148 research outputs found
FRW Universe Models in Conformally Flat Spacetime Coordinates. III: Universe models with positive spatial curvature
We deduce general expressions for the line element of universe models with
positive spatial curvature described by conformally flat spacetime coordinates.
Models with dust, radiation and vacuum energy are exhibited. Discussing the
existence of particle horizons we show that there is continual annihilation of
space, matter and energy in a dust and radiation dominated universe, and
continual creation in a LIVE domined universe when conformal time is used in
Friedmann-Robertson-Walker models with positive spatial curvature. A general
procedure is given for finding coordinates to be used in Penrose diagrams. We
also calculate the age and the redshift of some universe models using conformal
time.Comment: 22 pages, 9 figure
Kinematic cosmology in conformally flat spacetime
In a recent series of papers Endean examines the properties of spatially
homogeneous and isotropic (FLRW) cosmological models filled with dust in the
``conformally flat spacetime presentation of cosmology'' (CFS cosmology). This
author claims it is possible to resolve a certain number of the difficulties
the standard model exhibits when confronted to observations, if the theoretical
predictions are obtained in the special framework of CFS cosmology. As a
by-product of his analysis Endean claims that no initial (big-bang) nor final
(big-crunch) singularities occur in the closed FLRW model. In this paper we
show up the fallacious arguments leading to Endean's conclusions and we
consistently reject his CFS cosmology.Comment: 5 pages, LaTeX2e, aas2pp4.sty, cases.sty, to be published in the
Astrophysical Journa
FRW Universe Models in Conformally Flat Spacetime Coordinates. I: General Formalism
The 3-space of a universe model is defined at a certain simultaneity. Hence
space depends on which time is used. We find a general formula generating all
known and also some new transformations to conformally flat spacetime
coordinates. A general formula for the recession velocity is deduced.Comment: 12 page
FRW Universe Models in Conformally Flat Spacetime Coordinates. II: Universe models with negative and vanishing spatial curvature
We deduce general expressions for the line element of universe models with
negative and vanishing spatial curvature described by conformally flat
spacetime coordinates. The empty Milne universe model and models with dust,
radiation and vacuum energy are exhibited. Discussing the existence of particle
horizons we show that there is continual creation of space, matter and energy
when conformal time is used in Friedmann-Robertson-Walker models with negative
spatial curvature.Comment: 25 pages, 12 figure
On the Conformal forms of the Robertson-Walker metric
All possible transformations from the Robertson-Walker metric to those
conformal to the Lorentz-Minkowski form are derived. It is demonstrated that
the commonly known family of transformations and associated conformal factors
are not exhaustive and that there exists another relatively less well known
family of transformations with a different conformal factor in the particular
case that K = -1. Simplified conformal factors are derived for the special case
of maximally-symmetric spacetimes. The full set of all possible
cosmologically-compatible conformal forms is presented as a comprehensive
table. A product of the analysis is the determination of the set-theoretical
relationships between the maximally symmetric spacetimes, the Robertson-Walker
spacetimes, and functionally more general spacetimes. The analysis is preceded
by a short historical review of the application of conformal metrics to
Cosmology.Comment: Historical review added. Accepted by J. Math. Phy
On the kinematics of a corotating relativistic plasma stream in the perpendicular rotator model of a pulsar magnetosphere
An investigation of the kinematics of a rotating relativistic plasma stream
in the perpendicular rotator model of the pulsar magnetosphere is presented. It
is assumed that the plasma (ejected from the pulsar) moves along the pulsar
magnetic field lines and also corotates with them. The field lines are
considered to be radial straight lines, located in the plane which is
perpendicular to the pulsar rotation axis. The necessity of taking particle
inertia into account is discussed. It is argued that the "massless"
("force-free") approximation cannot be used for the description of this
problem. The frame selection is discussed and it is shown that it is convenient
to discuss the problem in the noninertial frame of ZAMOs (Zero Angular Momentum
Observers). The equation of motion and the exact set of equations describing
the behaviour of a relativistic plasma stream in the pulsar magnetosphere is
obtained. The possible relevance of this investigation for the understanding of
the formation process of a pulsar magnetosphere is discussed.Comment: Plain LaTe
A Kinematical Approach to Conformal Cosmology
We present an alternative cosmology based on conformal gravity, as originally
introduced by H. Weyl and recently revisited by P. Mannheim and D. Kazanas.
Unlike past similar attempts our approach is a purely kinematical application
of the conformal symmetry to the Universe, through a critical reanalysis of
fundamental astrophysical observations, such as the cosmological redshift and
others. As a result of this novel approach we obtain a closed-form expression
for the cosmic scale factor R(t) and a revised interpretation of the space-time
coordinates usually employed in cosmology. New fundamental cosmological
parameters are introduced and evaluated. This emerging new cosmology does not
seem to possess any of the controversial features of the current standard
model, such as the presence of dark matter, dark energy or of a cosmological
constant, the existence of the horizon problem or of an inflationary phase.
Comparing our results with current conformal cosmologies in the literature, we
note that our kinematic cosmology is equivalent to conformal gravity with a
cosmological constant at late (or early) cosmological times. The cosmic scale
factor and the evolution of the Universe are described in terms of several
dimensionless quantities, among which a new cosmological variable delta emerges
as a natural cosmic time. The mathematical connections between all these
quantities are described in details and a relationship is established with the
original kinematic cosmology by L. Infeld and A. Schild. The mathematical
foundations of our kinematical conformal cosmology will need to be checked
against current astrophysical experimental data, before this new model can
become a viable alternative to the standard theory.Comment: Improved version, with minor changes. 58 pages, including 7 figures
and one table. Accepted for publication in General Relativity and Gravitation
(GERG
Predictors of long-term pain and disability in patients with low back pain investigated by magnetic resonance imaging: A longitudinal study
<p>Abstract</p> <p>Background</p> <p>It is possible that clinical outcome of low back pain (LBP) differs according to the presence or absence of spinal abnormalities on magnetic resonance imaging (MRI), in which case there could be value in using MRI findings to refine case definition of LBP in epidemiological research. We therefore conducted a longitudinal study to explore whether spinal abnormalities on MRI for LBP predict prognosis after 18 months.</p> <p>Methods</p> <p>A consecutive series of patients aged 20-64 years, who were investigated by MRI because of mechanical LBP (median duration of current episode 16.2 months), were identified from three radiology departments, and those who agreed completed self-administered questionnaires at baseline and after a mean follow-up period of 18.5 months (a mean of 22.2 months from MRI investigation). MRI scans were assessed blind to other clinical information, according to a standardised protocol. Associations of baseline MRI findings with pain and disability at follow-up, adjusted for treatment and for other potentially confounding variables, were assessed by Poisson regression and summarised by prevalence ratios (PRs) with their 95% confidence intervals (CIs).</p> <p>Results</p> <p>Questionnaires were completed by 240 (74%) of the patients who had agreed to be followed up. Among these 111 men and 129 women, 175 (73%) reported LBP in the past four weeks, 89 (37%) frequent LBP, and 72 (30%) disabling LBP. In patients with initial disc degeneration there was an increased risk of frequent (PR 1.3, 95%CI 1.0-1.9) and disabling LBP (PR 1.7, 95%CI 1.1-2.5) at follow-up. No other associations were found between MRI abnormalities and subsequent outcome.</p> <p>Conclusions</p> <p>Our findings suggest that the MRI abnormalities examined are not major predictors of outcome in patients with LBP. They give no support to the use of MRI findings as a way of refining case definition for LBP in epidemiological research.</p
The association between subgroups of MRI findings identified with latent class analysis and low back pain in 40-year old Danes
Background: Research into the clinical importance of spinal MRI findings in patients with low back pain (LBP) has primarily focused on single imaging findings, such as Modic changes or disc degeneration, and found only weak associations with the presence of pain. However, numerous MRI findings almost always co-exist in the lumbar spine and are often present at more than one lumbar level. It is possible that multiple MRI findings are more strongly associated with LBP than single MRI findings. Latent Class Analysis is a statistical method that has recently been tested and found useful for identifying latent classes (subgroups) of MRI findings within multivariable datasets. The purpose of this study was to investigate the association between subgroups of MRI findings and the presence of LBP in people from the general population. Methods: To identify subgroups of lumbar MRI findings with potential clinical relevance, Latent Class Analysis was initially performed on a clinical dataset of 631 patients seeking care for LBP. Subsequently, 412 participants in a general population cohort (the ‘Backs on Funen’ project) were statistically allocated to those existing subgroups by Latent Class Analysis, matching their MRI findings at a segmental level. The subgroups containing MRI findings from the general population were then organised into hypothetical pathways of degeneration and the association between subgroups in the pathways and the presence of LBP was tested using exact logistic regression. Results: Six subgroups were identified in the clinical dataset and the data from the general population cohort fitted the subgroups well, with a median posterior probability of 93%–100%. These six subgroups described two pathways of increasing degeneration on upper (L1-L3) and lower (L4-L5) lumbar levels. An association with LBP was found for the subgroups describing severe and multiple degenerative MRI findings at the lower lumbar levels but none of the other subgroups were associated with LBP
Dental general anaesthetic receipt among Australians aged 15+ years, 1998–1999 to 2004–2005
Background Adults receive dental general anaesthetic (DGA) care when standard dental treatment is not possible. Receipt of DGA care is resource-intensive and not without risk. This study explores DGA receipt among 15+-year-old Australians by a range of risk indicators. Methods DGA data were obtained from Australia's Hospital Morbidity Database from 1998–1999 to 2004–2005. Poisson regression modeling was used to examine DGA rates in relation to age, sex, Indigenous status, location and procedure. Results The overall DGA rate was 472.79 per 100,000 (95% CI 471.50–474.09). Treatment of impacted teeth (63.7%) was the most common reason for DGA receipt, followed by dental caries treatment (12.4%), although marked variations were seen by age-group. After adjusting for other covariates, DGA rates among 15–19-year-olds were 13.20 (95% CI 12.65–13.78) times higher than their 85+-year-old counterparts. Females had 1.46 (95% CI 1.45–1.47) times the rate of their male counterparts, while those living in rural/remote areas had 2.70 (95% CI 2.68–2.72) times the rate of metropolitan-dwellers. DGA rates for non-Indigenous persons were 4.88 (95% CI 4.73–5.03) times those of Indigenous persons. The DGA rate for 1+ extractions was 461.9 per 100,000 (95% CI 460.6–463.2), compared with a rate of 23.6 per 100,000 (95% CI 23.3–23.9) for 1+ restorations. Conclusion Nearly two-thirds of DGAs were for treatment of impacted teeth. Persons aged 15–19 years were disproportionately represented among those receiving DGA care, along with females, rural/remote-dwellers and those identifying as non-Indigenous. More research is required to better understand the public health implications of DGA care among 15+-year-olds, and how the demand for receipt of such care might be reduced.Lisa M Jamieson and Kaye F Roberts-Thomso
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