Notes on Ghost Dark Energy
  We study a phenomenological dark energy model which is rooted in the
Veneziano ghost of QCD. In this dark energy model, the energy density of dark
energy is proportional to Hubble parameter and the proportional coefficient is
of the order $\Lambda^3_{QCD}$, where $\Lambda_{QCD}$ is the mass scale of QCD.
The universe has a de Sitter phase at late time and begins to accelerate at
redshift around $z_{acc}\sim0.6$. We also fit this model and give the
constraints on model parameters, with current observational data including
SnIa, BAO, CMB, BBN and Hubble parameter data. We find that the squared sound
speed of the dark energy is negative, which may cause an instability. We also
study the cosmological evolution of the dark energy with interaction with cold
dark matter.
