4,793 research outputs found
Contract to extend the usefullness of cytogenetic methodology as a research technique and as a biomedical monitoring procedure Quarterly progress report 1 Jan. - 30 Jun. 1966
Automatic cytogenetic analysis system with digital computer, scanning device, and microscope for mitotic cell detection and classificatio
[Extend the Usefullness of Cytogenetic Methodology as a Research Technique and as a Biomedical Monitoring Procedure through the Utilization of Automatic Electronic Scanning and Computer Analysis of Chromosomes]
Automatic microscope and computer program for chromosome analysi
Measure and Probability in Cosmology
General relativity has a Hamiltonian formulation, which formally provides a
canonical (Liouville) measure on the space of solutions. In ordinary
statistical physics, the Liouville measure is used to compute probabilities of
macrostates, and it would seem natural to use the similar measure arising in
general relativity to compute probabilities in cosmology, such as the
probability that the universe underwent an era of inflation. Indeed, a number
of authors have used the restriction of this measure to the space of
homogeneous and isotropic universes with scalar field matter
(minisuperspace)---namely, the Gibbons-Hawking-Stewart measure---to make
arguments about the likelihood of inflation. We argue here that there are at
least four major difficulties with using the measure of general relativity to
make probability arguments in cosmology: (1) Equilibration does not occur on
cosmological length scales. (2) Even in the minisuperspace case, the measure of
phase space is infinite and the computation of probabilities depends very
strongly on how the infinity is regulated. (3) The inhomogeneous degrees of
freedom must be taken into account (we illustrate how) even if one is
interested only in universes that are very nearly homogeneous. The measure
depends upon how the infinite number of degrees of freedom are truncated, and
how one defines "nearly homogeneous." (4) In a universe where the second law of
thermodynamics holds, one cannot make use of our knowledge of the present state
of the universe to "retrodict" the likelihood of past conditions.Comment: 43 pages, 2 figure
New Charged Black Holes with Conformal Scalar Hair
A new class of four-dimensional, hairy, stationary solutions of the
Einstein-Maxwell-Lambda system with a conformally coupled scalar field is
constructed in this paper. The metric belongs to the Plebanski-Demianski family
and hence its static limit has the form of the charged C-metric. It is shown
that, in the static case, a new family of hairy black holes arises. They turn
out to be cohomogeneity-two, with horizons that are neither Einstein nor
homogenous manifolds. The conical singularities in the C-metric can be removed
due to the back reaction of the scalar field providing a new kind of regular,
radiative spacetime. The scalar field carries a continuous parameter
proportional to the usual acceleration present in the C-metric. In the
zero-acceleration limit, the static solution reduces to the dyonic
Bocharova-Bronnikov-Melnikov-Bekenstein solution or the dyonic extension of the
Martinez-Troncoso-Zanelli black holes, depending on the value of the
cosmological constant.Comment: Published versio
How red is a quantum black hole?
Radiating black holes pose a number of puzzles for semiclassical and quantum
gravity. These include the transplanckian problem -- the nearly infinite
energies of Hawking particles created near the horizon, and the final state of
evaporation. A definitive resolution of these questions likely requires robust
inputs from quantum gravity. We argue that one such input is a quantum bound on
curvature. We show how this leads to an upper limit on the redshift of a
Hawking emitted particle, to a maximum temperature for a black hole, and to the
prediction of a Planck scale remnant.Comment: 3 pages, essay for the Gravity Research Foundatio
Asymptotic Symmetries of Rindler Space at the Horizon and Null Infinity
We investigate the asymptotic symmetries of Rindler space at null infinity
and at the event horizon using both systematic and ad hoc methods. We find that
the approaches that yield infinite-dimensional asymptotic symmetry algebras in
the case of anti-de Sitter and flat spaces only give a finite-dimensional
algebra for Rindler space at null infinity. We calculate the charges
corresponding to these symmetries and confirm that they are finite, conserved,
and integrable, and that the algebra of charges gives a representation of the
asymptotic symmetry algebra. We also use relaxed boundary conditions to find
infinite-dimensional asymptotic symmetry algebras for Rindler space at null
infinity and at the event horizon. We compute the charges corresponding to
these symmetries and confirm that they are finite and integrable. We also
determine sufficient conditions for the charges to be conserved on-shell, and
for the charge algebra to give a representation of the asymptotic symmetry
algebra. In all cases, we find that the central extension of the charge algebra
is trivial.Comment: 37 pages, 4 figures. Version 3: New Section 5 adde
On Cosmological Implication of the Trace Anomaly
We establish a connection between the trace anomaly and a thermal radiation
in the context of the standard cosmology. This is done by solving the covariant
conservation equation of the stress tensor associated with a conformally
invariant quantum scalar field. The solution corresponds to a thermal radiation
with a temperature which is given in terms of a cut-off time excluding the
spacetime regions very close to the initial singularity. We discuss the
interrelation between this result and the result obtained in a two-dimensional
schwarzschild spacetime.Comment: 8 pages, no figure
Monitoring the Thermal Power of Nuclear Reactors with a Prototype Cubic Meter Antineutrino Detector
In this paper, we estimate how quickly and how precisely a reactor's
operational status and thermal power can be monitored over hour to month time
scales, using the antineutrino rate as measured by a cubic meter scale
detector. Our results are obtained from a detector we have deployed and
operated at 25 meter standoff from a reactor core. This prototype can detect a
prompt reactor shutdown within five hours, and monitor relative thermal power
to three percent within seven days. Monitoring of short-term power changes in
this way may be useful in the context of International Atomic Energy Agency's
(IAEA) Reactor Safeguards Regime, or other cooperative monitoring regimes.Comment: 10 pages, 9 figure
Gravity-induced vacuum dominance
It has been widely believed that, except in very extreme situations, the
influence of gravity on quantum fields should amount to just small,
sub-dominant contributions. This view seemed to be endorsed by the seminal
results obtained over the last decades in the context of renormalization of
quantum fields in curved spacetimes. Here, however, we argue that this belief
is false by showing that there exist well-behaved spacetime evolutions where
the vacuum energy density of free quantum fields is forced, by the very same
background spacetime, to become dominant over any classical energy-density
component. This semiclassical gravity effect finds its roots in the infrared
behavior of fields on curved spacetimes. By estimating the time scale for the
vacuum energy density to become dominant, and therefore for backreaction on the
background spacetime to become important, we argue that this vacuum dominance
may bear unexpected astrophysical and cosmological implications.Comment: To appear in Phys. Rev. Lett
Stability of pulse-like earthquake ruptures
Pulse-like ruptures arise spontaneously in many elastodynamic rupture
simulations and seem to be the dominant rupture mode along crustal faults.
Pulse-like ruptures propagating under steady-state conditions can be
efficiently analysed theoretically, but it remains unclear how they can arise
and how they evolve if perturbed. Using thermal pressurisation as a
representative constitutive law, we conduct elastodynamic simulations of
pulse-like ruptures and determine the spatio-temporal evolution of slip, slip
rate and pulse width perturbations induced by infinitesimal perturbations in
background stress. These simulations indicate that steady-state pulses driven
by thermal pressurisation are unstable. If the initial stress perturbation is
negative, ruptures stop; conversely, if the perturbation is positive, ruptures
grow and transition to either self-similar pulses (at low background stress) or
expanding cracks (at elevated background stress). Based on a dynamic
dislocation model, we develop an elastodynamic equation of motion for slip
pulses, and demonstrate that steady-state slip pulses are unstable if their
accrued slip is a decreasing function of the uniform background stress
. This condition is satisfied by slip pulses driven by thermal
pressurisation. The equation of motion also predicts quantitatively the growth
rate of perturbations, and provides a generic tool to analyse the propagation
of slip pulses. The unstable character of steady-state slip pulses implies that
this rupture mode is a key one determining the minimum stress conditions for
sustainable ruptures along faults, i.e., their ``strength''. Furthermore, slip
pulse instabilities can produce a remarkable complexity of rupture dynamics,
even under uniform background stress conditions and material properties
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