764 research outputs found
Bulk Viscosity, Decaying Dark Matter, and the Cosmic Acceleration
We discuss a cosmology in which cold dark-matter particles decay into
relativistic particles. We argue that such decays could lead naturally to a
bulk viscosity in the cosmic fluid. For decay lifetimes comparable to the
present hubble age, this bulk viscosity enters the cosmic energy equation as an
effective negative pressure. We investigate whether this negative pressure is
of sufficient magnitude to account fo the observed cosmic acceleration. We show
that a single decaying species in a flat, dark-matter dominated cosmology
without a cosmological constant cannot reproduce the observed
magnitude-redshift relation from Type Ia supernovae. However, a delayed bulk
viscosity, possibly due to a cascade of decaying particles may be able to
account for a significant fraction of the apparent cosmic acceleration.
Possible candidate nonrelativistic particles for this scenario include sterile
neutrinos or gauge-mediated decaying supersymmetric particles.Comment: 7 pages, 4 figure
The X-Ray Concentration-Virial Mass Relation
We present the concentration (c)-virial mass (M) relation of 39 galaxy
systems ranging in mass from individual early-type galaxies up to the most
massive galaxy clusters, (0.06-20) x 10^{14} M_sun. We selected for analysis
the most relaxed systems possessing the highest quality data currently
available in the Chandra and XMM public data archives. A power-law model fitted
to the X-ray c-M relation requires at high significance (6.6 sigma) that c
decreases with increasing M, which is a general feature of CDM models. The
median and scatter of the c-M relation produced by the flat, concordance LCDM
model (Omega_m=0.3, sigma_8=0.9) agrees with the X-ray data provided the sample
is comprised of the most relaxed, early forming systems, which is consistent
with our selection criteria. Holding the rest of the cosmological parameters
fixed to those in the concordance model the c-M relation requires 0.76< sigma_8
<1.07 (99% conf.), assuming a 10% upward bias in the concentrations for early
forming systems. The tilted, low-sigma_8 model suggested by a new WMAP analysis
is rejected at 99.99% confidence, but a model with the same tilt and
normalization can be reconciled with the X-ray data by increasing the dark
energy equation of state parameter to w ~ -0.8. When imposing the additional
constraint of the tight relation between sigma_8 and Omega_m from studies of
cluster abundances, the X-ray c-M relation excludes (>99% conf.) both open CDM
models and flat CDM models with Omega_m ~1. This result provides novel evidence
for a flat, low-Omega_m universe with dark energy using observations only in
the local (z << 1) universe. Possible systematic errors in the X-ray mass
measurements of a magnitude ~10% suggested by CDM simulations do not change our
conclusions.Comment: Accepted for Publication in ApJ; 13 pages, 4 figures; minor
clarifications and updates; correlation coefficients corrected in Table 1
(correct values were used in the analysis in previous versions); conclusions
unchange
The Absence of Adiabatic Contraction of the Radial Dark Matter Profile in the Galaxy Cluster A2589
We present an X-ray analysis of the radial mass profile of the radio-quiet
galaxy cluster A2589 between 0.015-0.25 r_vir using an XMM-Newton observation.
Except for a ~16 kpc shift of the X-ray center of the R=45-60 kpc annulus,
A2589 possesses a remarkably symmetrical X-ray image and is therefore an
exceptional candidate for precision studies of its mass profile by applying
hydrostatic equilibrium. The total gravitating matter profile is well described
by the NFW model (fractional residuals <~10%) with c_vir=6.1 +/- 0.3 and M_vir
= 3.3 +/- 0.3 x 10^{14} M_sun (r_vir = 1.74 +/- 0.05 Mpc) in excellent
agreement with LCDM. When the mass of the hot ICM is subtracted from the
gravitating matter profile, the NFW model fitted to the resulting dark matter
(DM) profile produces essentially the same result. However, if a component
accounting for the stellar mass (M_*) of the cD galaxy is included, then the
NFW fit to the DM profile is substantially degraded in the central r ~50 kpc
for reasonable M_*/L_V. Modifying the NFW DM halo by adiabatic contraction
arising from the early condensation of stellar baryons in the cD galaxy further
degrades the fit. The fit is improved substantially with a Sersic-like model
recently suggested by high resolution N-body simulations but with an inverse
Sersic index, alpha ~0.5, a factor of ~3 higher than predicted. We argue that
neither random turbulent motions nor magnetic fields can provide sufficient
non-thermal pressure support to reconcile the XMM mass profile with adiabatic
contraction of a CDM halo assuming reasonable M_*/L_V. Our results support the
scenario where, at least for galaxy clusters, processes during halo formation
counteract adiabatic contraction so that the total gravitating mass in the core
approximately follows the NFW profile.Comment: 15 pages, 11 figures, accepted for publication in ApJ. Minor changes
to match published versio
Clinical outcome of muscle invasive carcinoma urinary bladder patients treated with bladder preservation protocol
Background: To evaluate the outcome in terms of bladder preservation in muscle invasive urinary bladder patients treated with radical radiotherapy.Methods: A total of sixty patients with muscle invasive bladder cancer who were treated between 2011 and 2013 with bladder preservation protocol were identified. Thirty-two patients were stage T2 disease, rest were T3 and T4. Initial TURBT was reported complete in 24 of the total 60 patients. All patients received 60 to 64Gy to the whole bladder.Results: The follow-up range was 3 to 72 months. One year follow-up proportion was 78%. The median progression free survival was 40months. Early stage (T2) and advanced stage (T3 and T4) had progression free survival of 87.8% and 48.5% respectively at one year follow-up with a p value of 0.001. Thirty-six patients who had an incomplete Transurethral resection of bladder tumor (TURBT) initially had a statistically significant lower PFS compared to those patients who had a complete TURBT with a p value of 0.029. Twenty-nine patients were disease free with an intact bladder during the follow-up period providing a bladder preservation rate of 48%.Conclusions: A bladder-conserving protocol with radiotherapy provides encouraging results with nearly half of the patients able to retain a disease-free bladder. Proper patient selection with early stage disease and those with complete TURBT may further improve the bladder preservation rates
Vacuum Stability with Tachyonic Boundary Higgs Masses in No-Scale Supersymmetry or Gaugino Mediation
No-scale supersymmetry or gaugino mediation augmented with large negative
Higgs soft masses at the input scale provides a simple solution to the
supersymmetric flavor problem while giving rise to a neutralino LSP. However,
to obtain a neutralino LSP it is often necessary to have tachyonic input Higgs
soft masses that can give rise to charge-and-color-breaking (CCB) minima and
unbounded-from-below (UFB) directions in the low energy theory. We investigate
the vacuum structure in these theories to determine when such problematic
features are present. When the standard electroweak vacuum is only metastable,
we compute its lifetime under vacuum tunneling. We find that vacuum
metastability leads to severe restrictions on the parameter space for larger
, while for smaller , only minor
restrictions are found. Along the way, we derive an exact bounce solution for
tunneling through an inverted parabolic potential.Comment: 18 Pages, 5 Figure
Probing the Dark Matter and Gas Fraction in Relaxed Galaxy Groups with X-ray observations from Chandra and XMM
We present radial mass profiles within 0.3 r_vir for 16 relaxed galaxy
groups-poor clusters (kT range 1-3 keV) selected for optimal mass constraints
from the Chandra and XMM data archives. After accounting for the mass of hot
gas, the resulting mass profiles are described well by a two-component model
consisting of dark matter (DM), represented by an NFW model, and stars from the
central galaxy. The stellar component is required only for 8 systems, for which
reasonable stellar mass-to-light ratios (M/L_K) are obtained, assuming a Kroupa
IMF. Modifying the NFW dark matter halo by adiabatic contraction does not
improve the fit and yields systematically lower M/L_K. In contrast to previous
results for massive clusters, we find that the NFW concentration parameter
(c_vir) for groups decreases with increasing M_vir and is inconsistent with no
variation at the 3 sigma level. The normalization and slope of the c_vir-M_vir
relation are consistent with the standard LambdaCDM cosmological model with
sigma_8 = 0.9. The small intrinsic scatter measured about the c_vir-M_vir
relation implies the groups represent preferentially relaxed, early forming
systems. The mean gas fraction (f =0.05 +/- 0.01) of the groups measured within
an overdensity Delta=2500 is lower than for hot, massive clusters, but the
fractional scatter (sigma_f/f=0.2) for groups is larger, implying a greater
impact of feedback processes on groups, as expected.Comment: Accepted for Publication in ApJ; 30 pages, 9 figures. No changes from
previous versio
ESL Student Perceptions of VLE Effectiveness at a University in South Korea
The purpose of this study is to determine students’ perception of the advantages, effects on language skills, suggestions for improvement, and limitations regarding the use of a VLE (Blackboard) and their differences according to gender, year, number of Blackboard courses taken, and computer literacy. The respondents of this study were 686 randomly selected university students enrolled in English classes at the University of Suwon in South Korea. An adapted survey questionnaire consisting of 33 items was administered to the students. The Mean was used to determine the students’ perceptions in the four areas followed by t-test and ANOVA to determine the differences in the students’ perceptions. The results showed that the students had a somewhat disagree rating in the areas of Advantages, Language, and Limitations and somewhat agree rating in the area of Suggestions. Significant differences were found in the students’ perceptions in the four areas when grouped according to gender and computer literacy; a significant difference was found in the area of Limitations when grouped according to year; and no significant differences were found according to number of Blackboard courses taken
The effect of Staphylococcus aureus carriage in late pregnancy on antibody levels to staphylococcal toxins in cord blood and breast milk.
We investigated the effect of carriage of Staphylococcus aureus in the later stages of pregnancy on levels of antibody specific to the S. aureus toxins, staphylococcal enterotoxin B (SEB), staphylococcal enterotoxin C (SEC) and toxic shock syndrome toxin-1 (TSST-1), in cord blood and breast milk and also explored the relationship between levels of antibody in antenatal serum and cord blood. Nasopharyngeal swabs and stool samples were collected on two occasions, from 96 women, during the last 6 weeks of pregnancy. Samples were cultured and S. aureus isolates were identified. Antenatal and cord blood samples from the same women and their infants were analysed for IgG antibody to SEB, SEC and TSST-1 by enzyme-linked immunosorbent assay. Breast milk samples were analysed for IgA antibody to the same toxins. We found that S. aureus carriage in pregnancy is common and exposure to a toxin-producing isolate boosts immunity. Over 89% of women and infants have some protective antibody to the toxins, and antitoxin IgG levels are higher in cord blood samples compared with antenatal samples. Levels of cord blood IgG and breast milk IgA specific for the staphylococcal toxins vary. Some infants lack protection and could be at risk of toxin-induced disease
Gamma-Ray Bursts via the Neutrino Emission from Heated Neutron Stars
A model is proposed for gamma-ray bursts based upon a neutrino burst of about
10^52 ergs lasting a few seconds above a heated collapsing neutron star. This
type of thermal neutrino burst is suggested by relativistic hydrodynamic
studies of the compression, heating, and collapse of close binary neutron stars
as they approach their last stable orbit, but may arise from other sources as
well. We present a hydrodynamic simulation of the formation and evolution of
the pair plasma associated with such a neutrino burst. This pair plasma leads
to the production of ~10^51 - 10^52 ergs in gamma-rays with spectral and
temporal properties consistent with observed gamma-ray bursts.Comment: Final version. 30 pages, 10 figures, 6 tables, accepted for
publication in The Astrophysical Journa
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