11,319 research outputs found
Direct Determinations of the Redshift Behavior of the Pressure, Energy Density, and Equation of State of the Dark Energy and the Acceleration of the Universe
One of the goals of current cosmological studies is the determination of the
expansion and acceleration rates of the universe as functions of redshift, and
the determination of the properties of the dark energy that can explain these
observations. Here the expansion and acceleration rates are determined directly
from the data, without the need for the specification of a theory of gravity,
and without adopting an a priori parameterization of the form or redshift
evolution of the dark energy. We use the latest set of distances to SN standard
candles from Riess et al. (2004), supplemented by data on radio galaxy standard
ruler sizes, as described by Daly and Djorgovski (2003, 2004). We find that the
universe transitions from acceleration to deceleration at a redshift of about
0.4. The standard "concordance model" provides a reasonably good fit to the
dimensionless expansion rate as a function of redshift, though it fits the
dimensionless acceleration rate as a function of redshift less well. The
expansion and acceleration rates are then combined with a theory of gravity to
determine the pressure, energy density, and equation of state of the dark
energy as functions of redshift. Adopting General Relativity as the correct
theory of gravity, the redshift trends for the pressure, energy density, and
equation of state of the dark energy out to redshifts of about one are
determined, and are found to be generally consistent with the concordance
model.Comment: 8 pages, 5 figures. Invited presentation at Coral Gables 200
Thrombotic variables and risk of idiopathic venous thromboembolism in women aged 45-64 years - Relationships to hormone replacement therapy
Hormone replacement therapy (HRT) has been shown to increase the relative risk of idiopathic venous thromboembolism (VTE) about threefold in several observational studies and one randomised controlled trial. Whether or not this relative risk is higher in women with underlying thrombophilia phenotypes, such as activated protein C (APC) resistance, is unknown. We therefore restudied the participants in a case-control study of the relationship between the use of HRT and the occurrence of idiopathic VTE in women aged 45-64 years. After protocol exclusions, 66 of the cases in the original study and 163 of the controls were studied. Twenty haematological variables relevant to risk of VTE were analysed, including thrombotic states defined from the literature. The relative risk of VTE showed significant associations with APC resistance (OR 4.06; 95% CI 1.62, 10.21); low antithrombin (3.33; 1.15, 9.65) or protein C (2.93; 1.06, 8.14); and high coagulation factor IX (2.34: 1.26, 1.35), or fibrin D-dimer (3.84; 1.99, 7.32). HRT use increased the risk of VTE in women without any of these thrombotic static; (OR 4.09; 95% CI 1.26, 13.30). A similar effect of HRT use on the relative risk of VTE was also found in women with prothrombotic states. Thus for example, the combination of HRT use and APC resistance increased the risk of VTE about 13-fold compared with women of similar age without either APC resistance or HRT use (OR 13.27; 95%, CI 4.30, 40.97). We conclude that the combination of HRT use and thrombophilias (especially if multiple) increases the relative risk of VTE substantially; hence women known to have thrombophilias (especially if multiple) should be counselled about this increased risk prior to prescription of HRT. However. HRT increases the risk of VTE about fourfold even in women without any thrombotic abnormalities: possible causes are discussed
The Relationship Between Beam Power and Radio Power for Classical Double Radio Sources
Beam power is a fundamental parameter that describes, in part, the state of a
supermassive black hole system. Determining the beam powers of powerful
classical double radio sources requires substantial observing time, so it would
be useful to determine the relationship between beam power and radio power so
that radio power could be used as a proxy for beam power. A sample of 31
powerful classical double radio sources with previously determined beam and
radio powers are studied; the sources have redshifts between about 0.056 and
1.8. It is found that the relationship between beam power, Lj, and radio power,
P, is well described by Log(Lj) = 0.84 Log(P) + 2.15, where both L_j and P are
in units of 10^(44) erg/s. This indicates that beam power is converted to radio
power with an efficiency of about 0.7%. The ratio of beam power to radio power
is studied as a function of redshift; there is no significant evidence for
redshift evolution of this ratio over the redshift range studied. The
relationship is consistent with empirical results obtained by Cavagnolo et al.
(2010) for radio sources in gas rich environments, which are primarily FRI
sources, and with the theoretical predictions of Willott et al. (1999).Comment: 6 pages, 2 figures, 2 tables; accepted for publication in MNRA
Water balance complexities in ephemeral catchments with different land uses: Insights from monitoring and distributed hydrologic modeling
Although ephemeral catchments are widespread in arid and semiarid climates, the relationship of their water balance with climate, geology, topography, and land cover is poorly known. Here we use 4 years (2011–2014) of rainfall, streamflow, and groundwater level measurements to estimate the water balance components in two adjacent ephemeral catchments in south-eastern Australia, with one catchment planted with young eucalypts and the other dedicated to grazing pasture. To corroborate the interpretation of the observations, the physically based hydrological model CATHY was calibrated and validated against the data in the two catchments. The estimated water balances showed that despite a significant decline in groundwater level and greater evapotranspiration in the eucalypt catchment (104–119% of rainfall) compared with the pasture catchment (95–104% of rainfall), streamflow consistently accounted for 1–4% of rainfall in both catchments for the entire study period. Streamflow in the two catchments was mostly driven by the rainfall regime, particularly rainfall frequency (i.e., the number of rain days per year), while the downslope orientation of the plantation furrows also promoted runoff. With minimum calibration, the model was able to adequately reproduce the periods of flow in both catchments in all years. Although streamflow and groundwater levels were better reproduced in the pasture than in the plantation, model-computed water balance terms confirmed the estimates from the observations in both catchments. Overall, the interplay of climate, topography, and geology seems to overshadow the effect of land use in the study catchments, indicating that the management of ephemeral catchments remains highly challenging
Improved Constraints on the Acceleration History of the Universe and the Properties of the Dark Energy
We extend and apply a model-independent analysis method developed earlier by
Daly & Djorgovski to new samples of supernova standard candles, radio galaxy
and cluster standard rulers, and use it to constrain physical properties of the
dark energy as functions of redshift. Similar results are obtained for the
radio galaxy and supernova data sets. The first and second derivatives of the
distance are compared directly with predictions in a standard model based on
General Relativity. The good agreement indicates that General Relativity
provides an accurate description of the data on look-back time scales of about
ten billion years. The first and second derivatives are combined to obtain the
acceleration parameter, assuming only the validity of the Robertson-Walker
metric, independent of a theory of gravity and of the physical nature of the
dark energy. The acceleration of the universe at the current epoch is indicated
by the analysis. The effect of non-zero space curvature on q(z) is explored. We
solve for the pressure, energy density, equation of state, and potential and
kinetic energy of the dark energy as functions of redshift assuming that
General Relativity is the correct theory of gravity, and the results indicate
that a cosmological constant in a spatially flat universe provides a good
description of each of these quantities over the redshift range from zero to
about one. We define a new function, the dark energy indicator, in terms of the
first and second derivatives of the coordinate distance and show how this can
be used to measure deviations of w from -1 and to obtain a new and independent
measure of Omega.Comment: 46 pages, submitted for publicatio
A Detailed Study of the Lobes of Eleven Powerful Radio Galaxies
Radio lobes of a sample of eleven very powerful classical double radio
galaxies were studied. Each source was rotated so that the symmetry axis of the
source was horizontal, and vertical cross-sectional cuts were taken across the
source at intervals of one beam size. These were used to study the
cross-sectional surface brightness profiles, the width of each slice, radio
emissivity as a function of position across each slice, the first and second
moments, and the average surface brightness, minimum energy magnetic field
strength, and pressure of each slice. A Gaussian provides a good description of
the surface brightness profile of cross-sectional slices. The Gaussian FWHM as
a function of distance from the hot spot first increases and then decreases
with distance from the hot spot. The width as a function of distance from the
hot spot is highly symmetric on each side of the source. The radio emissivity
is often close to flat across a slice, indicating a roughly constant emissivity
and pressure for that slice. Some slices show variations in radio emissivity
that indicate an ``edge-peaked'' pressure profile for that slice; these often
occur in slices near the local maxima of the bridge width. The emissivity does
not exhibit any signature of emission from a jet. The first moment is generally
quite close to zero indicating only small excursions of the ridge line from the
symmetry axis of the source. The second moment indicates the same source shape
as is found using the Gaussian FWHM. The average magnetic field strength and
pressure decrease with increasing distance from the hot spot, reaching a
roughly constant value at a location that is typically just before the location
of a local maximum of the bridge width. These results are interpreted in terms
of a heuristic model for the radio lobes.Comment: 102 pages, 136 figures, accepted for publication in ApJ Supplement
Serie
Cosmological Studies with Radio Galaxies and Supernovae
Physical sizes of extended radio galaxies can be employed as a cosmological
"standard ruler", using a previously developed method. Eleven new radio
galaxies are added to our previous sample of nineteen sources, forming a sample
of thirty objects with redshifts between 0 and 1.8. This sample of radio
galaxies are used to obtain the best fit cosmological parameters in a
quintessence model in a spatially flat universe, a cosmological constant model
that allows for non-zero space curvature, and a rolling scalar field model in a
spatially flat universe. Results obtained with radio galaxies are compared with
those obtained with different supernova samples, and with combined radio galaxy
and supernova samples. Results obtained with different samples are consistent,
suggesting that neither method is seriously affected by systematic errors. Best
fit radio galaxy and supernovae model parameters determined in the different
cosmological models are nearly identical, and are used to determine
dimensionless coordinate distances to supernovae and radio galaxies, and
distance moduli to the radio galaxies. The distance moduli to the radio
galaxies can be combined with supernovae samples to increase the number of
sources, particularly high-redshift sources, in the samples. The constraints
obtained here with the combined radio galaxy plus supernovae data set in the
rolling scalar field model are quite strong. The best fit parameter values
suggest a value of omega is less than about 0.35, and the model parameter alpha
is close to zero; that is, a cosmological constant provides a good description
of the data. We also obtain new constraints on the physics of engines that
power the large-scale radio emission.Comment: 32 pages. Accepted for publication in the Astrophysical Journa
Global Cosmological Parameters Determined Using Classical Double Radio Galaxies
A sample of 20 powerful extended radio galaxies with redshifts between zero
and two were used to determine constraints on global cosmological parameters.
Data for six radio sources were obtained from the VLA archive, analyzed, and
combined with the sample of 14 radio galaxies used previously by Guerra & Daly
to determine cosmological parameters. The results are consistent with our
previous results, and indicate that the current value of the mean mass density
of the universe is significantly less than the critical value. A universe with
of unity is ruled out at 99.0% confidence, and the best fitting
values of in matter are and
assuming zero space curvature and zero cosmological
constant, respectively. Note that identical results obtain when the low
redshift bin, which includes Cygnus A, is excluded; these results are
independent of whether the radio source Cygnus A is included. The method does
not rely on a zero-redshift normalization.
The radio properties of each source are also used to determine the density of
the gas in the vicinity of the source, and the beam power of the source. The
six new radio sources have physical characteristics similar to those found for
the original 14 sources. The density of the gas around these radio sources is
typical of gas in present day clusters of galaxies. The beam powers are
typically about .Comment: 39 pages includes 21 figures, accepted to Ap
Mechanism of droplet-formation in a supersonic microfluidic spray device
Spray drying is an approach employed in automotive, food, and pharmaceutical industries as a robust and cost efficient liquid atomization technique offering direct control over droplet dimensions. The majority of commercially available spray nozzles are designed for large throughput spray drying applications or uniform surface coating, but microfluidic nebulizers have recently been developed as small scale alternatives. Here, we explore the physical parameters that define the droplet size and formation under supersonic flow conditions commonly found in microfluidic spray drying systems. We examined the spray nozzle operation using high speed imaging and laser scattering measurements, which allowed us to describe the spray regimes and droplet size distributions. It was determined that by using this spray nozzle device, droplets with diameters of 4–8 μm could be generated. Moreover, we show that the supersonic de Laval nozzle model can be used to predict the average droplet size. Our approach can be used as a platform for interfacing fluid microprocessing with gas phase detection and characterization
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