Physics Letters B 779 (2018) 342–347
Contents lists available at ScienceDirect
Physics Letters B
www.elsevier.com/locate/physletb
Stability issues of black hole in non-local gravity
Yun Soo Myung
a,∗
, Young-Jai Park
b
a
Institute of Basic Sciences and Department of Computer Simulation, Inje University, Gimhae 50834, Republic of Korea
b
Department of Physics, Sogang University, Seoul 04107, Republic of Korea
a r t i c l e i n f o a b s t r a c t
Article history:
Received
22 November 2017
Received
in revised form 11 February 2018
Accepted
12 February 2018
Available
online 14 February 2018
Editor:
N. Lambert
We discuss stability issues of Schwarzschild black hole in non-local gravity. It is shown that the stability
analysis of black hole for the unitary and renormalizable non-local gravity with γ
2
=−2γ
0
cannot
be performed in the Lichnerowicz operator approach. On the other hand, for the unitary and non-
renormalizable
case with γ
2
= 0, the black hole is stable against the metric perturbations. For non-unitary
and renormalizable local gravity with γ
2
=−2γ
0
= const (fourth-order gravity), the small black holes are
unstable against the metric perturbations. This implies that what makes the problem difficult in stability
analysis of black hole is the simultaneous requirement of unitarity and renormalizability around the
Minkowski spacetime.
© 2018 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
It turns out that the infinite derivative gravity (non-local grav-
ity)
is ghost-free and renormalizable around the Minkowski space-
time
background when one chooses the exponential form of an
entire function [1,2]. We note that renormalizability can be easily
checked by showing the finiteness of the Newtonian potential at
the origin from the propagator [3–6].
On
the other hand, all Ricci-flat spacetimes including Schwarzs-
child
black hole are exact solutions for non-local gravitational the-
ories [7].
In order to check that the Schwarzschild black hole ex-
ists
really in the unitary (ghost-free) and renormalizable non-local
gravity, one has to perform the stability analysis of the black hole.
If the black hole solution passes the stability test, one may save
that black hole. Recently, it has shown that the Schwarzschild black
hole is stable against linear perturbations for a subclass of uni-
tary
non-local gravity with γ
2
= 0[8]. However, this case is not a
renormalizable gravity around Minkowski spacetime. Although the
non-locality (operators with infinitely many derivatives) is needed
to have a ghost-free and renormalizable gravity, the presence of
higher derivative gravity may make the black hole unsustainable.
This implies that non-locality may not be a good tool to cure the
black hole solutions.
In
this work, we wish to discuss stability issues of Schwarzs-
child
black hole in non-local gravity. We derive a linearized equa-
*
Corresponding author.
E-mail
addresses: ysmyung @inje .ac .kr (Y.S. Myung), yjpark @sogang .ac .kr
(Y.-J. Park).
tion (21) which governs the stability of black hole for the unitary
and renormalizable non-local gravity with γ
2
=−2γ
0
. However,
we could not perform the stability analysis of black hole in the
Lichnerowicz operator approach because the appearance of en-
tire
function in the linearized equation (34). On the other hand,
for a unitary and non-renormalizable gravity with γ
2
= 0, it has
shown that the black hole is stable against the metric perturba-
tions.
This is possible because this case reduces to the Einstein
gravity or f (R) gravity [9], which are surely independent of the
entire function [8]. Next, for the non-unitary and renormalizable
local gravity with γ
2
=−2γ
0
= const (fourth-order gravity) [10],
using the Gregory–Laflamme black string instability [11], the small
black holes are unstable against the Ricci tensor perturbations. This
contrasts to the conventional stability analysis of black hole in
Einstein gravity or f (R) gravity. It implies that the simultaneous
requirement for unitarity and renormalizability makes the stability
analysis difficult.
2. Non-local gravity
A non-local gravity in four dimensions is generally defined
by [8]
S
g
=
2
κ
2
d
4
x
|g|
R + Rγ
0
()R + R
μν
γ
2
()R
μν
+ V
g
,
(1)
where κ
2
= 32π G, the d’Alembertian = g
μν
∇
μ
∇
ν
, and the po-
tential
term V
g
is at least cubic in the curvature and at least
quadratic in the Ricci tensor. Hereafter, we choose V
g
= 0for
simplicity. The non-local gravity with γ
2
= 0is unitary and non-
https://doi.org/10.1016/j.physletb.2018.02.023
0370-2693/
© 2018 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
.