NULL ENERGY CONDITION AND DARK ENERGY MODELS
Null Energy Condition (NEC) requires the equation of state (EoS) of the universe wu satisfy wu ≥ -1, which implies, for instance in a universe with matter and dark energy dominating wu = w m Ω m + w de Ω de = w de Ω de ≥ -1. In this paper we study constraints on the dark energy models from the requirement of the NEC. We will show that with Ω de ~ 0.7, w de < -1 at present epoch is possible. However, NEC excludes the possibility of w de < -1 forever as happened in the Phantom model, but if w de < -1 stays for a short period of time as predicted in the Quintom theory, NEC can be satisfied. We take three examples of Quintom models of dark energy, namely the phenomenological EoS, the two-scalar-field model and the single scalar model with a modified Dirac–Born–Infeld (DBI) Lagrangian to show how this happens.