1,320 research outputs found
A randomized double-blind study of the effect of triiodothyronine on cardiac function and morbidity after coronary bypass surgery
AbstractBackground: Although triiodothyronine deficiency has been described after cardiopulmonary bypass, data supporting its use have been conflicting. A double-blind, randomized, placebo-controlled study was undertaken to further define the effect of triiodothyronine on hemodynamics and outcome after coronary artery bypass grafting. Methods: A total of 170 patients undergoing elective coronary artery bypass grafting were enrolled and completed the study from November 1996 through March 1998. On removal of the aortic crossclamp, patients were randomized to receive either intravenous triiodothyronine (0.4 μg/kg bolus plus 0.1 μg/kg infusion administered over a 6-hour period, n = 81) or placebo (n = 89). Outcome variables included hemodynamic profile and inotropic drug/pressor requirements at several time points (mean ± standard error of the mean), perioperative morbidity (arrhythmia/ischemia/infarction), and mortality. Results: Despite similar baseline characteristics, patients randomized to triiodothyronine had a higher cardiac index and lower inotropic requirements after the operation. Subjects receiving triiodothyronine demonstrated a significantly lower incidence of postoperative myocardial ischemia (4% vs 18%, P = .007) and pacemaker dependence (14% vs 25%, P = .013). Seven patients in the placebo group required postoperative mechanical assistance (intra-aortic balloon pump, n = 4; left ventricular assist device, n = 3), compared with none in the triiodothyronine group (P = .01). There were 2 deaths in the placebo group and no deaths in the triiodothyronine group. Conclusions: Parenteral triiodothyronine given after crossclamp removal during elective coronary artery bypass grafting significantly improved postoperative ventricular function, reduced the need for treatment with inotropic agents and mechanical devices, and decreased the incidence of myocardial ischemia. The incidence of atrial fibrillation was slightly decreased, and the need for postoperative pacemaker support was reduced. (J Thorac Cardiovasc Surg 1999;117:1128-35
Clipping the Cosmos: The Bias and Bispectrum of Large Scale Structure
A large fraction of the information collected by cosmological surveys is
simply discarded to avoid lengthscales which are difficult to model
theoretically. We introduce a new technique which enables the extraction of
useful information from the bispectrum of galaxies well beyond the conventional
limits of perturbation theory. Our results strongly suggest that this method
increases the range of scales where the relation between the bispectrum and
power spectrum in tree-level perturbation theory may be applied, from k_max ~
0.1 h/Mpc to ~ 0.7 h/Mpc. This leads to correspondingly large improvements in
the determination of galaxy bias. Since the clipped matter power spectrum
closely follows the linear power spectrum, there is the potential to use this
technique to probe the growth rate of linear perturbations and confront
theories of modified gravity with observation.Comment: 5 pages, 2 figures. To appear in Physical Review Letter
Signals for Lorentz Violation in Post-Newtonian Gravity
The pure-gravity sector of the minimal Standard-Model Extension is studied in
the limit of Riemann spacetime. A method is developed to extract the modified
Einstein field equations in the limit of small metric fluctuations about the
Minkowski vacuum, while allowing for the dynamics of the 20 independent
coefficients for Lorentz violation. The linearized effective equations are
solved to obtain the post-newtonian metric. The corresponding post-newtonian
behavior of a perfect fluid is studied and applied to the gravitating many-body
system. Illustrative examples of the methodology are provided using bumblebee
models. The implications of the general theoretical results are studied for a
variety of existing and proposed gravitational experiments, including lunar and
satellite laser ranging, laboratory experiments with gravimeters and torsion
pendula, measurements of the spin precession of orbiting gyroscopes, timing
studies of signals from binary pulsars, and the classic tests involving the
perihelion precession and the time delay of light. For each type of experiment
considered, estimates of the attainable sensitivities are provided. Numerous
effects of local Lorentz violation can be studied in existing or near-future
experiments at sensitivities ranging from parts in 10^4 down to parts in
10^{15}.Comment: 46 pages two-column REVTeX, accepted in Physical Review
Equilibrium and nonequilibrium properties associated with the chiral phase transition at finite density in the Gross-Neveu Model
We study the dynamics of the chiral phase transition at finite density in the
Gross-Neveu (GN) model in the leading order in large-N approximation. The phase
structure of the GN model in this approximation has the property that there is
a tricritical point at a fixed temperature and chemical potential separating
regions where the chiral transition is first order from that where it is second
order. We consider evolutions starting in local thermal and chemical
equilibrium in the massless unbroken phase for conditions pertaining to
traversing a first or second order phase transition. We assume boost invariant
kinematics and determine the evolution of the order parameter , the
energy density and pressure as well as the effective temperature, chemical
potential and interpolating number densities as a function of the proper time
. We find that before the phase transition, the system behaves as if it
were an ideal fluid in local thermal equilibrium with equation of state
. After the phase transition, the system quickly reaches its true
broken symmetry vacuum value for the fermion mass and for the energy density.
The single particle distribution functions for Fermions and anti-Fermions go
far out of equilibrium as soon as the plasma traverses the chiral phase
transition. We have also determined the spatial dependence of the "pion"
Green's function as a function of the proper time.Comment: 39 pages, 23 figure
The self-force on a static scalar test-charge outside a Schwarzschild black hole
The finite part of the self-force on a static scalar test-charge outside a
Schwarzschild black hole is zero. By direct construction of Hadamard's
elementary solution, we obtain a closed-form expression for the minimally
coupled scalar field produced by a test-charge held fixed in Schwarzschild
spacetime. Using the closed-form expression, we compute the necessary external
force required to hold the charge stationary. Although the energy associated
with the scalar field contributes to the renormalized mass of the particle (and
thereby its weight), we find there is no additional self-force acting on the
charge. This result is unlike the analogous electrostatic result, where, after
a similar mass renormalization, there remains a finite repulsive self-force
acting on a static electric test-charge outside a Schwarzschild black hole. We
confirm our force calculation using Carter's mass-variation theorem for black
holes. The primary motivation for this calculation is to develop techniques and
formalism for computing all forces - dissipative and non-dissipative - acting
on charges and masses moving in a black-hole spacetime. In the Appendix we
recap the derivation of the closed-form electrostatic potential. We also show
how the closed-form expressions for the fields are related to the infinite
series solutions.Comment: RevTeX, To Appear in Phys. Rev.
Instantons and unitarity in quantum cosmology with fixed four-volume
We find a number of complex solutions of the Einstein equations in the
so-called unimodular version of general relativity, and we interpret them as
saddle points yielding estimates of a gravitational path integral over a space
of almost everywhere Lorentzian metrics on a spacetime manifold with topology
of the "no-boundary" type. In this setting, the compatibility of the
no-boundary initial condition with the definability of the quantum measure
reduces reduces to the normalizability and unitary evolution of the no-boundary
wave function \psi. We consider the spacetime topologies R^4 and RP^4 # R^4
within a Taub minisuperspace model with spatial topology S^3, and the spacetime
topology R^2 x T^2 within a Bianchi type I minisuperspace model with spatial
topology T^3. In each case there exists exactly one complex saddle point (or
combination of saddle points) that yields a wave function compatible with
normalizability and unitary evolution. The existence of such saddle points
tends to bear out the suggestion that the unimodular theory is less divergent
than traditional Einstein gravity. In the Bianchi type I case, the
distinguished complex solution is approximately real and Lorentzian at late
times, and appears to describe an explosive expansion from zero size at T=0.
(In the Taub cases, in contrast, the only complex solution with nearly
Lorentzian late-time behavior yields a wave function that is normalizable but
evolves nonunitarily, with the total probability increasing exponentially in
the unimodular "time" in a manner that suggests a continuous creation of new
universes at zero volume.) The issue of the stability of these results upon the
inclusion of more degrees of freedom is raised.Comment: 32 pages, REVTeX v3.1 with amsfonts. (v2: minor typos etc corrected.
On the trispectrum as a gaussian test for cosmology
In the standard model for structure formation, bound objects originate from
the gravitational collapse of small perturbations arising from quantum
fluctuations with random phases. In other scenarios, based on defects,
structures are seeded by localized energy density. In principle, it is possible
to differentiate between these models on the basis of their statistical
properties; only in the former case is the initial density field an
almost-perfect random gaussian field. In this paper, we investigate the use of
the trispectrum of the galaxy density field, which is the connected four-point
function in Fourier space, as a discriminant between gaussian and non-gaussian
models. It has the advantage of having only weak non-linear growth. We define a
related statistic which, as a test of the gaussian hypothesis, is
independent of cosmology, the power spectrum and biasing, in real space, and
which is, in principle, a measure of the departure from gaussian statistics.
For galaxy redshift surveys, the statistic depends on cosmology and bias only
through the potentially observable parameter . We compute the expected
errors on the estimate of , and demonstrate with numerical simulations
that it can be a useful discriminant of models, with the important proviso that
any bias is linear on large scales. Whether it is the most effective method is
uncertain and depends on the nature of the departure from gaussianity.Comment: to appear in ApJ, 28 pages, 5 figure
The Multi-Object, Fiber-Fed Spectrographs for SDSS and the Baryon Oscillation Spectroscopic Survey
We present the design and performance of the multi-object fiber spectrographs
for the Sloan Digital Sky Survey (SDSS) and their upgrade for the Baryon
Oscillation Spectroscopic Survey (BOSS). Originally commissioned in Fall 1999
on the 2.5-m aperture Sloan Telescope at Apache Point Observatory, the
spectrographs produced more than 1.5 million spectra for the SDSS and SDSS-II
surveys, enabling a wide variety of Galactic and extra-galactic science
including the first observation of baryon acoustic oscillations in 2005. The
spectrographs were upgraded in 2009 and are currently in use for BOSS, the
flagship survey of the third-generation SDSS-III project. BOSS will measure
redshifts of 1.35 million massive galaxies to redshift 0.7 and Lyman-alpha
absorption of 160,000 high redshift quasars over 10,000 square degrees of sky,
making percent level measurements of the absolute cosmic distance scale of the
Universe and placing tight constraints on the equation of state of dark energy.
The twin multi-object fiber spectrographs utilize a simple optical layout
with reflective collimators, gratings, all-refractive cameras, and
state-of-the-art CCD detectors to produce hundreds of spectra simultaneously in
two channels over a bandpass covering the near ultraviolet to the near
infrared, with a resolving power R = \lambda/FWHM ~ 2000. Building on proven
heritage, the spectrographs were upgraded for BOSS with volume-phase
holographic gratings and modern CCD detectors, improving the peak throughput by
nearly a factor of two, extending the bandpass to cover 360 < \lambda < 1000
nm, and increasing the number of fibers from 640 to 1000 per exposure. In this
paper we describe the original SDSS spectrograph design and the upgrades
implemented for BOSS, and document the predicted and measured performances.Comment: 43 pages, 42 figures, revised according to referee report and
accepted by AJ. Provides background for the instrument responsible for SDSS
and BOSS spectra. 4th in a series of survey technical papers released in
Summer 2012, including arXiv:1207.7137 (DR9), arXiv:1207.7326 (Spectral
Classification), and arXiv:1208.0022 (BOSS Overview
Critical Exponents, Hyperscaling and Universal Amplitude Ratios for Two- and Three-Dimensional Self-Avoiding Walks
We make a high-precision Monte Carlo study of two- and three-dimensional
self-avoiding walks (SAWs) of length up to 80000 steps, using the pivot
algorithm and the Karp-Luby algorithm. We study the critical exponents
and as well as several universal amplitude ratios; in
particular, we make an extremely sensitive test of the hyperscaling relation
. In two dimensions, we confirm the predicted
exponent and the hyperscaling relation; we estimate the universal
ratios , and (68\% confidence
limits). In three dimensions, we estimate with a
correction-to-scaling exponent (subjective 68\%
confidence limits). This value for agrees excellently with the
field-theoretic renormalization-group prediction, but there is some discrepancy
for . Earlier Monte Carlo estimates of , which were , are now seen to be biased by corrections to scaling. We estimate the
universal ratios and ; since , hyperscaling holds. The approach to
is from above, contrary to the prediction of the two-parameter
renormalization-group theory. We critically reexamine this theory, and explain
where the error lies.Comment: 87 pages including 12 figures, 1029558 bytes Postscript
(NYU-TH-94/09/01
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