arXiv:1705.04923v2 [gr-qc] 16 May 2017 Thermodynamic stability, Q-Φ criticality, microstructure of charged AdS black holes in f (R) gravity and solution categorization Gao-Ming Deng1,2∗, Yong-Chang Huang1† 1 Institute of theoretical physics, Beijing University of Technology, Beijing 100124, China 2 Institute of theoretical physics, China West Normal University, Nanchong 637002, China Abstract The electric charge Q and its potential Φ are employed as state parameters to further investigate properties of charged AdS black holes in f (R) gravity, involving the thermodynamic stability, criticality and microstructure. Attempt in this new picture is launched from both macroscopic and microscopic perspectives. In the macroscopic view, the charged AdS black holes in f (R) gravity is thermally unstable and qualitatively behaves like Van der Waals fluid. However, of especial interest, it is when the temperature T >Tc that the striking Q-Φ “oscillation” arises. Moreover, unlike the ordinary case in general relativity, phase structures of the black holes in f (R) theory exhibit an interesting dependence on gravity modification parameters. Microscopic analyses are implemented by using the state space geometry. The Ruppeiner curvature shares the same divergence with the heat capacity. Although the charged f (R) AdS black hole possesses the property as ideal Bose gas, it manifests a completely opposite microscopic behaviour to Van der Waals system. In addition, we distinguish black hole solutions by performing a novel classification scenario. ∗ † [email protected] [email protected] 1 1 Introduction There is now a general consensus that our Universe is experiencing an accelerating expansion [1]. To give a persuasive theoretical interpretation of such puzzling acceleration, f (R) theory has been advanced as a promising candidate. This theory has been widely applied in gravitation and cosmology [2, 3]. Especially, it has succeeded in mimicking the historical evolution of the cosmology. For more comprehensive reviews, see, e.g. [4, 5]. Since f (R) theory is considered as an intriguing modification of general relativity, some distinctive properties from Einstein’s gravity are highly expected. Further studies of this theory, for example, black hole solutions and the thermodynamics etc., are of particular interest [6–8]. In general, it is a challenge to construct an exact f (R) black hole solution due to high nonlinearity of field equations. Sheykhi achieved a higher dimensional black hole solution [9] in f (R) gravity with a conformally invariant Maxwell field, abbreviated here as f (R)-CIM black hole solution. Motivated by these facts, this present work will specialize in further investigating the thermodynamic properties of the charged f (R)-CIM AdS black holes [9], such as the thermodynamic stability(instability), criticality and microstructure. Black hole thermodynamics has been a long-standing and fascinating subject [10–14], which provides interesting clues to the underlying structure of quantum gravity. In particular, promoted by the development of the AdS/CFT correspondence [15, 16], the investigation of black hole phase transitions in AdS spacetime has received popular attentions [17–22]. The pioneering study of these transitions can be traced back to the well-known Hawking-Page phase transition [23], which describes a first-order phase transition between Schwarzschild AdS black holes and the thermal AdS space. Inspired by the AdS/CFT, Witten has explained such an appealing phenomenon as a confinement/deconfinement transition in the dual quark gluon plasma [16]. For Reissner-Nordström AdS black holes, Van der Waals-like phase transition was first observed by Chamblin et al. [24, 25]. Soon afterwards, Cvetic et al. extended to elaborate phase transitions in gauged supergravity theory [26] and triggered popular discussions in various theories like massive gravity [27, 28]. Caldarelli, Dolan, Banerjee et al. later disclosed similar behaviours in the rotating case [17, 29, 30]. In Ref. [31], Kubizňák, Mann completed the analogy of charged AdS black hole and obtained the same critical exponents as that in Van der Waals fluid. Likewise, Hendi et al. [32] generalized this analogy to higher dimensional Reissner-Nordström AdS black holes and depicted the coexistence curves of large-small charged black holes. Up to now, it is found that the Van der Waals-like phase transition widely exists such as in BTZ black holes [33], Gauss-Bonnet black holes [34, 35], Born-Infeld black holes [36, 37]. It is noteworthy that a great many previous phase structure analyses in AdS spacetime were based on a popular proposal [38], which identifies the cosmology constant Λ and its conjugate quantity as thermodynamic variables [31, 39–41], 1 i.e., pressure P and volume V , respectively. As a result, the temperature T and the newly-introduced variables P , V were chosen as sate parameters to delineate phase structures and critical behaviours in various AdS spacetime backgrounds [42–46]. Nevertheless, aside from the variables P and V , if there exits another pair of parameters available to describe black hole thermodynamics, such as electric charge Q and its potential Φ. If so, some underlying thermodynamical properties are expected. The answer is positive. For charged black holes, electric charge Q is a natural intrinsic character. That is to say, a black hole is absolutely endowed with electric charge(s) as long as it’s charged. In fact, Davies [47] has implied the feasibility of state parameters (T, Q, Φ) in describing black hole thermodynamics. Consequently, our one attempt is focused on choosing (T, Q, Φ) as state parameters to describe the physical picture of black hole phase transitions in AdS background. In particular, we will put this idea into practice with the charged f (R)-CIM AdS black hole [9]. A well-defined statistical description still unclear, analyzing from diverse perspectives has an important physical significance. After all, the more directions we view a problem from, the more comprehensive and transparent picture we capture. In this sense, our approach provides one more alternative candidate other than (P, V ) description. The thermodynamic geometry was initiated by Weinhold [48] and has been developed by Ruppeiner, Aman, Quevedo et al. [49–53]. Ruppeiner has argued that the Riemannian geometry could give insight into the underlying statistical mechanical system [50]. In this picture, divergence of the scalar curvature R is closely related to phase transition. By calculating R, microscopic interactions and the resulting macroscopic phase transition can be linked. More interestingly, in Ref. [54], Ruppeiner pointed out that the sign of the curvature may be an indicator of the interactions among the microstructure of thermodynamic system. For instance, the curvature R<0 manifests the interaction as repulsive [55, 56], corresponding to ideal Fermi gases, and conversely, R>0 manifests the interaction as attractive, corresponding to ideal Bose gases. We will also further detect the stability and possible properties of the microstructure for f (R)-CIM black holes [9] from the geometrical perspective of thermodynamics. The remainders of this Letter are outlined as follows. In the next section, we briefly introduce the charged f (R)-CIM AdS black hole [9] and discuss its thermodynamics. In sec. 3, both the local and global stabilities are analyzed, we concentrate on employing (T, Q, Φ) as state parameters to investigate the critical behaviours in detail. Sec. 4 is devoted to adopting the thermodynamic geometry to further study the phase transition and underlying properties of microstructure. In sec. 5, we perform a novel classification [57, 58] of the black hole configurations depending on the number of phase transitions. Finally, the summary and discussions are given in sec. 6. 2 Charged f (R)-CIM AdS black holes and thermodynamics 2 To begin with, we briefly review the charged f (R)-CIM AdS black hole solution [9] and its thermodynamics. Behaviours of the charged f (R)-CIM AdS black hole are governed by the line element ds2 = −N(r)dt2 + with N(r) = 1 − dr 2 + r 2 dθ2 + sin2 θdφ2 , N(r) Q2 R0 2 2M + 2 − r , ξr r 12 (1) ξ = 1 + f ′ (R0 ) , where the constant curvature scalar R0 is defined as R0 = 4Λ, Q and M are the mass and electric charge of the black hole, respectively. Note that, the cosmology constant Λ usually defines the spacetime background [59], for the AdS sapcetime, Λ<0. If r+ denotes the radius of the outer event horizon, by setting N(r) = 0, the mass of the black hole are given by Q2 R0 2 ξr+ 1+ 2 − . (2) M= r 2 r+ 12 + The electric potential, entropy and temperature can be calculated as Φ= T = 3 ξQ , r+ 2 S = πξr+ , 4 2 −R0 r+ + 4r+ − 4Q2 N ′ (r+ ) = . 3 4π 16πr+ (3) (4) Instability and critical behaviours in Q-Φ plane This section concentrates on exploring the instability and criticality. Interestingly, analyses will be done by employing (T, Q, Φ) as state parameters. We first derive the equation of state (EoS). Combining Eqs.(3) and (4), the EoS has the following form Q= i p 2Φ h 2 ξ 2 T 2 − R (Φ2 − ξ 2 ) . −4πξT + 16π 0 R0 ξ 2 (5) We express the electric charge Q as a function of the potential Φ and temperature T . Moreover, Q relates to the parameters ξ and R0 as well. Behaviours of the electric charge Q can be witnessed intuitively by plotting Q-Φ diagram under different temperature, see Figure 1. As is depicted by gray isotherms in the left diagram, there exists an interval where electric charge Q 3 Q Q T1 0.24 T5 > T4 > T3 > T2 > T1 T2 T > Tc 0.22 T3 D Q2 0.20 A QA T4 C E 0.18 T5 Q1 B 0.16 0.4 0.6 0.8 F 1.0 0.4 0.5 0.6 0.7 0.8 0.9 1.0 F Figure 1: (color online) . Q-Φ diagram (isotherm) for different temperature. We have set ξ=1.5 and R0 =−8. The right diagram exhibits a gray isotherm of the left diagram. increases as potential Φ increases. More details have been presented in the right diagram. Obviously, one Q corresponds three possible values of Φ when Q1 < Q ) > 0, which < Q2 . Along the curve segment between the points B and D, ( ∂Q ∂Φ T does not satisfy the thermodynamical equilibrium condition and implies that the black holes are in a thermally unstable phase. Fortunately, as is presented in the left diagram, the isotherm will undergo a noticeable phase transition with a further decrease of the temperature. Next, we will proceed to investigate phase transitions and critical behaviours of the charged f (R)-CIM AdS black hole [9]. Taking into account the derivative of equation (5), 2 ∂ Q ∂Q = 0, = 0, (6) ∂Φ T ∂Φ2 T one obtains the critical point as √ −R0 Tc = √ , 3 2π Qc = √ 1 , −3R0 ξ Φc = √ , 6 (7) where critical entities have been labelled with subscript “c”, R0 =4Λ takes negative values for the AdS backgrounds. These critical values are in agreement with the outcomes in P -V description [60]. This fact implies validity of such a description in Q-Φ phase plane. The existence of the solution confirms that the charged f (R)-CIM AdS black hole is indeed thermally unstable. With the temperature decreasing, the black hole undergoes a phase transition at the critical point (Tc , Qc , Φc ). The physical picture is shown in Figure 2. It is apparent that this Q-Φ diagram qualitatively resembles the P -V behaviours of a Van der Waals liquid-gas system. An (gray solid) isotherm of a f (R)-CIM AdS black hole has a local minimum and maximum located at Φ1 and Φ2 respectively (seen from the top-left diagram of Figure 2). As is mentioned above, along the curve segment ) > 0, which means that the black holes are in a between the two points, ( ∂Q ∂Φ T 4 Q Q T11 0.24 0.24 T14 > T13 > Tc1 > T12 > T11 T12 0.22 T24 > T23 > Tc2 > T22 > T21 T21 0.22 Tc1 T22 HTc1 , Qc1 , Fc1 L Qc1 0.20 0.20 T13 Tc2 HTc2 , Qc2 , Fc2 L Qc2 0.18 0.18 T23 T14 0.16 T24 0.16 F1 0.2 0.4 Fc1 F2 0.6 0.8 1.0 1.2 Fc2 F 0.2 (a) ξ = 1.5, R0 = −8 0.4 0.6 0.8 1.0 1.2 F (b) ξ = 1.5, R0 = −10 Q Q T31 0.24 0.24 T44 > T43 > Tc4 > T42 > T41 T34 > T33 > Tc3 > T32 > T31 T32 0.22 T41 0.22 Tc3 T42 HTc3 , Qc3 , Fc3 L Qc3 0.20 0.20 T33 Tc4 Qc4 0.18 HTc4 , Qc4 , Fc4 L 0.18 T43 T34 0.16 0.16 0.14 0.04 Fc3 0.06 0.08 0.10 0.12 0.14 0.16 0.14 0.04 F (c) ξ = 0.2, R0 = −8 T44 Fc4 0.06 0.08 0.10 0.12 0.14 0.16 F (d) ξ = 0.2, R0 = −10 Figure 2: (color online) . Q-Φ diagram (isotherm) for different temperature. The temperature of isotherms decreases from bottom to top. The critical isotherm T = Tc is depicted by the red dashed line, the black dot denotes the critical point (Tc , Qc , Φc ). Each diagram corresponds to different ξ and R0 . thermally unstable phase, while the ( ∂Q ) < 0 parts denote that the black holes ∂Φ T stably exist in the form of small black holes or the big case. In fact, the unphysical “oscillating” part of the isotherm (blue dashed curve in the right diagram of Figure 1) can be replaced with an isocharge (red solid) line segment AE based on the Maxwell’s equal area law (the shaded area below the isocharge line equals to the above shaded part). Interestingly, differing from P -V description, the striking “oscillation” presents when the temperature is higher than the critical value. When the temperature comes down and reaches the critical temperature Tc , the local minimum and maximum located at Φ1 and Φ2 approach and eventually merge into the critical potential Φc1 . Proceeding with lower temperature, T < Tc , along the isotherm (blue solid curves in Figure 2), the electric charge Q monotonously decreases with the potential Φ, the black hole exists stably. It is worth emphasizing that, unlike the ordinary case in general relativity, Figure 2 exhibits an interesting dependence of phase structures on the parameters ξ and R0 . The remarkable influence of R0 on isotherms is shown in the horizonal direction, while the effect of ξ is listed in the vertical direction. As a characteristic thermodynamical quantity, the heat capacity CQ at constant charge plays a crucial role in measuring the local stability against a small 5 perturbation. With Eqs.(3) and (4) in hand, CQ can be written as 2 4 2 2ξπr+ R0 r+ − 4r+ + 4Q2 ∂S = . CQ = T 4 2 ∂T Q R0 r+ + 4r+ − 12Q2 (8) The heat capacity CQ varies with Q and the radius of the even horizon r+ . We illustrate the relation between them in Figure 3. In general, the local stability Figure 3: (color online) . Heat capacity CQ with r+ and Q for ξ=1.5, R0 =−8. can be reflected by the heat capacity. In other words, the positive heat capacity ensures stable existence of a black hole, while the negative sign means that a black hole cannot establish a thermal equilibrium with the environment. In addition, divergences signal the onset of a phase transition where the physics of the system undergoes an abrupt change. Observed from Figure 3, when Q takes a small value, there exists two sharp divergences at the points r1 and r2 . The heat capacity CQ takes positive values for r+ <r1 or r+ >r2 , while it is negative for r1 <r+ <r2 . Actually, r1 and r2 represent two phase transition points. It is at the point r+ =r1 that a small stable black hole with CQ > 0 completes an evolution and grows up to an intermediate unstable black hole with CQ < 0. Proceeding with evolution, at the point r+ =r2 , the intermediate unstable black hole develops into a large stable one. When electric charge Q increases, the phase transition points located at r1 and r2 degenerate into one, eventually, disappear. To better understand this interesting evolution process, we plot CQ versus r+ diagram in Figure 4. It is evident that divergent behaviour vanishes and a f (R)-CIM AdS black hole ends with a large stable state when Q>Qc . The heat capacity CQ reflects the local stability of the black holes, while the global thermodynamical properties of physical systems are characterized by the Gibbs free energy. In what follows, we will step forward to discuss the Gibbs free 6 CQ Q<Qc 60 Q=Qc Q>Qc 30 0.4 r1 rc r2 0.8 r+ 1.2 -30 Figure 4: (color online) . CQ versus r+ for different electric charge. The red dashed isocharge corresponds to the case of critical charge Qc . We have set ξ=1.5 and R0 =−8. energy. In an extended phase space which the cosmology constant Λ is included in, the Gibbs free energy can be defined as 3 3Q2 ξ R0 r+ , (9) + r+ + G = M − TS = 4 12 r+ where M has the meaning of the enthalpy H [38–40], r+ = r+ (T, Q) acts as a function of electric charge Q and temperature T through Eq.(4). The behaviour of the Gibbs free energy G is depicted in the left diagram of Figure 5. Of particular G G 0.6 2.0 0.5 Q=1.5Qc1 1.5 Q=Qc1 1.0 0.4 0.3 Q=0.8Qc1 0.2 L=0.4Lc Q=0.6Qc1 0.5 L=0.8Lc L=0.6Lc L=1.5Lc L=Lc 0.1 0.0 0.10 0.15 0.20 0.25 0.0 T 0.02 0.04 0.06 0.08 Figure 5: (color online) . The Gibbs free energy G versus T for (left): different charge Q, fixed R0 =4Λ=−8, (right): different Λ, fixed Q=1. We have set ξ=1.5. The red solid curve corresponds to the critical case in both diagrams. interest, in terms of the thermodynamical variables (T, Q, Φ), G-T diagram also displays a characteristic “swallow tail” analogous to Van der Waals fluid. This interesting behaviour reveals that there exists a first order phase transition for the charged f (R)-CIM AdS black hole. Paralleling with the (T, Q, Φ) description, we also plot G-T curves for different Λ when Q is fixed in the right diagram of Figure 5. 7 T By now, we have surveyed the local and global stabilities of the charged f (R)CIM AdS black hole, and investigated its critical behaviours from the macroscopic perspective. The results strongly imply that this black hole thermodynamics system qualitatively behaves like the Van der Waals fluid when adopting the state parameters (T, Q, Φ). It is noteworthy that, unlike P -V description [60], it is in the region of T >Tc in Q-Φ plane that the thermodynamic instability arises. Next, we will turn to disclose the microscopic properties by using the thermodynamic state space geometry. 4 Thermodynamic geometry and microstructure A systematic microscopic statistical description of black holes is still a lack. According to Ruppeiner’s proposal, the Riemannian geometry could give insight into the underlying statistical mechanical system. Inspired by close connection with phase transitions, thermodynamic geometry has been widely used for black hole research [42, 61–65]. The metric as defined by Ruppeiner [50] has the following form ∂ 2 S(x) ds2R = − i j dxi dxj , (10) ∂x ∂x where xi denotes the extensive variables of the system. Practically, it’s convenient to adopt Weinhold’s definition [48] ds2W = − ∂ 2 U(x) dxi dxj , ∂xi ∂xj (11) where U is the internal energy of the system. It is found that these two definitions are conformally related with each other ds2R = 1 2 ds . T W (12) As for the charged f (R)-CIM AdS black hole, U = M − QΦ, and we define the coordinates x1 = S, x2 = Φ. Combining Eqs.(2-4), the Ruppeiner metric can be calculated as 4 2 R0 r+ + 4r+ − 4Q2 , 2 4 2 2πξr+ (−R0 r+ + 4r+ − 4Q2 ) 8Qr+ = = gΦS , 4 2 ξ (−R0 r+ + 4r+ − 4Q2 ) 4 16πr+ . = 4 2 ξ (−R0 r+ + 4r+ − 4Q2 ) gSS = (13) gSΦ (14) gΦΦ (15) Programming with Mathematica 10.2, one can obtain the scalar curvature as R= 4 ξπ (−R0 r+ + 2 4r+ 8∆ , 4 2 − 4Q2 ) (R0 r+ + 4r+ − 12Q2 ) 2 8 (16) with 2 2 2 4 4 2 4 ∆ = 4Q4 2 − 5R0 r+ − Q2 r+ 16 − 18R0 r+ − 3R02 r+ + r+ 8 − 6R0 r+ − R02 r+ . It is clear that, considering the nonextremal condition (4), the scalar curvature (16) diverges when 4 2 R0 r+ + 4r+ − 12Q2 = 0 . (17) Obviously, it shares the same denominator with the heat capacity CQ in Eq.(8). Hence the curvature diverges exactly at the point where the heat capacity diverges. One can appreciate this consistency in Figure 6. Perhaps more interRCQ 800 CQ -r+ R-r+ 600 400 200 r+ 0.4 rc 0.6 0.8 Figure 6: (color online) . CQ versus r+ and R versus r+ for ξ=1.5, R0 =−8. The scalar curvature shares the same divergence point with the heat capacity CQ . estingly, the charged f (R)-CIM AdS black holes possess the property as ideal Bose gases. As is alluded in sec. 1, Ruppeiner claimed that different sign of the curvature manifests different interaction among the microstructure of the thermodynamic system [54]. For ideal Bose gas, the scalar curvature R>0 [55, 56] and goes to positive infinity at critical point, and it corresponds to Bose-Einstein condensation. While for ideal Fermi gas, R<0 [55, 56] and diverges to negative infinity at critical point, which appeared as the Pauli’s exclusion principle forbidding two particles to stay in the same state with unlimited repulsive force. For the Van der Waals liquid-gas system, R is always nonpositive, and this fact means that attractive force cannot exist in such system. In contrast with Van der Waals liquid-gas system, the charged f (R)-CIM AdS black hole shows a remarkable difference in microscopic property. Figure 6 intuitively displays that the scalar curvature is always nonnegative and positively diverges. Just as ideal Bose gas, R>0 signals that attractive interaction is permitted in the microscopic system of the charged f (R)-CIM AdS black hole. 9 5 Classification of black hole solutions in terms of the number of phase transitions Recently, Cembranos et al. [8, 57, 58] adopted a novel classification based on the number of phase transitions to distinguish black hole solutions. For the charged f (R)-CIM AdS black holes, as is aforementioned, phase transition was triggered at the point where the heat capacity diverges, namely p 2 −1 ± 1 + 3R0 Q2 4 2 2 R0 r+ + 4r+ − 12Q2 = 0 ⇒ r+ = , (18) R0 where R0 =4Λ takes negative values for the AdS backgrounds. According to the distinctive classification scenario [8,57,58], Eq.(18) allow us to classify the charged f (R)-CIM AdS black hole solutions into two classes, in detail, 1 , Eq.(18) has two different solutions • Slow black holes. When 0 < Q < √−3R 0 corresponding to signs “ ± ” respectively. It means that there exist two different phase transition points for these black hole configurations. We refer to these kinds of solutions as slow black holes. 1 2 , the equation about r+ in Eq.(18) • Fast black holes. When Q > √−3R 0 have no solution, which means there is no any phase transition occurrence for these black hole configurations. Consequently, we call these kinds of solutions as fast black holes. In Figure 7, we schematically display the regions in which black hole behaves as the “slow” or “f ast” black hole respectively. It should be pointed out that, Q 0.5 0.4 II Fast BH 0.3 0.2 I Slow BH L-6 -5 -4 -3 -2 0.1 -1 Figure 7: (color online) . The distribution region of the slow black holes and f ast black holes. according to the analysis above, this classification scenario has nothing to do with the gravitation modification parameter ξ for the charged f (R)-CIM AdS black holes, while strongly depends on the electric charge Q and the cosmology constant Λ. 10 6 Discussions and conclusions In this Letter, we choose the intrinsic quantities (Q, Φ) as state parameters to further investigated the thermodynamical stability, critical behaviours and phase structure of the charged AdS black hole in f (R) theory. This work, on the one hand, provided one more alternative candidate other than (P, V ) description, on the other hand, disclosed some underlying properties which contribute to better understanding f (R) gravity theory. A point worth stressing is that not only does the f (R)-CIM AdS black hole possess critical phenomena akin to Van der Waals liquid-gas system, but also some fascinating differences between them were dug out in this Letter, no matter from macroscopic or microscopic perspective. The detailed calculations and graphical illustrations confirmed that the charged f (R)-CIM AdS black hole is thermally unstable and it undergoes a first order phase transition. In the macroscopic view, the charged f (R)-CIM AdS black hole qualitatively shares some critical behaviours with Van der Waals liquid-gas system, including the characteristic Q-Φ “oscillation” curves and G-T “swallow tail” phenomenon. However, unlike Van der Waals fluid, the Q-Φ “oscillation” arises in the range of T >Tc . As a modification theory, f (R) gravity is highly expected to find some distinctive properties from general relativity. Excitingly, differing from black holes in Einstein’s gravity, phase structures of the black holes in f (R) theory exhibit an interesting dependence on the modification parameters ξ and R0 . From the microscopic perspective, we adopted the thermodynamic geometry and discussed the Ruppeiner scalar curvature R. It turns out that both the curvature R and heat capacity CQ share the same divergence with each other. This fact implies that the information of phase transition is contained in the curvature. Furthermore, the charged f (R)-CIM AdS black hole behaves as ideal Bose gas. In detail, the result that scalar curvature R>0 and diverges to positive infinity corresponds to attractive interaction among the microscopic system. But for Van der Waals liquid-gas system, it presents a completely opposite outcome. Finally, we classified the black hole solutions into two types based on the number of phase transitions, namely, the slow and f ast black holes. 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