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 with w_0<-1/3. For minimally coupled dark energy fluids, thermodynamic stability requires that 3w-dln |w|/dln age 0 and positiveness of entropy that wge -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.
Highlights
Between the end of the twenty century and the beginning of the twenty one’s, a large amount of observational data revealed that the Universe is currently expanding at an accelerated rate [1,2,3,4]
By considering the content of the Universe as a perfect fluid, it was shown in Ref. [28] that the cosmic fluids should necessarily have a thermodynamic stability
In the case of a perfect fluid with an equation of state (EoS) parameter w = p/ρ, this implies that the inequality w − 1 d ln |w| ≥ 0 3 d ln a must be satisfied
Summary
Between the end of the twenty century and the beginning of the twenty one’s, a large amount of observational data revealed that the Universe is currently expanding at an accelerated rate [1,2,3,4] This accelerated expansion can be explained if a cosmological constant is added to the Einstein field equations. Since baryonic and cold dark matter are pressureless ( pm = 0), and the pressure of relativistic matter is ργ /3, the Universe must contain an additional source term with a pressure sufficiently negative to ensure the validity of (1) This unknown source was dubbed dark energy (DE)
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