198 research outputs found
The Moduli of Reducible Vector Bundles
A procedure for computing the dimensions of the moduli spaces of reducible,
holomorphic vector bundles on elliptically fibered Calabi-Yau threefolds X is
presented. This procedure is applied to poly-stable rank n+m bundles of the
form V + pi* M, where V is a stable vector bundle with structure group SU(n) on
X and M is a stable vector bundle with structure group SU(m) on the base
surface B of X. Such bundles arise from small instanton transitions involving
five-branes wrapped on fibers of the elliptic fibration. The structure and
physical meaning of these transitions are discussed.Comment: 33+1 page
Vector Bundle Moduli and Small Instanton Transitions
We give the general presciption for calculating the moduli of irreducible,
stable SU(n) holomorphic vector bundles with positive spectral covers over
elliptically fibered Calabi-Yau threefolds. Explicit results are presented for
Hirzebruch base surfaces B=F_r. The transition moduli that are produced by
chirality changing small instanton phase transitions are defined and
specifically enumerated. The origin of these moduli, as the deformations of the
spectral cover restricted to the ``lift'' of the horizontal curve of the
M5-brane, is discussed. We present an alternative description of the transition
moduli as the sections of rank n holomorphic vector bundles over the M5-brane
curve and give explicit examples. Vector bundle moduli appear as gauge singlet
scalar fields in the effective low-energy actions of heterotic superstrings and
heterotic M-theory.Comment: 52 pages, LATEX, corrected typo
Vector Bundle Moduli Superpotentials in Heterotic Superstrings and M-Theory
The non-perturbative superpotential generated by a heterotic superstring
wrapped once around a genus-zero holomorphic curve is proportional to the
Pfaffian involving the determinant of a Dirac operator on this curve. We show
that the space of zero modes of this Dirac operator is the kernel of a linear
mapping that is dependent on the associated vector bundle moduli. By explicitly
computing the determinant of this map, one can deduce whether or not the
dimension of the space of zero modes vanishes. It is shown that this
information is sufficient to completely determine the Pfaffian and, hence, the
non-perturbative superpotential as explicit holomorphic functions of the vector
bundle moduli. This method is illustrated by a number of non-trivial examples.Comment: 81 pages, LaTeX, corrected typo
From Big Crunch to Big Bang
We consider conditions under which a universe contracting towards a big
crunch can make a transition to an expanding big bang universe. A promising
example is 11-dimensional M-theory in which the eleventh dimension collapses,
bounces, and re-expands. At the bounce, the model can reduce to a weakly
coupled heterotic string theory and, we conjecture, it may be possible to
follow the transition from contraction to expansion. The possibility opens the
door to new classes of cosmological models. For example, we discuss how it
suggests a major simplification and modification of the recently proposed
ekpyrotic scenario.Comment: 16 pages, compressed and RevTex file, including three postscript
figure files. Minor changes, version to appear in Physical Review
Linear and non-linear perturbations in dark energy models
I review the linear and second-order perturbation theory in dark energy
models with explicit interaction to matter in view of applications to N-body
simulations and non-linear phenomena. Several new or generalized results are
obtained: the general equations for the linear perturbation growth; an
analytical expression for the bias induced by a species-dependent interaction;
the Yukawa correction to the gravitational potential due to dark energy
interaction; the second-order perturbation equations in coupled dark energy and
their Newtonian limit. I also show that a density-dependent effective dark
energy mass arises if the dark energy coupling is varying.Comment: 12 pages, submitted to Phys. Rev; v2: added a ref. and corrected a
typ
Constraining Scale-Dependent Non-Gaussianity with Future Large-Scale Structure and the CMB
We forecast combined future constraints from the cosmic microwave background
and large-scale structure on the models of primordial non-Gaussianity. We study
the generalized local model of non-Gaussianity, where the parameter f_NL is
promoted to a function of scale, and present the principal component analysis
applicable to an arbitrary form of f_NL(k). We emphasize the complementarity
between the CMB and LSS by using Planck, DES and BigBOSS surveys as examples,
forecast constraints on the power-law f_NL(k) model, and introduce the figure
of merit for measurements of scale-dependent non-Gaussianity.Comment: 28 pages, 8 figures, 2 tables; v2: references update
A noninvasive hemoglobin monitor in the pediatric intensive care unit
Background Critically ill pediatric patients frequently require hemoglobin monitoring. Accurate noninvasive Hb (SpHb) would allow practitioners to decrease anemia from repeated blood draws, traumatic blood draws, and a decreased number of laboratory Hb (LabHb) medical tests. The Food and Drug Administration has approved the Masimo Pronto SpHb and associated Rainbow probes; however, its use in the pediatric intensive care unit (PICU) is controversial. In this study, we define the degree of agreement between LabHb and SpHb using the Masimo Pronto SpHb Monitor and identify clinical and demographic conditions associated with decreased accuracy. Materials and methods We performed a prospective, observational study in a large PICU at an academic medical center. Fifty-three pediatric patients (30-d and 18-y-old), weighing >3 kg, admitted to the PICU from January-April 2013 were examined. SpHb levels measured at the time of LabHb blood draw were compared and analyzed. Results Only 83 SpHb readings were obtained in 118 attempts (70.3%) and 35 readings provided a result of "unable to obtain." The mean LabHb and SpHb were 11.1 g/dL and 11.2 g/dL, respectively. Bland-Altman analysis showed a mean difference of 0.07 g/dL with a standard deviation of ±2.59 g/dL. Pearson correlation is 0.55, with a 95% confidence interval between 0.38 and 0.68. Logistic regression showed that extreme LabHb values, increasing skin pigmentation, and increasing body mass index were predictors of poor agreement between SpHb and LabHb (P < 0.05). Separately, increasing body mass index, hypoxia, and hypothermia were predictors for undetectable readings (P < 0.05). Conclusions The Masimo Pronto SpHb Monitor provides adequate agreement for the trending of hemoglobin levels in critically ill pediatric patients. However, the degree of agreement is insufficient to be used as the sole indicator for transfusion decisions and should be used in context of other clinical parameters to determine the need for LabHb in critically ill pediatric patients
Looking Beyond Inflationary Cosmology
In spite of the phenomenological successes of the inflationary universe
scenario, the current realizations of inflation making use of scalar fields
lead to serious conceptual problems which are reviewed in this lecture. String
theory may provide an avenue towards addressing these problems. One particular
approach to combining string theory and cosmology is String Gas Cosmology. The
basic principles of this approach are summarized.Comment: invited talk at "Theory Canada 1" (Univ. of British Columbia,
Vancouver, Canada, June 2 - 4, 2005) (references updated
Fundamental constants and tests of general relativity - Theoretical and cosmological considerations
The tests of the constancy of the fundamental constants are tests of the
local position invariance and thus of the equivalence principle. We summarize
the various constraints that have been obtained and then describe the
connection between varying constants and extensions of general relativity. To
finish, we discuss the link with cosmology, and more particularly with the
acceleration of the Universe. We take the opportunity to summarize various
possibilities to test general relativity (but also the Copernican principle) on
cosmological scales.Comment: Proceedings of the workshop ``The nature of gravity, confronting
theory and experiment in space'', ISSI, Bern, october 200
The acceleration of the universe and the physics behind it
Using a general classification of dark enegy models in four classes, we
discuss the complementarity of cosmological observations to tackle down the
physics beyond the acceleration of our universe. We discuss the tests
distinguishing the four classes and then focus on the dynamics of the
perturbations in the Newtonian regime. We also exhibit explicitely models that
have identical predictions for a subset of observations.Comment: 18 pages, 18 figure
- …