1,273 research outputs found
Proper Size of the Visible Universe in FRW Metrics with Constant Spacetime Curvature
In this paper, we continue to examine the fundamental basis for the
Friedmann-Robertson-Walker (FRW) metric and its application to cosmology,
specifically addressing the question: What is the proper size of the visible
universe? There are several ways of answering the question of size, though
often with an incomplete understanding of how far light has actually traveled
in reaching us today from the most remote sources. The difficulty usually
arises from an inconsistent use of the coordinates, or an over-interpretation
of the physical meaning of quantities such as the so-called proper distance
R(t)=a(t)r, written in terms of the (unchanging) co-moving radius r and the
universal expansion factor a(t). In this paper, we use the five non-trivial FRW
metrics with constant spacetime curvature (i.e., the static FRW metrics, but
excluding Minkowski) to prove that in static FRW spacetimes in which expansion
began from an initial signularity, the visible universe today has a proper size
equal to R_h(t_0/2), i.e., the gravitational horizon at half its current age.
The exceptions are de Sitter and Lanczos, whose contents had pre-existing
positions away from the origin. In so doing, we confirm earlier results showing
the same phenomenon in a broad range of cosmologies, including LCDM, based on
the numerical integration of null geodesic equations through an FRW metric.Comment: Accepted for publication in Classical and Quantum Gravit
The use of social-studies materials in the teaching of renaissance art in the senior high school art class.
Thesis (Ed.M.)--Boston Universit
Developing ODP student placements
In a similar way to nursing and other health professions, elements of the education of operating department practitioners (ODPs) can only be contextualised by clinical practice involvement (Stockhausen and Strutt, 2005; Higginson, 2006; Morgan, 2006). The importance of high-quality placement experiences for all UK health professions is widely acknowledged (Quality Assurance Agency, 2001). Prior to 2009, students on the Diploma in Higher Education ODP programme undertook four clinical placements in the same Trust or organisation. At the time of the project there was a shortfall in the number of placements available and, although the university explored placing students in different clinical areas, such as private healthcare providers and new areas in the NHS, none could provide the full learning experience for students to achieve the required competencies. The course team developed a system that uses placements in a variety of settings and ensures students can gain the required outcomes. The new approach involves auditing for individual placements, instead of for the full course duration. Students are now informed throughout recruitment and selection events that they should be undertaking placements in a minimum of three different organisations. Students now move to a different organisation with every experience, to gain a variety of clinical knowledge. This has resulted in the greater use of clinical placements and the development of new areas for students to gain experience. The benefits of students’ experiences have increased, as they can develop skills and understanding of the ODP role from different perspectives
Tropical Pacific spatial trend patterns in observed sea level: internal variability and/or anthropogenic signature?
In this study we focus on the sea level trend pattern observed by satellite altimetry in the tropical Pacific over the 1993–2009 time span (i.e. 17 yr). Our objective is to investigate whether this 17-yr-long trend pattern was different before the altimetry era, what was its spatio-temporal variability and what have been its main drivers. We try to discriminate the respective roles of the internal variability of the climate system and of external forcing factors, in particular anthropogenic emissions (greenhouse gases and aerosols). On the basis of a 2-D past sea level reconstruction over 1950–2009 (based on a combination of observations and ocean modelling) and multi-century control runs (i.e. with constant, preindustrial external forcing) from eight coupled climate models, we have investigated how the observed 17-yr sea level trend pattern evolved during the last decades and centuries, and try to estimate the characteristic time scales of its variability. For that purpose, we have computed sea level trend patterns over successive 17-yr windows (i.e. the length of the altimetry record), both for the 60-yr long reconstructed sea level and the model runs. We find that the 2-D sea level reconstruction shows spatial trend patterns similar to the one observed during the altimetry era. The pattern appears to have fluctuated with time with a characteristic time scale of the order of 25–30 yr. The same behaviour is found in multi-centennial control runs of the coupled climate models. A similar analysis is performed with 20th century coupled climate model runs with complete external forcing (i.e. solar plus volcanic variability and changes in anthropogenic forcing). Results suggest that in the tropical Pacific, sea level trend fluctuations are dominated by the internal variability of the ocean–atmosphere coupled system. While our analysis cannot rule out any influence of anthropogenic forcing, it concludes that the latter effect in that particular region is stillhardly detectable
A Magnetic Dynamo Origin For The Sub-mm Excess In Sgr A*
The sub-mm bump observed in the spectrum of Sgr A* appears to indicate the
existence of a compact emitting component within several Schwarzschild radii,
, of the nucleus at the Galactic Center. This is interesting in view of
the predicted circularized flow within , based on detailed
multi-dimensional hydrodynamic simulations of Bondi-Hoyle accretion onto this
unusual object. In this paper, we examine the physics of magnetic field
generation by a Keplerian dynamo subject to the conditions pertaining to Sgr
A*, and show that the sub-mm bump can be produced by thermal synchrotron
emission in this inner region. This spectral feature may therefore be taken as
indirect evidence for the existence of this circularization. In addition, the
self-Comptonization of the sub-mm bump appears to produce an X-ray flux
exceeding that due to bremsstrahlung from this region, which may account for
the X-ray counterpart to Sgr A* discovered recently by {\it Chandra}. However,
the required accretion rate in the Keplerian flow is orders of magnitude
smaller than that predicted by the Bondi-Hoyle simulations. We speculate that
rapid evaporation, in the form of a wind, may ensue from the heating associated
with turbulent mixing of gas elements with large eccentricity as they settle
down into a more or less circular (i.e., low eccentricity) trajectory. The
spectrum of Sgr A* longward of mm may be generated outside of the
Keplerian flow, where the gas is making a transition from a quasi-spherical
infall into a circularized pattern.Comment: 40 pages, 9 figure
Polarimetric Imaging of the Massive Black Hole at the Galactic Center
The radio source Sgr A* in the Galactic center emits a polarized spectrum at
millimeter and sub-millimeter wavelengths that is strongly suggestive of
relativistic disk accretion onto a massive black hole. We use the
well-constrained mass of Sgr A* and a magnetohydrodynamic model of the
accretion flow to match both the total flux and polarization from this object.
Our results demonstrate explicitly that the shift in the position angle of the
polarization vector, seen at wavelengths near the peak of the mm to sub-mm
emission from this source, is a signal of relativistic accretion flow in a
strong gravitational field. We provide maps of the polarized emission to
illustrate how the images of polarized intensity from the vicinity of the black
hole would appear in upcoming observations with very long baseline radio
interferometers (VLBI). Our results suggest that near-term VLBI observations
will be able to directly image the polarized Keplerian portion of the flow near
the horizon of the black hole.Comment: 12 pages, 2 figures, Accepted for publciation in ApJ Letter
Accretion Disk Evolution With Wind Infall I. General Solution and Application to Sgr A*
The evolution of an accretion disk can be influenced significantly by the
deposition of mass and angular momentum by an infalling Bondi-Hoyle wind. Such
a mass influx impacts the long-term behavior of the disk by providing
additional sources of viscosity and heating. In this paper, we derive and solve
the disk equations when these effects are taken into account. We present a
survey of models with various wind configurations and demonstrate that the disk
spectrum may then differ substantially from that of a standard alpha-disk. In
particular, it is likely that a wind-fed disk has a significant infrared bump
due to the deposition of energy in its outer region. We apply some of the
results of our calculations to the Galactic Center black hole candidate Sgr A*
and show that if a fossil disk is present in this source, it must have a very
low viscosity parameter (alpha<10^-4) and the Bondi-Hoyle wind must be
accreting with a very high specific angular momentum to prevent it from
circularizing in the inner disk region where its impact would be most
noticeable.Comment: accepted for The Astrophysical Journal, AAS LaTex, 20 pages, also
available at http://www.astro.umd.edu/~hfalcke/publications.html#wintercep
Plasma Perturbations and Cosmic Microwave Background Anisotropy in the Linearly Expanding Milne-like Universe
We expose the scenarios of primordial baryon-photon plasma evolution within
the framework of the Milne-like universe models. Recently, such models find a
second wind and promise an inflation-free solution of a lot of cosmological
puzzles including the cosmological constant one. Metric tensor perturbations
are considered using the five-vectors theory of gravity admitting the Friedmann
equation satisfied up to some constant. The Cosmic Microwave Background (CMB)
spectrum is calculated qualitatively.Comment: 20 page
Analisis Komitmen Organisasi Melalui Faktor Quality of Work Life (Studi Kasus Fakultas Ekonomi Dan Manajemen Institut Pertanian Bogor)
The purpose of this study were (1) to analyze the Quality of work life (QWL)condition in FEM, (2) to analyze the relationship between organizational commitment with the Quality of work life (QWL) in FEM, (3) to analyze Quality of Work Life (QWL) factors was the most influential on organizational commitment in the FEM. Type of data used in this study are primary and secondary data. Census method was the sampling method was used in this research. Processing and data analysis in this study was using SPSS software, Minitab, and Microsoft Excel. Results from this study indicated that the WQL has been pretty good in the FEM IPB. The relationship between quality of work life with organizational commitment has been good with a Spearman correlation showed a positive relationship, significant and rather strong. Quality of work life factors that most influence on organizational commitment is the integration of work environment factors with coefficients of a high standard compared to other factors that QWL
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