Several Issues Concerning Early Universe And Dark Energy

This thesis is devoted to the investigation of some issues of early universe and darkenergy.First of all, we study the non-Gaussianities of cosmic primordial perturbation. Weintroduce the cosmic perturbation theory, and then we turn to study a general multi-fieldin?ationary theory. We calculate the 4 point correlation function of cosmic primordialperturbation in this model. We find the main contribution comes from two parts, oneis contact diagram, the other is exchange scalar diagram. We find the contribution ofthese two parts both are important. We also analyze the momentum configuration andnon linear parameter. Beyond the 4 point correlation function calculation, we calcu-late the 5 point and 6 point correlation function in some in?ationary models. Severalobservational prediction is extracted.We investigate an alternative model to in?ation, i.e., a matter bounce model. Weuse ghost condensation theory to realize a bounce from matter contracting universe tobig bang universe. We calculate the primordial perturbation spectrum in this model, andfind the ghost condensation field produces a blue spectrum, which grows slower thanmatter perturbation, hence the scale invariant perturbation of matter sector is preserved.We make a phenomenological investigation to dark energy issue during cosmic latetime evolution. We calculate the stability of the perturbation of holographic dark energy,and investigate the fates of our universe in some holographic dark energy models withdifferent interacting term. We introduce a holographic gas model, the entropy of gas isproportional to the boundary area of the system. We apply this holographic gas modelto cosmology, and find it and accelerate universe. Our holographic gas model can be acandidate of dark energy.