11 research outputs found
Potentials between D-Branes in a Supersymmetric Model of Space-Time Foam
We study a supersymmetric model of space-time foam with two stacks each of
eight D8-branes with equal string tensions, separated by a single bulk
dimension containing D0-brane particles that represent quantum fluctuations.
The ground-state configuration with static D-branes has zero vacuum energy,
but, when they move, the interactions among the D-branes and D-particles due to
the exchanges of strings result in a non-trivial, positive vacuum energy. We
calculate its explicit form in the limits of small velocities and large or
small separations between the D-branes and/or the D-particles. This non-trivial
vacuum energy appears as a central charge deficit in the non-critical stringy
model describing perturbative string excitations on a moving D-brane.
These calculations enable us to characterise the ground state of the
D-brane/D-particle system, and provide a framework for discussing brany
inflation and the possibility of residual Dark Energy in the present-day
Universe.Comment: 26 pages Latex, four eps figures incorporated, minor typos corrected,
no effects on conclusion
Liouville Cosmology at Zero and Finite Temperatures
We discuss cosmology in the context of Liouville strings, characterized by a
central-charge deficit Q^2, in which target time is identified with (the
world-sheet zero mode of the) Liouville field: Q-Cosmology. We use a specific
example of colliding brane worlds to illustrate the phase diagram of this
cosmological framework. The collision provides the necessary initial
cosmological instability, expressed as a departure from conformal invariance in
the underlying string model. The brane motion provides a way of breaking
target-space supersymmetry, and leads to various phases of the brane and bulk
Universes. Specifically, we find a hot metastable phase for the bulk string
Universe soon after the brane collision in which supersymmetry is broken, which
we describe by means of a subcritical world-sheet sigma model dressed by a
space-like Liouville field, representing finite temperature (Euclidean time).
This phase is followed by an inflationary phase for the brane Universe, in
which the bulk string excitations are cold. This is described by a
super-critical Liouville string with a time-like Liouville mode, whose zero
mode is identified with the Minkowski target time. Finally, we speculate on
possible ways of exiting the inflationary phase, either by means of subsequent
collisions or by deceleration of the brane Universe due to closed-string
radiation from the brane to the bulk. While phase transitions from hot to cold
configurations occur in the bulk string universe, stringy excitations attached
to the brane world remain thermalized throughout, at a temperature which can be
relatively high. The late-time behaviour of the model results in
dilaton-dominated dark energy and present-day acceleration of the expansion of
the Universe, asymptoting eventually to zero.Comment: 59 pages LaTeX, 4 figure
Defective angiogenesis and fatal embryonic hemorrhage in mice lacking core 1–derived O-glycans
The core 1 β1-3-galactosyltransferase (T-synthase) transfers Gal from UDP-Gal to GalNAcα1-Ser/Thr (Tn antigen) to form the core 1 O-glycan Galβ1-3GalNAcα1-Ser/Thr (T antigen). The T antigen is a precursor for extended and branched O-glycans of largely unknown function. We found that wild-type mice expressed the NeuAcα2-3Galβ1-3GalNAcα1-Ser/Thr primarily in endothelial, hematopoietic, and epithelial cells during development. Gene-targeted mice lacking T-synthase instead expressed the nonsialylated Tn antigen in these cells and developed brain hemorrhage that was uniformly fatal by embryonic day 14. T-synthase–deficient brains formed a chaotic microvascular network with distorted capillary lumens and defective association of endothelial cells with pericytes and extracellular matrix. These data reveal an unexpected requirement for core 1–derived O-glycans during angiogenesis
A supersymmetric D-brane Model of Space-Time Foam
We present a supersymmetric model of space-time foam with two stacks of eight
D8-branes with equal string tensions, separated by a single bulk dimension
containing D0-brane particles that represent quantum fluctuations in the
space-time foam. The ground state configuration with static D-branes has zero
vacuum energy. However, gravitons and other closed-string states propagating
through the bulk may interact with the D0-particles, causing them to recoil and
the vacuum energy to become non zero. This provides a possible origin of dark
energy. Recoil also distorts the background metric felt by energetic massless
string states, which travel at less than the usual (low-energy) velocity of
light. On the other hand, the propagation of chiral matter anchored on the D8
branes is not affected by such space-time foam effects.Comment: 33 pages, latex, five figure