Crossing the Phantom Divide with Parameterized Post-Friedmann Dark Energy

Understand

Dark energy models with a single scalar field cannot cross the equation of state divide set by a cosmological constant.

• More general models that allow crossing require additional degrees of freedom to ensure gravitational stability.
• We show that a parameterized post-Friedmann description of cosmic acceleration provides a simple but accurate description of multiple scalar field crossing models.
• Moreover the prescription provides a well controlled approximation for a wide range of "smooth" dark energy models.