Study On Holographie Dark Energy Model And Emergent Phenomenon Of Cosmic Space

In recent years, numerous cosmological observations have disclosed the current accelerated expansion of our universe. For the cause of the accelerating expansion, now many scholars ascribe it to the unknown dark energy. With cosmological observations progress dark energy scenario becomes great challenge and has attracted a great deal of attention. Another new hot issue is that the viewpoint of spatial emergence is used to describe the universe. This thesis mainly focuses on the study of holographic dark energy models and emergent phenomenon of cosmic spaceIn this thesis, firstly the background of cosmology and the standard cosmological model are briefly reviewed; and then we summarize concisely some fashionable dark energy models.In chapter three, we construct (m,n)-type holographic dark energy models at a phenomenological level, which can be viewed as a generalization of agegraphic models with the conformal-like age as the holographic characteristic size. For some values of (m,n) the holographic dark energy can automatically evolve across ω=-1into a phantom phase even without introducing an interaction between the dark energy and background matter. Our construction is also applicable to the holographic dark energy with generalized future event horizon as the characteristic size. Finally we address the issue on the stability of our model and show that they are generally stable under the scalar perturbation.In addition, intrigued by the holographic principle, Padmanabhan recently proposed a novel idea saying that our cosmic space is emergent as cosmic time progresses. In particular, the expansion rate of the universe is related to the difference between the surface degrees of freedom on the holographic horizon and the bulk degrees of freedom inside. In chapter four, we generalize this interesting paradigm to brane world, scalar-tensor gravity and f(R) theory, and find that in the cosmological setting the Friedmann equations can be successfully derived respectively. So the holographic nature of this novel idea has been further testified in a more general setting.