# Some remarks on the thermodynamics of charged AdS black holes with cloud of strings and quintessence

- 94 Downloads

## Abstract

In this paper we investigate the combined effects of the cloud of strings and quintessence which surround a Reissner–Nordstrom AdS black hole on its thermodynamics. We treat the cosmological constant as a thermodynamic pressure whose conjugate variable is the volume. It is shown how the cloud of strings affect the thermodynamic variables, as for example, temperature, entropy, internal energy and heat capacity. The critical expressions for temperature, pressure and specific volume are obtained and the critical exponents calculated. Concerning the heat engine, we calculate its efficiency and show how this quantity is influenced by the presence of the cloud of strings and quintessence.

## 1 Introduction

Studies concerning a formalism to obtain the solutions of the Einstein equations corresponding to a black hole surrounded by a cloud of strings were initiated by Letelier [1]. These studies were motivated, originally, by the fact that the nature can be represented, in principle, by a collection of extended objects, like one-dimensional strings, rather than of point particles, in a more appropriate way.Among various results obtained by Letelier [1], as for example, for plane, cylindrical and spherical symmetries, the one of our interest concerns the spherical symmetry and corresponds to a generalization of the Schwarzschild solution, more specifically, corresponds to the Schwarzschild black hole surrounded by a spherically symmetric cloud of strings [1]. In other words, it corresponds to a modification of Schwarzschild solution by taking a cloud of strings as an additional source of the gravitational field. Concerning this solution, if we consider the mass of the central body (black hole) equal to zero, we have only a cloud of strings, in which case we have no horizon, but just a naked singularity at \(r=0\). Returning to the combined sources, black hole and cloud of strings, we can conclude that the presence of the cloud of strings produces an effect of global origin associated with a solid deficit angle which depends on the parameter that codifies the presence of the cloud, similarly to what happens in the spacetime of a global monopole [2]. Additionally, the presence of the cloud produces a profound influence on the horizon structure, enlarging the radius of the horizon event as compared with the Schwarzschild radius. Thus, the effects of global origin associated with the solid deficit angle, which is determined by the presence of the cloud of strings, will produce possible astrophysical consequences, which justify the investigation of the role played by the cloud of strings in this scenario.

The astronomical observations confirming that the universe experiences an accelerated expansion constitutes one of the most important discoveries of the era of the observational cosmology [3]. This fact implies that at large scale structure, there exists a gravitationally repulsive energy component which indicates the existence of a negative pressure. One of the possible origins of this negative pressure is associated to a fluid that permeates the universe, with energy density constant in time and spatially uniform. In this case, the pressure is proportional to the energy density, with the proportionality constant assuming values in the interval (− 1, − 1/3), in order to have the desired cosmic acceleration. This kind of scenario corresponds to what is called quintessence or quintessence dark energy [4]. This permeates the universe and consequently surrounds a black hole and certainly produces profound astrophysical consequences. Therefore, it is important to know the solution of the Einstein equations for black holes surrounded by quintessence as was done by Kiselev [4].

The black hole surrounded by quintessence, in the way obtained by Kiselev [4] has been extensively studied in the literature even though this solution is not free of problems that deserve further discussion. We can cite, for example, the correspondence of the Keselev stress-energy tensor with the scalar model for quintessence and the fact that, if we consider the black hole-free solution, the metric is not homogeneous, as expected for cosmological models.

Black hole thermodynamics is one of the topics in gravitational physics which becomes an active and important area of research since the works of Bekenstein [5, 6] and Hawking [7, 8, 9] in the 1970’s up to now, in part, due to the window it opens to understand the connection between quantum mechanics and gravity, providing a deep insight in which concerns the understanding of the nature of quantum gravity.

The study of the thermodynamical features of charged AdS black holes showed that there exists a similarity between these astrophysical objects and liquid-gas systems. In particular, Reissner–Nordstrom AdS black holes present first-order phase transitions whose critical behavior is similar to the ones experienced by a system in condensed matter [10, 11]. Investigations concerning this topic have enhanced after the suggestion that the cosmological constant should be treated as the thermodynamic variable [12, 13, 14, 15, 16], which is interpreted as the thermodynamic pressure with the thermodynamic volume being the conjugate quantity. Taking this fact into account, more attention has been done to the study of black hole thermodynamics of AdS black holes [15, 16, 17, 18, 19, 20, 21, 22, 23, 24, 25, 26, 27, 28], especially with respect to charged AdS black holes by the reason that there is an analogy between these objects and liquid-gas systems, phase transitions appear [17] and these must be investigated in order to find the whole phase structure, through the determination of the critical points associated with the thermodynamical quantities, but now, taking into account not only the quintessence, but also the presence of a cloud of strings.

Another idea that appears in the framework of black hole thermodynamics is related to the heat engine [29]. In this case, the efficiency can be determined analogously to a traditional heat engine [29, 30, 31, 32, 33]. Taking the pressure and the volume as dynamical variables, it is possible to extract mechanical work by using the *pdV* term. Similarly to what happens in classical thermodynamics, a heat engine is defined by a closed cycle in the pV plane [29]. In this scenario, the efficiency of the heat engines has been analyzed in different black hole spacetimes [26, 29, 34], among others.

In this paper, we analyze the thermodynamics of a Reissner–Nordstrom AdS black hole surrounded by a cloud of strings and quintessence, by calculating different thermodynamic variables, determining their critical values as well as the efficiency of a heat engine.

It is already known that a cloud of strings decreases the real and imaginary parts of the quasinormal frequencies of a black hole surrounded by quintessence, approaching the system from the astrophysical scenario [35].

In the present work, it will be shown that the presence of the cloud of strings gives rise to important issues in the thermodynamics of the black hole, if we compare with the one surrounded only by quintessence [36]. The cloud of strings parameter decreases the thermodynamic quantities as temperature, entropy and internal energy of the black hole. Furthermore, it increases the efficiency of the black hole considered as a heat engine. These important effects are extensively discussed in this paper.

This paper is organized as follows. In Sect. 2, we present the metric and the event horizons. In Sect. 3, we discuss the thermodynamics. In Sect. 4, the critical values for some thermodynamic variables and critical exponents are determined. In Sect. 5, we discuss how the black hole considered can be treated as a heat engine. Finally, in Sect. 6, we present the concluding remarks.

## 2 The metric and event horizons

In this section, we obtain the metric corresponding to the spacetime generated by a charged static black hole with cosmological constant and surrounded by a cloud of strings and quintessence.

*M*are normalization constants, which should be related to the quintessence and mass, respectively. With this solution, the spacetime metric is given by

*M*, surrounded by quintessence, with density \(\rho _q\) given by

*Q*, as well as, the cosmological constant. In this case, Eq. (5) turns into [4, 26, 36, 37]

*a*is an integration constant which is related to the presence of the cloud of strings. Using this mass–energy configuration, Letelier determined the spacetime metric of a black hole surrounded by a cloud of strings, which is written as [1]

*f*(

*r*) is shown for different values of the parameters

*a*and \(\alpha \), which codify the presence of the cloud of strings and the quintessence, respectively.

## 3 Black hole thermodynamics

In this section, we investigate the black hole thermodynamics, analyzing the thermodynamic potentials and intensive parameters.

*T*is the Hawking temperature [8, 9], whose behavior as a function of entropy is shown in Fig. 3, and is given by

*V*, they are obtained by

*V*is constant,

*S*is also constant, which means that, for this black hole, isochoric processes are also adiabatic ones, due to the relations between the entropy and the volume.

*a*, except for \(a = 0.9\), in which case this transition is suppressed.

*p*and

*V*, we find the equation of state

*p*x

*V*are shown in Figs. 5, 6 and 7.

*a*and \(\alpha \), are extensive thermodynamic parameters [26]. As a consequence, the first law of thermodynamics must be modified to

*a*, respectively.

## 4 Critical thermodynamic variables

In this section, let us investigate the critical thermodynamic variables and determine the critical exponents.

*a*, are represented in Tables 1, 2 and 3. The diagram

*p*–

*V*represented in Fig. 6 shows the critical behavior for different values of the parameter

*a*. In this case, the critical value of the temperature, \(T_c\), is 0.14472.

Behaviour of the critical parameters \(v_c\), \(T_c\), \(p_c\) and \(\frac{p_c v_c}{T_c}\) for different values of *a* and for \(Q=1\), \(\alpha = 0.01\) and \(\omega _q = -2/3\)

| \(v_c\) | \(T_c\) | \(p_c\) | \(\frac{p_c v_c}{T_c}\) |
---|---|---|---|---|

0 | 4.89898 | 0.18647 | 0.00332 | 0.08711 |

0.05 | 5.15682 | 0.16389 | 0.00298 | 0.09392 |

0.1 | 5.44331 | 0.14312 | 0.00266 | 0.10112 |

0.15 | 5.76350 | 0.12409 | 0.00234 | 0.10876 |

0.2 | 6.12372 | 0.10673 | 0.00203 | 0.11688 |

0.25 | 6.53197 | 0.0097 | 0.00175 | 0.12555 |

Behaviour of the critical parameters \(v_c\), \(T_c\), \(p_c\) e \(\frac{p_c v_c}{T_c}\) for different values of \(\alpha \) and for \(Q=1\), \(a = 0.1\) and \(\omega _q = -2/3\)

\(\alpha \) | \(v_c\) | \(T_c\) | \(p_c\) | \(\frac{p_c v_c}{T_c}\) |
---|---|---|---|---|

0.01 | 5.44331 | 0.14312 | 0.00266 | 0.10112 |

0.05 | 5.44331 | 0.13676 | 0.00266 | 0.10583 |

0.1 | 5.44331 | 0.12880 | 0.00266 | 0.11237 |

0.2 | 5.44331 | 0.11288 | 0.00266 | 0.12821 |

0.3 | 5.44331 | 0.09697 | 0.00266 | 0.14926 |

Behaviour of the critical parameters \(v_c\), \(T_c\), \(p_c\) e \(\frac{p_c v_c}{T_c}\) for different values of \(\alpha \) and for \(Q=1\), \(a = 0.25\) and \(\omega _q = -2/3\)

\(\alpha \) | \(v_c\) | \(T_c\) | \(p_c\) | \(\frac{p_c v_c}{T_c}\) |
---|---|---|---|---|

0.01 | 6.53197 | 0.09097 | 0.00154 | 0.11048 |

0.05 | 6.53197 | 0.08461 | 0.00154 | 0.11880 |

0.1 | 6.53197 | 0.07665 | 0.00154 | 0.13113 |

0.2 | 6.53197 | 0.06073 | 0.00154 | 0.16550 |

0.3 | 6.53197 | 0.04482 | 0.00154 | 0.22427 |

*l*, for a small one,

*s*, and that, during the phase transition, the pressure is kept constant, Li [36] verified that

From these results we conclude that the critical exponents are exactly the same obtained for a Reissner–Nordström-AdS black hole surrounded by quintessence [36] Thus, the presence of the cloud of strings does not modify the values of the critical exponents.

## 5 Black hole as heat engine

*W*), transferring the residual heat (\(Q_C\)) to a cold reservoir. The efficiency of a heat engine (\(\eta \)) is, thus, defined as the ratio between the work it produces and the heat it receives, which is expressed by

*W*, being given exactly by the area of the cycle in the

*p*–

*V*plane. As a black hole can be considered a heat engine with the thermodynamic potentials determined in the last section, in what follows we will consider that it operates in two different cycles.

*S*constant) are also isochoric and the cycle represented in Fig. 10 (in which \(1 \rightarrow 2\) and \(3 \rightarrow 4\) are isothermic processes) will produce the maximal efficiency.

*a*.

Note that, when the parameter *a* increases, the efficiency increases as shown in Fig. 11. On the other hand, Fig. 12 show us that, when *a* increases, the ratio between the efficiency of the engine and the maximal efficiency decreases.

## 6 Concluding remarks

We considered the cosmological constant as the thermodynamic pressure and analyzed the thermodynamics of the Reissner–Nordstrom AdS black hole surrounded by a cloud of strings and quintessence, for the specific case with \(\omega _q= -2/3\), and obtained analytic expressions for the thermodynamical quantities as well as for their values in the critical points. The thermodynamic quantities as temperature, entropy, internal energy, and heat capacity are analogous to the corresponding expressions when only the quintessence is present [36]. In fact, the role played by the cloud of strings is to decrease these quantities as compared to both cases, namely, when only the quintessence is present.

These effects due to the cloud of strings are shown in Fig. 2, for the mass parameter, and in Fig 3, for the Hawking temperature. Otherwise, the analytic expression corresponding to the internal energy, given by Eq. (29), and the one corresponding to the heat capacity at constant pressure, given by Eq. (30), show us that the effect of the cloud of strings is to decrease the values of these quantities as compared with the case in which only the quintessence is present [36].

In which concerns the critical Hawking temperature, critical pressure and critical specific volume, given by Eqs. (41), (42) and (43), respectively, they are affected by the presence of the cloud of strings, which decreases the values of these quantities when compared with the case where only the quintessence is present. Note that, when the cloud of strings is absent, the results coincide with the ones already obtained in the literature [36]. It is worth calling attention to the fact that the critical specific volume, \(v_c\), increases when the parameter which codifies the presence of the cloud of strings, *a*, increases, as shown in Table 1. On the other hand, when *a* increases, the critical temperature, \(T_c\), and the critical pressure, \(p_c\), decreases, as shown in Table 1.

When we consider the effect of the normalization factor, as shown in Tables 2 and 3, we conclude that in the presence of the cloud of strings, for the values assumed for this factor, the critical specific volume and the critical pressure are, approximately, constants, while the critical temperature varies. Therefore, the ration \(p_c v_c/T_c\) is not constant, but depends on the normalization factor, \(\alpha \), as well as on the parameter \(\omega _q\), which, in this paper, we fixed such that \(\omega _q = -2/3\). It is worth calling attention to the fact that the critical exponents have the same values as the ones obtained when only the quintessence is present [36], which means that the presence of the cloud of strings does not affect these parameters.

As to the heat engine, we investigated the effects of the cloud of strings, for fixed \(\omega _q\), on its efficiency. The efficiency, as shown in Fig. 11, increases when the parameter *a* increases, for fixed value of \(\omega _q\). Thus, the presence of the cloud of strings improves the heat engine efficiency which will increases as the parameter *a* grows. On the other hand, the ratio \(\eta /\eta _c\) decreases with the parameter *a* which codifies the presence of the cloud of strings.

The results obtained, in fact, show us how the thermodynamics of the system under consideration is affected by the presence of the cloud of strings as well as by the quintessence.

## Notes

### Acknowledgements

V. B. Bezerra is partially supported by Conselho Nacional de Desenvolvimento Científico e Tecnológico (CNPq) through the research Project no. 305835/2016-5.

## References

- 1.P.S. Letelier, Phys. Rev. D
**20**(6), 1294 (1979)ADSMathSciNetCrossRefGoogle Scholar - 2.M. Barriola, A. Vilenkin, Phys. Rev. Lett.
**63**(4), 341 (1989)ADSCrossRefGoogle Scholar - 3.S. Perlmutter, G. Aldering, G. Goldhaber, R. Knop, P. Nugent, P. Castro, S. Deustua, S. Fabbro, A. Goobar, D.E. Groom et al., Astrophys. J.
**517**(2), 565 (1999)ADSCrossRefGoogle Scholar - 4.V.V. Kiselev, Class. Quantum Grav.
**20**(6), 1187 (2003)ADSCrossRefGoogle Scholar - 5.J.D. Bekenstein, Lett. Nuovo Cimento (1971–1985)
**4**(15), 737 (1972)ADSCrossRefGoogle Scholar - 6.J.D. Bekenstein, Phys. Rev. D
**7**(8), 2333 (1973)ADSMathSciNetCrossRefGoogle Scholar - 7.S.W. Hawking, Nature
**248**(5443), 30 (1974)ADSCrossRefGoogle Scholar - 8.S.W. Hawking, Comm. Math. Phys.
**43**(3), 199 (1975)ADSMathSciNetCrossRefGoogle Scholar - 9.S.W. Hawking, Phys. Rev. D
**13**(2), 191 (1976)ADSCrossRefGoogle Scholar - 10.A. Chamblin, R. Emparan, C.V. Johnson, R.C. Myers, Phys. Rev. D
**60**(6), 064018 (1999)ADSMathSciNetCrossRefGoogle Scholar - 11.A. Chamblin, R. Emparan, C.V. Johnson, R.C. Myers, Phys. Rev. D
**60**(10), 104026 (1999)ADSMathSciNetCrossRefGoogle Scholar - 12.M.M. Caldarelli, G. Cognola, D. Klemm, Class. Quantum Grav.
**17**(2), 399 (2000)ADSCrossRefGoogle Scholar - 13.W. Shuang, W. Shuang-Qing, X. Fei, D. Lin, Chin. Phys. Lett.
**23**(5), 1096 (2006)ADSCrossRefGoogle Scholar - 14.Y. Sekiwa, Phys. Rev. D
**73**(8), 084009 (2006)ADSMathSciNetCrossRefGoogle Scholar - 15.D. Kastor, S. Ray, J. Traschen, Class. Quantum Grav.
**26**(19), 195011 (2009)ADSCrossRefGoogle Scholar - 16.B.P. Dolan, Class. Quantum Grav.
**28**(12), 125020 (2011)ADSCrossRefGoogle Scholar - 17.D. Kubizňák, R.B. Mann, J. High Energy Phys.
**2012**(7), 33 (2012)ADSCrossRefGoogle Scholar - 18.R.G. Cai, L.M. Cao, L. Li, R.Q. Yang, J. High Energy Phys.
**2013**(9), 5 (2013)CrossRefGoogle Scholar - 19.R. Banerjee, D. Roychowdhury, Phys. Rev. D
**85**(10), 104043 (2012)ADSCrossRefGoogle Scholar - 20.H. Liu, X.H. Meng, Mod. Phys. Lett. A
**31**(37), 1650199 (2016)ADSCrossRefGoogle Scholar - 21.D.C. Zou, S.J. Zhang, B. Wang, Phys. Rev. D
**89**(4), 044002 (2014)ADSCrossRefGoogle Scholar - 22.B.P. Dolan, Phys. Rev. D
**84**(12), 127503 (2011)ADSCrossRefGoogle Scholar - 23.M. Cvetič, G. Gibbons, D. Kubizňák, C. Pope, Phys. Rev. D
**84**(2), 024037 (2011)ADSCrossRefGoogle Scholar - 24.E. Spallucci, A. Smailagic, Phys. Lett. B
**723**(4–5), 436 (2013)ADSMathSciNetCrossRefGoogle Scholar - 25.S. Hendi, R. Mann, S. Panahiyan, B.E. Panah, Phys. Rev. D
**95**(2), 021501 (2017)ADSMathSciNetCrossRefGoogle Scholar - 26.H. Liu, X.H. Meng, Eur. Phys. J. C
**77**(8), 556 (2017)ADSCrossRefGoogle Scholar - 27.J.X. Mo, W.B. Liu, Eur. Phys. J. C
**74**(4), 2836 (2014)ADSCrossRefGoogle Scholar - 28.R. Zhao, H.H. Zhao, M.S. Ma, L.C. Zhang, Eur. Phys. J. C
**73**(12), 2645 (2013)ADSCrossRefGoogle Scholar - 29.C.V. Johnson, Class. Quantum Grav.
**31**(20), 205002 (2014)ADSCrossRefGoogle Scholar - 30.J.X. Mo, F. Liang, G.Q. Li, J. High Energy Phys.
**2017**(3), 10 (2017)CrossRefGoogle Scholar - 31.M.R. Setare, H. Adami, Gen. Relativ. Grav.
**47**(11), 133 (2015)ADSCrossRefGoogle Scholar - 32.C.V. Johnson, Entropy
**18**(4), 120 (2016)ADSCrossRefGoogle Scholar - 33.J. Sadeghi, K. Jafarzade. arXiv preprint. arXiv:1504.07744 (2015)
- 34.C.V. Johnson, Class. Quantum Grav.
**33**(21), 215009 (2016)ADSCrossRefGoogle Scholar - 35.J. Toledo, V. Bezerra, Int. J. Mod. Phys. D
**28**, 1950023 (2019)ADSCrossRefGoogle Scholar - 36.G.Q. Li, Phys. Lett. B
**735**, 256 (2014)ADSCrossRefGoogle Scholar - 37.W. Xu, Y. Wu, EPL (Europhys. Lett.)
**121**(4), 40001 (2018)ADSCrossRefGoogle Scholar - 38.R.A. Hennigar, F. McCarthy, A. Ballon, R.B. Mann, Class. Quantum Grav.
**34**(17), 175005 (2017)ADSCrossRefGoogle Scholar

## Copyright information

**Open Access**This article is distributed under the terms of the Creative Commons Attribution 4.0 International License (http://creativecommons.org/licenses/by/4.0/), which permits unrestricted use, distribution, and reproduction in any medium, provided you give appropriate credit to the original author(s) and the source, provide a link to the Creative Commons license, and indicate if changes were made.

Funded by SCOAP^{3}