Physics Letters B 747 (2015) 212–216
Contents lists available at ScienceDirect
Physics Letters B
www.elsevier.com/locate/physletb
Holography, probe branes and isoperimetric inequalities
Frank Ferrari
a,∗
, Antonin Rovai
b,c
a
Service de Physique Théorique et Mathématique, Université Libre de Bruxelles and International Solvay Institutes, Campus de la Plaine,
CP
231, B-1050 Bruxelles, Belgium
b
Arnold Sommerfeld Center for Theoretical Physics, Ludwig-Maximilians-Universität München, Theresienstrasse 37, D-80333 München, Germany
c
Center for Theoretical Physics, Massachusetts Institute of Technology, Cambridge, MA 02139, USA
a r t i c l e i n f o a b s t r a c t
Article history:
Received
3 April 2015
Received
in revised form 26 May 2015
Accepted
2 June 2015
Available
online 5 June 2015
Editor:
L. Alvarez-Gaumé
In many instances of holographic correspondences between a d-dimensional boundary theory and a (d +
1)-dimensional bulk, a direct argument in the boundary theory implies that there must exist a simple
and precise relation between the Euclidean on-shell action of a (d −1)-brane probing the bulk geometry
and the Euclidean gravitational bulk action. This relation is crucial for the consistency of holography, yet
it is non-trivial from the bulk perspective. In particular, we show that it relies on a nice isoperimetric
inequality that must be satisfied in a large class of Poincaré–Einstein spaces. Remarkably, this inequality
follows from theorems by Lee and Wang.
© 2015 The Authors. 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
Consider a holographic correspondence between a (d + 1)-di-
mensional
bulk gravitational theory on a conformally compact
manifold M and a d -dimensional field theory on its compact
boundary X = ∂ M.
1
Assume that the correspondence follows by
considering the near-horizon limit of a large number N of BPS
(d −1)-branes (mentioned simply as branes in the following) [1].
The boundary field theory has N colors and can be interpreted as
living on these branes. It is then natural to study the physics as-
sociated
with probe branes in the bulk geometry. These branes are
clearly special since, in some sense, they make up the bulk holo-
graphic
space itself. Such studies have appeared many times in the
literature; particularly instructive results were discussed, for exam-
ple,
in [2].
Recently,
a precise construction of the probe brane action S
b
from the point of view of the boundary field theory was pro-
posed [3].
The main motivation in [3] is to provide purely field
theoretic tools to study holography in a wide range of models. It is
shown that the probe action naturally describes the motion of the
brane in a higher-dimensional holographic bulk space, including in
the case of the pure Yang–Mills theory where a fifth dimension
*
Corresponding author.
E-mail
addresses: frank.ferrari@ulb.ac.be (F. Ferrari), arovai@mit.edu (A. Rovai).
1
Physically relevant non-compact manifolds can usually be obtained by taking
the large volume limit of a compact manifold.
automatically emerges [3]. In particular, the details of the bulk ge-
ometry
can be read off from the probe action [4].
The
construction in [3] implies an interpretation of the probe
brane that seems to depart from the standard lore, which relates
the presence of a probe brane in the bulk to some Higgsing of
the gauge group on the boundary. Instead, the Euclidean partition
function for K probe branes in the bulk is shown in [3] to com-
pute
exactly the ratio Z
N+K
/Z
N
between the Euclidean partition
functions of the boundary theory for N + K and N colors respec-
tively,
Z
N+K
Z
N
=
D e
−S
b
()
, (1.1)
where we have denoted by the degrees of freedom living on
the brane. This point of view has many interesting consequences
and seems consistent with the notion of Highly Effective Action
described in [5], which corresponds to the special case N = K = 1.
The
aim of the present work is to understand, from the bulk
perspective, one of the simplest consequence of Eq. (1.1). Assume
that the free energy − ln Z
N
scales as N
γ
F at large N, for some ex-
ponent
γ , with corrections o(N
γ −1
) (for example, in the standard
gauge theories considered in [3], γ = 2 and the corrections are of
order O (N
0
) =o(N)). Then ln( Z
N+1
/Z
N
) =−γ N
γ −1
(F +o(1)). On
the other hand, in the large N limit, the probe brane action S
b
is
very large (for example, it is proportional to N in gauge theory).
The right-hand side of (1.1) is then dominated by configurations
minimizing S
b
. If we denote by S
∗
b
the minimum value of S
b
, we
obtain in this way ln( Z
N+1
/Z
N
) =−S
∗
b
. If, moreover, we use the
http://dx.doi.org/10.1016/j.physletb.2015.06.004
0370-2693/
© 2015 The Authors. 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
.