Dark Energy And The Accelerating Expansion Of The Universe

Recent observation of Type Ia supernovae provide evidence for the acceleration of present universe, which leads to the possibility that the universe is entering an inflationary epoch. On the basis of reviewing Kaluza-Klein theory and Space-Time-Matter theory , the dissertation emphasizes on simulating present universe under a "big bounce" model, which contains a time variable cosmological "constant" that is derived from a higher dimension and manifests itself in 4D spacetime as dark energy. By properly choosing the two arbitrary functions contained in the model, a simple exact solution can be easily obtained in which the evolution of the universe is divided into several stages. Before the big bounce, the universe contracts and after the bounce, the universe expands. In the early time after the bounce, the expansion of the universe is decelerating. In the late time after the bounce, the dark energy overtakes the dark matter and the expansion enters an accelerating stage. When time tends to infinity, the contribution of dark energy tends to two third of the total energy density of the universe, in agreement with observations. Some discussion about big bounce singularity is also included.