Advances in High Energy Physics

Volume 2017 (2017), Article ID 9687976, 6 pages

https://doi.org/10.1155/2017/9687976

## Thermodynamic Analysis of Gravitational Field Equations in Lyra Manifold

Research Institute for Astronomy and Astrophysics of Maragha (RIAAM), P.O. Box 55134-441, Maragha, Iran

Correspondence should be addressed to H. Moradpour

Received 26 April 2017; Revised 18 June 2017; Accepted 16 July 2017; Published 14 August 2017

Academic Editor: Elias C. Vagenas

Copyright © 2017 H. Moradpour et al. This is an open access article distributed under the Creative Commons Attribution License, which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited. The publication of this article was funded by SCOAP^{3}.

#### Abstract

Considering the Einstein field equations in Lyra manifold and applying the unified first law of thermodynamics as well as the Clausius relation to the apparent horizon of FRW universe, we find the entropy of apparent horizon in Lyra manifold. In addition, the validity of second law of thermodynamics and its generalized form are also studied. Finally, we use the first law of thermodynamics in order to find the horizon entropy of static spherically symmetric spacetimes. Some results of considering (anti)de-Sitter and Schwarzschild metrics have also been addressed.

#### 1. Introduction

Inasmuch as some observational data are not suitably described by the general relativity theory, physicists try to get gravitational theories in more agreement with nature [1–5]. The Riemannian geometry is the backbone of most of these theories, and it has been shown that the system horizon (as the boundary of system) has entropy, and indeed, there is a deep connection between the thermodynamics laws and the gravitational field equations [6–33]. This mutual relation between gravity and thermodynamics may help us in providing a description for the primary inflationary era as well as the current accelerating phase of universe [34]. In addition, due to such relation, geometry and the universe material content may approach a thermodynamic equilibrium [35].

In fact, the dark energy candidates, as the source of the current phase of universe, may modify the horizon entropy and thus its thermodynamics in both gravitational and cosmological setups [36–44]. Moreover, it seems that such modifications to the thermodynamics of system are in line with probable nonextensive thermodynamic properties of spacetime and the current universe [45–47].

A generalization of the Riemannian manifold has been introduced by Lyra [48] which modifies the Einstein field equations [49]. Changing Riemannian manifold by the Lyra geometry, one can also find a new scalar-tensor gravity in which both the tensor and scalar fields have geometrical interpretations [50]. Various aspects of the Einstein field equations in the Lyra manifold have been studied [51–61]. Using the Einstein field equations in Lyra manifold, Karade and Borikar have studied thermodynamic equilibrium of a spherical gravitating fluid [62].

Here, using the Einstein field equations in Lyra manifold, we are going to study the mutual relation between the field equations and the thermodynamics laws. Moreover, we are interested in obtaining the effects of Lyra generalization of Riemannian manifold on some thermodynamic properties of gravitating systems. In order to achieve this goal, applying the thermodynamics laws to the apparent horizon of FRW universe, we find an expression for the horizon entropy in Section 2. In addition, the validity of second and generalized second laws of thermodynamics will also be investigated in the next section. In Section 3, we use the Einstein field equations in Lyra manifold as well as the unified first law of thermodynamics in order to obtain the generalization of the Misner-Sharp mass [63] in this theory. Moreover, we use the obtained mass relation, field equations, and the first law of thermodynamics to find the entropy of static spherically symmetric horizons. A summary is presented in Section 4. For the sake of simplicity, we consider in our study.

#### 2. Thermodynamics of the Apparent Horizon in Lyra Manifold

In the Lyra manifold, the Einstein field equations are written aswhere denotes the displacement vector field of Lyra manifold, and we set Einstein gravitational coupling constant to one [48, 54]. As it is obvious, due to the geometrical field , the Einstein gravitational field equations in Lyra and Riemannian manifolds differ from each other. Here, we are interested in investigating the effects of the displacement vector field on the apparent horizon entropy of the FRW universe and thus its thermodynamics.

In the cosmological setup, there are two common cases for vector including and , where is constant, which help us in describing the current accelerating universe [51, 52]. Here, we focus on the general case of and find the entropy of apparent horizon.

The FRW universe is described bywhere is scale factor. In addition, denotes the curvature parameter corresponding to closed, flat, and open universes, respectively. Therefore, , and if universe is filled by an energy-momentum source as , then by defining the Hubble parameter as , one obtainsfor the Friedmann equations in the Lyra geometry. Here, and are the energy density and pressure of the isotropic fluid which supports the geometry, respectively. Moreover, here and from here onwards, dot denotes the derivative with respect to time. One can also use the Friedmann equations in order to findIn addition, since the apparent horizon of FRW universe is located atwe can rewrite (4) in the form of

In order to obtain continuity equation in our setup, one can define an energy-momentum tensor corresponding to the displacement vector field asleading towhere are the energy density and isotropic pressure corresponding to the displacement vector field of . If we define the state parameter corresponding to the Lyra displacement vector field as , then we have meaning that the displacement vector field acts as a stiff matter. Moreover, since is negative for real nonzero values of , the weak energy condition is not respected by a real nonzero . In this manner, we have also for meaning that the real Lyra displacement vector field induces a negative pressure into the background. This vector field may help us to build a model for an accelerating universe [51, 56] which can avoid the singularity, entropy, and horizon problems [53]. It is also worth mentioning that for imaginary Lyra displacement vector fields we have meaning that the weak energy condition is satisfied. Therefore, since such fields do not lead to negative pressure, as the basic property of the accelerating universe, this kind of Lyra displacement vector field cannot be used to describe the inflationary and current phases of universe.

Let us define the total energy-momentum tensor () as in which and . In this manner, if , then the condition is respected. Moreover, rewriting the Einstein field equations in Lyra manifold as , one can easily obtain that the condition is satisfied whenever the Hubble parameter meets the condition, or equally should be a real quantity.

The Bianchi identity () implies which leads tofor the continuity equation. This equation can also be decomposed intowhenever there is no interaction between and the energy-momentum source.

##### 2.1. The Entropy of Apparent Horizon

The Cai-Kim temperature of the apparent horizon and the projection of the four-dimensional energy-momentum tensor on the normal direction of the two-dimensional sphere with radius are as follows:respectively [20, 21, 64]. Here, is the work density. Additionally, the energy flux () crossing the system boundary (the apparent horizon) is defined as , and therefore, simple calculations lead towhere we used (the Clausius relation) and (aerial volume) relations to obtain this equation [20, 21, 32, 64]. Now, inserting (6) into this equation and integrating the results, one can easily show thatwhere is the horizon entropy in the Einstein framework (or equally the Bekenstein entropy) if the Einstein field equations are written as . Indeed, since , we have and thus meaning that, at the limit, the results of the Einstein theory are covered. Therefore, the second term on the RHS of this equation is the correction term to the horizon entropy due to the displacement vector filed of considered in the Lyra geometry.

If one writes the Einstein field equations as , then simple calculations lead to for the horizon entropy. Moreover, it has also been shown that if the Einstein field equations are modified as , where includes both the ordinary energy-momentum tensor () and correction terms (), then one obtainsfor the horizon entropy [39–44]. Here, and denote the energy density and pressure components of , respectively. It is useful to mention here that, irrespective of a mutual interaction between various parts of , the above result is available [43, 44]. Therefore, one must expect that replacing by in (16), we should get (15). As we have previously shown, in our case . Thus, replacing by and inserting into (16), one can easily obtain (15) for the horizon entropy.

Finally, it is worthwhile mentioning that since we have not used (10) and (11) to obtain (15), this equation is valid irrespective of existence or absence of an interaction between Lyra displacement vector field and the energy-momentum source. This is in agreement with the properties of (16).

##### 2.2. The Second and Generalized Second Laws of Thermodynamics

From (14), it is obvious that, independent of , the second law of thermodynamics, stated as [6], is met if the energy-momentum tensor satisfies the condition. Therefore, if we define the state parameter as , then the condition is covered for sources of positive energy density () only if we have .

Moreover, the generalized second law of thermodynamics (GSLT) states the total entropy of system (), including the horizon entropy () and the entropy of fluid () which supports the geometry, should not decrease or equally it has to respect the condition [7, 8]. In order to find term, we use the first law of thermodynamics [65]where is the temperature corresponding to the supporter fluid. Inserting the relation into (17), one finds thatwhere we also used the relation [35]. Now, for noninteracting case, using (10) and (14) and the above result, we obtainfor the total entropy changes. Therefore, for a noninteracting expanding universe in which , GSLT is satisfied if the dominant fluid respects the condition. It is also obvious that, for spacetimes in that , we have meaning that, only in the flat and closed universe, GSLT is satisfied for a fluid respecting the condition. In addition, for an open universe, GSLT is met if we have . Moreover, considering an interacting universe in which (10) is not valid, we use the original form of the continuity equation (9) and follow the recipe which led to (19) in order to getfor the total entropy. Therefore, GSLT is valid whenever we have . It has been shown that a constant may help us in providing a description for the current acceleration phase of the universe expansion and thus the cosmological constant [51]. Therefore, we now focus on the case, leading tomeaning that GSLT is met when we have yielding for a flat universe () of constant Hubble parameter (). In obtaining the latter result, we used the relation.

#### 3. Thermodynamics of the Static Horizon

We consider a four-dimensional static, spherically symmetric spacetime with a horizon located at and described by the metric

Inserting metric (22) in (1), the and components of the Einstein tensors can be written aswhere prime denotes derivative with respect to . We consider a perfect fluid with which fills the spacetime. Therefore, since both the Einstein and energy-momentum tensors are diagonal, only one component of the Lyra displacement vector field may be nonzero and its other components should be zero. We consider a time-like Lyra displacement vector field as , where can be a function of or a constant [61, 62], and insert (23) into (1) to obtain [62]

One can use (13) as well as the relation in order to obtain [33]which leads toand thuswhere we also used the relation to find the above result. We finally obtainfor the Misner-Sharp mass confined to radius in this theory. Therefore, for total mass of a black hole, we get covering the Einstein result () in the appropriate limit of [11, 63]. Although we obtained in the limit, since we set the Einstein constant to one (), we have and thus in agreement with previous studies [11, 33, 63]. Substituting (23) into (1), one easily finds thatwhich finally leads to where we used . From (28) and (30), one gets and , respectively. It means that, in order to have meaningful expressions for and , the term should not diverge. Now, using these results as well as the first law of thermodynamics (), we obtainand thereforewhere is the Bekenstein entropy. Again, since , it is easy to obtain the familiar form of the Bekenstein entropy as . Moreover, because we have , we finally obtainfor the horizon entropy, covering the Einstein result at the appropriate limit of . In order to have a well-defined entropy, the second term of RHS of this equation should not be divergent meaning that, at the limit, the zero-zero component of Lyra time-like displacement vector field should meet the condition.

Finally, let us study the case of , where is arbitrary. Since metric is a spherically symmetric static metric and thus the Einstein tensor is static, for spherically symmetric static energy-momentum tensors, may be a constant or a function of and thereforeleading tofor and (Schwarzschild), respectively. In obtaining the above results, we assumed that is constant. It is obvious that, for real values of , and we have if and only if . In addition, it is also easy to verify that, for real values of , entropy is always positive and nonzero for an anti-de-Sitter universe, where .

#### 4. Summary

Throughout this work, applying the unified first law of thermodynamics to the apparent horizon of FRW universe and using the Clausius relation as well as the Cai-Kim temperature, we could obtain a relation for the effects of Lyra displacement vector on the entropy of apparent horizon. The validity of second and generalized second laws of thermodynamics has also been studied.

Moreover, considering the Einstein field equations in the Lyra manifold for a spherically symmetric static metric together with the first law of thermodynamics, an expression for the entropy of the static event horizon has also been obtained. Finally, the horizon entropy for (anti)de-Sitter and Schwarzschild black holes in the Lyra manifold was also investigated.

#### Conflicts of Interest

The authors declare that there are no conflicts of interest regarding the publication of this paper.

#### Acknowledgments

The work of H. Moradpour has been supported financially by Research Institute for Astronomy & Astrophysics of Maragha (RIAAM) under Project no. 1/4717-170.

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