problem. It should also be pointed out that the coupled quintessence models [@couple] provide this problem with a more natural solution. However, for quintessence models with flat potentials, the quintessence field has to be nearly massless and one thus expects radiative corrections to destabilize the ratio between this mass and the other known scales of physics. In addition, for the cosmological constant and many quintessence models, the event horizon would lead to a potential incompatibility with the string theory.

Other designs on dark energy roughly include k-essence [@kess], quiessence (or “X-matter”) [@xmatter], brane world [@brane], tachyon [@tachyon], generalized Chaplygin gas (GCG) [@chaplygin; @gcgtest], holographic dark energy [@holography; @holoSN], and so forth. The quiessence or X-matter component is simply characterized by a constant, non-positive equation of state $w_X$ ($w_X<-1/3$ is a necessary condition to make the Universe accelerate). In general, in a Friedmann-Robertson-Walker (FRW) background with the presence of cold dark matter (CDM), an arbitrary but constant $w_X$ for dark energy from the range $(-1,0)$ can be achieved by using a scalar field with a hyperbolic sine potential [@xmatter]. It may be noted that in principle the value of $w_X$ may be even less than $-1$. In fact, by fitting the 