7 research outputs found
The `s-rule' exclusion principle and vacuum interpolation in worldvolume dynamics
We show how the worldvolume realization of the Hanany-Witten effect for a
supersymmetric D5-brane in a D3 background also provides a classical
realization of the `s-rule' exclusion principle. Despite the supersymmetry, the
force on the D5-brane vanishes only in the D5 `ground state', which is shown to
interpolate between 6-dimensional Minkowski space and an -invariant
geometry. The M-theory analogue of these results is briefly
discussed.Comment: 25 pages, 9 figures, LaTeX JHEP styl
Sectional Curvature Bounds in Gravity: Regularisation of the Schwarzschild Singularity
A general geometrical scheme is presented for the construction of novel
classical gravity theories whose solutions obey two-sided bounds on the
sectional curvatures along certain subvarieties of the Grassmannian of
two-planes. The motivation to study sectional curvature bounds comes from their
equivalence to bounds on the acceleration between nearby geodesics. A universal
minimal length scale is a necessary ingredient of the construction, and an
application of the kinematical framework to static, spherically symmetric
spacetimes shows drastic differences to the Schwarzschild solution of general
relativity by the exclusion of spacelike singularities.Comment: 20 pages, 1 figure, REVTeX4, updated reference
Accelerating Cosmologies and a Phase Transition in M-Theory
M-theory compactifies on a seven-dimensional time-dependent hyperbolic or
flat space to a four-dimensional FLRW cosmology undergoing a period of
accelerated expansion in Einstein conformal frame. The strong energy condition
is violated by the scalar fields produced in the compactification, as is
necessary to evade the no-go theorem for time-independent compactifications.
The four-form field strength of eleven-dimensional supergravity smoothly
switches on during the period of accelerated expansion in hyperbolic
compactifications, whereas in flat compactifications, the three-form potential
smoothly changes its sign. For small acceleration times, this behaviour is like
a phase transition of the three-form potential, during which the cosmological
scale factor approximately doubles.Comment: 10 pages, 4 figures, REVTeX4, some references adde
Canonical differential geometry of string backgrounds
String backgrounds and D-branes do not possess the structure of Lorentzian
manifolds, but that of manifolds with area metric. Area metric geometry is a
true generalization of metric geometry, which in particular may accommodate a
B-field. While an area metric does not determine a connection, we identify the
appropriate differential geometric structure which is of relevance for the
minimal surface equation in such a generalized geometry. In particular the
notion of a derivative action of areas on areas emerges naturally. Area metric
geometry provides new tools in differential geometry, which promise to play a
role in the description of gravitational dynamics on D-branes.Comment: 20 pages, no figures, improved journal versio
Classical limit of quantum gravity in an accelerating universe
A one-parameter deformation of Einstein?Hilbert gravity with an inverse
Riemann curvature term is derived as the classical limit of quantum gravity
compatible with an accelerating universe. This result is based on the
investigation of semi-classical theories with sectional curvature bounds which
are shown not to admit static spherically symmetric black holes if otherwise of
phenomenological interest. We discuss the impact on the canonical quantization
of gravity, and observe that worldsheet string theory is not affected.Comment: 11 pages, no figure
Geometry of manifolds with area metric: multi-metric backgrounds
We construct the differential geometry of smooth manifolds equipped with an
algebraic curvature map acting as an area measure. Area metric geometry
provides a spacetime structure suitable for the discussion of gauge theories
and strings, and is considerably more general than Lorentzian geometry. Our
construction of geometrically relevant objects, such as an area metric
compatible connection and derived tensors, makes essential use of a
decomposition theorem due to Gilkey, whereby we generate the area metric from a
finite collection of metrics. Employing curvature invariants for multi-metric
backgrounds we devise a class of gravity theories with inherently stringy
character, and discuss gauge matter actions.Comment: 34 pages, REVTeX4, journal versio