50,992 research outputs found
Graviton scattering in matrix theory and supergravity
I briefly review recent work on the comparison between two and three graviton
scattering in supergravity and matrix theoryComment: Talk given at the TMR meeting, Kerkyria, September 1998, to appear in
the proceeding
Tunelling with a Negative Cosmological Constant
The point of this paper is see what light new results in hyperbolic geometry
may throw on gravitational entropy and whether gravitational entropy is
relevant for the quantum origin of the univeres. We introduce some new
gravitational instantons which mediate the birth from nothing of closed
universes containing wormholes and suggest that they may contribute to the
density matrix of the universe. We also discuss the connection between their
gravitational action and the topological and volumetric entropies introduced in
hyperbolic geometry. These coincide for hyperbolic 4-manifolds, and increase
with increasing topological complexity of the four manifold. We raise the
questions of whether the action also increases with the topological complexity
of the initial 3-geometry, measured either by its three volume or its Matveev
complexity. We point out, in distinction to the non-supergravity case, that
universes with domains of negative cosmological constant separated by
supergravity domain walls cannot be born from nothing. Finally we point out
that our wormholes provide examples of the type of Perpetual Motion machines
envisaged by Frolov and Novikov.Comment: 36 pages, plain TE
Graphene and the Zermelo Optical Metric of the BTZ Black Hole
It is well known that the low energy electron excitations of the curved
graphene sheet are solutions of the massless Dirac equation on a 2+1
dimensional ultra-static metric on . An externally
applied electric field on the graphene sheet induces a gauge potential which
could be mimicked by considering a stationary optical metric of the Zermelo
form, which is conformal to the BTZ black hole when the sheet has a constant
negative curvature. The Randers form of the metric can model a magnetic field,
which is related by a boost to an electric one in the Zermelo frame. We also
show that there is fundamental geometric obstacle to obtaining a model that
extends all the way to the black hole horizon.Comment: 10 pages Latex, no figures, substantial revisions, relation between
magnetic and electric fields and Randers and Zermelo forms clarifie
Cones, Tri-Sasakian Structures and Superconformal Invariance
In this note we show that rigid N=2 superconformal hypermultiplets must have
target manifolds which are cones over tri-Sasakian metrics. We comment on the
relation of this work to cone-branes and the AdS/CFT correspondence.Comment: 10 pages, Latex2
On the Heegaard Floer homology of Dehn surgery and unknotting number
n this thesis we generalise three theorems from the literature on Heegaard Floer
homology and Dehn surgery: one by Ozsv Ìath and Szab Ìo on deficiency symmetries in
half-integral
L
-space surgeries, and two by Greene which use Donaldsonâs diagonali-
sation theorem as an obstruction to integral and half-integral
L
-space surgeries. Our
generalisation is two-fold: first, we eliminate the
L
-space conditions, opening these
techniques up for use with much more general 3-manifolds, and second, we unify the
integral and half-integral surgery results into a broader theorem applicable to non-
zero rational surgeries in
S
3
which bound sharp, simply connected, negative-definite
smooth 4-manifolds. Such 3-manifolds are quite common and include, for example, a
huge number of Seifert fibred spaces.
Over the course of the first three chapters, we begin by introducing background
material on knots in 3-manifolds, the intersection form of a simply connected 4-
manifold, Spin- and Spin
c
-structures on 3- and 4-manifolds, and Heegaard Floer ho-
mology (including knot Floer homology). While none of the results in these chapters
are original, all of them are necessary to make sense of what follows. In Chapter 4,
we introduce and prove our main theorems, using arguments that are predominantly
algebraic or combinatorial in nature. We then apply these new theorems to the study
of unknotting number in Chapter 5, making considerable headway into the extremely
difficult problem of classifying the 3-strand pretzel knots with unknotting number
one. Finally, in Chapter 6, we present further applications of the main theorems,
ranging from a plan of attack on the famous Seifert fibred space realisation problem
to more biologically motivated problems concerning rational tangle replacement. An
appendix on the implications of our theorems for DNA topology is provided at the
end.Open Acces
Supersymmetric, cold and lukewarm black holes in cosmological Einstein-Maxwell theory
In flat space, the extreme Reissner-Nordstr\o m (RN) black hole is
distinguished by its coldness (vanishing Hawking temperature) and its
supersymmetry. We examine RN solutions to Einstein-Maxwell theory with a
cosmological constant , classifying the cold black holes and, for
positive , the ``lukewarm" black holes at the same temperature as the
de Sitter thermal background. For negative , we classify the
supersymmetric solutions within the context of gauged supergravity. One
finds supersymmetric analogues of flat-space extreme RN black holes, which for
nonzero differ from the cold black holes. In addition, there is an
exotic class of supersymmetric solutions which cannot be continued to flat
space, since the magnetic charge becomes infinite in that limit.Comment: (18 pp., plain tex
Axion-Dilaton Black Holes
In this talk some essential features of stringy black holes are described. We
consider charged four-dimensional axion-dilaton black holes. The Hawking
temperature and the entropy of all solutions are shown to be simple functions
of the squares of supercharges, defining the positivity bounds. Spherically
symmetric and multi black hole solutions are presented. The extreme solutions
have some unbroken supersymmetries. Axion-dilaton black holes with zero entropy
and zero area of the horizon form a family of stable particle-like objects,
which we call holons. We discuss the possibility of splitting of nearly extreme
black holes into holons.Comment: 8 pages, LATEX, (Talk presented at the TEXAS/PASCOS conference,
Berkeley, December 1992
The Rotating Dyonic Black Holes Of Kaluza-Klein Theory
The most general electrically and magnetically charged rotating black hole
solutions of 5 dimensional \KK\ theory are given in an explicit form. Various
classical quantities associated with the black holes are derived. In
particular, one finds the very surprising result that the gyromagnetic and
gyroelectric ratios can become {\tenit arbitrarily large}. The thermodynamic
quantities of the black holes are calculated and a Smarr-type formula is
obtained leading to a generalized first law of black hole thermodynamics. The
properties of the extreme solutions are investigated and it is shown how they
naturally separate into two classes. The extreme solutions in one class are
found to have two unusual properties: (i). Their event horizons have zero
angular velocity and yet they have non-zero ADM angular momentum. (ii). In
certain circumstances it is possible to add angular momentum to these extreme
solutions without changing the mass or charges and yet still maintain an
extreme solution. Regarding the extreme black holes as elementary particles,
their stability is discussed and it is found that they are stable provided they
have sufficient angular momentum.Comment: 28 pages, LaTeX with 3 PostScript figure
A String and M-theory Origin for the Salam-Sezgin Model
An M/string-theory origin for the six-dimensional Salam-Sezgin chiral gauged
supergravity is obtained, by embedding it as a consistent Pauli-type reduction
of type I or heterotic supergravity on the non-compact hyperboloid times . We can also obtain embeddings of larger, non-chiral,
gauged supergravities in six dimensions, whose consistent truncation yields the
Salam-Sezgin theory. The lift of the Salam-Sezgin (Minkowski)
ground state to ten dimensions is asymptotic at large distances to the
near-horizon geometry of the NS5-brane.Comment: Latex, 18 pages; minor correction
Extreme Domain Wall--Black Hole Complementarity in N=1SUPERGRAVITY with a General Dilaton Coupling
We study supersymmetric (extreme) domain walls in four-dimensional (4d) N=1
supergravity theories with a general dilaton coupling . Type I
walls, which are static, planar (say, in () plane) configurations,
interpolate between Minkowski space-time and a vacuum with a varying dilaton
field. We classify their global space-time with respect to the value of the
coupling . supergravity with , an effective theory
from superstrings, provides a dividing line between the theories with
, where there is a naked (planar) singularity on one side of the
wall, and the theories with , where the singularity of the of the
wall is covered by the horizon. The global space-time (in ) direction) of
the extreme walls with the coupling is the same as the global
space-time (in ( direction) of the extreme magnetically charged black
holes with the coupling .Comment: 11 pages (3 figures available from
ftp://dept.physics.upenn.edu/pub/UPR-600-T/), UPR-600-T revised to be
compatible with the published versio
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