Statefinder diagnostic is a useful method which can differ one dark energy model from each others. In this letter, we apply this method to a holographic dark energy model from Ricci scalar curvature, called the Ricci dark energy model(RDE). We plot the evolutionary trajectories of this model in the statefinder parameter-planes, and it is found that the parameter of this model plays a significant role from the statefinder viewpoint. In a very special case , the statefinder diagnostic fails to discriminate LCDM and RDE models, thus we apply a new diagnostic called the Om diagnostic proposed recently to this model in this case in Appendix A and it works well.

We investigate the viscous Ricci dark energy (RDE) model by assuming that there is bulk viscosity in the linear barotropic fluid and the RDE. In the RDE model without bulk viscosity, the universe is younger than some old objects at some redshifts. Since the age of the universe should be longer than any objects in the universe, the RDE model suffers the age problem, especially when we consider the object APM 08279 + 5255 at z = 3.91, whose age is t = 2.1 Gyr. In this letter, we find that once the viscosity is taken into account, this age problem is alleviated.Recent observations like CMB anisotropy, supernovae and galaxies clustering have strongly indicated that our universe is spatial flat and there exists a exotic cosmic fluid called dark energy with negative pressure, which constitute about two thirds of the total energy of the universe. The dark energy is characterized by its equation of state w, which lies very close to −1, probably being below −1 indicated by the present data.Many candidates including the cosmological constant, quintessence, phantom, quintom, holographic dark energy, etc. have been proposed to explain the acceleration. However, people still do not understand what's dark energy so far. Ricci dark energy, which is a kind of holographic dark energy [1] taking the square root of the inverse Ricci scalar as its infrared cutoff, has been proposed by Gao et al. [2], and this model is also phenomenologically viable. Assuming the black hole is formed by gravitation collapsing of the perturbation in the universe, the maximal black hole can be formed is determined by the casual connection scale R CC given by the "Jeans" scale of the perturbations. For tensor perturbations, i.e. gravitational perturbations, R −2 CC = M ax(Ḣ + 2H 2 , −Ḣ) for a flat universe, where H =ȧ/a is the Hubble parameter, and according to the ref. [3], only in the case of R −2 CC =Ḣ + 2H 2 , it could be consistent with the current cosmological observations when the vacuum density appears as an independently conserved energy component. As we know, in flat FRW universe, the Ricci scalar is R = 6(Ḣ + 2H 2 ), which means the R CC ∝ R and if one choices the casual connection scale R CC as the IR cutoff, the Ricci dark energy model is also obtained. For recent progress on Ricci dark energy and holographic dark energy, see ref.[4][5] [6]. The energy density of RDE in flat universe readswhere we have set 8πG = 1 and α is a dimensionless parameter which will determine the evolution behavior * Electronic address: fengcj@itp.ac.cn † Electronic address: kychz@shnu.edu.cn of RDE. Dissipative processes in the universe including bulk viscosity, shear viscosity and heat transport have been conscientiously studied [7]. The general theory of dissipation in relativistic imperfect fluid was put on a firm foundation by Eckart[8], and, in a somewhat different formulation, by Landau and Lifshitz[9]. This is only the first order deviation from equilibrium and may has a causality problem, the full causal theory was developed by Isreal and Stewart[10], and...

Recently, the vacuum energy of the QCD ghost in a time-dependent background is proposed as a kind of dark energy candidate to explain the acceleration of the Universe. In this model, the energy density of the dark energy is proportional to the Hubble parameter H, which is the Hawking temperature on the Hubble horizon of the Friedmann-Robertson-Walker (FRW) Universe. In this paper, we generalized this model and choice the Hawking temperature on the so-called trapping horizon, which will coincides with the Hubble temperature in the context of flat FRW Universe dominated by the dark energy component. We study the thermodynamics of Universe with this kind of dark energy and find that the entropy-area relation is modified, namely, there is an another new term besides the area term.