12,847 research outputs found
Cosmography: Cosmology without the Einstein equations
How much of modern cosmology is really cosmography? How much of modern
cosmology is independent of the Einstein equations? (Independent of the
Friedmann equations?) These questions are becoming increasingly germane -- as
the models cosmologists use for the stress-energy content of the universe
become increasingly baroque, it behoves us to step back a little and carefully
disentangle cosmological kinematics from cosmological dynamics. The use of
basic symmetry principles (such as the cosmological principle) permits us to do
a considerable amount, without ever having to address the vexatious issues of
just how much "dark energy", "dark matter", "quintessence", and/or "phantom
matter" is needed in order to satisfy the Einstein equations. This is the
sub-sector of cosmology that Weinberg refers to as "cosmography", and in this
article I will explore the extent to which cosmography is sufficient for
analyzing the Hubble law and so describing many of the features of the universe
around us.Comment: 7 pages; uses iopart.cls setstack.sty. Based on a talk presented at
ACRGR4, the 4th Australasian Conference on General Relativity and
Gravitation, Monash University, Melbourne, January 2004. To appear in the
proceedings, in General Relativity and Gravitatio
Hybridized polymer matrix composite
Under certain conditions of combined fire and impact, graphite fibers are released to the atmosphere by graphite fiber composites. The retention of graphite fibers in these situations is investigated. Hybrid combinations of graphite tape and cloth, glass cloth, and resin additives are studied with resin systems. Polyimide resins form the most resistant composites and resins based on simple novolac epoxies the least resistant of those tested. Great improvement in the containment of the fibers is obtained in using graphite/glass hybrids, and nearly complete prevention of individual fiber release is made possible by the use of resin additives
High pressure transport study of non-Fermi liquid behaviour in U2Pt2In and U3Ni3Sn4
The strongly correlated metals U2Pt2In and U3Ni3Sn4 show pronounced non-Fermi
liquid (NFL) phenomena at ambient pressure. Here we review single-crystal
electrical resistivity measurements under pressure (p <= 1.8 GPa) conducted to
investigate the stability of the NFL phase. For tetragonal U2Pt2In (I||a) we
observe a rapid recovery of the Fermi-liquid T^2-term with pressure. The
Fermi-liquid temperature varies as T_FL ~ p-p_c, where p_c= 0 is a critical
pressure. The analysis within the magnetotransport theory of Rosch provides
evidence for the location of U2Pt2In at a zero pressure antiferromagnetic
quantum critical point (QCP). In the case of cubic U3Ni3Sn4 we find T_FL ~
(p-p_c)^1/2. The analysis provides evidence for an antiferromagnetic QCP in
U3Ni3Sn4 at a negative pressure p_c= -0.04+-0.04 GPa.Comment: 6 pages (4 figures); to appear in Proc. of Int. Conf. PPHMF-IV (20-25
Oct. 2001, Santa Fe
Modelling Planck-scale Lorentz violation via analogue models
Astrophysical tests of Planck-suppressed Lorentz violations had been
extensively studied in recent years and very stringent constraints have been
obtained within the framework of effective field theory. There are however
still some unresolved theoretical issues, in particular regarding the so called
"naturalness problem" - which arises when postulating that Planck-suppressed
Lorentz violations arise only from operators with mass dimension greater than
four in the Lagrangian. In the work presented here we shall try to address this
problem by looking at a condensed-matter analogue of the Lorentz violations
considered in quantum gravity phenomenology. Specifically, we investigate the
class of two-component BECs subject to laser-induced transitions between the
two components, and we show that this model is an example for Lorentz
invariance violation due to ultraviolet physics. We shall show that such a
model can be considered to be an explicit example high-energy Lorentz
violations where the ``naturalness problem'' does not arise.Comment: Talk given at the Fourth Meeting on Constrained Dynamics and Quantum
Gravity (QG05), Cala Gonone (Sardinia, Italy) September 12-16, 200
Radiation from collapsing shells, semiclassical backreaction and black hole formation
We provide a detailed analysis of quantum field theory around a collapsing
shell and discuss several conceptual issues related to the emission of
radiation flux and formation of black holes. Explicit calculations are
performed using a model for a collapsing shell which turns out to be
analytically solvable. We use the insights gained in this model to draw
reliable conclusions regarding more realistic models. We first show that any
shell of mass which collapses to a radius close to will emit
approximately thermal radiation for a period of time. In particular, a shell
which collapses from some initial radius to a final radius
(where ) without forming a black hole,
will emit thermal radiation during the period . Later on (), the flux from such a
shell will decay to zero exponentially. We next study the effect of
backreaction computed using the vacuum expectation value of the stress tensor
on the collapse. We find that, in any realistic collapse scenario, the
backreaction effects do \emph{not} prevent the formation of the event horizon.
The time at which the event horizon is formed is, of course, delayed due to the
radiated flux -- which decreases the mass of the shell -- but this effect is
not sufficient to prevent horizon formation. We also clarify several conceptual
issues and provide pedagogical details of the calculations in the Appendices to
the paper.Comment: 26 pages, 6 figures, revtex4; v2 -- minor reformatting, some typos
fixed, one reference added, to appear in PR
Bounding the Hubble flow in terms of the w parameter
The last decade has seen increasing efforts to circumscribe and bound the
cosmological Hubble flow in terms of model-independent constraints on the
cosmological fluid - such as, for instance, the classical energy conditions of
general relativity. Quite a bit can certainly be said in this regard, but much
more refined bounds can be obtained by placing more precise constraints (either
theoretical or observational) on the cosmological fluid. In particular, the use
of the w-parameter (w=p/rho) has become increasingly common as a surrogate for
trying to say something about the cosmological equation of state. Herein we
explore the extent to which a constraint on the w-parameter leads to useful and
nontrivial constraints on the Hubble flow, in terms of constraints on density
rho(z), Hubble parameter H(z), density parameter Omega(z), cosmological
distances d(z), and lookback time T(z). In contrast to other partial results in
the literature, we carry out the computations for arbitrary values of the space
curvature k in [-1,0,+1], equivalently for arbitrary Omega_0 <= 1.Comment: 15 page
The effect of root and shoot damage on the growth of tea plants.
Experiments were carried out under various conditions of elevation and climate, in which young vegetatively propagated plants of selected tea clones were subjected to pruning operations at or after transplanting. Generally the removal of a substantial portion of shoot, root or both caused a temporary set back in growth and, in many instances, increased mortality. The subsequent reduction in total growth depended on the amount initially removed, except when only the immature apical part of the shoot was cut off, in which case the plants grew as well as the untreated ones. Retarded root growth and mortality as a consequence of pruning were associated with a marked depletion of the carbohydrate reserves of the roots, which was more serious when the original level (in Ceylon primarily determined by altitude) was low. Bringing a young tea plant into bearing as soon as possible requires the induction of lateral branching while avoiding growth restriction as far as possible; the practice of bending was found to be less detrimental to growth than other methods involving repeated pinching of apical shoots, cutting across at about two-thirds of the height of the plant, centering and modifications of these methods.-Inst. hort. Plant Breed., Wageningen. (Abstract retrieved from CAB Abstracts by CABI’s permission
Theft and Deterrence
We report results from economic experiments of decisions that are best described as petty larceny, with high school and college students who can anonymously steal real money from each other. Our design allows exogenous variation in the rewards of crime, and the penalty and probability of detection. We find that the probability of stealing is increasing in the amount of money that can be stolen, and that it is decreasing in the probability of getting caught and in the penalty for getting caught. Furthermore, the impact of the certainty of getting caught is larger when the penalty is bigger, and the impact of the penalty is bigger when the probability of getting caught is larger.crime, punishment, incentives, deterrence, juvenile, arrest, risk, larceny
Gait and electromyographic analysis of patients recovering after limb-saving surgery
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