I'm always glad to read papers about dark energy. Dark energy can be described by its equation of state w. There's a model of dark energy with w<-1 called phantom energy. In the framework of GR, phantom energy violates the dominant energy condition. This seems a fatal blow to phantom energy. Generalizing, this paper says that in GR, all dark energy models with w<-1 violate the dominant energy condition.
But if you're a fan of phantom energy, this paper offers a hope: throw General Relativity to the trash bin, and worship a near relative called Brans-Dicke theory. This paper says that in the context of Brans-Dicke theory, phantom energy doesn't violate the dominant energy condition
Separate names with a comma.