56 research outputs found
All conformally flat pure radiation metrics
The complete class of conformally flat, pure radiation metrics is given,
generalising the metric recently given by Wils.Comment: 7 pages, plain Te
Double-Kasner Spacetime: Peculiar Velocities and Cosmic Jets
In dynamic spacetimes in which asymmetric gravitational collapse/expansion is
taking place, the timelike geodesic equation appears to exhibit an interesting
property: Relative to the collapsing configuration, free test particles undergo
gravitational "acceleration" and form a double-jet configuration parallel to
the axis of collapse. We illustrate this aspect of peculiar motion in simple
spatially homogeneous cosmological models such as the Kasner spacetime. To
estimate the effect of spatial inhomogeneities on cosmic jets, timelike
geodesics in the Ricci-flat double-Kasner spacetime are studied in detail.
While spatial inhomogeneities can significantly modify the structure of cosmic
jets, we find that under favorable conditions the double-jet pattern can
initially persist over a finite period of time for sufficiently small
inhomogeneities.Comment: 37 pages, 5 figures; v2: minor typos correcte
On the Symmetries of the Edgar-Ludwig Metric
The conformal Killing equations for the most general (non-plane wave)
conformally flat pure radiation field are solved to find the conformal Killing
vectors. As expected fifteen independent conformal Killing vectors exist, but
in general the metric admits no Killing or homothetic vectors. However for
certain special cases a one-dimensional group of homotheties or motions may
exist and in one very special case, overlooked by previous investigators, a
two-dimensional homethety group exists. No higher dimensional groups of motions
or homotheties are admitted by these metrics.Comment: Plain TeX, 7 pages, No figure
Geodesic motion in the Kundt spacetimes and the character of envelope singularity
We investigate geodesics in specific Kundt type N (or conformally flat)
solutions to Einstein's equations. Components of the curvature tensor in
parallelly transported tetrads are then explicitly evaluated and analyzed. This
elucidates some interesting global properties of the spacetimes, such as an
inherent rotation of the wave-propagation direction, or the character of
singularities. In particular, we demonstrate that the characteristic envelope
singularity of the rotated wave-fronts is a (non-scalar) curvature singularity,
although all scalar invariants of the Riemann tensor vanish there.Comment: 21 pages, 3 figures. To appear in Class. Quantum Gra
Nonvanishing Local Scalar Invariants even in VSI Spacetimes with all Polynomial Curvature Scalar Invariants Vanishing
VSI (`vanishing scalar invariant') spacetimes have zero values for all total
scalar contractions of all polynomials in the Riemann tensor and its covariant
derivatives. However, there are other ways of concocting local scalar
invariants (nonpolynomial) from the Riemann tensor that need not vanish even in
VSI spacetimes, such as Cartan invariants. Simple examples are given that
reduce to the squared amplitude for a linearized monochromatic plane
gravitational wave. These nonpolynomial local scalar invariants are also
evaluated for non-VSI spacetimes such as Schwarzschild and Kerr and are
estimated near the surface of the earth. Similar invariants are defined for
null fluids and for electromagnetic fields.Comment: 9 pages, LaTeX, proofread version accepted 2009 January 14 for
publication in Classical and Quantum Gravit
Invariant classification and the generalised invariant formalism: conformally flat pure radiation metrics, with zero cosmological constant
Metrics obtained by integrating within the generalised invariant formalism
are structured around their intrinsic coordinates, and this considerably
simplifies their invariant classification and symmetry analysis. We illustrate
this by presenting a simple and transparent complete invariant classification
of the conformally flat pure radiation metrics (except plane waves) in such
intrinsic coordinates; in particular we confirm that the three apparently
non-redundant functions of one variable are genuinely non-redundant, and easily
identify the subclasses which admit a Killing and/or a homothetic Killing
vector. Most of our results agree with the earlier classification carried out
by Skea in the different Koutras-McIntosh coordinates, which required much more
involved calculations; but there are some subtle differences. Therefore, we
also rework the classification in the Koutras-McIntosh coordinates, and by
paying attention to some of the subtleties involving arbitrary functions, we
are able to obtain complete agreement with the results obtained in intrinsic
coordinates. In particular, we have corrected and completed statements and
results by Edgar and Vickers, and by Skea, about the orders of Cartan
invariants at which particular information becomes available.Comment: Extended version of GRG publication, with some typos etc correcte
Explicit Kundt type II and N solutions as gravitational waves in various type D and O universes
A particular yet large class of non-diverging solutions which admits a
cosmological constant, electromagnetic field, pure radiation and/or general
non-null matter component is explicitly presented. These spacetimes represent
exact gravitational waves of arbitrary profiles which propagate in background
universes such as Minkowski, conformally flat (anti-)de Sitter, Edgar-Ludwig,
Bertotti-Robinson, and type D (anti-)Nariai or Plebanski-Hacyan spaces, and
their generalizations. All possibilities are discussed and are interpreted
using a unifying simple metric form. Sandwich and impulsive waves propagating
in the above background spaces with different geometries and matter content can
easily be constructed. New solutions are identified, e.g. type D pure radiation
or explicit type II electrovacuum waves in (anti-)Nariai universe. It is also
shown that, in general, there are no conformally flat Einstein-Maxwell fields
with a non-vanishing cosmological constant.Comment: 17 pages, LaTeX 2e. v2: added two references concerning generalized
Kerr-Schild transformations, minor changes in the tex
Women’s preference for laparoscopic or abdominal hysterectomy
In the present study, women’s preferences on advantages and disadvantages of laparoscopic hysterectomy (LH) and abdominal hysterectomy (AH) have been studied. Patients’ preferences were evaluated in individual, structured interviews in women scheduled for hysterectomy and questionnaires in nurses. Forty-three patients and 39 nurses were included. After general information, 84% of patients and 74% of nurses preferred LH over AH. This preference did not change after supplying more detailed information or after hysterectomy. The avoidance of complications was indicated as the most important factor in the decision. More than half of the women evaluated a difference of 1% as the maximum acceptable risk of major complications. When confronted with scenarios based on current evidence, both patients and nurses prefer LH over AH. This study supports further implementation of LH in clinical practice. The actual major complication rate in hysterectomy, however, is perceived as high
Educational level, prevalence of hysterectomy, and age at amenorrhoea: a cross-sectional analysis of 9536 women from six population-based cohort studies in Germany
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