Holographic Ricci dark energy: Current observational constraints,
  quintom feature, and the reconstruction of scalar-field dark energy
  In this work, we consider the cosmological constraints on the holographic
Ricci dark energy proposed by Gao et al. [Phys. Rev. D 79, 043511 (2009)], by
using the observational data currently available. The main characteristic of
holographic Ricci dark energy is governed by a positive numerical parameter
$\alpha$ in the model. When $\alpha<1/2$, the holographic Ricci dark energy
will exhibit a quintomlike behavior; i.e., its equation of state will evolve
across the cosmological-constant boundary $w=-1$. The parameter $\alpha$ can be
determined only by observations. Thus, in order to characterize the evolving
feature of dark energy and to predict the fate of the universe, it is of
extraordinary importance to constrain the parameter $\alpha$ by using the
observational data. In this paper, we derive constraints on the holographic
Ricci dark energy model from the latest observational data including the Union
sample of 307 type Ia supernovae, the shift parameter of the cosmic microwave
background given by the five-year Wilkinson Microwave Anisotropy Probe
observations, and the baryon acoustic oscillation measurement from the Sloan
Digital Sky Survey. The joint analysis gives the best-fit results (with
1$\sigma$ uncertainty): $\alpha=0.359^{+0.024}_{-0.025}$ and $\Omega_{\rm
m0}=0.318^{+0.026}_{-0.024}$. That is to say, according to the observations,
the holographic Ricci dark energy takes on the quintom feature. Finally, in
light of the results of the cosmological constraints, we discuss the issue of
the scalar-field dark energy reconstruction, based on the scenario of the
holographic Ricci vacuum energy.
