8,195 research outputs found
Classical resolution of singularities in dilaton cosmologies
For models of dilaton-gravity with a possible exponential potential, such as
the tensor-scalar sector of IIA supergravity, we show how cosmological
solutions correspond to trajectories in a 2D Milne space (parametrized by the
dilaton and the scale factor). Cosmological singularities correspond to points
at which a trajectory meets the Milne horizon, but the trajectories can be
smoothly continued through the horizon to an instanton solution of the
Euclidean theory. We find some exact cosmology/instanton solutions that lift to
black holes in one higher dimension. For one such solution, the singularities
of a big crunch to big bang transition mediated by an instanton phase lift to
the black hole and cosmological horizons of de Sitter Schwarzschild spacetimes.Comment: 24 pages, 2 figure
Guidance, flight mechanics and trajectory optimization. Volume 1 - Coordinate systems and time measure
Coordinate measuring system for flight control, and trajectory optimizatio
Higher-dimensional resolution of dilatonic black hole singularities
We show that the four-dimensional extreme dilaton black hole with dilaton
coupling constant can be interpreted as a {\it completely
non-singular}, non-dilatonic, black -brane in dimensions provided
that is {\it odd}. Similar results are obtained for multi-black holes and
dilatonic extended objects in higher spacetime dimensions. The non-singular
black -brane solutions include the self-dual three brane of ten-dimensional
N=2B supergravity and a multi-fivebrane solution of eleven-dimensional
supergravity. In the case of a supersymmetric non-dilatonic -brane solution
of a supergravity theory, we show that it saturates a bound on the energy per
unit -volume.Comment: 27 pages, R/94/28, UCSBTH-94-35 (Comments added to the discussion
section
Cosmological D-instantons and Cyclic Universes
For models of gravity coupled to hyperbolic sigma models, such as the
metric-scalar sector of IIB supergravity, we show how smooth trajectories in
the `augmented target space' connect FLRW cosmologies to non-extremal
D-instantons through a cosmological singularity. In particular, we find closed
cyclic universes that undergo an endless sequence of big-bang to big-crunch
cycles separated by instanton `phases'. We also find `big-bounce' universes in
which a collapsing closed universe bounces off its cosmological singularity to
become an open expanding universe.Comment: 21 pages, 4 figures. v2: minor change
Guidance, flight mechanics and trajectory optimization. Volume 11 - Guidance equations for orbital operations
Mathematical formulation of guidance equations and solutions for orbital space mission
Domain Walls on the Brane
We show that all branes admit worldvolume domain wall solutions. We find one
class of solutions for which the tension of the brane changes discontinuously
along the domain wall. These solutions are not supersymmetric. We argue that
there is another class of domain wall solutions which is supersymmetric. A
particular case concerns supersymmetric domain wall solutions on IIB D-5- and
NS-5-branes.Comment: 18 pages, Tex, uses phyzz
Variability of Young Massive Stars in the Galactic Super Star Cluster Westerlund 1
This paper presents the first optical variability study of the Westerlund 1
super star cluster in search of massive eclipsing binary systems. A total of
129 new variable stars have been identified, including the discovery of 4
eclipsing binaries that are cluster members, 1 additional candidate, 8 field
binaries, 19 field delta Scuti stars, 3 field W UMa eclipsing binaries, 13
other periodic variables and 81 long period or non-periodic variables. These
include the known luminous blue variable, the B[e] star, 11 Wolf-Rayet stars,
several supergiants, and other reddened stars that are likely members of
Westerlund 1. The bright X-ray source corresponding to the Wolf-Rayet star
WR77o (B) is found to be a 3.51 day eclipsing binary. The discovery of a
reddened detached eclipsing binary system implies the first identification of
main-sequence stars in Westerlund 1.Comment: 28 pages, 9 figures, 3 tables; accepted for publication in The
Astronomical Journal. Full-resolution version and color image of the cluster
are available at http://www.dtm.ciw.edu/bonanos/Westerlund
Dilaton Domain Walls and Dynamical Systems
Domain wall solutions of -dimensional gravity coupled to a dilaton field
with an exponential potential are shown
to be governed by an autonomous dynamical system, with a transcritical
bifurcation as a function of the parameter when . All
phase-plane trajectories are found exactly for , including
separatrices corresponding to walls that interpolate between and
adS_{d-1} \times\bR, and the exact solution is found for . Janus-type
solutions are interpreted as marginal bound states of these ``separatrix
walls''. All flat domain wall solutions, which are given exactly for any
, are shown to be supersymmetric for some superpotential ,
determined by the solution.Comment: 30 pp, 11 figs, significant revision of original. Minor additional
corrections in version to appear in journa
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