Some Issues On The Late-Time Cosmology

In the end of 1990 s, two independent projects simultaneously obtained results suggesting a acceleration in the expansion of the Universe by observing distant type Ia supernova. After that, this weird phenomenon is identified by other observations such as cosmic microwave background radiation(CMBR),large scale structure(LSS) and baryon acoustic oscillations(BAO).This acceleration is totally unexpected because the gravity among the usual matter is an attraction. Thus, a lot of models were proposed in order to take the mystery out of this unexpected discovery, and today,theorists also keep on working on this issue which already becomes the topic in cosmology. On the other hand, the cosmology has developed into the accurate era with the help of the huge advancement in observation technologies and the egregious power of data disposal in computer. For example, the parameters in the cosmology model can be constrained into a very tiny range by data fitting technology. Under the circumstances, in this paper we discuss three models in which the Universe could expand with an acceleration, and two of those models are made to fit the observation data; And we discuss some issues on the thermodynamics on the trapping horizon. In the first part, we give a brief introduction to process in modern cosmology, including the observation evidence for the acceleration and the the proposal models. In the second part, we give a brief introduction to details about the data fitting technology such as the method, observation data and computer program. Then we give out the data fitting results of the Ghost dark energy model and a new class of parametrization for dark energy without divergence. In the third part, we discuss another proposal model in which the gravity is modified to the vector-tensor theory. We give out the dynamical behavior of this model, and we find there would be bifurcation if the parameters are in some particular range. We give out the parameters range in which the Universe could expand with an acceleration. In the Fourth part, we discuss some issues on the thermodynamics on the trapping horizon. We find a thermodynamics origin of the Cardassian model which couldn’t be explained fundamentally before.We give out the entropy on the trapping horizon in Ghost dark energy model, and find the entropy is in accordance with the one from other fundamental theory.