7,242 research outputs found
The Limits of Quintessence
We present evidence that the simplest particle-physics scalar-field models of
dynamical dark energy can be separated into distinct behaviors based on the
acceleration or deceleration of the field as it evolves down its potential
towards a zero minimum. We show that these models occupy narrow regions in the
phase-plane of w and w', the dark energy equation-of-state and its
time-derivative in units of the Hubble time. Restricting an energy scale of the
dark energy microphysics limits how closely a scalar field can resemble a
cosmological constant. These results, indicating a desired measurement
resolution of order \sigma(w')\approx (1+w), define firm targets for
observational tests of the physics of dark energy.Comment: 4 pages, 2 figure
Dark Energy Scaling from Dark Matter to Acceleration
The dark sector of the Universe need not be completely separable into
distinct dark matter and dark energy components. We consider a model of early
dark energy in which the dark energy mimics a dark matter component in both
evolution and perturbations at early times. Barotropic aether dark energy
scales as a fixed fraction, possibly greater than one, of the dark matter
density and has vanishing sound speed at early times before undergoing a
transition. This gives signatures not only in cosmic expansion but in sound
speed and inhomogeneities, and in number of effective neutrino species. Model
parameters describe the timing, sharpness of the transition, and the relative
abundance at early times. Upon comparison with current data, we find viable
regimes in which the dark energy behaves like dark matter at early times: for
transitions well before recombination the dark energy to dark matter fraction
can equal or exceed unity, while for transitions near recombination the ratio
can only be a few percent. After the transition, dark energy goes its separate
way, ultimately driving cosmic acceleration and approaching a cosmological
constant in this scenario.Comment: 10 pages, 8 figure
Plasma Wakefield Acceleration with a Modulated Proton Bunch
The plasma wakefield amplitudes which could be achieved via the modulation of
a long proton bunch are investigated. We find that in the limit of long bunches
compared to the plasma wavelength, the strength of the accelerating fields is
directly proportional to the number of particles in the drive bunch and
inversely proportional to the square of the transverse bunch size. The scaling
laws were tested and verified in detailed simulations using parameters of
existing proton accelerators, and large electric fields were achieved, reaching
1 GV/m for LHC bunches. Energy gains for test electrons beyond 6 TeV were found
in this case.Comment: 9 pages, 7 figure
The Paths of Quintessence
The structure of the dark energy equation of state phase plane holds
important information on the nature of the physics. We explain the bounds of
the freezing and thawing models of scalar field dark energy in terms of the
tension between the steepness of the potential vs. the Hubble drag.
Additionally, we extend the phase plane structure to modified gravity theories,
examine trajectories of models with certain properties, and categorize regions
in terms of scalar field hierarchical parameters, showing that dark energy is
generically not a slow roll phenomenon.Comment: 12 pages, 7 figures; matches PRD versio
Phantom Accretion by Five Dimensional Charged Black Hole
This paper deals with the dynamical behavior of phantom field near five
dimensional charged black hole. We formulate equations of motion for
steady-state spherically symmetric flow of phantom fluids. It is found that
phantom energy accretes onto black holes for . Further, the location of
critical point of accretion are evaluated that leads to mass to charge ratio
for 5D charged black hole. This ratio implies that accretion cannot transform a
black hole into a naked singularity. We would like to mention here that this
work is an irreducible extension of 4D charged black hole.Comment: 8 pages, accepted for publication in Mod. Phys. Lett.
Improving access and quality of care in a TB control programme
Objectives. To use a quality improvement approach to improve access to and quality of tuberculosis (TB) diagnosis and care in Cape Town.Methods. Five HIV/AIDS/sexually transmitted infections/TB (HAST) evaluations were conducted from 2008 to 2010, with interviews with 99 facility managers and a folder review of over 850 client records per evaluation cycle. The data were used in a local quality improvement process: sub-district workshops identified key weaknesses and facility managers drew up action plans. Lessons learnt and successful strategies were shared at quarterly districtwide HIV/TB meetings.Results. Geographical access was good, but there were delays in treatment commencement times. Access for high-risk clients improved significantly with intensified TB case finding made routine in both the HIV counselling and testing and antiretroviral treatment (ART) services (p<0.01 for both). Access for children in contact with an infectious case has improved but is still low (42% investigated and treated). Quality of care was mostly high atbaseline (adherence to treatment protocols 95%). Measurement of body mass index improved from 20% to 62%. The assessment of contraception improved from 27% to 58%. Care for co-infected clients showed improved use of customised HIV stationery and increased assessment for ART eligibility.Conclusions. The HAST audit contributed to the improved TB cure rates by supplementing routine information and involving sub-district managers, facility managers and facility staff in a quality improvement process that identified local opportunities for programme strengthening
Effect of plasma inhomogeneity on plasma wakefield acceleration driven by long bunches
Effects of plasma inhomogeneity on self-modulating proton bunches and
accelerated electrons were studied numerically. The main effect is the change
of the wakefield wavelength which results in phase shifts and loss of
accelerated particles. This effect imposes severe constraints on density
uniformity in plasma wakefield accelerators driven by long particle bunches.
The transverse two stream instability that transforms the long bunch into a
train of micro-bunches is less sensitive to density inhomogeneity than are the
accelerated particles. The bunch freely passes through increased density
regions and interacts with reduced density regions.Comment: 7 pages, 10 figure
Non-Gaussian features of primordial magnetic fields in power-law inflation
We show that a conformal-invariance violating coupling of the inflaton to
electromagnetism produces a cross correlation between curvature fluctuations
and a spectrum of primordial magnetic fields. According to this model, in the
case of power-law inflation, a primordial magnetic field is generated with a
nearly flat power spectrum and rms amplitude ranging from nG to pG. We study
the cross correlation, a three-point function of the curvature perturbation and
two powers of the magnetic field, in real and momentum space. The
cross-correlation coefficient, a dimensionless ratio of the three-point
function with the curvature perturbation and magnetic field power spectra, can
be several orders of magnitude larger than expected as based on the amplitude
of scalar metric perturbations from inflation. In momentum space, the
cross-correlation peaks for flattened triangle configurations, and is three
orders of magnitude larger than the squeezed triangle configuration. These
results suggest likely methods for distinguishing the observational signatures
of the model.Comment: 15 pages, 2 figure
Plant canopy shape and the influences on UV exposures to the canopy
The solar spectra at selected sites over hemispherical, conical and pinnacle plant canopy models has been evaluated with a dosimetric technique. The irradiance at the sites varies by up to a factor of 0.31 compared to the irradiance on a horizontal plane. The biologically effective (UVBE) exposures evaluated with the dosimetric technique at sites over the plant canopy are up to 19% of that on a horizontal plane. Compared to a spectroradiometer, the technique provides a more practicable method of measuring the UVBE exposures at multiple sites over a plant canopy. Usage of a dosimeter at one site to provide the exposures at that site for different sun angles introduces an error of more than 50%. Knowledge of the spectra allowed the UV and UVBE exposures to be calculated at each site along with the exposures to the entire canopies. These were dependent on the sun angle and the canopy shape. For plant damage, the UVBE was a maximum of about 1.4 mJ cm-2/min. Compared to the hemispherical canopy, the UVBE exposure for generalised plant damage was 45% less for the pinnacle canopy and 23% less for the conical canopy. The canopy exposures could not be determined from measurements of the ambient exposure
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