2,957,725 research outputs found
A Spin-Statistics Theorem for Certain Topological Geons
We review the mechanism in quantum gravity whereby topological geons,
particles made from non-trivial spatial topology, are endowed with nontrivial
spin and statistics. In a theory without topology change there is no
obstruction to ``anomalous'' spin-statistics pairings for geons. However, in a
sum-over-histories formulation including topology change, we show that
non-chiral abelian geons do satisfy a spin-statistics correlation if they are
described by a wave function which is given by a functional integral over
metrics on a particular four-manifold. This manifold describes a topology
changing process which creates a pair of geons from .Comment: 21 pages, Plain TeX with harvmac, 3 figures included via eps
Large Fluctuations in the Horizon Area and what they can tell us about Entropy and Quantum Gravity
We evoke situations where large fluctuations in the entropy are induced, our
main example being a spacetime containing a potential black hole whose
formation depends on the outcome of a quantum mechanical event. We argue that
the teleological character of the event horizon implies that the consequent
entropy fluctuations must be taken seriously in any interpretation of the
quantal formalism. We then indicate how the entropy can be well defined despite
the teleological character of the horizon, and we argue that this is possible
only in the context of a spacetime or ``histories'' formulation of quantum
gravity, as opposed to a canonical one, concluding that only a spacetime
formulation has the potential to compute --- from first principles and in the
general case --- the entropy of a black hole. From the entropy fluctuations in
a related example, we also derive a condition governing the form taken by the
entropy, when it is expressed as a function of the quantal density-operator.Comment: 35 pages, plain Tex, needs mathmacros.tex and msmacros.te
Making Clean Energy with a Kerr Black Hole: a Tokamak Model for Gamma-Ray Bursts
In this paper we present a model for making clean energy with a Kerr black
hole. Consider a Kerr black hole with a dense plasma torus spinning around it.
A toroidal electric current flows on the surface of the torus, which generates
a poloidal magnetic field outside the torus. On the surface of the tours the
magnetic field is parallel to the surface. The closed magnetic field lines
winding around the torus compress and confine the plasma in the torus, as in
the case of tokamaks. Though it is unclear if such a model is stable, we look
into the consequences if the model is stable. If the magnetic field is strong
enough, the baryonic contamination from the plasma in the torus is greatly
suppressed by the magnetic confinement and a clean magnetosphere of
electron-positron pairs is built up around the black hole. Since there are no
open magnetic field lines threading the torus and no accretion, the power of
the torus is zero. If some magnetic field lines threading the black hole are
open and connect with loads, clean energy can be extracted from the Kerr black
hole by the Blandford-Znajek mechanism.
The model may be relevant to gamma-ray bursts. The energy in the Poynting
flux produced by the Blandford-Znajek mechanism is converted into the kinetic
energy of the electron-positron pairs in the magnetosphere around the black
hole, which generates two oppositely directed jets of electron-positron pairs
with super-high bulk Lorentz factors. The jets collide and interact with the
interstellar medium, which may produce gamma-ray bursts and the afterglows.Comment: 14 pages, 1 figure, accepted by Ap
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Instabilities and soot formation in spherically expanding, high pressure, rich, iso-octane-air flames
Flame instabilities, cellular structures and soot formed in high pressure, rich, spherically expanding iso-octane-air flames have been studied experimentally using high speed Schlieren cinematography, OH fluorescence, Mie scattering and laser induced incandescence. Cellular structures with two wavelength ranges developed on the flame surface. The larger wavelength cellular structure was produced by the Landau-Darrieus hydrodynamic instability, while the short wavelength cellular structure was produced by the thermal-diffusive instability. Large negative curvature in the short wavelength cusps caused local flame quenching and fracture of the flame surface. In rich flames with equivalence ratio φ > 1.8, soot was formed in a honeycomb-like structure behind flame cracks associated with the large wavelength cellular structure induced by the hydrodynamic instability. The formation of soot precursors through low temperature pyrolysis was suggested as a suitable mechanism for the initiation of soot formation behind the large wavelength flame cracks
Boundary Effects in the One Dimensional Coulomb Gas
We use the functional integral technique of Edwards and Lenard to solve the
statistical mechanics of a one dimensional Coulomb gas with boundary
interactions leading to surface charging. The theory examined is a one
dimensional model for a soap film. Finite size effects and the phenomenon of
charge regulation are studied. We also discuss the pressure of disjunction for
such a film. Even in the absence of boundary potentials we find that the
presence of a surface affects the physics in finite systems. In general we find
that in the presence of a boundary potential the long distance disjoining
pressure is positive but may become negative at closer interplane separations.
This is in accordance with the attractive forces seen at close separations in
colloidal and soap film experiments and with three dimensional calculations
beyond mean field. Finally our exact results are compared with the predictions
of the corresponding Poisson-Boltzmann theory which is often used in the
context of colloidal and thin liquid film systems.Comment: 28 pages, LATEX2e, 11 figures, uses styles[12pt] resubmission because
of minor corrections to tex
How Future Space-Based Weak Lensing Surveys Might Obtain Photometric Redshifts Independently
We study how the addition of on-board optical photometric bands to future
space-based weak lensing instruments could affect the photometric redshift
estimation of galaxies, and hence improve estimations of the dark energy
parameters through weak lensing. Basing our study on the current proposed
Euclid configuration and using a mock catalog of galaxy observations, various
on-board options are tested and compared with the use of ground-based
observations from the Large Synoptic Survey Telescope (LSST) and Pan-STARRS.
Comparisons are made through the use of the dark energy Figure of Merit, which
provides a quantifiable measure of the change in the quality of the scientific
results that can be obtained in each scenario. Effects of systematic offsets
between LSST and Euclid photometric calibration are also studied. We find that
adding two (U and G) or even one (U) on-board optical band-passes to the
space-based infrared instrument greatly improves its photometric redshift
performance, bringing it close to the level that would be achieved by combining
observations from both space-based and ground-based surveys while freeing the
space mission from reliance on external datasets.Comment: Accepted for publication in PASP. A high-quality version of Fig 1 can
be found on http://www.ap.smu.ca/~sawicki/DEphoto
On the duration of long GRBs: effects of black hole spin
In the frame of the collapsar model for long gamma ray bursts (GRBs), we
investigate the formation of a torus around a spinning BH and we check what
rotational properties a progenitor star must have in order to sustain torus
accretion over relatively long activity periods. We also study the time
evolution of the BH spin parameter. We take into account the coupling between
BH mass, its spin parameter and the critical specific angular momentum of
accreting gas, needed for the torus to form. The large BH spin reduces the
critical angular momentum which in turn can increase the GRB duration with
respect to the Schwarzschild BH case. We quantify this effect and estimate the
GRB durations in three cases: when a hyper accreting torus operates or a BH
spins very fast or both. We show under what conditions a given progenitor star
produces a burst that can last as short as several seconds and as long as
several hundred of seconds. Our models indicate that it is possible for a
single collapse to produce three kinds of jets: (1) a very short, lasting
between a fraction of a second and a few seconds, 'precursor' jet, powered only
by a hyper accreting torus before the BH spins up, (2) an 'early' jet, lasting
several tens of seconds and powered by both hyper accretion and BH rotation,
and (3) a 'late' jet, powered only by the spinning BH.Comment: 13 pages; 9 figures; ApJ in pres
Microscopic Analysis of Low-Frequency Flux Noise in YBaCuO Direct Current Superconducting Quantum Interference Devices
We use low-temperature scanning electron microscopy combined with SQUID
detection of magnetic flux to image vortices and to investigate low-frequency
flux noise in YBaCuO thin film SQUIDs. The low-frequency flux noise
shows a nonlinear increase with magnetic cooling field up to 60 T. This
effect is explained by the surface potential barrier at the SQUID hole. By
correlating flux noise data with the spatial distribution of vortices, we
obtain information on spatial fluctuations of vortices on a microscopic scale,
e.g. an average vortex hopping length of approximately 10 nm.Comment: submitted to Applied Physics Letter
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