We start by providing other examples of theories leading to regular billiards, focusing first
on pure gravity in any number of (
) spacetime dimensions. In this case, there are
scale factors
and the relevant walls are the symmetry walls, Equation (2.48
),
Accordingly, in the case of pure gravity in any number of spacetime dimensions, one finds also that the
billiard region is regular. This provides new examples of Coxeter billiards, with Coxeter groups ,
which are also Kac–Moody billiards since the Coxeter groups are the Weyl groups of the Kac–Moody
algebras
.
Let us review the conditions that must be fulfilled in order to get a Kac–Moody billiard and let us
emphasize how restrictive these conditions are. The billiard region computed from any theory coupled to
gravity with dilatons in
dimensions always defines a convex polyhedron in a
-dimensional hyperbolic space
. In the general case, the dihedral angles between
adjacent faces of
can take arbitrary continuous values, which depend on the dilaton couplings, the
spacetime dimensions and the ranks of the
-forms involved. However, only if the dihedral angles are
integer submultiples of
(meaning of the form
for
) do the reflections in
the faces of
define a Coxeter group. In this special case the polyhedron is called a
Coxeter polyhedron. This Coxeter group is then a (discrete) subgroup of the isometry group of
.
In order for the billiard region to be identifiable with the fundamental Weyl chamber of a
Kac–Moody algebra, the Coxeter polyhedron should be a simplex, i.e., bounded by
walls in a
-dimensional space. In general, the Coxeter polyhedron need not be a
simplex.
There is one additional condition. The angle between two adjacent faces
and
is given, in
terms of the Coxeter exponents, by
These conditions are very restrictive and hence gravitational theories which can be mapped to a Kac–Moody algebra in the BKL-limit are rare.
Consider for instance the action (2.1) for gravity coupled to a single three-form in
spacetime
dimensions. We assume
since in lower dimensions the 3-form is equivalent to a scalar (
) or
has no degree of freedom (
).
Theorem: Whenever a -form (
) is present, the curvature wall is subdominant as it can be
expressed as a linear combination with positive coefficients of the electric and magnetic walls of the
-forms. (These walls are all listed in Section 2.5.)
Proof: The dominant electric wall is (assuming the presence of a dilaton)
while one of the magnetic wall reads so that the dominant curvature wall is just the sum It follows that in the case of gravity coupled to a single three-form in spacetime dimensions,
the relevant walls are the symmetry walls, Equation (2.48
),
However, it is only for that the billiard is a Coxeter billiard. In all the other spacetime
dimensions, the angle between the relevant
-form wall and the symmetry wall that does
not intersect it orthogonally is not an integer submultiple of
. More precisely, the angle
between
is easily verified to be an integer submultiple of only for
, for which it is equal to
.
From the point of view of the regularity of the billiard, the spacetime dimension is thus
privileged. This is of course also the dimension privileged by supersymmetry. It is quite intriguing that
considerations a priori quite different (billiard regularity on the one hand, supersymmetry on the other
hand) lead to the same conclusion that the gravity-3-form system is quite special in
spacetime
dimensions.
For completeness, we here present the wall system relevant for the special case of . We obtain
ten dominant wall forms, which we rename
,
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