Discovery of the accelerating Universe through Super Novae Ia observation about a decade ago has resulted in a paradigm shift in cosmological research. The present monograph is a consequence of theoretical investigation of dark energy, the agent supposed to be responsible for accelerating the Universe. Two main features of this work are Einstein s general relativistic approach and time-varying models of phenomenological character. It has been possible successfully to match the theoretical models with the observational data regarding the age of the Universe. Observational data of time...