Research Article | Open Access

# The Thermodynamic Relationship between the RN-AdS Black Holes and the RN Black Hole in Canonical Ensemble

**Academic Editor:**Juan José Sanz-Cillero

#### Abstract

In this paper, by analyzing the thermodynamic properties of charged AdS black hole and asymptotically flat space-time charged black hole in the vicinity of the critical point, we establish the correspondence between the thermodynamic parameters of asymptotically flat space-time and nonasymptotically flat space-time, based on the equality of black hole horizon area in the two different types of space-time. The relationship between the cavity radius (which is introduced in the study of asymptotically flat space-time charged black holes) and the cosmological constant (which is introduced in the study of nonasymptotically flat space-time) is determined. The establishment of the correspondence between the thermodynamics parameters in two different types of space-time is beneficial to the mutual promotion of different time-space black hole research, which is helpful to understand the thermodynamics and quantum properties of black hole in space-time.

#### 1. Introduction

The AdS black hole solution in four-dimensional space-time is an accurate black hole solution of the Einstein equation with negative cosmological constant in asymptotic AdS space-time [1]. This solution has the same thermodynamic characteristics as the black hole solution in asymptotically flat space-time; that is, the black hole entropy is equal to a quarter of the event horizon area, while the corresponding thermodynamics quantity satisfies the law of thermodynamics of black hole. It is well known that, if taken as a thermodynamic system, the asymptotically flat black hole does not meet the requirements of thermodynamic stability due to its negative heat capacity. However, compared with the asymptotically flat space-time black hole, the AdS black hole can be in thermodynamic equilibrium and stable state, because the heat capacity of the system is positive when the system parameters take certain values.

Therefore, the thermodynamics of AdS charged black holes, in particular its phase transition, in -dimensional anti-de Sitter space-time was firstly discussed and extensively investigated in [2, 3], which discovered the first-order phase transition in the charged nonrotating RN-AdS black hole space-time. Recently, increasing attention has been paid to the possibility that the cosmological constant could be an independent thermodynamic parameter (pressure), and the first law of thermodynamics of AdS black hole may also be established with terms. For instance, the critical properties of AdS black hole were firstly studied in [4], which found that the phase transition and critical behavior of RN-AdS black hole are similar to those of the general van der Waals-Maxwell system. More specifically, the RN-AdS black hole exhibits the same (liquid-gas phase transition) critical phase transition behavior and critical exponent as van der Waals-Maxwell system. The phase transition and critical behavior of various black holes in the extended phase space of AdS have also been extensively studied in the literature [5–23], which showed very similar phase diagrams in different black hole systems.

The asymptotically flat black holes cannot reach thermodynamic stability, due to the inevitable so-called Hawking radiation. In order to obtain a better understanding of the thermodynamic properties and phase transition of black holes, we must ensure that the black hole can achieve stability in the sense of thermodynamics. According to the previous results obtained by York (1986) [24], achieving thermodynamic stability for asymptotically flat black hole system also depends on the effect of environments; that is, one needs to consider the ensemble system. Different from the general thermodynamic system, the self-gravitational system has inhomogeneity in space, which makes it necessary to determine the corresponding thermodynamic quantities and their characteristic values.

The local thermodynamic stability of self-gravitational systems can be analyzed by considering the extreme value of the Helmholtz free energy of the system. When it reaches to a minimum value, the corresponding system is at least locally stable. According to the methods extensively studied in the literature [25–31], the extreme value of the free energy of gravitational systems can be derived from the action I; that is, the partition function of the system at the zero-order approximation can be calculated by using the Gibbons-Hawking Euclidean action [32]Combining this with the Helmholtz free energy from the equation , we can obtainwhere , , and , respectively, denote the radius, temperature, and electric charge of the cavity, , and is the radius of the black hole horizon. and are the internal energy and entropy of the black hole in the cavity. Therefore, when , , and are determined, the only variable for the system is . The thermal equilibrium conditions of the black hole and environment can be determined by the following equation:The condition where the free energy reaches to its minimum value is that the system is at local equilibrium state. In order to reach to the thermal equilibrium, it should satisfy the following criteria:Utilizing the method described above, the literatures [26–32] have studied the charged black hole and black branes, obtained the requirements to meet the thermodynamic equilibrium conditions, and discussed the phase transition and critical phenomena. More recently, Eune et al. (2015) investigated the phase transition based on the corrections of Schwarzschild black hole radiation temperature [33]. More specifically, both of the charged black hole and the radiation field outside the black hole were considered in their work, under the condition that they are both in the equilibrium state.

On the other hand, Reissner-Nordstrom (RN) black hole and RN-AdS black hole are the exact solutions of the Einstein equation. The main difference between AdS space-time and asymptotically flat space-time is the famous Hawking-Page phase transition [1]; that is, the AdS background provides a natural constraint box, which makes it possible to form a thermal equilibrium between large stable black hole and hot gas. In the recent study of the phase transition* of* the RN black hole, in order to meet the requirement of thermodynamic stability, one needs to artificially add a cavity concentric in the horizon of the black hole. However, the determination of the specific value of the radius of the cavity is still to be done. In the previous studies of phase transient of RN and RN-AdS black holes, it was found that both of the two types of black holes exhibit the same (liquid-gas phase transition) properties as van der Waals-Maxwell system, which is also consistent with our finding through the comparison between the phase transition curves of these two kinds of black holes. Therefore, the consistency of the thermodynamic stable phase and phase transition between the cavity asymptotically flat black hole and the black hole in the AdS space seems to indicate a more profound connotation [25, 26]. One of the roles played by the cavity and the AdS space is to ensure the conservation of the degree of freedom within a certain system. Naturally, the discussion of the following problems is the main motivation of our analysis: Does a duality of a gravitational theory and a nongravitational theory exist in the cavity setting? Are these two types of black holes inherently connected? If so, can the cavity radius introduced* for* the RN black hole be determined by the thermodynamic properties of the RN-AdS black hole?

In this paper, by comparing the phase transition curves of the RN black hole with the RN-AdS black hole, we establish the equivalent thermodynamic relations of the two kinds of black holes. We also discuss the relationship between the radius of cavity introduced into the RN black hole, the black hole horizon radius, and the cosmological constant. Finally, we investigate the equivalent thermodynamic quantities of two kinds of black holes, which provides theoretical basis for the further exploration of their internal relation.

#### 2. Thermodynamic Properties of Flat Space-Time Charged Black Hole

To begin with, we review the thermodynamic properties of RN black holes. The metric of a charged RN black hole is given bywhereThe corresponding action is expressed as [21, 25]Here is the temperature of the cavity and . is the radius of concentricity outside a black hole horizon , which can be obtained from the following equation:The corresponding reduction quantities are defined asNote that the relation of always holds (given , ) and the reduced action takes the form asTherefore, we can obtainwhere the function of the reciprocal of temperature isThe condition for the RN black hole and cavity to reach thermal equilibrium isThus the reciprocal of the black hole radiation temperature can be written as

The critical charge and critical radius of a black hole can be determined by the following conditions:Combing (12) and (15), one can easily derive the critical charge and the critical radius of black hole as [25]The characteristic behavior curve of with respect to is shown in Figure 1.

#### 3. Criticality of Charged AdS Black Holes

To start with, we review some basic thermodynamic properties of the spherical RN-AdS black holes. In the framework of Schwarzschild-like coordinates, the metric and the field readwhere stands for the standard element on and the function is given bywhere is the AdS length scale and is related to the cosmological constant as . The radius of the black hole horizon, , satisfies the following equation:and the black hole radiation temperature readsIn order to explicitly illustrate the critical phenomenon of RN black hole, we present the diagram in Figure 2, from which the phase transition point of system could be obtained asThese results are well consistent with those obtained in the previous analysis [4].

Moreover, the state parameters of a certain system should satisfy the first law of thermodynamicswhere the potential , thermodynamic volume , and pressure of the black hole, respectively, are expressed as

#### 4. Thermodynamic Relationship of Two Different Types of Space-Time Black Holes

As can be seen from Figures 1 and 2, for the two types of black holes, the curves of the reciprocal of temperature with respect to the radius of black hole horizon are quite similar to that for the van der Waals-Maxwell gas-liquid phase transition (the corresponding critical exponents can be calculated at the critical point). Moreover, similar to the cases in the AdS space and dS space, the specific heat capacity of the black hole in the asymptotically flat space can be expressed as [29], where denotes the critical temperature. These common thermodynamic properties give us a hint about the possibility of making the thermodynamic properties of the black hole in these different types of space-time well consistent with each other, only by adjusting some specific parameters. Therefore, a cavity radius, which may ensure the thermodynamic equilibrium stability of a system, is introduced in our discussion of the thermodynamic properties of charged black holes in the flat space-time. Meanwhile, there is no definite value for the radius in our analysis, which provides us with the possibility of adjusting this parameter to obtain similar thermodynamic properties from the black holes in two different types of space-time and finally derive the relationship between the radius of the cavity and the radius of the black hole or the cosmological constant .

It is generally believed that if the corresponding state parameters in two thermodynamic systems behave the same as the selected independent variables, these two systems will have the same thermodynamic properties. In order to obtain similar thermodynamic properties of the black holes in two different types of space-time, we ensure that, with the change of horizon radius of black hole, the temperature and entropy of the two systems are kept to be equal. That is, the radii of the two black holes are equal to each other in two different types of space-time, which leads to the similar entropy in the two systems. In the vicinity of the critical point, the temperature of a black hole in the space-time varies synchronously with the radius of the horizon. Based on the assumption that the entropy and temperature of a black hole in two different types of space-time change the same with the horizon radius, we can obtain the relationship between the radius of the cavity introduced in the flat space-time and the radius of the black hole horizon and the cosmological constant.

Supposing the radii of the black hole horizon in the two different types of space-time are both , which can be, respectively, determined by (8) and (19) for RN black holes and RN-AdS black holes, the common entropy of these two systems is expressed asAs can be seen from the analysis in Section 3, both of the two types of black holes show the same characteristics as van der Waals-Maxwell system (fluid-gas phase transition), the critical points of which are, respectively, given by (16) and (21). It is apparent that (24) implies the critical point of RN black holes and RN-AdS black holes are determined when the black hole radius is the same in two different types of space-time, while (21) shows the critical value of the RN-AdS black hole is dependent on AdS length scale . Therefore, when the horizon position and charge are given for such black hole, the radiation temperature depends on the cosmological constant. On the other hand, it can be seen from (13) that when and are fixed, the temperature of RN black holes depends on the cavity radius .

To start with, when the black hole horizon is the same in the two different types of space-time, we should determine the requirement of coincidence of critical curves, which is also the requirement to make . With the definition of , the combination of (13) and (20) givesto obtain equal temperature in two different types of space-time, where , . The quantitative relation between and is plotted in Figure 3. Because the black hole horizon position is the same for both types of black holes, the curve also quantifies the relation between and . We can get when , with which the critical curves of RN black hole and of RN-*AdS* black hole coincide with the critical curve of black hole.

Secondly, when , at the critical point , , supposing there is a deviation of the charge to the threshold, we can get and from the following expression:It is straightforward to obtainSubstituting , into the above equation, we can get the relationship between and and (26) can be rewritten asWhen is given, one can get the value of from (27). Denoting in (28), we can obtain the curve deviating from the critical charge. The results are plotted in Figure 4, in which the blue, red, and black lines, respectively, correspond to the three cases with the critical charge, above the critical charge, and under critical charge. It is obvious that when the relation between and satisfies (25) and (28), the critical curves of RN black holes and of RN-AdS black holes coincide, which implies the equal temperature of the two different types of space-time. Therefore, similar to the definition of entropy in (24), the heat capacity of the two systems can also be written as

We remark here that, in the discussion above, the cavity radius is introduced as a state parameter to test the thermodynamic properties near the critical point of the RN black hole in the asymptotically flat space-time. This procedure follows the study of the thermodynamic properties near the critical point of the RN-AdS black hole, where the cosmological constant is taken as the state parameter in the thermodynamic system. Our results demonstrate that the change of the two systems is the same near the critical point, as long as the variables and introduced in the two systems satisfy (25) and (28). As is shown in Figures 1 and 2, the radius of the cavity is a one-to-one correspondence with the cosmological constant and thus could be the dual of the cosmological constant in nonasymptotic flat space-time.

#### 5. Conclusion

It is well known that black hole is an ideal system to understand the nature and behavior of quantum gravity. On one hand, black hole provides an ideal model to study all kinds of interesting behaviors of classical gravitation (under the sense of general relativity); on the other hand, it can be regarded as a macroscopic quantum system with unique thermodynamic properties (the entropy, temperature, and holographic properties of gravitation are quantum), which provides an important window to probe the quantum gravity. More importantly, a better understanding of the black hole singularity, cosmological singularity, and cosmological inflation needs a basic theory of space-time gravitation. Up to now there is still no mature theory to precisely describe the quantum characteristics of the asymptotic flat black holes and nonasymptotically flat space-time black holes. However, string theory firstly provided the microcosmic explanations for the entropy of some AdS black holes, which predicted the existence of M-theory and some duality relations (especially AdS/CFT correspondence) and realized the holographic properties of gravitational systems [34].

In this paper, by setting the black hole horizon at the same value and assuming the temperature near the critical point is equal in two different types of space-time, we have discussed the relationship between the radius of the cavity and the cosmological constant . The former parameter is always introduced in studying the thermodynamics of the charged black hole in the flat space-time, while the latter is related to the nonflat space-time. Our results may provide a theoretical basis for exploring the internal relations of thermodynamics in the black space. Moreover, by establishing the correspondence between the thermodynamic parameters of black holes in the asymptotically flat space-time and nonasymptotically flat space-time, we hope to seek the correspondence between the quantum properties of the black hole in two different types of space-time, which is beneficial to understand the thermodynamics and quantum properties of black holes in different space-time.

#### Conflicts of Interest

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

#### Acknowledgments

This work was supported by the Young Scientists Fund of the National Natural Science Foundation of China (Grant nos. 11605107 and 11503001), in part by the National Natural Science Foundation of China (Grant no. 11475108), by Program for the Innovative Talents of Higher Learning Institutions of Shanxi, the Natural Science Foundation of Shanxi Province, China (Grant no. 201601D102004) and the Natural Science Foundation for Young Scientists of Shanxi Province, China (Grant no. 201601D021022), and by the Natural Science Foundation of Datong city (Grant no. 20150110).

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#### Copyright

Copyright © 2017 Yu-Bo Ma 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}.