Physics Letters B 773 (2017) 596–600
Contents lists available at ScienceDirect
Physics Letters B
www.elsevier.com/locate/physletb
Stability of Schwarzschild singularity in non-local gravity
Gianluca Calcagni
a,∗
, Leonardo Modesto
b
a
Instituto de Estructura de la Materia, CSIC, Serrano 121, 28006 Madrid, Spain
b
Department of Physics, Southern University of Science and Technology, Shenzhen 518055, China
a r t i c l e i n f o a b s t r a c t
Article history:
Received
6 July 2017
Received
in revised form 2 September 2017
Accepted
7 September 2017
Available
online 12 September 2017
Editor:
M. Cveti
ˇ
c
In a previous work, it was shown that all Ricci-flat spacetimes are exact solutions for a large class of non-
local
gravitational theories. Here we prove that, for a subclass of non-local theories, the Schwarzschild
singularity is stable under linear perturbations. Thus, non-locality may be not enough to cure all the sin-
gularities
of general relativity. Finally, we show that the Schwarzschild solution can be generated by the
gravitational collapse of a thin shell of radiation.
© 2017 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 and main result
Although there are more theories of quantum gravity than we
can count with the fingers of our hands [1–5], all share some com-
mon
goals. First, to unify in one way or another the laws of quan-
tum
mechanics with a microscopic description of the gravitational
force. Second, to do so in a self-consistent and predictive way,
something which experts translate into the requirement of finite-
ness
or at least renormalizability. Third, to solve some problems
left open in general relativity, namely, the presence of singularities
in the interior of black holes or at the birth of the universe (big
bang). Fourth, to make testable predictions in the near future.
It
is in this spirit, but with special attention to the third point of
this agenda, that non-locality (i.e., operators with infinitely many
derivatives) was proposed as a mechanism to replace the big-
bang
singularity with a bounce [6]. Pursuing this hope, black-hole-
or
cosmology-related gravitational models with non-local opera-
tors
were built [6–28] together with full-fledged non-local classical
and quantum gravities [29–43].
1
(For gravity in a local higher-
derivative
limit, see [44].) In the majority of these works, the
typical non-local operator, inspired by what found in string field
theory, is ∼ exp(−2/M
2
), the exponential of the Laplace–Beltrami
operator 2 := ∇
μ
∇
μ
, where M is the characteristic energy scale of
the theory. Indeed, many results confirmed the role of non-locality
in the singularity-resolution mechanism [12,15,16,18,20,25], but
*
Corresponding author.
E-mail
addresses: calcagni@iem.cfmac.csic.es (G. Calcagni),
lmodesto@sustc.edu.cn (L. Modesto).
1
There is also a whole literature on cosmological and field-theory models with
non-local operators acting only on the matter sector. The reader can find more in-
formation
in the papers cited above.
others were not so optimistic. For instance, singular cosmological
solutions can still exist in certain general non-local theories [11]
and
conformal invariance may be an ingredient even more impor-
tant
than non-locality to remove infinities [11,26,43].
In
this paper, we argue that non-locality is not enough. De-
spite
positive results on the removal of the big-bang singularity,
black-hole singularities may be more difficult to tame. There are
many results about approximate singularity-free black holes in
non-local gravity [45–53], and the existence of the exact classical
Schwarzschild black-hole solution in non-local gravitational theo-
ries
was noted in [54]. Here we make one step further and show
that this solution is stable against linear perturbations. This means
that, once formed, singular spherically symmetric black holes sur-
vive
just like in general relativity [55].
The
theory and the Schwarzschild solution are briefly reviewed
in, respectively, sections 2 and 3. Stability of the solution is
discussed in section 4, while in section 5 we show that the
Schwarzschild solution can be the result of the gravitational col-
lapse
of matter. Conclusions are in section 6.
2. The theory
A general class of theories compatible with unitarity and super-
renormalizability
of finiteness has the following structure in four
dimensions:
S
g
=
2
κ
2
d
4
x
|g|
R + Rγ
0
(2)R + R
μν
γ
2
(2)R
μν
+ V
g
,
(1)
where κ
2
= 32π G and the “potential” term V
g
is at least cubic
in the curvature and at least quadratic in the Ricci tensor. In this
http://dx.doi.org/10.1016/j.physletb.2017.09.018
0370-2693/
© 2017 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
.