10,559 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
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
Pseudo-Supersymmetry and the Domain-Wall/Cosmology Correspondence
The correspondence between domain-wall and cosmological solutions of gravity
coupled to scalar fields is explained. Any domain wall solution that admits a
Killing spinor is shown to correspond to a cosmology that admits a
pseudo-Killing spinor: whereas the Killing spinor obeys a Dirac-type equation
with hermitian `mass'-matrix, the corresponding pseudo-Killing spinor obeys a
Dirac-type equation with a anti-hermitian `mass'-matrix. We comment on some
implications of (pseudo)supersymmetry.Comment: 11 pages, contribution to the proceedings of IRGAC 2006;v3: minor
change
Cosmology as Relativistic Particle Mechanics: From Big Crunch to Big Bang
Cosmology can be viewed as geodesic motion in an appropriate metric on an
`augmented' target space; here we obtain these geodesics from an effective
relativistic particle action. As an application, we find some exact (flat and
curved) cosmologies for models with N scalar fields taking values in a
hyperbolic target space for which the augmented target space is a Milne
universe. The singularities of these cosmologies correspond to points at which
the particle trajectory crosses the Milne horizon, suggesting a novel
resolution of them, which we explore via the Wheeler-deWitt equation.Comment: 17 pages, 3 figures, references and comments adde
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
The coupling between pulsation and mass loss in massive stars
To what extent can pulsational instabilities resolve the mass-loss problem of
massive stars? How important is pulsation in structuring and modulating the
winds of these stars? What role does pulsation play in redistributing angular
momentum in massive stars? Although I cannot offer answers to these questions,
I hope at the very least to explain how they come to be asked.Comment: 12 pages, 1 figure, to appear in proceedings of "Unsolved Problems in
Stellar Physics" conference (Cambridge, UK, July 2007
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
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