%0 Journal Article
%T Necessity of Dark Energy from Thermodynamic Arguments
%A H. Moradpour
%A A. Sheykhi
%A N. Riazi
%A B. Wang
%J Advances in High Energy Physics
%D 2014
%I Hindawi Publishing Corporation
%R 10.1155/2014/718583
%X Considering the cosmic fluid as a quasi-static thermodynamic system, the status of the generalized second law of thermodynamics is investigated and the valid range of the equation of state parameter is derived for a few important cosmological models. Our study shows that the satisfaction of the laws of thermodynamics in these cosmological models requires the existence of some kind of energy in our universe with . In other words, the existence of a dark energy component, or equivalently modified gravity theory, is unavoidable if the cosmological model is to approach thermal equilibrium in late times. 1. Introduction Thermodynamical nature of Einstein¡¯s theory of general relativity was first disclosed by Jacobson [1] who showed that the hyperbolic second order partial differential field equations of gravity can be derived by applying the first law of thermodynamics on any local Rindler horizon. Generalization of this method to gravity, by introducing the entropy generation term due to nonequilibrium nature of spacetime, was investigated in [2]. More attempts to reveal the connection between thermodynamics and various theories of gravity can be found in [3¨C8]. An elegant example is the derivation of the Friedmann equations as a consequence of the validity of the first law of thermodynamics on the apparent horizon of the Friedmann-Robertson-Walker (FRW) universe [9]. Recently, an entropic origin for gravity was proposed by Verlinde [10]. He argued that the laws of gravity are not fundamental and in particular they emerge as an entropic force caused by the changes in the information associated with the positions of material bodies. Verlinde¡¯s derivation of Newton¡¯s law of gravitation at the very least offers a strong analogy with the well-understood statistical approach. Therefore, this derivation opens a new window to understanding gravity from first principles. The entropic approach to gravity has arisen a lot of enthusiasm, recently (see, e.g., [11¨C34] and references therein). The studies have also been generalized to higher order gravity theories such as Gauss-Bonnet and Lovelock gravity [35]. All these attempts indicate a hope to achieve a deeper connection between gravity and thermodynamics. From the second law of thermodynamics, we know that every closed system moves towards its maximum entropy state which is an equilibrium state. This leads to the conclusion that the second derivative of the entropy should be negative [36]. The assumption that the second derivative of entropy is negative comes from the fact that while the entropy is increasing as the
%U http://www.hindawi.com/journals/ahep/2014/718583/