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 Λ 3 QCD , where Λ 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 ∼ 0.6. 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.

In this paper we study the possibly existing anisotropy in the accelerating expansion Universe by use of the full sample of Union2 data. Using the hemisphere comparison method to search for a preferred direction, we take the deceleration parameter q0 as the diagnostic to quantify the anisotropy level in the wCDM model. We find that the maximum accelerating expansion direction is (l, b) = (314 •−13 • +20 • , 28 •−33 • +11 • ), with the maximum anisotropy level of ∆q0,max/q0 = 0.79 −0.28 +0.27 , and that the anisotropy is more prominent when only low redshift data (z ≤ 0.2) are used. We also discuss this issue in the CP L parameterized model, showing a similar result.PACS numbers: 98.80.Es, 98.80.Jk

The difference between vacuum energy of quantum fields in Minkowski space and in Friedmann-Robterson-Walker universe might be related to the observed dark energy. The vacuum energy of the Veneziano ghost field introduced to solve the U (1) A problem in QCD is of the form, H + O(H 2 ). Based on this, we study the dynamical evolution of a phenomenological dark energy model whose energy density is of the form αH + βH 2 . In this model, the universe approaches to a de Sitter phase at late times. We fit the model with current observational data including SnIa, BAO, CMB, BBN, Hubble parameter and growth rate of matter perturbation. It shows that the universe begins to accelerate at redshift z ∼ 0.75 and this model is consistent with current data. In particular, this model fits the data of growth factor well as the ΛCDM model.