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 omega(Z). We focus on the value of redshift Z* at which the cosmic late time acceleration begins and a(Z*) = 0. Four omega(Z) are studied, including the well-known CPL model and a new model that has advantages when describing the entire expansion era, infinity > Z > -1, such that the future 0 > Z > -1 is also sensibly described. If dark energy is represented by a CC model with omega = -1, the present ranges for Omega(Lambda)(t(0)) and Omega(m)(t(0)) imply that Z* = 0.743 with 4% error. We discuss the possible implications of a model-independent measurement of Z* with better accuracy.