Browsing by Author "Dunsby, Peter Klaus"

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    A disformally coupled quintessence mimicking the ΛCDM background
    (2022) Dusoye, Avishek; Dunsby, Peter Klaus; de la Cruz-Dombriz, Alvaro; Dunsby, P K S; Nunes, N J
    Although the currently-accepted Concordance model of the Universe has been very successful observationally, it cannot resolve two main issues. Firstly, it cannot untangle the unknown nature of the cosmological constant in the Einstein Field Equations, which is responsible for the accelerated cosmological expansion. Secondly, it cannot explain the σ8 tension, which occurs because the constraints upon galactic clustering by the Cosmic Microwave Background Planck experiments diverge from the large-scale measurement by the Dark Energy Survey. As an alternative to the cosmological constant, this thesis will be using a scalar field, namely the quintessence. Our studied cosmological model assumes that the quintessence is coupled with a generic fluid. It also assumes a theory of gravity with two geometries. The gravitational geometry describes the curvature of space-time while the physical geometry describes the propagation of matter fields. The conformal transformation, which relates the gravitational metric and the physical metric, is extended here to a disformal transformation. In this thesis, the disformally coupled quintessence model mimics the expansion history of the Concordance model, in order to reproduce its observational success and yet have additional degrees of freedom to attempt to address those two issues. Using this approach, the quintessential potential is not specified. The dynamical system for our model is analysed using phase portraits for various studied scenarios. We investigate the expansion history of the DCQ model, where the quintessence couples disformally with dark matter (Scenario I). Our investigation confirms that the quintessential mass influences the disformal characteristics of the dynamical system. Furthermore, the evolution of the density perturbations for the disformally coupled dark matter is reviewed. A disformal effect due to the quintessential mass is seen in the growth rate of the cosmological structures on large scales. The disformal parameter renders no appreciable effect on the evolution of total matter perturbation. A Bayesian analysis of the relevant parameters for the perturbative model (i.e., conformal parameter and quintessential mass) is then carried out using the Redshift Space Distortion data to constrain the best-fit parameters, which might elucidate the σ8 tension. The best fit set of parameters indicates that the data prefers the model to behave conformally.
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    Cyclic universes & direct detection of cosmic expansion by holonomy in the McVittie spacetime
    (2019) Campbell, Mariam; Dunsby, Peter Klaus
    This dissertation consists of two parts. They are separate ideas, but both fall into the context of General Relativity using dynamical systems. Part one is titled Cyclic Universes. It is shown that a Friedmann model with positive spatial sections and a decaying dark energy term admits cyclic solutions which is shown graphically by the use of phase planes. Coupling the modified Friedmann model to a scalar field model with cross-sectional terms in order to model the reheating phase in the early universe, it is found that there is a violation of the energy condition, i.e. when the universe is in the contracting phase and re-collapses again. We suspect that the cause for this violation is due to the asymmetry of the solution of w together with the cross-sectional terms at the bounce preceding slow-roll inflation. Part two is titled Thought Experiment to Directly Detect Cosmic Expansion by Holonomy. Two thought experiments are proposed to directly measure the expansion of the universe by the parallel transfer of a vector around a closed loop in a curved spacetime. Generally, expansion would cause a measurable deficit angle between the vector’s initial and final positions. Using the McVittie spacetime (which describes a spherically symmetric object in an expanding universe) as a backdrop to perform these experiments it is shown that the expansion of the universe can be directly detected by measuring changes in the components of a gyroscopic spin axis. We find these changes to be small but large enough (∆S ∼ 10−7 ) to be measured if the McVittie spacetime were a representation of our universe.
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    Dark matter searches with cosmic-ray detectors and the Square Kilometre Array
    (2020) Méndez, Isla Miguel Alfonso; De La Cruz Dombriz, Alvaro; Dunsby, Peter Klaus
    Beyond gravitational evidence for dark matter, a set of search techniques are employed in the present thesis within the particle dark matter paradigm. Under the possibility of dark matter annihilating into particles of the Standard Model of Particle Physics, we study the products of annihilation with cosmic-ray detectors, such as AMS, Fermi-LAT and PAMELA, and radio telescopes, such as the SKA. In this work, we focus on the positron fraction measured in the Solar System due to dark matter annihilating in the dark matter galactic halo, but also on radio signals from the Milky Way and dwarf spheroidal galaxies. Our main purpose is to constrain the dark matter parameter space under the light of the latest experimental data for cosmic-rays and the new sensitivities reached in radio astronomy. Furthermore, we discuss some of the most promising locations and synchrotron frequencies to search for dark matter with masses around the TeV scale. The analysis presented in this thesis lies in setting constraints on modelindependent dark matter. However, some specific dark matter candidates in the context of extra-dimensional theories are considered as well. Indeed, brane fluctuations, dubbed branons, are new degrees of freedom appearing in flexible brane-world models. These new fields behave as standard weakly interacting massive particles with a significant associated thermal relic density and would explain dark matter observational features.
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    Probing the cosmological dynamics of a logarithmic f (R) theory of gravity
    (2015) Aboelhassan, Sheref Nasereldin; Dunsby, Peter Klaus
    In this thesis we make a contribution in the area of Extended Theories of Gravity (ETG) by studying the dynamics of the R ln R model. We draw attention to the importance of introducing complete alternative theories of gravity and studying the possible geometrical origin of Dark Energy (unknown form of energy), which is commonly thought to be responsible for the present epoch of accelerated expansion that our universe undergoes. The first chapter of the thesis is an introduction to one of the most successful models in the realm of cosmology i.e. the CDM model. At the end of the first chapter we give a brief discussion on the ground breaking news from the BICEP2 experiment. However, part of the community argues that the BICEP2 results cannot be ascribed to a primordial gravitational waves. In fact, the recent dust map was released by Planck's team lowered the chances that the signal detected by BICEP2 team can be due to primordial gravitational waves. The second chapter introduces a review of the motivations that stimulated researchers to pursue new theories of gravity. Moreover, we revisit the mathematical basis of the f (R) theory of gravity and explain how a viable model of f (R) can account for the most bizarre phenomenon in the universe, namely Dark Energy. The third chapter is dedicated to the basis and the techniques of the theory of dynamical systems. The fifth chapter includes some original work on the R ln R model dynamics in two different versions (compact and non-compact phase space). In this thesis, we show that the R ln R model cannot be considered as a viable model since it has serious flaws. These flaws will be addressed, in detail, in chapter 5.
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    Shear-free perfect fluid theorems in general relativity
    (2023) Sikhonde, Muzikayise Edward; Dunsby, Peter Klaus; Ellis George
    We present a detailed method for proving shear-free perfect fluid theorems in General Relativity. This method uses the (1+3)-covariant formalism to establish the consistency of the Einstein gravitational field equations under the barotropic shear-free perfect fluid condition. Using a Mathematica package xTensor, we were able to prove the following cases: the case where the pressure is constant, the acceleration vector is parallel to the vorticity, the components of a rescaled acceleration vector field orthogonal to the vorticity are basic and the case where the dot product of the rescaled acceleration vector field and the unit vorticity vector is basic, leading to the existence of a Killing vector along the vorticity