3 research outputs found
A tale of two superpotentials: Stability and Instability in Designer Gravity
We investigate the stability of asymptotically anti-de Sitter gravity coupled to tachyonic scalar fields with mass at or slightly above the Breitenlohner-Freedman bound. The boundary conditions in these ``designer gravity'' theories are defined in terms of an arbitrary function W. Previous work had suggested that the energy in designer gravity is bounded below if i) W has a global minimum and ii) the scalar potential admits a superpotential P. More recently, however, certain solutions were found (numerically) to violate the proposed energy bound. We resolve the discrepancy by observing that a given scalar potential can admit two possible branches of the corresponding superpotential, P_{\pm}. When there is a P_- branch, we rigorously prove a lower bound on the energy; the P_+ branch alone is not sufficient. Our numerical investigations i) confirm this picture, ii) confirm other critical aspects of the (complicated) proofs, and iii) suggest that the existence of P_- may in fact be necessary (as well as sufficient) for the energy of a designer gravity theory to be bounded below
Stability in Designer Gravity
We study the stability of designer gravity theories, in which one considers
gravity coupled to a tachyonic scalar with anti-de Sitter boundary conditions
defined by a smooth function W. We construct Hamiltonian generators of the
asymptotic symmetries using the covariant phase space method of Wald et al.and
find they differ from the spinor charges except when W=0. The positivity of the
spinor charge is used to establish a lower bound on the conserved energy of any
solution that satisfies boundary conditions for which has a global minimum.
A large class of designer gravity theories therefore have a stable ground
state, which the AdS/CFT correspondence indicates should be the lowest energy
soliton. We make progress towards proving this, by showing that minimum energy
solutions are static. The generalization of our results to designer gravity
theories in higher dimensions involving several tachyonic scalars is discussed.Comment: 29 page
Asymptotic generators of fermionic charges and boundary conditions preserving supersymmetry
We use a covariant phase space formalism to give a general prescription for
defining Hamiltonian generators of bosonic and fermionic symmetries in
diffeomorphism invariant theories, such as supergravities. A simple and general
criterion is derived for a choice of boundary condition to lead to conserved
generators of the symmetries on the phase space. In particular, this provides a
criterion for the preservation of supersymmetries. For bosonic symmetries
corresponding to diffeomorphisms, our prescription coincides with the method of
Wald et al.
We then illustrate these methods in the case of certain supergravity theories
in . In minimal AdS supergravity, the boundary conditions such that the
supercharges exist as Hamiltonian generators of supersymmetry transformations
are unique within the usual framework in which the boundary metric is fixed. In
extended AdS supergravity, or more generally in the presence
of chiral matter superfields, we find that there exist many boundary conditions
preserving supersymmetry for which corresponding generators
exist. These choices are shown to correspond to a choice of certain arbitrary
boundary ``superpotentials,'' for suitably defined ``boundary superfields.'' We
also derive corresponding formulae for the conserved bosonic charges, such as
energy, in those theories, and we argue that energy is always positive, for any
supersymmetry-preserving boundary conditions. We finally comment on the
relevance and interpretation of our results within the AdS-CFT correspondence.Comment: 45 pages, Latex, no figures, v2: extended discussion of positive
energy theorem and explicit form of fermionic generators, references adde