451 research outputs found
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
High-Redshift Radio Galaxies as a Cosmological Tool: Exploration of a Key Assumption and Comparison with Supernova Results
There are many different approaches to using observations to constrain or
determine the global cosmological parameters that describe our universe.
Methods that rely upon a determination of the coordinate distance to
high-redshift sources are particularly useful because they do not involve
assumptions about the clustering properties of matter, or the evolution of this
clustering.
Two of the methods currently being used to determine the coordinate distance
to high-redshift sources are the radio galaxy method and the supernova method.
These methods are similar in their dependence on the coordinate distance. Here,
the radio galaxy method is briefly described and results are presented. One of
the underlying assumptions of the method is explored. In addition, the method
is compared and contrasted to the supernova method. The constraints imposed on
global cosmological parameters by radio galaxies are consistent with those
imposed by supernovae.
For a universe that is spatially flat with mean mass density in
non-relativistic matter and mean mass density 1- in quintessence,
radio galaxies alone indicate at 84 % confidence that the expansion of the
universe is accelerating at the current epoch. And, independent of whether or
not the universe is spatially flat, radio galaxies alone indicate at 95 %
confidence that must be less than 0.6 at the current epoch.Comment: 8 page
Early Permian Zircon Ages from the \u3cem\u3eP. confluens\u3c/em\u3e and \u3cem\u3eP. pseudoreticulata\u3c/em\u3e Spore-Pollen Zones in the Southern Bonaparte and Canning Basins, Northwestern Australia
The Pseudoreticulatispora confluensâP. pseudoreticulata spore-pollen zonal datum typically coincides with the end of widespread Permian glacial deposits in Western Australia. Although previously attributed to the mid-Sakmarian, chemical abrasion isotope dilution thermal ionisation mass spectrometry (TIMS) dating of zircons from volcanic tuffs in the Ditji Formation of the Bonaparte Basin and the Grant Group in the Canning Basin point to an Asselian age of about 295.25âMa for this datum. All dated zircons from the Ditji Formation came from petroleum well cuttings but the accompanying palynology was mostly from sidewall cores; however, all Grant Group samples were from conventional core. TIMS dates from the Ditji Formation range in age from 295.2 to 292.7âMa whereas the only productive tuff from the Grant Group yielded a 296.26âMa date. By comparison, there are no zircon dates to constrain the onset of glacial deposition in Australia. The Bonaparte Basin ages overlap with those for the Edie Tuff (296.1â294.5âMa) in Queenslandâs Galilee Basin, approximately 2000âkm to the southeast, which also lies close to the base of the P. pseudoreticulata Zone. To date the only fossil group within the P. confluens Zone in Western Australia to provide independent age control, albeit loosely, are goniatites from the northern Perth Basin (Uraloceras irwinense and Juresanites jacksoni) that have consistently been attributed to the Sakmarian; these require a reassessment of their affinity with Russian faunas and therefore to global stratotypes. The position of the CarboniferousâPermian boundary is elusive in Australia and will remain so until additional volcanic tuffs containing young datable zircons are found; however, spore-pollen and zircon dates from Namibia place this boundary within the P. confluens Zone
3-D Perturbations in Conformal Turbulence
The effects of three-dimensional perturbations in two-dimensional turbulence
are investigated, through a conformal field theory approach. We compute scaling
exponents for the energy spectra of enstrophy and energy cascades, in a strong
coupling limit, and compare them to the values found in recent experiments. The
extension of unperturbed conformal turbulence to the present situation is
performed by means of a simple physical picture in which the existence of small
scale random forces is closely related to deviations of the exact
two-dimensional fluid motion.Comment: Discussion of intermittency improved. Figure include
Structure of a Bathtub Vortex : Importance of the Bottom Boundary Layer
A bathtub vortex in a cylindrical tank rotating at a constant angular velocity [omega] is studied by meansof a laboratory experiment, a numerical experiment and a boundary layer theory. The laboratory and numerical experiments show that two regimes of vortices in the steady-state can occur depending on [omega] and the volume flux Q through the drain hole: when Q is large and [omega] is small, a potential vortex is formed in which angular momentum outside the vortex core is constant in the non-rotating frame. However, when Q is small or [omega] is large, a vortex is generated in which the angular momentum decreases with decreasing radius. Boundary layertheory shows that the vortex regimes strongly depend on the theoretical radial volume flux through the bottomboundary layer under a potential vortex : when the ratio of Q to the theoretical boundary-layer radial volume flux Qb (scaled by 2Ï R2([omega] Îœ)12 ) at the outer rim of the vortex core is larger than a critical value (of order 1), the radial flow in the interior exists at all radiiand Regime I is realized, where R is the inner radius of the tank and Îœ the kinematicviscosity.When the ratio is less than the critical value, the radial flow in the interior nearlyvanishes inside a critical radius and almost all of the radial volume flux occurs only in the boundary layer,resulting in Regime II in which the angular momentum is not constant with radius. This criterion is found to explain the results of the laboratory and numerical experiments very well
The Leech and the Physician: Biology, Etymology, and Medical Practice with Hirudinea medicinalis
The history of the word âleechâ and the practice of leeching reveal interconnected social histories. We give the linguistic and medical histories of the word, and explore its biology and clinical history. Our historical account extends from the earliest known record of leeching to current research. Despite historical variation in its reputation as a therapeutic technique, leeching remains useful today in a number of applications. Further investigation may well disclose even more uses for the leech, particularly for its enzymes with anesthetic, anticoagulant, and antimetastatic properties.Peer Reviewedhttp://deepblue.lib.umich.edu/bitstream/2027.42/42411/1/268-24-7-878_00240878.pd
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
Active Flow Control on Vertical Tail Models
Active flow control (AFC) subscale experiments were conducted at the Lucas Wind Tunnel of the California Institute of Technology. Tests were performed on a generic vertical tail model at low speeds. Fluidic oscillators were used at the trailing edge of the main element (vertical stabilizer) to redirect the flow over the rudder and delay or prevent flow separation. Side force increases in excess of 50% were achieved with a 2% momentum coefficient (C_ÎŒ) input. The results indicated that a collective C_ÎŒ of about 1% could increase the side force by 30â50%. This result is achieved by reducing the spanwise flow on the swept back wings that contributes to early flow separation near their tips. These experiments provided the technical backdrop to test the full-scale Boeing 757 vertical tail model equipped with a fluidic oscillator system at the National Full-scale Aerodynamics Complex 40-by 80-foot Wind Tunnel, NASA Ames Research Center. The C_ÎŒ is shown to be an important parameter for scaling a fluidic oscillator AFC system from subscale to full-scale wind tunnel tests. The results of these tests provided the required rationale to use a fluidic oscillator AFC configuration for a follow-on flight test on the Boeing 757 ecoDemonstrator
Active Flow Control on Vertical Tail Models
Active flow control (AFC) subscale experiments were conducted at the Lucas Wind Tunnel of the California Institute of Technology. Tests were performed on a generic vertical tail model at low speeds. Fluidic oscillators were used at the trailing edge of the main element (vertical stabilizer) to redirect the flow over the rudder and delay or prevent flow separation. Side force increases in excess of 50% were achieved with a 2% momentum coefficient (C(sub )) input. The results indicated that a collective C(sub ) of about 1% could increase the side force by 3050%. This result is achieved by reducing the spanwise flow on the swept back wings that contributes to early flow separation near their tips. These experiments provided the technical backdrop to test the full-scale Boeing 757 vertical tail model equipped with a fluidic oscillator system at the National Full-scale Aerodynamics Complex 40-by 80-foot Wind Tunnel, NASA Ames Research Center. The C(sub ) is shown to be an important parameter for scaling a fluidic oscillator AFC system from subscale to full-scale wind tunnel tests. The results of these tests provided the required rationale to use a fluidic oscillator AFC configuration for a follow-on flight test on the Boeing 757 ecoDemonstrator
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