1,721,018 research outputs found
Cosmic string cusps and their application to fast radio bursts
Full text linkThis 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
A study of chameleon-photon mixing from pulsars
No full textIncludes bibliographical references.A 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
Causal dynamical triangulations and the quest for quantum gravity
No full textQuantum Gravity by Causal Dynamical Triangulation has over the last few years emerged as a serious contender for a nonperturbative description of the theory. It is a nonperturbative implementation of the sum-over-histories, which relies on few ingredients and initial assumptions, has few free parameters and - crucially - is amenable to numerical simulations. It is the only approach to have demonstrated that a classical universe can be generated dynamically from Planckian quantum fluctuations. At the same time, it allows for the explicit evaluation of expectation values of invariants characterizing the highly nonclassical, short-distance behaviour of spacetime. As an added bonus, we have learned important lessons on which aspects of spacetime need to be fixed a priori as part of the background structure and which can be expected to emerge dynamically
Mathematical methods for classifying fast radio bursts
Full text linkThe recent extragalactic discovery of Fast Radio Bursts (FRBs) has ignited a whirlwind of research, and numerous fundamental questions about them are being studied. This thesis delves into this field, aiming to bridge the gap in our exploration of FRB populations. The FRBs are cosmic transients characterised by powerful millisecond-duration radio waves emanating from extragalactic distances. Their origins remain a mystery, fueling ongoing debate regarding progenitor models. Despite their ephemeral nature, the energy output of a single FRB can surpass the daily radiant energy emitted by a main sequence star in a radio band. While undoubtedly powerful astrophysical phenomena, their transient nature presents a significant challenge in pinpointing source locations. The discovery of the first FRB in 2001 was initially met with scepticism and attributed to instrumental error. However, advancements in telescope sensitivity and data analysis techniques have led to the detection of numerous subsequent FRBs, solidifying their status as a novel class of astronomical phenomenon. Unveiling the mysteries surrounding FRBs encompasses a multifaceted research endeavour. Astronomers are actively engaged in identifying their host environments, elucidating the mechanisms responsible for their tremendous energy release, and exploring potential sub-classifications within the FRB population. By deciphering the secrets of FRBs, we stand to gain invaluable insights into extreme astrophysical processes and the nature of the distant Universe. The Hydrogen Intensity Real-time eXperiment (HIRAX) is a next-generation instrument specifically designed to detect and localise FRB along with intensity mapping. To efficiently manage the high-volume data stream generated by HIRAX, a dedicated FRB processing pipeline is essential. We explore FRB rates with the HIRAX instrument in this thesis. We also discuss the FRB detection pipeline with this radio telescope
Modeling Compact Objects with Effective Field Theory
Full text linkIn 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
Going Beyond Counting First Authors in Author Co-citation Analysis
Full text linkThe present study examines one of the fundamental aspects of author co-citation analysis (ACA) - the way co-citation
counts are defined. Co-citation counting provides the data on which all subsequent statistical analyses and mappings
are based, and we compare ACA results based on two different types of co-citation counting - the traditional type that
only counts the first one among a cited work's authors on the one hand and a non-traditional type that takes into
account the first 5 authors of a cited work on the other hand. Results indicate that the picture produced through this non-traditional author co-citation counting contains more coherent author groups and is therefore considerably clearer. However, this picture represents fewer specialties in the research field being studied than that produced through the traditional first-author co-citation counting when the same number of top-ranked authors is selected and analyzed. Reasons for these effects are discussed
A study of vortex lattices and pulsar glitches
Full text linkIn 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
Collective effects in multi-field inflation
No full textWe present a new model of multi-field inflation in the limit when, N, the number of fields is very large. To implement this limit, we reformulate the problem in terms of a colourless bilocal field [(x; y) which encodes the collective degrees of freedom of the N scalar fields. As a concrete example, we apply the collective field theory formalism to the bosonic O(N) vector model with quartic self interaction minimally coupled to gravity and show how this may be used to model the quantum to classical transition out of inflation
Some numerical investigations in cosmology
Full text linkNumerical 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
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