Crossing of the Cosmological Constant Boundary - an Equation of State   Description
  The phenomenon of the dark energy transition between the quintessence regime ($w > -1$) and the phantom regime ($w < -1$), also known as the cosmological constant boundary crossing, is analyzed in terms of the dark energy equation of state. It is found that the dark energy equation of state in the dark energy models which exhibit the transition is {\em implicitly} defined. The generalizations of the the models explicitly constructed to exhibit the transition are studied to gain insight into the mechanism of the transition. It is found that the cancellation of the terms corresponding to the cosmological constant boundary makes the transition possible. 
Hrvoje Stefancic