Eur. Phys. J. C (2017) 77:629
DOI 10.1140/epjc/s10052-017-5217-7
Regular Article - Theoretical Physics
Noncommutative geometry inspired black holes in Rastall gravity
Meng-Sen Ma
1,2,a
, Ren Zhao
1,b
1
Institute of Theoretical Physics, Shanxi Datong University, Datong 037009, China
2
Department of Physics, Shanxi Datong University, Datong 037009, China
Received: 23 June 2017 / Accepted: 11 September 2017 / Published online: 20 September 2017
© The Author(s) 2017. This article is an open access publication
Abstract Under two different metric ansatzes, the non-
commutative geometry inspired black holes (NCBH) in the
framework of Rastall gravity are derived and analyzed. We
consider the fluid-type matter with the Gaussian-distribution
smeared mass density. Taking a Schwarzschild-like met-
ric ansatz, it is shown that the noncommutative geometry
inspired Schwarzschild black hole (NCSBH) in Rastall grav-
ity, unlike its counterpart in general relativity (GR), is not a
regular black hole. It has at most one event horizon. After
showing a finite maximal temperature, the black hole will
leave behind a point-like massive remnant at zero tempera-
ture. Considering a more general metric ansatz and a special
equation of state of the matter, we also find a regular NCBH
in Rastall gravity, which has a similar geometric structure
and temperature to that of NCSBH in GR.
Contents
1 Introduction ..................... 1
2 Rastall gravity .................... 2
3 Noncommutative geometry inspired black holes .. 2
4 Conclusion and discussion .............. 5
References ........................ 6
1 Introduction
General relativity (GR), which has many successful predic-
tions, is the most popular theory of gravity. The Einstein field
equation can be derived via the variational principle from a
total action (the gravitational action plus the matter action).
Due to the Bianchi identity, the covariant conservation of
the energy-momentum tensor, namely T
μν
;μ
= 0, is natu-
rally satisfied. In GR and many other theories of gravity, it
a
e-mail: mengsenma@gmail.com; ms_ma@sxdtdx.edu.cn
b
e-mail: zhao2969@sina.com
is assumed that the geometry and the matter fields are cou-
pled to each other in a minimal way. It has been shown that
when the geometry and the matter fields are coupled in a
non-minimal way, such as the direct curvature–matter cou-
pling, the covariant conservation of the energy-momentum
tensor may be violated [1–9]. In fact, the covariant conser-
vation of the energy-momentum tensor is just an assump-
tion and has not been generally tested by observation. Thus,
by relaxing the condition of covariant conservation of the
energy-momentum tensor, Rastall proposed a phenomeno-
logical gravitational model by considering T
μν
;μ
∝ R
;ν
with
R the Ricci scalar [10]. Rastall gravity has been employed to
study the cosmological consequences [11–13]. Various exact
solutions of Rastall gravity have also been derived in [14–18].
In Rastall gravity, the energy-momentum tensor can also
be derived from the Lagrangian of matter fields, namely
T
μν
=−2
δL
m
δg
μν
+ g
μν
L
m
. However, there is no the corre-
sponding equation of motion for the matter fields due to the
lack of a total action for Rastall gravity. For fluid-type mat-
ter, it is relatively simple to find the exact solution in Rastall
gravity because in this case one does not need to consider the
equation of motion of the matter fields, but only the equation
of state. In this paper, we will consider this kind of matter.
By considering the noncommutativity of spacetime, Nicol-
ini et al. studied several kinds of noncommutative geometry
inspired black holes (NCBH) in GR [19–23]. Noncommu-
tativity eliminates point-like structures in favor of smeared
objects. The conventional mass density of a point-like source
can be replaced by a smeared, Gaussian distribution:
ρ =
M
(4πθ)
3/2
exp
−
r
2
4θ
, (1.1)
where θ is a constant with dimension of length squared and
represents the noncommutativity of spacetime. The effect of
noncommutative geometry on gravity has been contained in
the matter source. As stated in [19], “the noncommutativity
is an intrinsic property of the manifold itself, rather than a
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