Vacuum stability of phantom field from the nonuniqueness of Lagrangian

Abstract

According to the nonuniqueness principle, the homogeneous scalar field Lagrangian can be expressed in various forms both standard and nonstandard ones. Therefore, the standard and all possible nonstandard Lagrangians can be linearly combined while the Klein-Gordon equation is still intact. This linear combination of Lagrangians is used to demonstrate that the energy density of homogeneous phantom field can possibly be bounded from below. The applications of this new Lagrangian in the nature of the ghost condensate and the equation of state with $w<-1$ are discussed.