12,286 research outputs found
The Number of States of Two Dimensional Critical String Theory
We discuss string theory vacua which have the wrong number of spacetime
dimensions, and give a crude argument that vacua with more than four large
dimensions are improbable. We then turn to two dimensional vacua, which naively
appear to violate Bekenstein's entropy principle. A classical analysis shows
that the naive perturbative counting of states is unjustified. All excited
states of the system have strong coupling singularities which prevent us from
concluding that they really exist. A speculative interpretation of the
classical solutions suggests only a finite number of states will be found in
regions bounded by a finite area. We also argue that the vacuum degeneracy of
two dimensional classical string theory is removed in quantum mechanics. The
system appears to be in a Kosterlitz-Thouless phase. This leads to the
conclusion that it is also improbable to have only two large spacetime
dimensions in string theory. However, we note that, unlike our argument for
high dimensions, our conclusions about the ground state have neglected two
dimensional quantum gravitational effects, and are at best incomplete.Comment: 12 pages, harvma
CPT and Other Symmetries in String/M Theory
We initiate a search for non-perturbative consistency conditions in M theory.
Some non-perturbative conditions are already known in Type I theories; we
review these and search for others. We focus principally on possible anomalies
in discrete symmetries. It is generally believed that discrete symmetries in
string theories are gauge symmetries, so anomalies would provide evidence for
inconsistencies. Using the orbifold cosmic string construction, we give some
evidence that the symmetries we study are gauged. We then search for anomalies
in discrete symmetries in a variety of models, both with and without
supersymmetry. In symmetric orbifold models we extend previous searches, and
show in a variety of examples that all anomalies may be canceled by a
Green-Schwarz mechanism. We explore some asymmetric orbifold constructions and
again find that all anomalies may be canceled this way. Then we turn to Type
IIB orientifold models where it is known that even perturbative anomalies are
non-universal. In the examples we study, by combining geometric discrete
symmetries with continuous gauge symmetries, one may define non-anomalous
discrete symmetries already in perturbation theory; in other cases, the
anomalies are universal. Finally, we turn to the question of CPT conservation
in string/M theory. It is well known that CPT is conserved in all string
perturbation expansions; here in a number of examples for which a
non-perturbative formulation is available we provide evidence that it is
conserved exactly.Comment: 52 pages.1 paragraph added in introduction to clarify assumption
Is There A String Theory Landscape
We examine recent claims of a large set of flux compactification solutions of
string theory. We conclude that the arguments for AdS solutions are plausible.
The analysis of meta-stable dS solutions inevitably leads to situations where
long distance effective field theory breaks down. We then examine whether these
solutions are likely to lead to a description of the real world. We conclude
that one must invoke a strong version of the anthropic principle. We explain
why it is likely that this leads to a prediction of low energy supersymmetry
breaking, but that many features of anthropically selected flux
compactifications are likely to disagree with experiment.Comment: 39 pages, Latex, ``Terminology surrounding the anthropic principle
revised to conform with accepted usage. More history of the anthropic
principle included. Various references added.
Ten Dimensional Black Hole and the D0-brane Threshold Bound State
We discuss the ten dimensional black holes made of D0-branes in the regime
where the effective coupling is large, and yet the 11D geometry is unimportant.
We suggest that these black holes can be interpreted as excitations over the
threshold bound state. Thus, the entropy formula for the former is used to
predict a scaling region of the wave function of the latter. The horizon radius
and the mass gap predicted in this picture agree with the formulas derived from
the classical geometry.Comment: 11 pages, harvmac; v2: typos corrected, argument for the convergence
of two integrals improved, v3: one ref. adde
Model validation for a noninvasive arterial stenosis detection problem
Copyright @ 2013 American Institute of Mathematical SciencesA current thrust in medical research is the development of a non-invasive method for detection, localization, and characterization of an arterial stenosis (a blockage or partial blockage in an artery). A method has been proposed to detect shear waves in the chest cavity which have been generated by disturbances in the blood flow resulting from a stenosis. In order to develop this methodology further, we use both one-dimensional pressure and shear wave experimental data from novel acoustic phantoms to validate corresponding viscoelastic mathematical models, which were developed in a concept paper [8] and refined herein. We estimate model parameters which give a good fit (in a sense to be precisely defined) to the experimental data, and use asymptotic error theory to provide confidence intervals for parameter estimates. Finally, since a robust error model is necessary for accurate parameter estimates and confidence analysis, we include a comparison of absolute and relative models for measurement error.The National Institute of Allergy and Infectious Diseases, the Air Force Office of Scientific Research, the Deopartment of Education and the Engineering and Physical Sciences Research Council (EPSRC)
On the Possibility of Large Axion Decay Constants
The decay constant of the QCD axion is required by observation to be small
compared to the Planck scale. In theories of "natural inflation," and certain
proposed anthropic solutions of the cosmological constant problem, it would be
interesting to obtain a large decay constant for axion-like fields from
microscopic physics. String theory is the only context in which one can
sensibly address this question. Here we survey a number of periodic fields in
string theory in a variety of string vacua. In some examples, the decay
constant can be parameterically larger than the Planck scale but the effective
action then contains appreciable harmonics of order . As a result,
these fields are no better inflaton candidates than Planck scale axions.Comment: 17 pages, no figures, minor change mad
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