1,549 research outputs found
Linearized supergravity from Matrix theory
We show that the linearized supergravity potential between two objects
arising from the exchange of quanta with zero longitudinal momentum is
reproduced to all orders in 1/r by terms in the one-loop Matrix theory
potential. The essential ingredient in the proof is the identification of the
Matrix theory quantities corresponding to moments of the stress tensor and
membrane current. We also point out that finite-N Matrix theory violates the
equivalence principle.Comment: 13 pages, LaTex, v2: additional comments mostly in section
The Bekenstein Formula and String Theory (N-brane Theory)
A review of recent progress in string theory concerning the Bekenstein
formula for black hole entropy is given. Topics discussed include p-branes,
D-branes and supersymmetry; the correspondence principle; the D- and M-brane
approach to black hole entropy; the D-brane analogue of Hawking radiation, and
information loss; D-branes as probes of black holes; and the Matrix theory
approach to charged and neutral black holes. Some introductory material is
included.Comment: 53 pages, LaTeX. v3: Typos fixed, minor updates, references added,
brief Note Added on AdS/CF
Stochastic Gravity: A Primer with Applications
Stochastic semiclassical gravity of the 90's is a theory naturally evolved
from semiclassical gravity of the 70's and 80's. It improves on the
semiclassical Einstein equation with source given by the expectation value of
the stress-energy tensor of quantum matter fields in curved spacetimes by
incorporating an additional source due to their fluctuations. In stochastic
semiclassical gravity the main object of interest is the noise kernel, the
vacuum expectation value of the (operator-valued) stress-energy bi-tensor, and
the centerpiece is the (stochastic) Einstein-Langevin equation. We describe
this new theory via two approaches: the axiomatic and the functional. The
axiomatic approach is useful to see the structure of the theory from the
framework of semiclassical gravity. The functional approach uses the
Feynman-Vernon influence functional and the Schwinger-Keldysh close-time-path
effective action methods which are convenient for computations. It also brings
out the open systems concepts and the statistical and stochastic contents of
the theory such as dissipation, fluctuations, noise and decoherence. We then
describe the application of stochastic gravity to the backreaction problems in
cosmology and black hole physics. Intended as a first introduction to this
subject, this article places more emphasis on pedagogy than completeness.Comment: 46 pages Latex. Intended as a review in {\it Classical and Quantum
Gravity
A Comment on the Geometric Entropy and Conical Space
It has been recently pointed out that a definition of the geometric entropy
using the partition function in a conical space does not in general lead to a
positive definite quantity. For a scalar field model with a non-minimal
coupling we clarify the origin of the anomalous behavior from the viewpoint of
the canonical formulation.Comment: No Figures. To appear in Classical and Quantum Gravit
How Not to Construct an Asymptotically de Sitter Universe
Observational evidence suggests that our universe is currently evolving
towards an asymptotically de Sitter future. Unfortunately and in spite of much
recent attention, various quantum, holographic and cosmological aspects of de
Sitter space remain quite enigmatic. With such intrigue in mind, this paper
considers the ``construction'' of a toy model that describes an asymptotically
de Sitter universe. More specifically, we add fluid-like matter to an otherwise
purely de Sitter spacetime, formulate the relevant solutions and then discuss
the cosmological and holographic implications. If the objective is to construct
an asymptotically de Sitter universe that is free of singularities and has a
straightforward holographic interpretation, then the results of this analysis
are decidedly negative. Nonetheless, this toy model nicely illustrates the
pitfalls that might be encountered in a more realistic type of construction.Comment: 25 pages, Latex; references and footnotes added (other minor changes
Comparing Brane Inflation to WMAP
We compare the simplest realistic brane inflationary model to recent
cosmological data, including WMAP 3-year cosmic microwave background (CMB)
results, Sloan Digital Sky Survey luminous red galaxies (SDSS LRG) power
spectrum data and Supernovae Legacy Survey (SNLS) Type 1a supernovae distance
measures. Here, the inflaton is simply the position of a -brane which is
moving towards a -brane sitting at the bottom of a throat (a warped,
deformed conifold) in the flux compactified bulk in Type IIB string theory. The
analysis includes both the usual slow-roll scenario and the Dirac-Born-Infeld
scenario of slow but relativistic rolling. Requiring that the throat is inside
the bulk greatly restricts the allowed parameter space. We discuss possible
scenarios in which large tensor mode and/or non-Gaussianity may emerge. Here,
the properties of a large tensor mode deviate from that in the usual slow-roll
scenario, providing a possible stringy signature. Overall, within the brane
inflationary scenario, the cosmological data is providing information about the
properties of the compactification of the extra dimensions.Comment: 45 pages 11 figure
Aspects of Quantum Gravity in de Sitter Spaces
In these lectures we give a review of recent attempts to understand quantum
gravity on de Sitter spaces. In particular, we discuss the holographic
correspondence between de Sitter gravity and conformal field theories proposed
by Hull and by Strominger, and how this may be reconciled with the
finite-dimensional Hilbert space proposal by Banks and Fischler. Furthermore we
review the no-go theorems that forbid an embedding of de Sitter spaces in
string theory, and discuss how they can be circumvented. Finally, some curious
issues concerning the thermal nature of de Sitter space are elucidated.Comment: 36+1 pages, 5 Postscript figures, introduction and section 6
extended, further references, final version to appear in JCA
Remarks on geometric entropy
The recently discussed notion of geometric entropy is shown to be related to
earlier calculations of thermal effects in Rindler space. The evaluation is
extended to de Sitter space and to a two-dimensional black hole.Comment: 7p.,uses jyTeX,MUTP/94/
Nonabelian Phenomena on D-branes
A remarkable feature of D-branes is the appearance of a nonabelian gauge
theory in the description of several (nearly) coincident branes. This
nonabelian structure plays an important role in realizing various geometric
effects with D-branes. In particular, the branes' transverse displacements are
described by matrix-valued scalar fields and so noncommutative geometry
naturally appears in this framework. I review the action governing this
nonabelian theory, as well as various related physical phenomena such as the
dielectric effect, giant gravitons and fuzzy funnels.Comment: Lecture at Leuven workshop on ``The quantum structure of spacetime
and the geometrical nature of fundamental interactions'' (September 13-19,
2002); ref.'s adde
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