1,563 research outputs found
The Hoop Conjecture in Spherically Symmetric Spacetimes
We give general sufficient conditions for the existence of trapped surfaces
due to concentration of matter in spherically symmetric initial data sets
satisfying the dominant energy condition. These results are novel in that they
apply and are meaningful for arbitrary spacelike slices, that is they do not
require any auxiliary assumptions such as maximality, time-symmetry, or special
extrinsic foliations, and most importantly they can easily be generalized to
the nonspherical case once an existence theory for a modified version of the
Jang equation is developed. Moreover, our methods also yield positivity and
monotonicity properties of the Misner-Sharp energy
A Stringy Correspondence Principle in Cosmology
We study a d-dimensional FRW universe, containing a perfect fluid with p = w
\rho and \frac{1} {d - 1} \le w \le 1, and find a correspondence principle
similar to that of Horowitz and Polchinski in the black hole case. This
principle follows quite generally from thermodynamics and the conservation of
energy momentum tensor, and can be stated along similar lines as in the black
hole case: ``When the temperature T of the universe becomes of order string
scale the universe state becomes a highly excited string state. At the
transition, the entropies and energies of the universe and strings differ by
factors of {\cal O}(1).'' Such a matching is absent for w \ne 1 if the
transition is assumed to be when the curvature or the horizon length is of
order string scale.Comment: 14 pages. V2: More references added and some minor textual
modifications mad
Dynamics of Extreme Black Holes and Massive String States
In a recent paper, Duff and Rahmfeld argued that certain massive
states of the four-dimensional heterotic string correspond to extreme black
hole solutions. We provide further, dynamical, evidence for this identification
by comparing the scattering of these elementary string states with that of the
corresponding extreme black holes, in the limit of low velocities.Comment: 18 pages, harvma
A Comment on String Solitons
We derive an exact string-like soliton solution of D=10 heterotic string
theory. The solution possesses instanton structure in the
eight-dimensional space transverse to the worldsheet of the soliton.Comment: 4 page
Brane Effects on Extra Dimensional Scenarios: A Tale of Two Gravitons
We analyze the propagation of a scalar field in multidimensional theories
which include kinetic corrections in the brane, as a prototype for
gravitational interactions in a four dimensional brane located in a (nearly)
flat extra dimensional bulk. We regularize the theory by introducing an
infrared cutoff given by the size of the extra dimensions and a physical
ultraviolet cutoff of the order of the fundamental Planck scale in the higher
dimensional theory. We show that, contrary to recent suggestions, the radius of
the extra dimensions cannot be arbitrarily large. Moreover, for finite radii,
the gravitational effects localized on the brane can substantially alter the
phenomenology of collider and/or table-top gravitational experiments. This
phenomenology is dictated by the presence of a massless graviton, with standard
couplings to the matter fields, and a massive graviton which couples to matter
in a much stronger way. While graviton KK modes lighter than the massive
graviton couple to matter in a standard way, the couplings to matter of the
heavier KK modes are strongly suppressed.Comment: 21 pages, latex2e, axodraw.sty, 2 figure
Four-dimensional High-Branes as Intersecting D-Branes
We show that a class of extremal four-dimensional supersymmetric
``high-branes'', i.e. string and domain wall solutions, can be interpreted as
intersections of four ten-dimensional Dirichlet branes. These solutions
are related, via -duality in ten dimensions, to the four-dimensional
extremal Maxwell/scalar black holes that are characterized by a scalar coupling
parameter with .Comment: 9 pages, Latex, typos and minor errors correcte
More on Four-Dimensional Extremal Black Holes
We consider an embedding of the extremal four-dimensional
Reissner-Nordstr\"om black hole into type string theory. The equivalent
type configuration, in the D-brane weak-coupling picture, is a bound
state of D1- and D5-branes threaded by fundamental type strings. The
bound state involves also a NSNS solitonic 5-brane, mimicking the role of the
Kaluza-Klein magnetic monopole. The statistical entropy derived by counting the
degeneracy of the BPS-saturated excitations of this bound state agrees
perfectly with the (semiclassical) Bekenstein-Hawking formula.Comment: 19 pages, using Latex, no figure
Acoustic Surface Wave Probing of Ceramics
We have developed a low frequency theory for scattering from surface cracks. For the case of halfpenny shaped surface cracks, we are able to relate the reflection coefficient of a Rayleigh wave incident on the crack with the crack size as well as the fracture stress of a sample with the crack in it. Comparisons of the theoretical predictions for the fracture stress with the actual fracture stresses for silicon nitride samples containing cracks with estimated radii ranging from 51 μm to 274 μm show excellent agreement, with less than 16% error. A qualitative study in the high frequency regime of the reflected echos from surface cracks in silicon nitride turbine blades has also been made
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