Viscous Unified Dark Fluid Models And Spherical Collapse

In the recent years, a mass of cosmic observations (such as type la supernova, cosmic microwave background radiation, and matter power spectra, etc.) strongly suggest that the present universe is undergoing an accelerating expansion, and this accelerated expansion is pushed by an unknown dark fluid which takes up about 96% of cosmic total energy. Usually this dark fluid is separated into two parts:dark matter and dark energy. However, the present gravitational probe cannot distinguish these two components effectively (this problem is dubbed as dark degeneracy), and so far there are no reliable physics principle to support any form of decomposition. This situation inspires scholars to put forward some unified models to study the universe. The common features of these models are that they regard the medium which permeating the universe as a perfect fluid, and that the perturbations are adiabatic. With the contradiction between the cosmological observations and the theoretical models coming out, some authors suggest that the non-adiabatic perturbations should be considered. That is to say, the medium of universe may be not a perfect fluid but a non-perfect fluid with viscosity. These viscous models are popular since they make the non-adiabatic perturbations become their intrinsic properties.In this paper, we will mainly study cosmological constrains on the viscous unified dark fluid models and investigate the effect of bulk viscosity on the formation of the large-scale structure of the universe. Firstly, we outline the development history, research contents and background of the cosmology. Then, the main astronomical observations and some familiar cosmological models are introduced. Our main researches are the third chapter, the forth chapter and the appendix. In the third chapter, we investigate a viscous generalized Chaplygin gas model, and consider the perturbations of bulk viscosity for the first time. Through observational constraints on VGCG model, we constrain the parameter space of this model, obtain the value of the new model parameter ?0 in 3? range, also discuss the effect of ?0 on the evolution of EOS and effective adiabatic sound speed. We study the spherical collapse of VGCG model in the forth chapter, analyze the impact of bulk viscosity on the the formation of large-scale structure (namely collapse process), and compare these results with the results from ACDM model. The last part is the summary and outlook. In addition, we introduce the theory of cosmological perturbations (including the perturbations of bulk viscosity) in the appendix, and deduce the density perturbations and the velocity perturbations equations.