We explore the properties of dark energy models for which the equation-of-state , w , defined as the ratio of pressure to energy density , crosses the cosmological-constant boundary w = -1 .
We adopt an empirical approach , treating the dark energy as an uncoupled fluid or a generalized scalar field .
We describe the requirements for a viable model , in terms of the equation-of-state and sound speed .
A generalized scalar field can not safely traverse w = -1 , although a pair of scalars with w > -1 and w < -1 will work .
A fluid description with a well-defined sound speed can also cross the boundary .
Contrary to expectations , such a crossing model does not instantaneously resemble a cosmological constant at the moment w = -1 since the density and pressure perturbations do not necessarily vanish .
But because a dark energy with w < -1 dominates only at very late times , and because the dark energy is not generally prone to gravitational clustering , then crossing the cosmological-constant boundary leaves no distinct imprint .