Advances in High Energy Physics

Volume 2018, Article ID 7124730, 8 pages

https://doi.org/10.1155/2018/7124730

## Energy Definition and Dark Energy: A Thermodynamic Analysis

^{1}Research Institute for Astronomy and Astrophysics of Maragha (RIAAM), P.O. Box 55134-441, Maragha, Iran^{2}Departamento de Física, Universidade Federal da Paraíba, Caixa Postal 5008, 58051-970 João Pessoa, PB, Brazil^{3}Institut de Mathématiques et de Sciences Physiques (IMSP), 01 BP 613 Porto-Novo, Benin

Correspondence should be addressed to H. Moradpour; moc.liamg@ruopdarom.nh

Received 19 May 2018; Revised 16 July 2018; Accepted 30 July 2018; Published 9 August 2018

Academic Editor: Elias C. Vagenas

Copyright © 2018 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

Accepting the Komar mass definition of a source with energy-momentum tensor and using the thermodynamic pressure definition, we find a relaxed energy-momentum conservation law. Thereinafter, we study some cosmological consequences of the obtained energy-momentum conservation law. It has been found out that the dark sectors of cosmos are unifiable into one cosmic fluid in our setup. While this cosmic fluid impels the universe to enter an accelerated expansion phase, it may even show a baryonic behavior by itself during the cosmos evolution. Indeed, in this manner, while behaves baryonically, a part of it, namely, which is satisfying the ordinary energy-momentum conservation law, is responsible for the current accelerated expansion.

#### 1. Introduction

Friedmann equations, the ordinary energy-momentum conservation law (OCL) (or the continuity equation), and its compatibility with the Bianchi identity (BI) are the backbone of the standard cosmology which forms the foundation of our understanding of cosmos [1]. Since on scales larger than about 100 Megaparsecs, cosmos is homogeneous and isotropic [1], the FRW metric is a suitable metric to study the cosmic evolution [1]. Relations between thermodynamics and Friedmann equations have been studied in various setups which help us in getting more close to the thermodynamic origin of space-time, gravity, and related topics [2–20].

A thermodynamic analysis can also lead to a better understanding of the origin of dark energy, responsible for the current accelerated universe [15–33]. In fact, there are thermodynamic and holographic approaches claiming that the cosmos expansion can be explained as an emergent phenomenon [18, 34–46]. Two key points in these approaches are the definition of energy and the form of the energy-momentum conservation law [14, 19, 34–38], and indeed their results are so sensitive to the energy definitions that have been employed [19].

In order to make the discussion clearer, consider a flat FRW universe with scale factor [1]filled by a prefect fluid source with energy density and pressure . Einstein field equations and energy-momentum conservation law (or equally the Bianchi identity) lead toforming the cornerstone of standard cosmology. Although, the third equation (OCL) is in full agreement with our observations on the cosmic fluid in matter and radiation dominated eras [1, 47], its validity for the current accelerated cosmos is questionable [48], which may encourage us to consider the relaxed types of OCL to describe the current universe [18, 49]. Combining the Friedmann equations with each other, we getwhich can finally be added to the time derivative of the first Friedmann equation to reach OCL. Therefore, although the third equation of (2) is generally obtained from the conservation law (), one can easily find it by only using the Friedmann equations, a result which is the direct consequence of the Einstein field equations satisfying the Bianchi identity. It means that two equations of the above set of equations (2) are indeed enough to study the cosmos, and the third equation will be automatically valid and inevitable in this situation.

From the viewpoint of thermodynamics, if OCL is valid, then by applying the thermodynamics laws to the apparent horizon, as the proper causal boundary located at [2–4]where is the comoving radius of apparent horizon, one can reach the Friedmann equations (2) [5–18]. Indeed, the validity of OCL is crucial, and its relaxation leads to modification of the Friedmann equations [14–16, 18]. These results propose that the cosmos is so close to its thermodynamic equilibrium state [19–28], motivating us to be more confident of thermodynamical quantities, such as pressure, to study the cosmos [19].

For a FRW background filled by a fluid whose energy density is at most a function of time, we may reachas the total energy confined to the volume and felt by a comoving observer with four velocities . Equation (5) is fully compatible with the combination of the Misner-Sharp mass, thermodynamics laws, and standard cosmology (2) [5–18].

Now, if we write , where is the comoving volume [1], using (5) and the thermodynamic pressure definitionthen we easily get OCL. In fact, it shows that the form of the conservation law depends on the energy definition. The above arguments also motivated the authors of [19] to use various energy definitions such as the Komar mass [50, 51] andinstead of (2), in order to study the relations between energy definitions, thermodynamics laws, and the cosmic evolution, which led to a model for the cosmic fluid consistent with observations [19].

The Einstein field equations also admit another mass definition, namely, the Komar mass [47, 50, 51]:which clearly differs from (5) unless we have a pressureless fluid [41, 47]. Although this energy definition has originally been obtained by using the Einstein field equations [47], it is in accordance with various thermodynamic and holographic approaches [34–46]. In fact, if we accept this energy definition as a basic equation and not a secondary equation [34–37], then there are various thermodynamic and holographic approaches employed to get the Friedmann and gravitational field equations in various theories by using the Komar mass definition [34–46].

Now, inserting the Komar energy definition into (6) and using , one can easily reach a new conservation law asmet by Komar mass, and we call it the Komar conservation law (KCL). For the pressureless systems, everything is consistent due to the fact that the above addressed energy definitions are equal. Similar types of this energy-momentum conservation law have also been obtained in theories which include a nonminimal coupling between the geometry and matter fields [14, 49, 52].

One basic consequence of the Komar mass is that not only , but both of and participate in building the system energy which leads to appropriate models for dark sectors of cosmos, if one uses (7) to model the cosmos [19]. On the other hand, (9) claims that if we use the Komar mass, then OCL and thus (7) are not valid and other modified Friedmann equations should be employed to describe the cosmos.

In the general relativity (GR) framework, where , the satisfaction of BI by the Einstein tensor () is equivalent to the satisfaction of OCL by . We saw that the form of conservation law depends on the energy definition (6), and indeed, if the Komar mass notion is used, then one can easily find that should satisfy KCL instead of OCL. Thus, in the GR framework, there is an inconsistency between the notions of BI (or equally OCL) and the Komar mass. This inconsistency together with the results of recent observations [48], admitting the break-down of OCL in the current accelerated era, motivates us to modify GR and, thus, the Friedmann equations by modifying the matter side of the field equations in the manner in which the modified matter part meets OCL, in agreement with the satisfaction of BI by the geometrical part. In summary, we think that OCL is very restrictive constraint on . Probably, OCL should not be applied to the whole of , and it should only be applied to some parts of , the parts which may modify GR in a compatible way with the current accelerated cosmos [18, 48, 49].

There is also an elegant consistency between (5) and (2); i.e., is the energy density in (5), and it also appears in the first Friedmann equation. On the other hand, (8) implies that the quantity () plays the role of energy density, while it is not present in the first Friedmann equation. In fact, if we define , then (9) can be rewritten asequivalent to a hypothetical fluid with an effective energy-momentum tensor of , meeting OCL (see [53] for a debate on effective energy-momentum tensor). In this manner, (5) for will be equal to (8) for . Indeed, the above arguments (specially (9)) claim that, even in the Einstein framework, OCL (2) may not be valid, if Komar mass is employed. In this situation, because OCL is the backbone of the Friedmann equations, we may conclude that the Friedmann equations (2) should be modified.

In summary, (i) the energy definition and the form of energy-momentum conservation law play crucial roles in getting the Friedmann equations in various theories [5–19]. (ii) We also showed that the form of energy-momentum conservation law depends on the energy definition, and if one uses the Komar mass, then KCL is obtained instead of OCL. (iii) The Komar mass is the backbone of some holographic and thermodynamic attempts to obtain field equations of various gravitational theories [34–46] motivating us to use KCL instead of OCL. (iv) Observational data admit the break-down of OCL in the current accelerated era [48]. It also motivates us to use the relaxed forms of OCL in order to describe the current universe. On the other hand, there are also various models for the current accelerated universe satisfying OCL [1]. Therefore, it seems that some inconsistency exists between the various definitions of energy, continuity, and Friedmann equations. One may conclude that the fluid obtained from the observation may not be the real cosmic fluid, and it indeed represents some less well-known or even unknown parts of the energy source which become important, effective, and tangible in the current cosmos. We want to say that the real cosmic fluid may satisfy conservation laws such as KCL instead of OCL, while some of its parts, responsible for the current accelerated expansion, may satisfy OCL.

As we mentioned, based on (8), one may conclude that pressure has some contribution to the energy density of system. The question that arises here is, what if this contribution can describe the current accelerated universe? In other words, are there combinations of and of a baryonic source which can play the role of dark energy? Here, we are going to find out the answers of the mentioned questions by studying some consequences of accepting (6), the Komar mass definition, and thus (9), in cosmological setups. In the next section, using thermodynamics laws, Bekenstein entropy, and (9), we address some models for dark energy. The third section is devoted to a summary and concluding remarks. Throughout this work, the unit of , where denotes the Boltzmann constant, has been employed.

#### 2. Komar Definition of Energy and the Friedmann Equations

For an energy-momentum source with , the amount of energy crossing the apparent horizon is evaluated as [8]where and denote the horizon area and the work density, respectively. is also the comoving area. After some calculations, one obtains [8, 14–17]combined with the Clausius relation (), to get [8, 15]where the additional minus sign in the Clausius relation is due to the direction of energy flux [8, 15]. Here, is the horizon entropy and the Cai-Kim temperature () is used to reach this equation [8, 15]. The role of energy-momentum conservation law in deriving the Friedmann equations is now completely clear. The effects of considering various approximations and temperatures have been studied in [15].

##### 2.1. Modified Friedmann Equations

Here, using some simple examples, we are going to show the effects of considering KCL in modifying the standard cosmology.

Case (). If OCL is valid, then (13) is reduced tocovering the first Friedmann equation whenever . In this situation, (9) implies thatwhere is the integration constant and we considered ( is the current value of the scale factor). In this manner, for , we reachinstead of (2). It is easy to check that for power law regimes in which , conservation law leads to and , which is nothing but the dust source, in full agreement with our previous results indicating that for a pressureless source everything is consistent. The geometrical parts of the two first equations of (16) lead us to the Einstein tensor. It is in fact the direct result of attributing the Bekenstein entropy to the horizon [8]. Therefore, mathematically, since the Einstein tensor obeys BI, the appearing and should also satisfy OCL, an expectation in full agreement with (15) and thus the third line of (16).

However, the general solution of the obtained conservation law (16) iswhere is the integration constant. It clearly covers the ordinary matter era whenever . From (15) and (17), we reachfor the current era. Thus, if we have and , where and denote the current values of the energy density and the equation of state (EoS) of dark energy, respectively, then the obtained fluid may theoretically generate the current accelerating universe whenever . Now, we can writeIn this manner, EoS () and the deceleration parameter () are evaluated asAt the limit, independently of the value of , we have and . Moreover, one can easily see that, at the limit, the consistent values with observations for and are also obtainable, depending on the value of . Despite this compatibility, there is a singularity at the behavior of and for , located at , meaning that this solution may be at most useable to study the era. However, as it is obvious from Figure 1, it does not lead to suitable and even for . Therefore, as we previously saw, only the case can meet all of the desired expectations; i.e., it is in agreement with the Friedmann equations, which both addressed energy definitions and thus the matter dominated era.