The Quintessence Field As A Perfect Cosmic Fluid Of Constant Pressure

The late-time accelerated expansion of the universe is one of the most challenging problems in modern physics and cosmology. Such a phenomenon implies probably that there is a special substance (called dark energy) in the universe providing suffi-cient negative pressure. The simplest candidate of dark energy is the cosmological constant.However, the physical explanation on cosmological constant encounters some fundamental obstacles like fine-tuning problem and coincidence problem. So far there have been many phenomenological models on dark energy, the unified dark matter (UDM) model that the Chaplygin gas model represents is among them. Despite their advantage of alleviating the coincidence problem in a simple manner, Chaplygin gas and its generalizations suffer from obstacles at the linear perturbation level. They can-not explain the structure formation in small scales if the model departs notably from the standard A-CDM model.In this thesis, we tried a possible method to solve the coincidence problem. The in-novation point in our model is the use of a quintessence scalar field which is equivalent (on background level) to a non-barotropic perfect fluid of constant pressure. The coin-cidence problem is alleviated by such a quintessence equation-of-state that interpolates between plateau of zero at large redshifts and plateau of minus one as the redshift ap-proaches to zero.The quintessence field is neither a unified dark matter nor a mixture of cosmological constant and cold dark matter, relying on the facts that the quintessence density contrasts of sub-horizon modes would undergo a period of late-time decline and the squared sound speeds of quintessence perturbations do not vanish. What a role does the quintessence play is a dynamic dark energy, its clustering could remarkably reduce the growth rate of the density perturbations of non-relativistic matters.