; Cai, Yi-Fu
; Zhang, Xinmin] Chinese Acad Sci, Inst High Energy Phys, Beijing 100049, Peoples R China
Null Energy Condition (NEC) requires the equation of state (EoS) of the universe w(u) satisfy w(u) >= -1, which implies, for instance in a universe with matter and dark energy dominating w(u) = w(m)Omega(m) + w(de)Omega(de) = w(de)Omega(de) >= -1. In this paper we study constraints on the dark energy models from the requirement of the NEC. We will show that with Omega(de) similar to 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 (DB1) Lagrangian to show how this happens.