265 research outputs found
Are Causality Violations Undesirable?
Causality violations are typically seen as unrealistic and undesirable
features of a physical model. The following points out three reasons why
causality violations, which Bonnor and Steadman identified even in solutions to
the Einstein equation referring to ordinary laboratory situations, are not
necessarily undesirable. First, a space-time in which every causal curve can be
extended into a closed causal curve is singularity free--a necessary property
of a globally applicable physical theory. Second, a causality-violating
space-time exhibits a nontrivial topology--no closed timelike curve (CTC) can
be homotopic among CTCs to a point, or that point would not be causally well
behaved--and nontrivial topology has been explored as a model of particles.
Finally, if every causal curve in a given space-time passes through an event
horizon, a property which can be called "causal censorship", then that
space-time with event horizons excised would still be causally well behaved.Comment: Accepted in October 2008 by Foundations of Physics. Latex2e, 6 pages,
no figures. Presented at a seminar at the Universidad Nacional Autonoma de
Mexico. Version 2 was co-winner of the QMUL CTC Essay Priz
Dark Matter Direct Detection Signals inferred from a Cosmological N-body Simulation with Baryons
We extract at redshift z=0 a Milky Way sized object including gas, stars and
dark matter (DM) from a recent, high-resolution cosmological N-body simulation
with baryons. Its resolution is sufficient to witness the formation of a
rotating disk and bulge at the center of the halo potential. The phase-space
structure of the central galactic halo reveals the presence of a dark disk
component, that is co-rotating with the stellar disk. At the Earth's location,
it contributes to around 25% of the total DM local density, whose value is
rho_DM ~ 0.37 GeV/cm^3. The velocity distributions also show strong deviations
from pure Gaussian and Maxwellian distributions, with a sharper drop of the
high velocity tail.
We give a detailed study of the impact of these features on the predictions
for DM signals in direct detection experiments. In particular, the question of
whether the modulation signal observed by DAMA is or is not excluded by limits
set by other experiments (CDMS, XENON and CRESST...) is re-analyzed and
compared to the case of a standard Maxwellian halo, in both the elastic and the
inelastic scattering scenarios. We find that the compatibility between DAMA and
the other experiments is improved. In the elastic scenario, the DAMA modulation
signal is slightly enhanced in the so-called channeling region, as a result of
several effects. For the inelastic scenario, the improvement of the fit is
mainly attributable to the departure from a Maxwellian distribution at high
velocity.Comment: 39 page
Efficacy of transoral fundoplication vs omeprazole for treatment of regurgitation in a randomized controlled trial.
Background
The aim of this randomized, crossover study was to determine if transoral fundoplication (TF) could further improve clinical outcomes in partial responders to high-dose (HD) proton-pump inhibitor (PPI) therapy and to evaluate durability of TF. Methods
In seven United States centers, patients with hiatal hernia ≤2 cm and abnormal esophageal acid exposure (EAE) were randomized to TF (n = 40) or HD PPIs (n = 23) group. At 6-month follow-up, PPI patients underwent crossover. We assessed clinical outcomes 6-month post TF in crossover patients (COP), as compared to 6-month of HD PPI therapy, and 12-month outcomes in patients initially randomized to TF. The primary outcome was symptom control evaluated by Reflux Disease Questionnaire and Reflux Symptom Index. Secondary outcomes included healing of esophagitis, normalization of EAE and PPI use after TF. We analyzed 21 COP and 39 TF patients. McNemar’s test or Fisher exact test was used to compare proportions. Results
Of 63 randomized patients, 3 were lost to follow-up, leaving 39 TF and 21 COP for analyses. In the COP, TF further improved control of regurgitation and of atypical symptoms achieved after six months of HD PPIs. Of 20 patients with GERD symptoms after six months of high-dose PPI therapy, 65% (13/20) reported global elimination of troublesome regurgitation and atypical symptoms post TF off PPIs; 67% (6/9) reported no troublesome regurgitation. Esophagitis further healed in 75% (6/8) of patients. Seventy-one percent of COP patients were off PPIs six months following TF. Normalization of EAE decreased from 52% after HD PPIs (on PPIs) to 33% after TF (off PPIs), p =0.388. In the original TF group, 12-month post TF, 77% of patients achieved complete symptom control, 82% ceased PPI therapy, 100% healed esophagitis and 45% normalized EAE. Conclusions
The results of this study indicate that in patients with incomplete symptom control on high-dose PPI therapy TF may provide further elimination of symptoms and esophagitis healing. In the original TF group, the clinical outcomes of TF remained stable between 6- and 12-month follow-up. Trial registration
Clinicaltrials.gov: NCT01647958
Rotating Black Branes in the presence of nonlinear electromagnetic field
In this paper, we consider a class of gravity whose action represents itself
as a sum of the usual Einstein-Hilbert action with cosmological constant and an
gauge field for which the action is given by a power of the Maxwell
invariant. We present a class of the rotating black branes with Ricci flat
horizon and show that the presented solutions may be interpreted as black brane
solutions with two event horizons, extreme black hole and naked singularity
provided the parameters of the solutions are chosen suitably. We investigate
the properties of the solutions and find that for the special values of the
nonlinear parameter, the solutions are not asymptotically anti-deSitter. At
last, we obtain the conserved quantities of the rotating black branes and find
that the nonlinear source effects on the electric field, the behavior of
spacetime, type of singularity and other quantities.Comment: 7 pages, 5 figures, to appear in EPJ
Charged BTZ-like Black Holes in Higher Dimensions
Motivated by many worthwhile paper about (2 + 1)-dimensional BTZ black holes,
we generalize them to to (n + 1)-dimensional solutions, so called BTZ-like
solutions. We show that the electric field of BTZ-like solutions is the same as
(2 + 1)-dimensional BTZ black holes, and also their lapse functions are
approximately the same, too. By these similarities, it is also interesting to
investigate the geometric and thermodynamics properties of the BTZ-like
solutions. We find that, depending on the metric parameters, the BTZ-like
solutions may be interpreted as black hole solutions with inner (Cauchy) and
outer (event) horizons, an extreme black hole or naked singularity. Then, we
calculate thermodynamics quantities and conserved quantities, and show that
they satisfy the first law of thermodynamics. Finally, we perform a stability
analysis in the canonical ensemble and show that the BTZ-like solutions are
stable in the whole phase space.Comment: 5 pages, two column format, one figur
The Similarity Hypothesis in General Relativity
Self-similar models are important in general relativity and other fundamental
theories. In this paper we shall discuss the ``similarity hypothesis'', which
asserts that under a variety of physical circumstances solutions of these
theories will naturally evolve to a self-similar form. We will find there is
good evidence for this in the context of both spatially homogenous and
inhomogeneous cosmological models, although in some cases the self-similar
model is only an intermediate attractor. There are also a wide variety of
situations, including critical pheneomena, in which spherically symmetric
models tend towards self-similarity. However, this does not happen in all cases
and it is it is important to understand the prerequisites for the conjecture.Comment: to be submitted to Gen. Rel. Gra
Hidden Voices of Black Men
Peer Reviewedhttp://deepblue.lib.umich.edu/bitstream/2027.42/66982/2/10.1177_002193479402500102.pd
Multidimensional Conservation Laws: Overview, Problems, and Perspective
Some of recent important developments are overviewed, several longstanding
open problems are discussed, and a perspective is presented for the
mathematical theory of multidimensional conservation laws. Some basic features
and phenomena of multidimensional hyperbolic conservation laws are revealed,
and some samples of multidimensional systems/models and related important
problems are presented and analyzed with emphasis on the prototypes that have
been solved or may be expected to be solved rigorously at least for some cases.
In particular, multidimensional steady supersonic problems and transonic
problems, shock reflection-diffraction problems, and related effective
nonlinear approaches are analyzed. A theory of divergence-measure vector fields
and related analytical frameworks for the analysis of entropy solutions are
discussed.Comment: 43 pages, 3 figure
The Formation and Evolution of Massive Stellar Clusters in IC 4662
We present a multiwavelength study of the formation of massive stellar
clusters, their emergence from cocoons of gas and dust, and their feedback on
surrounding matter. Using data that span from radio to optical wavelengths,
including Spitzer and Hubble ACS observations, we examine the population of
young star clusters in the central starburst region of the irregular Wolf-Rayet
galaxy IC 4662. We model the radio-to-IR spectral energy distributions of
embedded clusters to determine the properties of their HII regions and dust
cocoons (sizes, masses, densities, temperatures), and use near-IR and optical
data with mid-IR spectroscopy to constrain the properties of the embedded
clusters themselves (mass, age, extinction, excitation, abundance). The two
massive star-formation regions in IC 4662 are excited by stellar populations
with ages of ~ 4 million years and masses of ~ 3 x 10^5 M_sun (assuming a
Kroupa IMF). They have high excitation and sub-solar abundances, and they may
actually be comprised of several massive clusters rather than the single
monolithic massive compact objects known as Super Star Clusters (SSCs). Mid-IR
spectra reveal that these clusters have very high extinctions, A_V ~ 20-25 mag,
and that the dust in IC 4662 is well-mixed with the emitting gas, not in a
foreground screen.Comment: 7 pages, 11 figures, to appear in proceedings of the conference
"Young Massive Star Clusters: Initial Conditions and Environments ", held in
Granada, Spain, September 200
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