Browsing by Author "Dunsby, P K S"

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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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    Constraining scalar-tensor quintessence by cosmic clocks
    (2007) Capozziello, S; Dunsby, P K S; Piedipalumbo, E; Rubano, C
    Aims. We study scalar tensor theories of gravity with power law scalar field potentials as cosmological models for accelerating universe, using cosmic clocks. Methods. Scalar-tensor quintessence models can be constrained by identifying suitable cosmic clocks which allow one to select confidence regions for cosmological parameters. In particular, we constrain the characterizing parameters of non-minimally coupled scalar-tensor cosmological models which admit exact solutions of the Einstein field equations. Lookback time to galaxy clusters at low intermediate, and high redshifts is considered. The high redshift time-scale problem is also discussed in order to select other cosmic clocks such as quasars. Results. The presented models seem to work in all the regimes considered: the main feature of this approach is the fact that cosmic clocks are completely independent of each other, so that, in principle, it is possible to avoid bias due to primary, secondary and so on indicators in the cosmic distance ladder. Conclusions. We have used different methods to test cosmological models at low, intermediate and high redshift by different indicators: this seems to confirm independently these proposed dark energy models.
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    Cosmological perturbation theory and the variational principle in gravitation
    (2000) Laatz, C D; Ellis, GFR; Dunsby, P K S
    In this thesis firstly the theory of relativistic cosmological perturbations is studies, in the process being reviewed over the period 1960-1993. Secondly the variational principle, apropos of gravitation, is formulated and discussed. These two fields are then synthesised via a variational formulation of general relativity and cosmological perturbation theory. In the process new light is shed on Covariant Perturbation Theory via the development of generalised alternative variables, culminating in a unique variational formulation.