Abstract. In this paper, we revisit generalized Chaplygin gas (GCG) model as a unified dark matter and dark energy model. The energy density of GCG model is given as ρGCG/ρGCG0, where α and Bs are two model parameters which will be constrained by type Ia supernova as standard candles, baryon acoustic oscillation as standard rulers and the seventh year full WMAP data points. In this paper, we will not separate GCG into dark matter and dark energy parts any more as adopted in the literatures. By using Markov Chain Monte Carlo method, we find the result: α = 0.00126 +0.000970+0.00268 −0.00126−0.00126 and Bs = 0.775

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We investigate the validity of the thermodynamical properties of the universe in a new parametric model of dark energy with the equation of state w = w 0 +w 1 ·z(1+z)/(1+z 2 ). In the spatially homogeneous and isotropic universe, assuming that the temperature and entropy in cosmology is as in a black hole, we examine the thermodynamical properties of the universe bounded by the apparent horizon and the event horizon respectively. By analysis, we find that the first and the second laws of thermodynamics are valid inside the apparent horizon, while they break down inside the event horizon. Keywords: Dark energy; the thermodynamical properties; apparent horizon; event horizon. PACS Nos.: 98.80.-k, 98.80.Cq 885 Mod. Phys. Lett. A 2011.26:885-892. Downloaded from www.worldscientific.com by UNIVERSITY OF CALIFORNIA @ SAN DIEGO on 02/03/15. For personal use only. 886 L. Xing et al.

We explore an extension of the symmetric teleparallel gravity denoted the f (Q) theory, by considering a function of the nonmetricity invariant Q as the gravitational Lagrangian. Some interesting properties could be found in the f (Q) theory by comparing with the f (R) and f (T ) theories. The field equations are derived in the f (Q) theory. The cosmological application is investigated. In this theory the accelerating expansion is an intrinsic property of the universe geometry without need of either exotic dark energy or extra fields. And the state equation of the geometrical dark energy can cross over the phantom divide line in the f (Q) theory. In addition, the dynamical system method are investigated. It is shown that there are five critical points in the STG model for taking f (Q) = Q + αQ 2 . The critical points P4 and P5 are stable. P4 corresponds to the geometrical dark energy dominated de Sitter universe (w ef f tot =-1), while P5 corresponds to the matter dominated universe (w ef f tot =0). Given that P4 represents an attractor, the cosmological constant problems, such as the fine tuning problem, could be solved in the STG model.