JCAP09(2014)038
It is easy to see [45] that, outside the thick cod-2 brane, the slope function is constant
S(
ˆ
ξ) = ±S
+
≡
¯
λ
4M
4
6
for
ˆ
ξ ≷ ±l
2
(3.7)
and is determined only by the total amount of tension
¯
λ (it is independent of how the tension
is distributed inside the brane). Ther e for e, the thin lim i t of these solutions exists and is given
by the configurations where the tension
¯
λ is perfectly localized on C
2
and the components of
the embedding function read
Z(
ˆ
ξ) = sin
¯
λ
4M
4
6
|
ˆ
ξ| Y (
ˆ
ξ) = cos
¯
λ
4M
4
6
ˆ
ξ (3.8)
Note that the normal 1-form reads
2
¯n
A
(
ˆ
ξ) =
Y
′
(
ˆ
ξ), −Z
′
(
ˆ
ξ), 0, 0, 0, 0
(3.9)
and becomes discontinuous in the thin limit. The complete 6D spacetime which corresponds
to these thin limit configurations is the product of the 4D Minkowski space and a two di-
mensional cone of deficit angle α =
¯
λ/M
4
6
. When
¯
λ → 2πM
4 −
6
the deficit angle tends to 2π,
and the 2D cone tends to a degenerate cone (a half-line). Therefore there is an upper bound
¯
λ <
¯
λ
M
≡ 2πM
4
6
on the tension which we can put on the thin cod-2 brane.
3.2 Small perturbations in the bulk- b as ed approach
As we discuss in [
45], to find c ons i st ent soluti on s in the thin limit of the ribbon brane we
need either embedding functions which are cuspy, or a bulk metric which is di sc ontinuous,
or both. This is necessary to produce a delta function divergence i n the left hand side of the
junction conditions (which balanc es the delta function dive r ge nce on the right hand si de ),
while at the same time keeping the grav i t ati onal field on the thin cod-2 brane finite (gravity
regularization).
The choice to privilege a smooth bulk metric, without constraining the form of the
embedding, or t o privilege a smooth embedding, leaving the b ul k metric free to have dis-
conti nuities, is related to adopting a bulk-based or a brane-base d point of view. We su ggest
in [
45] that the bulk-base d approach have several advantages. In fact, i t permits to iden-
tify clearly the degrees of freedom which are responsible for the singularity (the embe ddi ng
functions), separating them from the degrees of freedom which are not (the bulk metric).
Moreover, the property of gravity being finite is mirrored by the fact that all the degrees of
freedom remain continuous in the thin limit, s o the regularity properties of the solutions ar e
tight l y linked to the regularity properties of the gravitational field. In addition, the global
geometry of the thin limit configurations is more transparent in the bulk-based approach, for
example in the pure tension case the deficit angle is directly connected to the slope of the
embedding. As we shall see, there is also a more technical reason in favour of this choice:
the bulk-based approach permits us to identify clearly the convergence properties of the
perturbative degrees of freedom in the thi n limit.
For these reasons, we adopt the bulk-based approach to study small perturbations
around the pure tension solutions, following closely the analysis of [
45].
2
As we discuss in [
45], this is the choice of the normal fo r m with the correct orientation.
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