Physics Letters B 805 (2020) 135416
Contents lists available at ScienceDirect
Physics Letters B
www.elsevier.com/locate/physletb
The geometry of RN-AdS fluids
Joy Das Bairagya, Kunal Pal, Kuntal Pal, Tapobrata Sarkar
∗
Department of Physics, Indian Institute of Technology, Kanpur 208016, India
a r t i c l e i n f o a b s t r a c t
Article history:
Received 20 December 2019
Accepted 5 April 2020
Available online 8 April 2020
Editor: N. Lambert
We establish the parameter space geometry of a fluid system characterized by two constants, whose
equation of state mimics that of the RN-AdS black hole. We call this the RN-AdS fluid. We study the
scalar curvature on the parameter space of this system, and show its equivalence with the RN-AdS black
hole, in the limit of vanishing specific heat at constant volume. Further, an analytical construction of the
Widom line is established. We also numerically study the behavior of geodesics on the parameter space
of the fluid, and find a geometric scaling relation near its second order critical point.
© 2020 The Author(s). Published by Elsevier B.V. This is an open access article under the CC BY license
(http://creativecommons.org/licenses/by/4.0/). Funded by SCOAP
3
.
1. Introduction
Black holes are singular solutions of general relativity that may
arise as the end stages of gravitational collapse. Understanding the
physics of black holes continues to be the focus of much attention.
With the nature of black hole microstates being elusive, a popu-
lar
line of research is to understand the macroscopic properties of
black holes, for example their thermodynamic properties, as these
often provide useful insights into the underlying coarse grained
structure of black holes. More recently, geometric methods have
been applied to such studies, and it has been claimed in the litera-
ture
that the nature of interactions between black hole microstates
can possibly be elucidated by these analyses.
The four laws of black hole mechanics which were formulated
in the early 70’s, are formally identical with the four laws of ther-
modynamics.
For black holes having electric charge Q and angu-
lar
momentum J with the corresponding potentials being and
respectively, these are given as: (i) The surface gravity κ on
the horizon of a stationary black hole of area A is a constant.
(ii) The
change in energy E of a black hole can be expressed as
dE = κdA/8π + dQ + dJ. (iii) The horizon area can never de-
crease
with time, i.e. dA/dt ≥ 0. (iv) It is impossible to have κ = 0.
Hawking formalized this by showing that black holes can radiate,
and that such radiation will have a temperature T = κ/2π . Then,
the four laws above are precisely the laws of thermodynamics,
with the entropy of the black hole identified with A/4. This for-
*
Corresponding author.
E-mail addresses: joydas@iitk.ac.in (J.D. Bairagya), kunalpal@iitk.ac.in (K. Pal),
kuntal@iitk.ac.in (K. Pal),
tapo@iitk.ac.in (T. Sarkar).
mal correspondence has been studied extremely well, for almost
five decades now.
Note that a pressure term is missing in the above discussion.
For a long time it was believed that a pressure (and a volume)
cannot be associated to black hole thermodynamics. This however
changed a few years back with the proposal of Kastor, Ray and
Traschen [1] that a (varying) cosmological constant can be identi-
fied
with the pressure of a black hole, with the conjugate volume
associated with the volume of the event horizon. In this formal-
ism,
the black hole mass has to be identified with the enthalpy
of the system, and this can be shown to satisfy the correspond-
ing
Smarr relation. This is popularly termed as the extended phase
space formalism in black hole thermodynamics. Charged black
holes appearing in theories of gravity with a (negative) cosmolog-
ical
constant i.e. the Reissner-Nordstrom-anti-de-Sitter (RN-AdS)
black holes, where this is primarily interesting will be the focus
of this work. Thermodynamics of RN-AdS black holes and their re-
semblance
with van der Waals (vdW) systems were analyzed in
the pioneering works of Chamblin, Emparan, Johnson and Myers
[2], [3]. In these works, an equation of state that gave the temper-
ature
as a function of the charge and the electric potential, was
used to study this behavior. In the context of thermodynamics of
the extended phase space, the results of [1]were used by Kubiznak
and Mann [4]to derive a relation between the pressure P
BH
and
its volume V of the RN-AdS black hole of charge Q and horizon
radius r
+
, via its Hawking temperature T
BH
. This reads
P
BH
=
k
B
T
BH
2l
2
p
r
+
−
¯
hc
8πl
2
p
r
2
+
+
¯
hcQ
2
8πl
2
p
r
4
+
, r
+
=
3V
4π
1
3
, (1)
https://doi.org/10.1016/j.physletb.2020.135416
0370-2693/© 2020 The Author(s). Published by Elsevier B.V. This is an open access article under the CC BY license (http://creativecommons.org/licenses/by/4.0/). Funded by
SCOAP
3
.