can be roughly summarized as the following several items: it is a kind of exotic energy form with sufficiently large negative pressure such that drives the Universe to undergo a period of accelerating expansion currently; it is spatially homogeneous and non-clustering; and it is in small part at the early times while dominates the Universe very recently.

The investigation of the nature of dark energy is an important mission in the modern cosmology. Much work has been done on this issue, and there is still a long way to go. Currently, the preferred candidates of dark energy are vacuum energy (or cosmological constant) and dynamical fields. The simplest form of dark energy is the cosmological constant $\Lambda$. A tiny positive cosmological constant which can naturally explain the current acceleration would encounter many theoretical problems, such as the “fine-tuning” problem and the “coincidence” problem. Another possible form of dark energy is provided by scalar fields. Dark energy can be attributed to the dynamics of a scalar field $\phi$, called quintessence [@quin], which convincingly realize the present accelerated expansion of the Universe by evolving slowly down its potential $V(\phi)$. The tracker version quintessence is to some extent likely to resolve the coincidence 