17,256 research outputs found
Critical Collapse in Einstein-Gauss-Bonnet Gravity in Five and Six Dimensions
Einstein-Gauss-Bonnet gravity (EGB) provides a natural higher dimensional and
higher order curvature generalization of Einstein gravity. It contains a new,
presumably microscopic, length scale that should affect short distance
properties of the dynamics, such as Choptuik scaling. We present the results of
a numerical analysis in generalized flat slice co-ordinates of self-gravitating
massless scalar spherical collapse in five and six dimensional EGB gravity near
the threshold of black hole formation. Remarkably, the behaviour is universal
(i.e. independent of initial data) but qualitatively different in five and six
dimensions. In five dimensions there is a minimum horizon radius, suggestive of
a first order transition between black hole and dispersive initial data. In six
dimensions no radius gap is evident. Instead, below the GB scale there is a
change in the critical exponent and echoing period.Comment: 21 pages, 39 figures, a couple of references and two new figures
adde
Cosmological Acceleration Through Transition to Constant Scalar Curvature
As shown by Parker and Raval, quantum field theory in curved spacetime gives
a possible mechanism for explaining the observed recent acceleration of the
universe. This mechanism, which differs in its dynamics from quintessence
models, causes the universe to make a transition to an accelerating expansion
in which the scalar curvature, R, of spacetime remains constant. This
transition occurs despite the fact that we set the renormalized cosmological
constant to zero. We show that this model agrees very well with the current
observed type-Ia supernova (SNe-Ia) data. There are no free parameters in this
fit, as the relevant observables are determined independently by means of the
current cosmic microwave background radiation (CMBR) data. We also give the
predicted curves for number count tests and for the ratio, w(z), of the dark
energy pressure to its density, as well as for dw(z)/dz versus w(z). These
curves differ significantly from those obtained from a cosmological constant,
and will be tested by planned future observations.Comment: 31 pages, 7 figures; to appear in ApJ. Corrected numerical results;
described quantum basis of theory; 18 references added; 2 figures adde
Do OB Runaway Stars Have Pulsar Companions?
We have conducted a VLA search for radio pulsars at the positions of 44
nearby OB runaway stars. The observations involved both searching images for
point sources of continuum emission and a time series analysis. Our mean flux
sensitivity to pulsars slower than 50 ms was 0.2 mJy. No new pulsars were found
in the survey. The size of the survey, combined with the high sensitivity of
the observations, sets a significant constraint on the probability, , of a
runaway OB star having an observable pulsar companion. We find \%
with 95\% confidence, if the general pulsar luminosity function is applicable
to OB star pulsar companions. If a pulsar beaming fraction of \onethird\ is
assumed, then we estimate that fewer than 20\% of runaway OB stars have neutron
star companions, unless pulsed radio emission is frequently obscured by the OB
stellar wind. Our result is consistent with the dynamical (or cluster) ejection
model for the formation of OB runaways. The supernova ejection model is not
ruled out, but is constrained by these observations to allow only a small
binary survival fraction, which may be accommodated if neutron stars acquire
significant natal kicks. According to Leonard, Hills and Dewey (1994), a 20\%
survival fraction corresponds to a 3-d kick velocity of 420 km s. This
value is in close agreement with recent revisions of the pulsar velocity
distribution.Comment: Submitted to the Astronomical Journal. 16 pages. Latex uses
aaspp4.sty. 3 postscript figures. Address correspondence to Colin Philp
([email protected]). Revision was to replace .ps file with latex fil
Meteorological factors in Earth-satellite propagation
Using the COMSTAR D/4 28.56 GHz beacon as a source, a differential gain experiment was performed by connecting a 5-meter paraboloidal antenna and a 0.6-meter paraboloidal antenna alternately to the same receiver. Substantial differential gain changes were observed during some, but not all, rain events. A site-diversity experiment was implemented which consists of two 28.56 GHz radiometers separated by 9 km. The look-angle corresponds to that of the D/4 beacon, and data were obtained with one radiometer during several weeks of concurrent beacon operation to verify the system calibration. A theoretical study of the effect of scattering from a nonuniform rain distribution along the path is under way to aid in interpreting the results of this experiment. An improved empirical site diversity-gain model was derived from data in the literature relating to 34 diversity experiments. Work on the experiment control and data acquisition system is continuing with a view toward future experiments
Ballistic spin transport in exciton gases
Traditional spintronics relies on spin transport by charge carriers, such as
electrons in semiconductor crystals. This brings several complications: the
Pauli principle prevents the carriers from moving with the same speed; Coulomb
repulsion leads to rapid dephasing of electron flows. Spin-optronics is a
valuable alternative to traditional spintronics. In spin-optronic devices the
spin currents are carried by electrically neutral bosonic quasi-particles:
excitons or exciton-polaritons. They can form highly coherent quantum liquids
and carry spins over macroscopic distances. The price to pay is a finite
life-time of the bosonic spin carriers. We present the theory of exciton
ballistic spin transport which may be applied to a range of systems where
bosonic spin transport has been reported, in particular, to indirect excitons
in coupled GaAs/AlGaAs quantum wells. We describe the effect of spin-orbit
interaction of electrons and holes on the exciton spin, account for the Zeeman
effect induced by external magnetic fields, long range and short range exchange
splittings of the exciton resonances. We also consider exciton transport in the
non-linear regime and discuss the definitions of exciton spin current,
polarization current and spin conductivity.Comment: 16 pages, 10 figures to be published in Phys. Rev.
Computer recommendations for an automatic approach and landing system for V/STOL aircraft. Volume 1 - Computer recommendations
Evaluation of digital computer for V/STOL aircraft automatic approach and landing syste
The Number of States of Two Dimensional Critical String Theory
We discuss string theory vacua which have the wrong number of spacetime
dimensions, and give a crude argument that vacua with more than four large
dimensions are improbable. We then turn to two dimensional vacua, which naively
appear to violate Bekenstein's entropy principle. A classical analysis shows
that the naive perturbative counting of states is unjustified. All excited
states of the system have strong coupling singularities which prevent us from
concluding that they really exist. A speculative interpretation of the
classical solutions suggests only a finite number of states will be found in
regions bounded by a finite area. We also argue that the vacuum degeneracy of
two dimensional classical string theory is removed in quantum mechanics. The
system appears to be in a Kosterlitz-Thouless phase. This leads to the
conclusion that it is also improbable to have only two large spacetime
dimensions in string theory. However, we note that, unlike our argument for
high dimensions, our conclusions about the ground state have neglected two
dimensional quantum gravitational effects, and are at best incomplete.Comment: 12 pages, harvma
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