Seeking evolution of dark energy

We study how observationally to distinguish between a cosmological constant (CC) and an evolving dark energy with equation of state ω(Z). We focus on the value of redshift Z* at which the cosmic late time acceleration begins and a(Z*)= 0. Four ω(Z) are studied, including the well-known CPL model and a new model that has advantages when describing the entire expansion era, ∞>Z>-1, such that the future 0>Z>-1 is also sensibly described. If dark energy is represented by a CC model with ω≡-1, the present ranges for ΩΛ (t0) and Ωm (t0) imply that Z*=0.743 with 4% error. We discuss the possible implications of a model-independent measurement of Z* with better accuracy. (orig.)