712 research outputs found
The Axiverse Extended: Vacuum Destabilisation, Early Dark Energy and Cosmological Collapse
A model is presented in the philosophy of the "String Axiverse" of Arvanitaki
et al (arXiv:0905.4720v2 [hep-th]) that incorporates a coupling of ultralight
axions to their corresponding moduli through the mass term. The light fields
roll in their potentials at late times and contribute to the dark sector energy
densities in the cosmological expansion. The addition of a coupling and extra
field greatly enrich the possible phenomenology of the axiverse. There are a
number of interesting phases where the axion and modulus components behave as
Dark Matter or Dark Energy and can have considerable and distinct effects on
the expansion history of the universe by modifying the equation of state in the
past or causing possible future collapse of the universe. In future such a
coupling may help to alleviate fine tuning problems for cosmological axions. We
motivate and present the model, and briefly explore its cosmological
consequences numerically.Comment: 13 pages, 17 figures, published in PRD. v3: corrected SUSY
interpretation of axion potential scal
Gravitational waves from an early matter era
We investigate the generation of gravitational waves due to the gravitational
instability of primordial density perturbations in an early matter-dominated
era which could be detectable by experiments such as LIGO and LISA. We use
relativistic perturbation theory to give analytic estimates of the tensor
perturbations generated at second order by linear density perturbations. We
find that large enhancement factors with respect to the naive second-order
estimate are possible due to the growth of density perturbations on sub-Hubble
scales. However very large enhancement factors coincide with a breakdown of
linear theory for density perturbations on small scales. To produce a
primordial gravitational wave background that would be detectable with LIGO or
LISA from density perturbations in the linear regime requires primordial
comoving curvature perturbations on small scales of order 0.02 for Advanced
LIGO or 0.005 for LISA, otherwise numerical calculations of the non-linear
evolution on sub-Hubble scales are required.Comment: 23 pages, 2 figure
Recommended from our members
Aseptic Barriers Allow a Clean Contact for Contaminated Stethoscope Diaphragms.
Objective:To determine whether a single-use stethoscope diaphragm barrier surface remains aseptic when placed on pathogen-contaminated stethoscopes. Methods:From May 31 to August 5, 2019, we tested 2 separate barriers using 3 different strains of 7 human pathogens, including extended-spectrum β-lactamase-producing Escherichia coli, methicillin-resistant Staphylococcus aureus, and vancomycin resistant Enterococcus faecium. Results:For all diaphragms with either of the 2 barriers tested, no growth was recorded for any of the pathogens. Stethoscopes with aseptic barriers remained sterile for up to 24 hours. These single-use barriers also provided aseptic surfaces when stethoscope diaphragms were inoculated with human specimens, including saliva, stool, urine, and sputum. Conclusion:Disposable aseptic diaphragm barriers may provide robust and efficient solutions to reduce transmission of pathogens via stethoscopes
Optimizing future imaging survey of galaxies to confront dark energy and modified gravity models
We consider the extent to which future imaging surveys of galaxies can
distinguish between dark energy and modified gravity models for the origin of
the cosmic acceleration. Dynamical dark energy models may have similar
expansion rates as models of modified gravity, yet predict different growth of
structure histories. We parameterize the cosmic expansion by the two
parameters, and , and the linear growth rate of density fluctuations
by Linder's , independently. Dark energy models generically predict
, while the DGP model . To determine
if future imaging surveys can constrain within 20 percent (or
), we perform the Fisher matrix analysis for a weak lensing
survey such as the on-going Hyper Suprime-Cam (HSC) project. Under the
condition that the total observation time is fixed, we compute the Figure of
Merit (FoM) as a function of the exposure time \texp. We find that the
tomography technique effectively improves the FoM, which has a broad peak
around \texp\simeq {\rm several}\sim 10 minutes; a shallow and wide survey is
preferred to constrain the parameter. While
cannot be achieved by the HSC weak-lensing survey alone, one can improve the
constraints by combining with a follow-up spectroscopic survey like WFMOS
and/or future CMB observations.Comment: 18 pages, typos correcte
High-Redshift Galaxies in Cold Dark Matter Models
We use hydrodynamic cosmological simulations to predict the star formation
properties of high-redshift galaxies (z=2-6) in five variants of the
inflationary cold dark matter scenario, paying particular attention to z=3, the
redshift of the largest "Lyman-break galaxy" (LBG) samples. Because we link the
star formation timescale to the local gas density, the rate at which a galaxy
forms stars is governed mainly by the rate at which it accretes cooled gas from
the surrounding medium. At z=3, star formation in most of the simulated
galaxies is steady on 200 Myr timescales, and the instantaneous star formation
rate (SFR) is correlated with total stellar mass. However, there is enough
scatter in this correlation that a sample selected above a given SFR threshold
may contain galaxies with a fairly wide range of masses. The redshift history
and global density of star formation in the simulations depend mainly on the
amplitude of mass fluctuations in the underlying cosmological model. The three
models whose mass fluctuation amplitudes agree with recent analyses of the
Lyman-alpha forest also reproduce the observed luminosity function of LBGs
reasonably well, though the dynamic range of the comparison is small and the
theoretical and observational uncertainties are large. The models with higher
and lower amplitudes appear to predict too much and too little star formation,
respectively, though they are not clearly ruled out. The intermediate amplitude
models predict SFR ~ 30-40 Msun/yr for galaxies with a surface density 1 per
arcmin^2 per unit redshift at z=3. They predict much higher surface densities
at lower SFR, and significant numbers of galaxies with SFR > 10 Msun/yr at z >=
5.Comment: Submitted to ApJ. 31 pages including 10 ps figures. Full resolution
version of Fig 2 available at
http://www.astronomy.ohio-state.edu/~dhw/Sph/zgal.fig2.ps.g
The Halo Occupation Distribution: Towards an Empirical Determination of the Relation Between Galaxies and Mass
We investigate galaxy bias in the framework of the ``Halo Occupation
Distribution'' (HOD), which defines the bias of a population of galaxies by the
conditional probability P(N|M) that a dark matter halo of virial mass M
contains N galaxies, together with prescriptions that specify the relative
spatial and velocity distributions of galaxies and dark matter within halos. By
populating the halos of a cosmological N-body simulation using a variety of HOD
models, we examine the sensitivity of different galaxy clustering statistics to
properties of the HOD. The galaxy correlation function responds to different
aspects of P(N|M) on different scales. Obtaining the observed power-law form of
xi(r) requires rather specific combinations of HOD parameters, implying a
strong constraint on the physics of galaxy formation; the success of numerical
and semi-analytic models in reproducing this form is entirely non-trivial.
Other clustering statistics such as the galaxy-mass correlation function, the
bispectrum, the void probability function, the pairwise velocity dispersion,
and the group multiplicity function are sensitive to different combinations of
HOD parameters and thus provide complementary information about galaxy bias. We
outline a strategy for determining the HOD empirically from redshift survey
data. This method starts from an assumed cosmological model, but we argue that
cosmological and HOD parameters will have non-degenerate effects on galaxy
clustering, so that a substantially incorrect cosmological model will not
reproduce the observations for any choice of HOD. Empirical determinations of
the HOD as a function of galaxy type from the 2dF and SDSS redshift surveys
will provide a detailed target for theories of galaxy formation, insight into
the origin of galaxy properties, and sharper tests of cosmological models.Comment: 60 pages + 21 eps figures. Replaced with accepted ApJ version. Minor
changes + added reference
Determination of the Baryon Density from Large Scale Galaxy Redshift Surveys
We estimate the degree to which the baryon density, , can be
determined from the galaxy power spectrum measured from large scale galaxy
redshift surveys, and in particular, the Sloan Digital Sky Survey. A high
baryon density will cause wiggles to appear in the power spectrum, which should
be observable at the current epoch. We assume linear theory on scales and do not include the effects of redshift distortions, evolution,
or biasing. With an optimum estimate of to ,
the uncertainties in are roughly 0.07 and 0.016 in flat
and open () cosmological models, respectively. This result
suggests that it should be possible to test for consistency with big bang
nucleosynthesis estimates of if we live in an open universe.Comment: 23 Pages, 10 Postscript figure
Cosmology in a String-Dominated Universe
The string-dominated universe locally resembles an open universe, and fits
dynamical measures of power spectra, cluster abundances, redshift distortions,
lensing constraints, luminosity and angular diameter distance relations and
microwave background observations. We show examples of networks which might
give rise to recent string-domination without requiring any fine-tuned
parameters. We discuss how future observations can distinguish this model from
other cosmologies.Comment: 17 pages including 4 figures, of which one is in colo
Endemic invasive amoebiasis in northern Australia
In October 2000, a 10-year-old Aboriginal boy from the Darwin region of the Northern Territory was referred to hospital with a 24-hour history of abdominal pain, initially generalised, but then localising to the right iliac fossa. The pain was accompanied by occasional vomiting, but no fever or diarrhoea was noted. At laparotomy, a gangrenous, unruptured appendix was removed. Postoperatively, the patient made a good recovery. Neither he nor any family members had travelled outside the Northern Territory.Histological sections of the surgical specimen showed changes typical of acute suppurative appendicitis. Closer examination, however, revealed numerous round-to-oval structures resembling trophozoites (see Box). When the possibility of invasive amoebiasis was raised, staining of the section with Entamoeba histolytica-specific sera confirmed the diagnosis. E. histolytica serology was negative
Real world costs and cost-effectiveness of Rituximab for diffuse large B-cell lymphoma patients: a population-based analysis.
BackgroundCurrent treatment of diffuse-large-B-cell lymphoma (DLBCL) includes rituximab, an expensive drug, combined with cyclophosphamide, doxorubicin, vincristine, and prednisone (CHOP) chemotherapy. Economic models have predicted rituximab plus CHOP (RCHOP) to be a cost-effective alternative to CHOP alone as first-line treatment of DLBCL, but it remains unclear what its real-world costs and cost-effectiveness are in routine clinical practice.MethodsWe performed a population-based retrospective cohort study from 1997 to 2007, using linked administrative databases in Ontario, Canada, to evaluate the costs and cost-effectiveness of RCHOP compared to CHOP alone. A historical control cohort (n = 1,099) with DLBCL who received CHOP before rituximab approval was hard-matched on age and treatment intensity and then propensity-score matched on sex, comorbidity, and histology to 1,099 RCHOP patients. All costs and outcomes were adjusted for censoring using the inverse probability weighting method. The main outcome measure was incremental cost per life-year gained (LYG).ResultsRituximab was associated with a life expectancy increase of 3.2 months over 5 years at an additional cost of 61,984 (95% CI 135,890) per LYG. The probability of being cost-effective was 90% if the willingness-to-pay threshold was 31,800/LYG) but increased to 110,100/LYG for patients ≥ 80 years old. We found that post-market survival benefits of rituximab are similar to or lower than those reported in clinical trials, while the costs, incremental costs and cost-effectiveness ratios are higher than in published economic models and differ by age.ConclusionsOur results showed that the addition of rituximab to standard CHOP chemotherapy was associated with improvement in survival but at a higher cost, and was potentially cost-effective by standard thresholds for patients <60 years old. However, cost-effectiveness decreased significantly with age, suggesting that rituximab may be not as economically attractive in the very elderly on average. This has important clinical implications regarding age-related use and funding decisions on this drug
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