Cosmological perturbations of interacting viscous dark fluids models

Abstract

This dissertation studies the cosmological models in which the dark-sector components (dust and dark energy) interact and allow energy to flow from one component to the other while considering the viscous effects. The models explored are the Viscous Interacting Dark Fluid (VIDF), Little-Rip (LR), Pseudo-Rip (PR), and Bounce Cosmology (BC) models. The models are examined in three linear coupling choices, with each coupling choice explored in two cases of bulk viscosity. These cases of bulk viscosity differ only by their dependencies of the energy density, that is one case bulk viscosity is proportional to dust while the other case is proportional to dark energy. Moreover, These cosmological models are such that when certain conditions are affected, they mimic the ΛCDM model. The impact of these models on cosmological events during the Universe expansion history is also investigated. In addition to the cosmological implications of these models for cosmic events, the feasibility of the model, based on both theoretical and observational considerations, is also examined. A previously developed Markov chain Monte-Carlo (MCMC) simulation is used to obtain the best-fit cosmological parameters using supernovae Type-Ia data. The predicted expansion history of these models is statistically compared to supernovae data and the lambda cold dark matter (ΛCDM) model, where only eight models managed to be accepted and the remainder rejected. It was seen that the VIDF models subscribe to the Quintensence dark energy models from the value obtained of Equation of state (EoS) parameter. Moreover, all the models seem to be in favour of a negative coupling strength, which translates into the energy flow going from dust into dark energy. However, it was seen that when dust decays into dark energy, the radiation-to-dust equality epoch occurs much earlier and, as a result, has a longer period of dust domination regime, unlike when the coupling strength is negligible or positive. In the Little-Rip models, there is a mixture of classes from phantom to null energy conditions, as well as energy transfer. Model 2, Case 1 and Model 3, Case 1, are defined by a quintessence dark energy phenomenon, while Model 2, Case 2 is defined by a null energy condition. As stated in the VIDF models, for a negative coupling strength, the radiation-dust equality occurs earlier and, as such, yields a longer period of dominance for dust. Moreover, Model 2, case 2, had characteristics comparable to the ΛCDM model; however, unlike the ΛCDM model, this model tends to have the information and details of the