74 research outputs found
How to Stop (Worrying and Love) the Bubble: Boundary Changing Solutions
We discover that a class of bubbles of nothing are embedded as time dependent
scaling limits of previous spacelike-brane solutions. With the right initial
conditions, a near-bubble solution can relax its expansion and open the compact
circle. Thermodynamics of the new class of solutions is discussed and the
relationships between brane/flux transitions, tachyon condensation and
imaginary D-branes are outlined. Finally, a related class of simultaneous
connected S-branes are also examined.Comment: 47 pages; v2 introduction to Weyl cards added, comments added,
references added, typos corrected, matches JHEP versio
D-branes in 2d Lorentzian black hole
We study D-branes in the Lorentzian signature 2D black hole string theory. We
use the technique of gauged WZW models to construct the associated boundary
conformal field theories. The main focus of this work is to discuss the
(semi-classical) world-volume geometries of the D-branes. We also discuss
comparison of our work with results in related gauged WZW models.Comment: 24 pages, 5 figures, uses JHEP3.cl
Non-Standard Intersections of S-Branes in D=11 Supergravity
We construct new intersecting S-brane solutions in 11-dimensional
supergravity which do not have supersymmetric analogs. They are obtained by
letting brane charges to be proportional to each other. Solutions fall into two
categories with respect to whether there is a non-diagonal term to be cancelled
in the field equations or not. In each case we show that they can be
constructed by using a simple set of rules which is similar to the harmonic
function rule of the usual static p-branes. Furthermore, we study an
intersection where the Chern-Simons term makes a non-zero contribution to the
field equations. We show that this configuration has a singularity like other
S-branes.Comment: 13 pages, 2 figures;v2 Section 2.2 is improved with new examples,
references added;v3 typos correcte
Regular S-Brane Backgrounds
We construct time-dependent S-brane solutions to the supergravity field
equations in various dimensions which (unlike most such geometries) do not
contain curvature singularities. The configurations we consider are less
symmetric than are earlier solutions, with our simplest solution being obtained
by a simple analytical continuation of the Kerr geometry. We discuss in detail
the global structure and properties of this background. We then generalize it
to higher dimensions and to include more complicated field configurations -
like non vanishing scalars and antisymmetric tensor gauge potentials - by the
usual artifice of applying duality symmetries.Comment: 22 pages, 3 figures. Typos in eq.(2.6) correcte
Mutated Hilltop Inflation : A Natural Choice for Early Universe
We propose a model of inflation with a suitable potential for a single scalar
field which falls in the wide class of hilltop inflation. We derive the
analytical expressions for most of the physical quantities related to inflation
and show that all of them represent the true behavior as required from a model
of inflation. We further subject the results to observational verification by
formulating the theory of perturbations based on our model followed by an
estimation for the values of those observable parameters. Our model is found to
be in excellent agreement with observational data. Thus, the features related
to the model leads us to infer that this type of hilltop inflation may be a
natural choice for explaining the early universe.Comment: 22 pages, 7 figures, 2 tables. Matches published version in JCA
S-Brane Thermodynamics
The description of string-theoretic s-branes at g_s=0 as exact worldsheet
CFTs with a (lambda cosh X^0) or (lambda e^(X^0)) boundary interaction is
considered. Due to the imaginary-time periodicity of the interaction under X^0
-> X^0 + 2 pi i, these configurations have intriguing similarities to black
hole or de Sitter geometries. For example, the open string pair production as
seen by an Unruh detector is thermal at temperature T = 1/4 pi. It is shown
that, despite the rapid time dependence of the s-brane, there exists an exactly
thermal mixed state of open strings. The corresponding boundary state is
constructed for both the bosonic and superstring cases. This state defines a
long-distance Euclidean effective field theory whose light modes are confined
to the s-brane. At the critical value of the coupling lambda=1/2, the boundary
interaction simply generates an SU(2) rotation by pi from Neumman to Dirichlet
boundary conditions. The lambda=1/2 s-brane reduces to an array of sD-branes
(D-branes with a transverse time dimension) on the imaginary time axis. The
long range force between a (bosonic) sD-brane and an ordinary D-brane is shown
from the annulus diagram to be 11/12 times the force between two D-branes. The
linearized time-dependent RR field F=dC produced by an sD-brane in superstring
theory is explicitly computed and found to carry a half unit of s-charge
Q_s=\int_S *F=1/2, where S is any transverse spacelike slice.Comment: 42 page
SD-brane gravity fields and rolling tachyons
S(pacelike)D-branes are objects arising naturally in string theory when
Dirichlet boundary conditions are imposed on the time direction. SD-brane
physics is inherently time-dependent. Previous investigations of gravity fields
of SD-branes have yielded undesirable naked spacelike singularities. We set up
the problem of coupling the most relevant open-string tachyonic mode to
massless closed-string modes in the bulk, with backreaction and Ramond-Ramond
fields included. We find solutions numerically in a self-consistent
approximation; our solutions are naturally asymptotically flat and
time-reversal asymmetric. We find completely nonsingular evolution; in
particular, the dilaton and curvature are well-behaved for all time. The
essential mechanism for spacetime singularity resolution is the inclusion of
full backreaction between the bulk fields and the rolling tachyon. Our analysis
is not the final word on the story, because we have to make some significant
approximations, most notably homogeneity of the tachyon on the unstable branes.
Nonetheless, we provide significant progress in plugging a gaping hole in prior
understanding of the gravity fields of SD-branes.Comment: References added. Analysis for much broader range of solutions
presented. Conclusions unchanged. Time-reversal symmetric examples ruled out,
new examples are provide
HI in the Outskirts of Nearby Galaxies
The HI in disk galaxies frequently extends beyond the optical image, and can
trace the dark matter there. I briefly highlight the history of high spatial
resolution HI imaging, the contribution it made to the dark matter problem, and
the current tension between several dynamical methods to break the disk-halo
degeneracy. I then turn to the flaring problem, which could in principle probe
the shape of the dark halo. Instead, however, a lot of attention is now devoted
to understanding the role of gas accretion via galactic fountains. The current
cold dark matter theory has problems on galactic scales, such as
the core-cusp problem, which can be addressed with HI observations of dwarf
galaxies. For a similar range in rotation velocities, galaxies of type Sd have
thin disks, while those of type Im are much thicker. After a few comments on
modified Newtonian dynamics and on irregular galaxies, I close with statistics
on the HI extent of galaxies.Comment: 38 pages, 17 figures, invited review, book chapter in "Outskirts of
Galaxies", Eds. J. H. Knapen, J. C. Lee and A. Gil de Paz, Astrophysics and
Space Science Library, Springer, in pres
The kinetic fragility of liquids as manifestation of the elastic softening
We show that the fragility , the steepness of the viscosity and relaxation
time close to the vitrification, increases with the degree of elastic
softening, i.e. the decrease of the elastic modulus with increasing
temperature, in universal way. This provides a novel connection between the
thermodynamics, via the modulus, and the kinetics. The finding is evidenced by
numerical simulations and comparison with the experimental data of glassformers
with widely different fragilities (), leading to a
fragility-independent elastic master curve extending over eighteen decades in
viscosity and relaxation time. The master curve is accounted for by a cavity
model pointing out the roles of both the available free volume and the cage
softness. A major implication of our findings is that ultraslow relaxations,
hardly characterised experimentally, become predictable by linear elasticity.
As an example, the viscosity of supercooled silica is derived over about
fifteen decades with no adjustable parameters.Comment: 7 pages, 6 figures; Added new results, improved the theoretical
sectio
Caracterização da estrutura anatômica, densidade básica e morfologia de cavacos da madeira de Eucalyptus grandis para a produção de painéis MDF
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