119 research outputs found
Zero-Mode Dynamics of String Webs
At sufficiently low energy the dynamics of a string web is dominated by zero
modes involving rigid motion of the internal strings. The dimension of the
associated moduli space equals the maximal number of internal faces in the web.
The generic web moduli space has boundaries and multiple branches, and for webs
with three or more faces the geometry is curved. Webs can also be studied in a
lift to M-theory, where a string web is replaced by a membrane wrapped on a
holomorphic curve in spacetime. In this case the moduli space is complexified
and admits a Kaehler metric.Comment: LaTeX, 17 pages, 5 eps figures; v2: references adde
Geometric Interpretation and Classification of Global Solutions in Generalized Dilaton Gravity
Two dimensional gravity with torsion is proved to be equivalent to special
types of generalized 2d dilaton gravity. E.g. in one version, the dilaton field
is shown to be expressible by the extra scalar curvature, constructed for an
independent Lorentz connection corresponding to a nontrivial torsion.
Elimination of that dilaton field yields an equivalent torsionless theory,
nonpolynomial in curvature. These theories, although locally equivalent exhibit
quite different global properties of the general solution. We discuss the
example of a (torsionless) dilaton theory equivalent to the --model.
Each global solution of this model is shown to split into a set of global
solutions of generalized dilaton gravity. In contrast to the theory with
torsion the equivalent dilaton one exhibits solutions which are asymptotically
flat in special ranges of the parameters. In the simplest case of ordinary
dilaton gravity we clarify the well known problem of removing the Schwarzschild
singularity by a field redefinition.Comment: 21 pages, 6 Postscript figure
Cosmological Models and Renormalization Group Flow
We study cosmological solutions of Einstein gravity with a positive
cosmological constant in diverse dimensions. These include big-bang models that
re-collapse, big-bang models that approach de Sitter acceleration at late
times, and bounce models that are both past and future asymptotically de
Sitter. The re-collapsing and the bounce geometries are all tall in the sense
that entire spatial slices become visible to a comoving observer before the end
of conformal time, while the accelerating big-bang geometries can be either
short or tall. We consider the interpretation of these cosmological solutions
as renormalization group flows in a dual field theory and give a geometric
interpretation of the associated c-function as the area of the apparent
cosmological horizon in Planck units. The covariant entropy bound requires
quantum effects to modify the early causal structure of some of our big-bang
solutions.Comment: 26 pages, 11 figures, v2: improved discussion of entropy bounds,
references added, v3: minor changes, reference adde
Open strings in relativistic ion traps
Electromagnetic plane waves provide examples of time-dependent open string
backgrounds free of corrections. The solvable case of open strings in
a quadrupolar wave front, analogous to pp-waves for closed strings, is
discussed. In light-cone gauge, it leads to non-conformal boundary conditions
similar to those induced by tachyon condensates. A maximum electric gradient is
found, at which macroscopic strings with vanishing tension are pair-produced --
a non-relativistic analogue of the Born-Infeld critical electric field. Kinetic
instabilities of quadrupolar electric fields are cured by standard atomic
physics techniques, and do not interfere with the former dynamic instability. A
new example of non-conformal open-closed duality is found. Propagation of open
strings in time-dependent wave fronts is discussed.Comment: 43 pages, 11 figures, Latex2e, JHEP3.cls style; v2: one-loop
amplitude corrected, open-closed duality proved, refs added, miscellaneous
improvements, see historical note in fil
Founder populations and their uses for breast cancer genetics
Numerous founder mutations have been reported in BRCA1 and BRCA2. For genetic screening of a population with a founder mutation, testing can be targeted to the mutation, allowing for a more rapid and less expensive test. In addition, more precise estimates of the prior probability of carrying a mutation and of the likelihood of a mutation carrier developing cancer should be possible. For a given founder mutation a large number of carriers are available, so that focused scientific studies of penetrance, expression, and genetic and environmental modifiers of risk can be performed. Finally, founder populations may be a powerful resource to localize additional breast cancer susceptibility loci, because of the reduction in locus heterogeneity
Strings between branes
D-brane configurations containing fundamental strings are constructed as
classical solutions of Yang-Mills theory. The fundamental strings in these
systems stretch between D-branes. In the case of D1-branes, this construction
gives smooth (classical) resolutions of string junctions and string networks.
Using a non-abelian Yang-Mills analysis of the string current, the string
charge density is computed and is shown to have support in the region between
the D-brane world-volumes. The 't Hooft-Polyakov monopole is analyzed using
similar methods, and is shown to contain D-strings whose flux has support off
the D-brane world-volume defined by the Higgs scalar field, when this field is
interpreted in terms of a transverse dimension. The constructions presented
here are used to give a qualitative picture of tachyon condensation in the
Yang-Mills limit, where fundamental strings and lower-dimensional D-branes
arise in a volume of space-time where brane-antibrane annihilation has
occurred.Comment: 35 pages, 16 eps figures, JHEP style; v2: a comment adde
Classical and Quantum Gravity in 1+1 Dimensions, Part I: A Unifying Approach
We provide a concise approach to generalized dilaton theories with and
without torsion and coupling to Yang-Mills fields. Transformations on the space
of fields are used to trivialize the field equations locally. In this way their
solution becomes accessible within a few lines of calculation only. In this
first of a series of papers we set the stage for a thorough global
investigation of classical and quantum aspects of more or less all available 2D
gravity-Yang-Mills models.Comment: 24 pages, no figures, some sign errors in Eqs. 52--59 have been
corrected (according to the Erratum
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
The holographic principle
There is strong evidence that the area of any surface limits the information
content of adjacent spacetime regions, at 10^(69) bits per square meter. We
review the developments that have led to the recognition of this entropy bound,
placing special emphasis on the quantum properties of black holes. The
construction of light-sheets, which associate relevant spacetime regions to any
given surface, is discussed in detail. We explain how the bound is tested and
demonstrate its validity in a wide range of examples.
A universal relation between geometry and information is thus uncovered. It
has yet to be explained. The holographic principle asserts that its origin must
lie in the number of fundamental degrees of freedom involved in a unified
description of spacetime and matter. It must be manifest in an underlying
quantum theory of gravity. We survey some successes and challenges in
implementing the holographic principle.Comment: 52 pages, 10 figures, invited review for Rev. Mod. Phys; v2:
reference adde
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