1,147 research outputs found
In vitro measurement of nucleus pulposus swelling pressure: A new technique for studies of spinal adaptation to gravity
Swelling of the intervertebral disc nucleus pulposus is altered by posture and gravity. We have designed and tested a new osmometer for in vitro determination of nucleus pulposus swelling pressure. The functional principle of the osmometer involves compressing a sample of nucleus pulposus with nitrogen gas until saline pressure gradients across a 0.45 microns Millipore filter are eliminated. Swelling pressure of both pooled dog and pooled pig lumbar disc nucleus pulposus were measured on the new osmometer and compared to swelling pressures determined using the equilibrium dialysis technique. The osmometer measured swelling pressures comparable to those obtained by the dialysis technique. This osmometer provides a rapid, direct, and accurate measurement of swelling pressure of the nucleus pulposus
Self-Gravitating Strings In 2+1 Dimensions
We present a family of classical spacetimes in 2+1 dimensions. Such a
spacetime is produced by a Nambu-Goto self-gravitating string. Due to the
special properties of three-dimensional gravity, the metric is completely
described as a Minkowski space with two identified worldsheets. In the flat
limit, the standard string is recovered. The formalism is developed for an open
string with massive endpoints, but applies to other boundary conditions as
well. We consider another limit, where the string tension vanishes in
geometrical units but the end-masses produce finite deficit angles. In this
limit, our open string reduces to the free-masses solution of Gott, which
possesses closed timelike curves when the relative motion of the two masses is
sufficiently rapid. We discuss the possible causal structures of our spacetimes
in other regimes. It is shown that the induced worldsheet Liouville mode obeys
({\it classically}) a differential equation, similar to the Liouville equation
and reducing to it in the flat limit. A quadratic action formulation of this
system is presented. The possibility and significance of quantizing the
self-gravitating string, is discussed.Comment: 55 page
Topology from the Simulated Sloan Digital Sky Survey
We measure the topology (genus curve) of the galaxy distribution in a mock
redshift catalog designed to resemble the upcoming Sloan Digital Sky Survey
(SDSS). The catalog, drawn from a large N-body simulation of a Lambda-CDM cos-
mological model, mimics the anticipated spectroscopic selection procedures of
the SDSS in some detail. Sky maps, redshift slices, and 3-D contour maps of the
mock survey reveal a rich and complex structure, including networks of voids
and superclusters that resemble the patterns seen in the CfA redshift survey
and the Las Campanas Redshift Survey (LCRS). The 3-D genus curve can be
measured from the simulated catalog with superb precision; this curve has the
general shape predicted for Gaussian, random phase initial conditions, but the
error bars are small enough to demonstrate with high significance the subtle
departures from this shape caused by non-linear gravitational evolution. These
distortions have the form predicted by Matsubara's (1994) perturbative anal-
ysis, but they are much smaller in amplitude. We also measure the 3-D genus
curve of the radial peculiar velocity field measured by applying distance-
indicator relations (with realistic errors) to the mock catalog. This genus
curve is consistent with the Gaussian random phase prediction, though it is of
relatively low precision because of the large smoothing length required to
overcome noise in the measured velocity field. Finally, we measure the 2-D
topology in redshift slices, similar to early slices from the SDSS and to
slices already observed in the LCRS. The genus curves of these slices are
consistent with the observed genus curves of the LCRS, providing further
evidence in favor of the inflationary CDM model with Omega_M~0.4. The catalog
is publicly available at http://www.astronomy.ohio-state.edu/~dhw/sdss.html.Comment: ASTeX 4.0 Preprint Style, 5 GIF figures (Figs 1, 2, 3a, 3b, 6; see
http://cfa-www.harvard.edu/~wcolley/SDSS_Top/ for PostScript versions), 7
PostScript figures. Figure 5 and Table 1 have minor corrections since
publicatio
A prescription for probabilities in eternal inflation
Some of the parameters we call ``constants of Nature'' may in fact be
variables related to the local values of some dynamical fields. During
inflation, these variables are randomized by quantum fluctuations. In cases
when the variable in question (call it ) takes values in a continuous
range, all thermalized regions in the universe are statistically equivalent,
and a gauge invariant procedure for calculating the probability distribution
for is known. This is the so-called ``spherical cutoff method''. In
order to find the probability distribution for it suffices to consider a
large spherical patch in a single thermalized region. Here, we generalize this
method to the case when the range of is discontinuous and there are
several different types of thermalized region. We first formulate a set of
requirements that any such generalization should satisfy, and then introduce a
prescription that meets all the requirements. We finally apply this
prescription to calculate the relative probability for different bubble
universes in the open inflation scenario.Comment: 15 pages, 5 figure
An ansatz for spacetimes of zero gravitational mass : global monopoles and textures
We propose a geometric ansatz, a restriction on Euclidean / Minkowski
distance in the embedding space being propotional to distance in the embedded
space, to generate spacetimes with vanishing gravitational mass (). It turns out that these spacetimes can represent
global monopoles and textures. Thus the ansatz is a prescription to generate
zero mass spacetimes that could describe topological defects, global monopoles
and textures.Comment: 9 pages, LaTeX versio
Preheating in an Expanding Universe: Analytic Results for the Massless Case
Analytic results are presented for preheating in both flat and open models of
chaotic inflation, for the case of massless inflaton decay into further
inflaton quanta. It is demonstrated that preheating in both these cases closely
resembles that in Minkowski spacetime. Furthermore, quantitative differences
between preheating in spatially-flat and open models of inflation remain of
order for the chaotic inflation initial conditions considered here.Comment: 15pp, revtex. No figures. Very minor revisions; forthcoming in Phys
Rev
Experiment K-6-03. Gravity and skeletal growth, part 1. Part 2: Morphology and histochemistry of bone cells and vasculature of the tibia; Part 3: Nuclear volume analysis of osteoblast histogenesis in periodontal ligament cells; Part 4: Intervertebral disc swelling pressure associated with microgravity
Bone area, bone electrophysiology, bone vascularity, osteoblast morphology, and osteoblast histogenesis were studied in rats associated with Cosmos 1887. The results suggest that the synchronous animals were the only group with a significantly larger bone area than the basal group, that the bone electrical potential was more negative in flight than in the synchronous rats, that the endosteal osteoblasts from flight rats had greater numbers of transitional Golgi vesicles but no difference in the large Golgi saccules or the alkaline phosphatase activity, that the perioteal vasculature in the shaft of flight rats often showed very dense intraluminal deposits with adjacent degenerating osteocytes as well as lipid accumulations within the lumen of the vessels and sometimes degeneration of the vascular wall (this change was not present in the metaphyseal region of flight animals), and that the progenitor cells decreased in flight rats while the preosteoblasts increased compared to controls. Many of the results suggest that the animals were beginning to recover from the effects of spaceflight during the two day interval between landing and euthanasia; flight effects, such as the vascular changes, did not appear to recover
Particle Spectrum Created Through Bubble Nucleation
Using the multi-dimensional wave function formalism, we investigate the
quantum state of a scalar field inside a true vacuum bubble nucleated through
false vacuum decay in flat spacetime. We developed a formalism which allows us
a mode-by-mode analysis. To demonstrate its advantage, we describe in detail
the evolution of the quantum state during the tunneling process in terms of
individual mode functions and interpret the result in the language of particle
creation. The spectrum of the created particles is examined based on quantum
field theory in the Milne universe.Comment: 14 pages, revtex file, 4 uuencoded compressed postscript figures
appended at the en
Gott Time Machines, BTZ Black Hole Formation, and Choptuik Scaling
We study the formation of BTZ black holes by the collision of point
particles. It is shown that the Gott time machine, originally constructed for
the case of vanishing cosmological constant, provides a precise mechanism for
black hole formation. As a result, one obtains an exact analytic understanding
of the Choptuik scaling.Comment: 6 pages, Late
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