1,670 research outputs found
Spectral geometry as a probe of quantum spacetime
Employing standard results from spectral geometry, we provide strong evidence
that in the classical limit the ground state of three-dimensional causal
dynamical triangulations is de Sitter spacetime. This result is obtained by
measuring the expectation value of the spectral dimension on the ensemble of
geometries defined by these models, and comparing its large scale behaviour to
that of a sphere (Euclidean de Sitter). From the same measurement we are also
able to confirm the phenomenon of dynamical dimensional reduction observed in
this and other approaches to quantum gravity -- the first time this has been
done for three-dimensional causal dynamical triangulations. In this case, the
value for the short-scale limit of the spectral dimension that we find is
approximately 2. We comment on the relevance of these results for the
comparison to asymptotic safety and Horava-Lifshitz gravity, among other
approaches to quantum gravity.Comment: 25 pages, 6 figures. Version 2: references to figures added,
acknowledgment added
Central spindle self-organization and cytokinesis in artificially activated sea urchin eggs
Author Posting. © Marine Biological Laboratory, 2016. This article is posted here by permission of Marine Biological Laboratory for personal use, not for redistribution. The definitive version was published in Biological Bulletin 230, no.2 (2016): 85-95.The ability of microtubules of the mitotic apparatus
to control the positioning and initiation of the cleavage
furrow during cytokinesis was first established from
studies on early echinoderm embryos. However, the identity
of the microtubule population that imparts cytokinetic signaling
is unclear. The two main––and not necessarily mutually
exclusive–– candidates are the central spindle and the
astral rays. In the present study, we examined cytokinesis in
ammonia-activated sea urchin eggs, which lack paternally
derived centrosomes and undergo mitosis mediated by unusual
anastral, bipolar mini-spindles. Live cell imaging and
immunolabeling for microtubules and the centralspindlin
constituent and kinesin-related protein, MKLP1, demonstrated
that furrowing in ammonia-activated eggs was associated
with aligned arrays of centralspindlin-linked, opposed
bundles of antiparallel microtubules. These autonomous, zipper-
like arrays were not associated with a mitotic apparatus,
but did possess characteristics similar to the central spindle
region of control, fertilized embryos. Our results highlight the
self-organizing nature of the central spindle region and its
ability to induce cytokinesis-like furrowing, even in the absence
of a complete mitotic apparatus.This research was
supported by student/faculty summer research grants from
the Dickinson College Research and Development Committee
to JHH; Laura and Arthur Colwin Summer Research
Fellowships from the MBL to JHH and CBS; a National
Science Foundation Major Research Instrumentation grant
to JHH (MRI-0320606); and a NSF collaborative research
grant to JHH (MCB-1412688) and to CBS (MCB-
1412734)
A Bell Inequality Analog in Quantum Measure Theory
One obtains Bell's inequalities if one posits a hypothetical joint
probability distribution, or {\it measure}, whose marginals yield the
probabilities produced by the spin measurements in question. The existence of a
joint measure is in turn equivalent to a certain causality condition known as
``screening off''. We show that if one assumes, more generally, a joint {\it
quantal measure}, or ``decoherence functional'', one obtains instead an
analogous inequality weaker by a factor of . The proof of this
``Tsirel'son inequality'' is geometrical and rests on the possibility of
associating a Hilbert space to any strongly positive quantal measure. These
results lead both to a {\it question}: ``Does a joint measure follow from some
quantal analog of `screening off'?'', and to the {\it observation} that
non-contextual hidden variables are viable in histories-based quantum
mechanics, even if they are excluded classically.Comment: 38 pages, TeX. Several changes and added comments to bring out the
meaning more clearly. Minor rewording and extra acknowledgements, now closer
to published versio
A Study of the Soft X-Ray Emission Lines in NGC 4151. I. Kinematic Properties of the Plasma Wind
We present our analysis of the narrow emission lines produced in the plasma regions within the bright active galactic nucleus of NGC 4151, from an ORBYTS research-with-schools public engagement project. Our goal was to test whether the properties of these plasma regions changed between XMM-Newton observations spanning 15 yr from 2000 to 2015, by measuring the outflow velocities and distances. From this study, we found that NGC 4151 has at least two to three plasma regions. There is no evidence of the outflowing wind properties changing as the velocities and distances are consistent throughout the observations
An axisymmetric evolution code for the Einstein equations on hyperboloidal slices
We present the first stable dynamical numerical evolutions of the Einstein
equations in terms of a conformally rescaled metric on hyperboloidal
hypersurfaces extending to future null infinity. Axisymmetry is imposed in
order to reduce the computational cost. The formulation is based on an earlier
axisymmetric evolution scheme, adapted to time slices of constant mean
curvature. Ideas from a previous study by Moncrief and the author are applied
in order to regularize the formally singular evolution equations at future null
infinity. Long-term stable and convergent evolutions of Schwarzschild spacetime
are obtained, including a gravitational perturbation. The Bondi news function
is evaluated at future null infinity.Comment: 21 pages, 4 figures. Minor additions, updated to agree with journal
versio
MRI-based anatomical characterisation of lower-limb muscles in older women
The ability of muscles to produce force depends, among others, on their anatomical features and it is altered by ageing-associated weakening. However, a clear characterisation of these features, highly relevant for older individuals, is still lacking. This study hence aimed at characterising muscle volume, length, and physiological cross-sectional area (PCSA) and their variability, between body sides and between individuals, in a group of post-menopausal women. Lower-limb magnetic resonance images were acquired from eleven participants (69 (7) y. o., 66.9 (7.7) kg, 159 (3) cm). Twenty-three muscles were manually segmented from the images and muscle volume, length and PCSA were calculated from this dataset. Personalised maximal isometric force was then calculated using the latter information. The percentage difference between the muscles of the two lower limbs was up to 89% and 22% for volume and length, respectively, and up to 84% for PCSA, with no recognisable pattern associated with limb dominance. Between-subject coefficients of variation reached 36% and 13% for muscle volume and length, respectively. Generally, muscle parameters were similar to previous literature, but volumes were smaller than those from in-vivo young adults and slightly higher than ex-vivo ones. Maximal isometric force was found to be on average smaller than those obtained from estimates based on linear scaling of ex-vivo-based literature values. In conclusion, this study quantified for the first time anatomical asymmetry of lower-limb muscles in older women, suggesting that symmetry should not be assumed in this population. Furthermore, we showed that a scaling approach, widely used in musculoskeletal modelling, leads to an overestimation of the maximal isometric force for most muscles. This heavily questions the validity of this approach for older populations. As a solution, the unique dataset of muscle segmentation made available with this paper could support the development of alternative population-based scaling approaches, together with that of automatic tools for muscle segmentation
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