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, $f_p$, of a runaway OB star having an observable pulsar companion. We find $f_p \le 6.5$\% 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$^{-1}$. 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