The dark energy-dominated Universe

In this paper we investigate the epochs in which the Universe started accelerating and when it began to become dark energy-dominated (i.e., the dynamics of the expansion of the Universe dominated by the dark energy). We provide analytic expressions to calculate the redshifts of these epochs as a function of density parameters. Moreover, we review and discuss cosmological models with a dark energy component, which can have an interesting characteristic, namely, they never stop accelerating. This holds even if the Universe is at present time either flat, open, or closed. If the dark energy is the cosmological constant the Universe will eventually end up undergoing an exponentially expansion phase, and the total density parameter converging to Omega = 1. This is exactly what is considered in inflationary scenario to generate the initial conditions for the big bang. One can then argue that the Universe begun with an inflationary phase and will end up with another inflationary phase. Thus, it follows that in both the early and the late Universe Omega -> 1. We also discuss the above issues in the context of the XCDM parametrization. (c) 2005 Elsevier B.V. All rights reserved.