Superstring theory posits that as complicated as nature may seem to the naive observer, the variety of observed phenomena may be explained by postulating that at the fundamental scale, matter is composed of lines of energy, namely strings. These oscillating lines would be elementary and would hence have no substructure. They are expected to be incredibly tiny, their line-like structure would become noticeable at scales close to the string scale (which may lie anywhere from the TeV scale all the way up to the Planck scale) and would appear to be point-like to the macroscopic observer. Internal consistency then also requires the presence of higher dimensional objects, namely D-branes, all of which conspire and combine in such a way so as to give rise to the observable Universe. Advances in cosmology suggest the early universe was much hotter and denser than is the Universe at present, that the Universe has expanded and continues to expand (exponentially in fact) at present. This in turn has led a number of theorists to point out the remarkable possibility that some of these strings or D-branes were also stretched with the expansion. The resulting macroscopic strings, the so-called cosmic strings, would potentially stretch across the entire Universe. Cosmic strings make their presence manifest by oscillating, scattering off other structures, by decaying, producing gravitational waves and so on, and this in turn hints at the available handles that may be used to observe them. Before we can hope to observe cosmic strings however, the first step is then clearly to understand these properties which determine their evolution. A number of approximate (classical) descriptions of cosmic strings have been constructed to date, but approximations break down, especially when potentially interesting things happen (e.g. close to cusps, i.e. points on the string that reach the speed of light) and can obscure the physics. Thankfully, one can go beyond these approximations: all properties of cosmic strings can be concisely and accurately contained or encoded in a single object, the so-called fundamental cosmic string vertex operator. In the present thesis I construct precisely this, covariant vertex operators for general cosmic strings and this is the first such construction. Cosmic strings, being macroscopic, are likely to exhibit classical behaviour in which case they would most accurately be described by a string theory analogue of the well known harmonic oscillator coherent states. By minimally extending the standard definition of coherent states, so as to include the string theory requirements, I go on to construct both open and closed covariant coherent state vertex operators. The naive construction of the latter requires the existence of a lightlike compactification of spacetime. When the lightlike winding states in the underlying Hilbert space are projected out, the resulting vertex operators have a classical interpretation and can consistently propagate in noncompact spacetime. Using the DDF map I identify explicitly the corresponding general lightcone gauge classical solutions around which the exact macroscopic quantum states are fluctuating. We go on to show that both the covariant gauge coherent vertex operators, the corresponding lightcone gauge coherent states and the classical solutions all share the same mass and angular momenta, which leads us to conjecture that the covariant and lightcone gauge states are different manifestations of the same state and share identical interactions. Apart from the coherent state vertices I also present a complete set of covariant mass eigenstate vertex operators and these may also be relevant in cosmic string evolution. Finally, I also present the first amplitude computation with the coherent states, the graviton emission amplitude (including the effects of gravitational backreaction) for a simple class of cosmic string loops. As a byproduct of the above, I find that the fundamental building blocks of arbitrarily massive covariant string states are given by elementary Schur polynomials (equivalently complete Bell polynomials). This construction enables one to address the aforementioned questions concerning the properties of cosmic strings, their cosmological signatures, and may lead to the first observations of such objects in the sky. This in turn would be a remarkable way of verifying Superstring theory as the framework underlying the structure of our Universe.

This thesis aims to give an approach to dealing with Hierarchy problems in theoretical physics, plaguing theories that span a wide range of energy scales. At present, any theory that is formulated to connect observations of the Early Universe to results in present day particle physics, exhibits the necessity of (at least one) unnaturally fine-tuned parameter. This has encouraged the sectioning of of many separate, highly specialized fields - each dealing with Effective Field Theories (EFTs) valid at a limited range of energy scales only. Here I describe an effort to connect different energy scales while dynamically accounting for hierarchies. This thesis discusses the appeal of pseudo-Goldstone bosons (pGBs) for the generation of scales in Early Universe cosmology. In particular, I will show how models with pGBs address the radiative instability of mass scales in quantum mechanical theories. I will start with an introduction to the two hierarchy problems that will be the primary focus of the thesis: the electroweak hierarchy problem, or the puzzle of the lightness of the Higgs mass; and the inflationary hierarchy problem, or the flatness of the inflaton potential demanded by the nearly scale invariant spectrum of the Cosmic Microwave Background. I will briefly introduce how pGBs arise, and can be described, using an example of a compact Special Orthogonal group SO(n) breaking to its largest coset SO(n - 1). I will then explore various models that address the electroweak and the inflationary hierarchy problem, using appropriate EFT tools such as the Callan-Coleman-Wess-Zumino mechanism and 5D approaches. I will discuss the relative strength of these models compared to existing models in the literature. After this discussion I will show that it is possible to address both hierarchy problems in a unified model, in which an inflaton decays into the Higgs field after inflation, in a process called reheating. This section will include a detailed derivation of the model, and will explore the regions of parameter space that lead to inflation, reheating, and electroweak symmetry breaking compatible with the relevant experimental data. This is followed by an excursion in which I will discuss non-compact models, based on SO(n;1)=SO(n) cosets. I will show how such setups can also give rise to inflation compatible with the current data, and discuss different scenarios for reheating. I will finish with an epilogue of the prospects of (holographic) Composite Higgs models - in which the Higgs is a pGB of the breaking of a strong compact symmetry - at particle colliders.

published_or_final_version / Physics / Master / Master of Philosophy

No description available.

No description available.

Capacitive resistivity (CR) is a novel geophysical technique for the non-intrusive characterisation of the shallow subsurface by electrical imaging. CR is capable of extending the scope of the conventional DC resistivity technique to the urban built environment and other settings where galvanic contact cannot be achieved or where high contact impedances result in poor data quality. Fundamentally, the CR technique is based upon the concept of capacitive coupling between sensors and the ground and a generalisation of the DC four-point array for measuring the resistivity of the subsurface at frequencies in the VLF range. This thesis provides a unified description of CR, including its physical principles, their theoretical formulation and practical implementation in geophysical instruments. In general, the transfer impedance across a capacitive array is a complex function of frequency and geometry. It is shown that a low induction number mode of operation exists where resistivity is proportional to the in-phase component of the transfer impedance. The quadrature component is generally sensitive to a combination of parameters including sensor elevation, dipole offset and ground resistivity. Under the low induction number regime, the electric field is quasi-stationary so that theoretical equivalence with the DC case is achieved and conventional DC interpretation schemes are applicable to CR data. A comprehensive parameter study undertaken in this thesis investigates the applicability of the technique under the specific conditions typically encountered in environmental and engineering site investigation surveys. In those circumstances, practical CR measurements are shown to be limited to an optimal frequency window between 1 kHz and 25 kHz. The condition of low induction numbers imposes further restrictions on the maximum dimensions of the sensor array and the minimum resistivity of the ground. However, a key finding of the parameter study is that even under the quasi-static regime, practical conditions may be such that substantial phase rotations may occur which are exclusively due to the capacitive nature of the technique. Modelling of sensor capacitances is used to demonstrate that the concept of point poles postulated in the quasi-static formulation of CR has a practical realisation in the form of plate-wire sensors. Subsequently, the fundamental concepts of CR are validated experimentally in a series of elementary surveys where the fully complex transfer impedance (amplitude and phase) is measured with a newly developed prototype CR instrument. It is shown that for assessments of shallow subsurface conditions with typical survey parameters and standard geometries, the observed responses are typically in-phase. However, it is also demonstrated that practical circumstances exist under which significant phase rotations can be observed. In such cases, an estimation of apparent resistivity using the in-phase component only is more appropriate than the magnitude-based calculation performed by existing commercial instruments. The nature of the CR technique facilitates the use of towed arrays that allow the dynamic collection of multi-offset apparent resistivity data without the disadvantages of galvanic coupling. This thesis examines the operational characteristics of towed CR arrays and compares data acquired with a range of instruments in a variety of different environments. It is shown that towed-array CR enables the collection of highly repeatable resistivity data at sampling intervals of the order of centimetres. Towing-induced noise is found to be much less problematic than previously found with DC towed-array techniques. It is also demonstrated that high-quality data can be obtained by towed-array CR on artificial surfaces such as tarmac or concrete. Consequently, the technique appears to be particularly suited for assessing the condition of engineered structures such as roads and pavements. Finally, it is demonstrated how multi-offset towed-array CR can be employed for electrical tomographic imaging of the shallow subsurface. Conventional DC resistivity interpretation schemes based on quasi-2D, 2D and 3D inversion algorithms are shown to be applicable to such datasets, provided that some elementary rules are observed with regard to the design of towed-array surveys. Real-time interpretation during data acquisition is shown to be feasible with a continuous vertical electrical sounding (CVES) technique based on a Zohdy-type inversion. Examples of 2D and 3D surveys over shallow targets show the superior quality and resolution of CR datasets compared with conventional DC resistivity.

Chan Kit Hung. / "August 2002." / Thesis (Ph.D.)--Chinese University of Hong Kong, 2002. / Includes bibliographical references (p. 132-152). / Electronic reproduction. Hong Kong : Chinese University of Hong Kong, [2012] System requirements: Adobe Acrobat Reader. Available via World Wide Web. / Mode of access: World Wide Web. / Abstracts in English and Chinese.

published_or_final_version / Physics / Master / Master of Philosophy

In this dissertation, we consider the variable chaplygin gas (VCG) model as derived from the Tachyon gas model and search for a sub-class of models that provide an adequate fit to the cosmic microwave background (CMB) observations. We find that, for an appropriate choice of VCG parameters, up to ~ 80% of the VCG collapses into a gravitationally bound condensate which behaves as matter; the evolution of the remaining VCG, as governed by its equation of state, brings about accelerated expansion at late times. In light of this high collapsed fraction, we approximate the VCG transfer function with that of cold dark matter. We show that we can sufficiently describe the VCG cosmology from decoupling to today in terms of a model in which the gravitationally bound condensate plays the role of cold dark matter and the remaining VCG takes the place of dark energy in the concordance model. We then compute the CMB temperature anisotropy spectrum for a subset of VCG models and proceed to find a best-fit model to the WMAP-9yr data [46]. Our best-fit model has a x² per degrees of freedom of 2.03. / Thesis (M.Sc.)-University of KwaZulu-Natal, Pietermaritzburg, 2013.

Thesis (Ph.D.)--Tufts University, 2005. / Adviser: Alexander Vilenkin. Submitted to the Dept. of Physics and Astronomy. Includes bibliographical references (leaf 99). Access restricted to members of the Tufts University community. Also available via the World Wide Web;