statistical tools can provide reasonable constraints on the dispersion measure of the Milky Way halo. Further, we expect that a modest increase in data (from fewer than 100 FRB detections to over 1000) will significantly tighten constraints, demonstrating that the technique we present may offer a valuable complement to other analyses in the near future. In Chapter 4, we study the fine time-frequency structure of the most famous FRB: FRB 121102. Here, we use autocorrelation functions to maximise the structure of 11 pulses detected with the MeerKAT radio telescope. The study is motivated by the low time-resolution of MeerKAT data, which presents a challenge to more traditional techniques. The burst profiles that are unveiled offer unique insight into the local environment of the FRB, including a possible deviation from the expected cold plasma dispersion relationship. The pulse features and their possible physical mechanisms are critically discussed in a bid to uncover the nature and origin of these transients.
- ItemOpen AccessCosmic string cusps and their application to fast radio bursts(2018) Gordin, Jake E. B.; Weltman, AmandaThis thesis concerns observational characteristics of two theoretical aspects of cosmic strings. The first is relativistic modification of cusps. Nambu-Goto strings generically develop cusps, regions of the string which emit coherent electromagnetic radiation when they decay. We point out that consideration of relativistic effects in the rest frame of the string cusp substantially reduces the cusp length, and therefore modifies the normally assumed power, rate, and time scale of any radiation bursts. The second is consideration of wiggly cosmic strings. Simulations imply a distribution of strings in an expanding universe develop small-scale structure called wiggles. We extend on a wiggly Polyakov formalism and show that wiggles prohibit cusp formation (barring ad-hoc fine tuning of initial conditions). We discuss these theoretical results in the context of using strings to explain fast radio bursts (FRBs). Cusp decay is a possible mechanism for sourcing FRBs. We show, however, that (1) consideration of relativistic effect leads to incompatibility with FRB data, and (2) the absence of cusps from “realistic” cosmic strings casts further doubt on the possibility of detecting cosmic strings via electromagnetic signatures.
- ItemOpen AccessGravitational collapse and the information loss problem(2015) Kesselly, Alton Vanie; Weltman, Amanda; Hellaby, CharlesThis thesis is intended to critically review the standard black holes. In this thesis, we used the intractability of the black hole Information loss problem and the current crisis stirred up by the black hole Firewall paradox to support the argument that nature is better off without black holes.
- ItemOpen AccessInvestigating the parameter space of viable models for f(R) gravity(2019) Kandhai, Sulona; Dunsby, Peter; de la Cruz, Alvaro; Weltman, AmandaThe accelerated expansion of spacetime intuitively points to the existence of new, unknown energy fields pervading the universe, but it is has also spurred the growth of the research field of modified gravity theories. Of these, f(R) theories of gravity is the first and simplest modification to General Relativity, and have been studied extensively for their astrophysical and cosmological predictions. Power law f(R) modifications have been shown to exhibit desirable characteristics, producing the late time accelerated expansion as well as satisfying local tests of gravity. However, there is wide degeneracy among models in this class, and they are known to suffer from cosmological instabilities, which could lead to curvature singularities at finite times. This thesis addresses questions directly relating to model degeneracy and sudden singularities. Cosmologies and cosmological perturbations, resulting from a general broken power law modification to GR are generated, studied and evolved. Simulations are performed using 1+3 space time decomposition of the field equations and a dynamical systems approach to f(R) cosmology. The parameter space of this model, which includes the HuSawicki [6], Starobinsky [96] and Miranda [7] f(R) forms as subclasses, is investigated. It is found that there are regions in the parameter space which are completely singular and bound by continuous curves. We also investigate regions of the parameter space in which the attractive nature of gravity is preserved, and find that these regions intersect. The results of a Markov Chain Monte Carlo analysis significantly narrowed the viable region of the exponent parameter space of the general power law f(R) model. Current cosmological distance data; SNIa (Union 2), BAO (6dFGS, BOSS, SDSS, WiggleZ) as well as the LRG power spectrum (SDSS DR9), were used to obtain these constraints. The best fits are compared with the ΛCDM model, and leads to the conclusion that this class is still a candidate for the gravitational interaction.
- ItemOpen AccessThe KLT relations in unimodular gravity(2016) Burger, Daniel; Weltman, Amanda; Murugan, Jeffrey; Ellis, George F RHere we initiate a systematic study of some of the symmetry properties of unimodular gravity, building on much of the known structure of general relativity, and utilizing the powerful technology developed in that context, such as the spinor helicity formal-ism. In particular, we show, up to five-points and tree-level, that the KLT relations of perturbative gravity hold for trace free or unimodular gravity. This work is in conjunction with a paper written with A. Welman, J. Murugan and G.F.R. Ellis (ARXIV: 1511.08517)
- ItemOpen AccessModeling Compact Objects with Effective Field Theory(2022) Martinez, Rodriguez Irvin Fabian; Weltman, AmandaIn this master's thesis we have developed a worldline Effective Field Theory of compact objects, by extending the model of spinning extended objects derived using the coset construction [1], from which one can derive the effective theory from symmetry principles. To massive spinning extended objects, we have added electromagnetic charge and the finite-size structure including dissipation, such that we describe charged spinning compact objects, the most general compact object allowed in a theory of gravity such as General Relativity with classical electrodynamics. To the derived effective action, we have matched the coefficients of the theory from the literature and obtained the leading order post-Newtonian expansion of our effective description of compact objects to show its predictability. We have expanded on the theoretical foundations of the effective theory for spinning extended objects by showing that the developed theory can be equivalent to the currently used theories as a special case. Nonetheless, the effective theory itself is more general and does not require additional degrees of freedom to be introduced, other than the ones derived from symmetries. We bring new results on the interaction and internal structure of charged spinning compact objects. On the numerical side, based on the Effective Field Theory reasoning, we have introduced a framework for evolving a compact object binary. Within this approach, we obtain the leading order waveform emitted by the binary during its late inspiral and compare it to a waveform from standard methodologies. Then, by performing illustrative numerical experiments of systems that the LIGO-Virgo observatories have already detected, we show the role of the stellar structure and their coefficients in the phase evolution of the waveform, as well as the order in which they arise and the sensitivity required for the gravitational wave observatories to measure them. If these coefficients are to be measured, tight constraints on fundamental physics can be placed.
- ItemOpen AccessPhase planes in the universe : chaotic cyclic universes and kicking Chameleons(2016) Platts, Emma; Weltman, Amanda; Ellis, George F RThis thesis consists of two main sections: chaotic cyclic cosmology and Chameleon gravity in the early universe. Both sections invoke a phase plane analysis as their commonality. The first explores a cyclic model, proposed by Ellis et al, that is in keeping with current observations. No exotic nor new physics is needed for the bounce nor the turnaround. The model is chaotic in nature and requires only that the universe is closed and that dark energy (at some time) decays. The second section contests the claim by Burrage et al. that Chameleon gravity is inconsistent in the early universe, unless constraints on its coupling mechanism are significantly increased. It is shown that the addition of a Dirac-Borne-Infeld (DBI) correction - a consistent, high energy modification - to the Chameleon dynamically renders it weakly coupled to matter. This is done without any fine-tuning and ensures the consistency of the Chameleon at all scales without infringing upon its crucial feature as a dark energy candidate: its elusive but prominent coupling to matter.
- ItemOpen AccessRelativistic neutron stars in general relativity and fourth order gravity(2021) Masetlwa, Nkosinathi; Mongwane, Bishop; van der Heyden, Kurt; Weltman, AmandaThis thesis investigates numerical instabilities arising from stiffness in the models of nonrotating, spherically symmetric single neutron star systems. The work deals with two distinct problems, each of which involves a stiff system of differential equations. In each case, we deal with stiffness by employing an IMEX Runge-Kutta scheme as opposed to the more computationally intensive fully implicit schemes or other adaptive Runge Kutta methods that may be impractical for partial differential equations. The first problem is focused on the mass-radius relation of a neutron star under a quadratic f(R) = R+αR2 theory for various realistic equations of state. This results in a coupled system of ODEs with stiff source terms which we discretize using an IMEX scheme. The observed maximum masses for different values of α, were consistent with the current neutron star maximum mass limit for some equations of state in both GR and beyond. In the second problem, we compute the frequencies of radial oscillations of neutron stars in the context of general relativity. This is achieved by linearly perturbing the ADM equations coupled to a matter source term. We discretize the resulting coupled system of PDEs with a third order WENO scheme in space and an IMEX scheme in time. We obtained 18 frequencies from the Fast Fourier Transform (FFT) of the evolved perturbation equations, which were consistent with the frequencies of the neutron star's Sturm-Liouville problem. The efficiency of the IMEX scheme as compared to other methods such as fully implicit schemes or adaptive methods makes it ideal for implementation in fully 3D numerical relativity codes for modified gravity.
- ItemOpen AccessSome numerical investigations in cosmology(2017) Walters, Anthony; Weltman, Amanda; Hellaby, CharlesNumerical simulations have become an indispensable tool for understanding the complex non-linear behavior of many physical systems. Here we present two numerical investigations in cosmology. The first is posed in the context of inhomogeneous solutions to General Relativity. We lay out formalism for calculating observables in an arbitrary spacetime, for an arbitrary placed observer. In particular, we calculate the area distance, redshift and transverse motion across the observers sky. We apply our method to the Szekeres metric, and develop code in MATLAB to implement it. We successfully demonstrate that the code works for the FLRW and LT special cases, and then investigate some Szekeres models with no spherical symmetry. The second project is posed in the context of chameleon gravity. Recently, it was argued that the conformal coupling of the chameleon to matter fields created an issue for early universe cosmology. As standard model degrees of freedom become non-relativistic in the early universe, the chameleon is attracted towards a "surfing" solution, so that it arrives at the potential minimum with too large a velocity. This leads to rapid variations in the chameleon's mass and excitation of high energy modes, casting doubts on the classical treatment at Big Bang Nucleosynthesis. We propose the DBI chameleon, a consistent high energy modification of the chameleon theory that dynamically renders it weakly coupled to matter during the early universe thereby avoiding the breakdown of calculability. We demonstrate this explicitly with numerical simulations.
- ItemOpen AccessA study of chameleon-photon mixing from pulsars(2012) Sikhonde, Muzikayise E; Weltman, AmandaA number of solutions to the dark energy problem have been proposed in literature, the simplest is the cosmological constant A. The cosmological constant lacks theoretical explanation for its extremely small value, thus dark energy is more generally modelled as a quintessence scalar field rolling down a flat potential.
- ItemOpen AccessThe gravity of modern amplitudes: using on-shell scattering amplitudes to probe gravityBurger, Daniel Johannes; Murugan, Jeff; Weltman, AmandaIn this thesis, we explore the use of on-shell scattering amplitudes as a way to understand various gravitational phenomena. We show that amplitudes are a viable way of studying certain aspects of gravity and showcase three such novel results here. First is the computation of the deflection angle of both light and gravitational waves due to a massive static body. We compute this from a purely on-shell amplitude perspective and find that the result is in complete agreement with the corresponding calculation in General Relativity. The second is the ability to derive classical results from the amplitudes. In this section we use on-shell scattering amplitudes to derive the perturbative metric of a rotating black hole in a generic form of Einstein gravity that has additional terms cubic in the Riemann tensor. We show that the metric we derive reduces to correct static metric in the zero angular momentum limit. We show that at first order in the coupling, the classical potential can be written to all orders in spin as a differential operator acting on the non-rotating potential. Further we compute the classical impulse and scattering angle of such a black hole. The third is the resolution of a classical discontinuity in N = 1 super gravity. Here we use on-shell methods for massive particles and use them to compute the supersymmetric version of the van Damme-Veltman-Zakharov (vDVZ) discontinuity. We construct the amplitudes of massive gravitinos (the superpartner of massive gravitons) and show that in the massless limit of the gravitinos there is the same discontinuity as found in massive gravity. This method sheds light on intricacies of the discontinuity that is obscured when handled classically.