MODELING TIME-VARYING DARK ENERGY WITH CONSTRAINTS FROM LATEST OBSERVATIONS

We introduce a set of two parameter models for the dark energy equation of state (FOS) w(z) to investigate time-varying dark energy. The models are classified into two types according to their boundary behaviors at the redshift, z = (0, infinity) and their local extremum properties. A joint analysis based on four observations (SNe + BAO + CMB + H-0) is carried out to constrain all the models. It is shown that all models get almost the same chi(2)(min) similar or equal to 469 and the cosmological parameters (Omega(M), h, Omega(b)h(2)) with the best-fit results (0.28, 0.70, 2.24), although the constraint results on two parameters (w(0), w(1)) and the allowed regions for the EOS w(z) are sensitive to different models and a given extra model parameter. For three of Type I models which have similar functional behaviors with the so-called CPL model, the constrained two parameters w(0) and w(1) have have negative correlation and are compatible with the ones in CPL model, and the allowed regions of w(z) get a narrow node at z similar to 0.2. The best-fit results from the most stringent constraints straints in Model Ia give (w(0), w(1)) = (-0.96(-0.21)(+0.26), -0.12(-0.89)(+0.61)) which may compare with the best-fit results (w(0), w(1)) = (-0.97(-0.18)(+0.22), -0.15-(+0.85)(1.33)) in the CPL model. For four of Type II models which. have logarithmic function forms and an extremum point, the allowed regions of w(z) are found to be sensitive to different models and a given extra parameter. It is interesting to obtain two models in which two parameters w(0) and w(1) are strongly correlative and appropriately reduced to one parameter by a linear relation w(1) proportional to (1 + w(0)).