The unphysical character of minimally coupled dark energy fluids

Abstract

Abstract It is well known that, in the context of general relativity, an unknown kind of matter that must violate the strong energy condition is required to explain the current accelerated phase of expansion of the Universe. This unknown component is called dark energy and is characterized by an equation of state parameter $$w=p/\rho $$ w=p/ρ with $$w_0<-1/3$$ w0<-1/3 . For minimally coupled dark energy fluids, thermodynamic stability requires that $$3w-d\ln |w|/d\ln a\ge 0$$ 3w-dln|w|/dlna≥0 and positiveness of entropy that $$w\ge -1$$ w≥-1 . In this paper we proof that we cannot obtain a differentiable function w(a) to represent the dark energy that satisfies these conditions during the entire history of the Universe.