163,571 research outputs found
A Note on Topological M5-branes and String-Fivebrane Duality
We derive the stability conditions for the M5-brane in topological M-theory
using kappa-symmetry. The non-linearly self-dual 3-form on the world-volume is
necessarily non-vanishing, as is the case also for the 2-form field strengths
on coisotropic branes in topological string theory. It is demonstrated that the
self-duality is consistent with the stability conditions, which are solved
locally in terms of a tensor in the representation 6 of SU(3) in G_2. The
double dimensional reduction of the M5-brane is the D4-brane, and its direct
reduction is an NS5-brane. We show that the equation of motion for the 3-form
on the NS5-brane wrapping a Calabi-Yau space is exactly the Kodaira-Spencer
equation, providing support for a string-fivebrane duality in topological
string theory.Comment: 11 pp, plain te
Stable Exact Solutions in Cosmological Models with Two Scalar Fields
The stability of isotropic cosmological solutions for two-field models in the
Bianchi I metric is considered. We prove that the sufficient conditions for the
Lyapunov stability in the Friedmann-Robertson-Walker metric provide the
stability with respect to anisotropic perturbations in the Bianchi I metric and
with respect to the cold dark matter energy density fluctuations. Sufficient
conditions for the Lyapunov stability of the isotropic fixed points of the
system of the Einstein equations have been found. We use the superpotential
method to construct stable kink-type solutions and obtain sufficient conditions
on the superpotential for the Lyapunov stability of the corresponding exact
solutions. We analyze the stability of isotropic kink-type solutions for string
field theory inspired cosmological models.Comment: 23 pages, v3:typos corrected, references adde
A Positive Cosmological Constant in String Theory Through Ads/CFT Wormholes
There are two important examples of physical systems which violate the strong
energy condition : Universes (like, it would seem, our own) with a positive
cosmological constant, and wormholes. We suggest that a positive cosmological
constant can be reconciled with string theory by considering wormholes in
string backgrounds. This is argued in two directions : first, we show that
brane-worlds with positive cosmological constants give rise to bulk
singularities which are best resolved by embedding the brane-world in an
AdS/CFT wormhole; and second, for the simplest kind of wormhole in an
asymptotically AdS space, we show that the IR stability of the matter needed to
keep the wormhole open requires the presence of a brane-world. UV stability
conditions then forbid a negative cosmological constant on the brane-world.Comment: 21 pages, references added, clarification of claim that string theory
on a wormhole background requires a positive observed cosmological constant;
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