2,897 research outputs found
Scale-invariance in gravity and implications for the cosmological constant
Recently a scale invariant theory of gravity was constructed by imposing a
conformal symmetry on general relativity. The imposition of this symmetry
changed the configuration space from superspace - the space of all Riemannian
3-metrics modulo diffeomorphisms - to conformal superspace - the space of all
Riemannian 3-metrics modulo diffeomorphisms and conformal transformations.
However, despite numerous attractive features, the theory suffers from at least
one major problem: the volume of the universe is no longer a dynamical
variable. In attempting to resolve this problem a new theory is found which has
several surprising and atractive features from both quantisation and
cosmological perspectives. Furthermore, it is an extremely restrictive theory
and thus may provide testable predictions quickly and easily. One particularly
interesting feature of the theory is the resolution of the cosmological
constant problem.Comment: Replaced with final version: minor changes to text; references adde
Scale-invariant gravity: Spacetime recovered
The configuration space of general relativity is superspace - the space of
all Riemannian 3-metrics modulo diffeomorphisms. However, it has been argued
that the configuration space for gravity should be conformal superspace - the
space of all Riemannian 3-metrics modulo diffeomorphisms and conformal
transformations. Recently a manifestly 3-dimensional theory was constructed
with conformal superspace as the configuration space. Here a fully
4-dimensional action is constructed so as to be invariant under conformal
transformations of the 4-metric using general relativity as a guide. This
action is then decomposed to a (3+1)-dimensional form and from this to its
Jacobi form. The surprising thing is that the new theory turns out to be
precisely the original 3-dimensional theory. The physical data is identified
and used to find the physical representation of the theory. In this
representation the theory is extremely similar to general relativity. The
clarity of the 4-dimensional picture should prove very useful for comparing the
theory with those aspects of general relativity which are usually treated in
the 4-dimensional framework.Comment: Replaced with final version: minor changes to tex
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In-situ Neutron Diffraction Studies of Various Metals on Engin-X at ISIS
The physical basis of self-organization of the mammalian oocyte spindle
To prepare gametes with the appropriate number of chromosomes, mammalian
oocytes undergo two sequential cell divisions. During each division, a large,
long-lived, microtubule-based organelle called the meiotic spindle assembles
around condensed chromosomes. Although meiotic spindles have been intensively
studied for several decades, as force-generating mechanical objects, they
remain very poorly understood. In materials physics, coarse-grained theories
have been essential in understanding the large-scale behavior of systems
composed of many interacting particles. It is unclear, however, if this
approach can succeed in capturing the properties of active, biochemically
complex, living materials like the spindle. Here, we show that a class of
models based on nematic liquid crystal theory can describe important aspects of
the organelle-scale structure and dynamics of spindles in living mouse oocytes.
Using our models to interpret quantitative polarization microscopy data, we
measure for the first time material properties relating to stress propagation
in living oocytes, including the nematic diffusivities corresponding to splay
and bend deformations. Unlike the reconstituted amphibian spindles that were
previously studied in vitro, nematic elastic stress is exponentially screened
in the microtubule network of living mammalian oocytes, with a screening length
of order one micron. This observation can be explained by the relatively high
volume fraction of embedded chromosomes in mammalian meiotic spindles, which
cause long voids in the microtubule network and so disrupt orientational stress
propagation
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The effect of cyclic-loading generated intergranular strains on the creep deformation of a polycrystalline material
Intergranular strains are generated due to the incompatible deformations at grain length-scales during plastic loading in a polycrystalline material. Estimating the effects of intergranular strains on the creep life of the material is of interest for accurate life prediction of high-temperature structural systems. In this study, the effect of the cyclic loading generated intergranular strains on the creep deformation behaviour of Type 316H austenitic stainless steel was studied using in-situ neutron diffraction. The load-controlled creep dwells introduced at various positions during tension-compression cyclic loading with different intergranular strain state but under the same applied stress showed markedly different behaviours. It is inferred that the intergranular strains are a significant contributor to the observed differences in creep deformation behaviour. Comparing the evolution of intergranular strains in various grain families during plastic and creep deformation, it was found that the grain families which deformed relatively more or less during plastic deformation behaved similarly during creep deformation. The present work shows that intergranular strains, which contribute to accelerating/decelerating creep deformation rates, need to be accounted for in current creep life assessment procedures, to obtain a more realistic creep deformation prediction under cyclic loading conditions
The physical gravitational degrees of freedom
When constructing general relativity (GR), Einstein required 4D general
covariance. In contrast, we derive GR (in the compact, without boundary case)
as a theory of evolving 3-dimensional conformal Riemannian geometries obtained
by imposing two general principles: 1) time is derived from change; 2) motion
and size are relative. We write down an explicit action based on them. We
obtain not only GR in the CMC gauge, in its Hamiltonian 3 + 1 reformulation but
also all the equations used in York's conformal technique for solving the
initial-value problem. This shows that the independent gravitational degrees of
freedom obtained by York do not arise from a gauge fixing but from hitherto
unrecognized fundamental symmetry principles. They can therefore be identified
as the long-sought Hamiltonian physical gravitational degrees of freedom.Comment: Replaced with published version (minor changes and added references
Origin of the Bauschinger effect in a polycrystalline material
There is a long and lively debate in the literature about the origin of the Bauschinger effect in polycrystalline materials, the most widely accepted explanation being the easier movement of dislocations during reverse loading causing the reduction of the yield stress. Other explanations include incompatible deformation at the grain scale and change of dislocation cell structures during forward and reverse loading, but recent publications show these phenomenological explanations of the Bauschinger effect are not holistic. In the experimental work presented here, we have investigated the role of micro residual lattice strain on the origin of the Bauschinger effect in type 316H austenitic stainless steel using in-situ neutron diffraction. Standard cylindrical specimens were tension-compression load cycled at room temperature with the loading interrupted at incrementally larger compressive and tensile strains followed by reloading to the tensile loop peak strain. Mirror symmetric compression-tension cyclic tests were also performed with tensile and compressive load interruptions followed by compressive reloading to the compressive loop peak strain. A strong correlation is demonstrated between the evolution of residual lattice strain in the grain families and the change in magnitude in macroscopic yield stress, peak stress and the shape of the yielding part of the stress-strain curve for both the cyclic tension yield and compression yield tests. This implies that the residual lattice strain generated by grain scale elastic and plastic deformation anisotropy is the primary source of the Bauschinger kinematic hardening effect observed in type 316H austenitic stainless steel
A multi-object spectral imaging instrument
We have developed a snapshot spectral imaging system which fits onto the side camera port of a commercial inverted microscope. The system provides spectra, in real time, from multiple points randomly selected on the microscope image. Light from the selected points in the sample is directed from the side port imaging arm using a digital micromirror device to a spectrometer arm based on a dispersing prism and CCD camera. A multi-line laser source is used to calibrate the pixel positions on the CCD for wavelength. A CMOS camera on the front port of the microscope allows the full image of the sample to be displayed and can also be used for particle tracking, providing spectra of multiple particles moving in the sample. We demonstrate the system by recording the spectra of multiple fluorescent beads in aqueous solution and from multiple points along a microscope sample channel containing a mixture of red and blue dye
Antinuclear antibodies (ANA) in chronic hepatitis C virus infection: correlates of positivity and clinical relevance.
We examined correlates of antinuclear antibody (ANA) positivity (ANA+) in individuals with chronic hepatitis C virus (HCV) infection and the effect of positivity on clinical outcome of HCV. Pretreatment sera from 645 patients from three centres in Sweden (n = 225), the UK (n = 207) and Italy (n = 213) were evaluated by indirect immunofluorescence on Hep-2 cells for ANA pattern and titre by a single laboratory. Liver biopsies were all scored by one pathologist. A total of 258 patients were subsequently treated with interferon monotherapy. There was a significant difference in the prevalence of ANA (1:40) by geographic location: Lund 4.4%, London 8.7%, Padova 10.3% [odds ratio (OR) = 0.66; 95% CI: 0.46-0.94; P = 0.023]. Duration of HCV infection, age at infection, current age, route of infection, viral genotype, alcohol consumption, fibrosis stage and inflammatory score were not correlated with ANA+ or ANA pattern. Female gender was correlated with ANA+ and this association persisted in multivariable analyses (OR = 3.0; P = 0.002). Increased plasma cells were observed in the liver biopsies of ANA-positive individuals compared with ANA-negative individuals, while a trend towards decreased lymphoid aggregates was observed [hazard ratio (HR) = 9.0, P = 0.037; HR = 0.291, P = 0.118, respectively]. No correlations were observed between ANA positivity and nonresponse to therapy (OR = 1.4; P = 0.513), although ANA+ was correlated with faster rates of liver fibrosis, this was not statistically significant (OR = 1.8; P = 0.1452). Low titre ANA+ should not be a contraindication for interferon treatment. Our observation of increased plasma cells in ANA+ biopsies might suggest B-cell polyclonal activity with a secondary clinical manifestation of increased serum immunoglobulins
2D mapping of plane stress crack-tip fields following an overload
The evolution of crack-tip strain fields in a thin (plane stress) compact tension sample following an
overload (OL) event has been studied using two different experimental techniques. Surface behaviour has been
characterised by Digital Image Correlation (DIC), while the bulk behaviour has been characterised by means of
synchrotron X-ray diffraction (XRD). The combination of both surface and bulk information allowed us to
visualise the through-thickness evolution of the strain fields before the OL event, during the overload event,
just after OL and at various stages after it. Unlike previous work, complete 2D maps of strains around the
crack-tip were acquired at 60m spatial resolution by XRD. The DIC shows less crack opening after overload
and the XRD a lower crack-tip peak stress after OL until the crack has grown past the compressive crack-tip
residual stress introduced by the overload after which the behaviour returned to that for the baseline fatigue
response. While the peak crack-tip stress is supressed by the compressive residual stress, the crack-tip stress
field changes over each cycle are nevertheless the same for all Kmax cycles except at OL
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