9,106 research outputs found
Cosmology as Geodesic Motion
For gravity coupled to N scalar fields with arbitrary potential V, it is
shown that all flat (homogeneous and isotropic) cosmologies correspond to
geodesics in an (N+1)-dimensional `augmented' target space of Lorentzian
signature (1,N), timelike if V>0, null if V=0 and spacelike if V<0.
Accelerating cosmologies correspond to timelike geodesics that lie within an
`acceleration subcone' of the `lightcone'. Non-flat (k=-1,+1) cosmologies are
shown to evolve as projections of geodesic motion in a space of dimension
(N+2), of signature (1,N+1) for k=-1 and signature (2,N) for k=+1. This
formalism is illustrated by cosmological solutions of models with an
exponential potential, which are comprehensively analysed; the late-time
behviour for other potentials of current interest is deduced by comparison.Comment: 26 pages, 2 figures, journal version with additional reference
Spin period change and the magnetic fields of neutron stars in Be X-ray binaries in the Small Magellanic Cloud
We report on the long-term average spin period, rate of change of spin period
and X-ray luminosity during outbursts for 42 Be X-ray binary systems in the
Small Magellanic Cloud. We also collect and calculate parameters of each system
and use these data to determine that all systems contain a neutron star which
is accreting via a disc, rather than a wind, and that if these neutron stars
are near spin equilibrium, then over half of them, including all with spin
periods over about 100 s, have magnetic fields over the quantum critical level
of 4.4x10^13 G. If these neutron stars are not close to spin equilibrium, then
their magnetic fields are inferred to be much lower, of the order of 10^6-10^10
G, comparable to the fields of neutron stars in low-mass X-ray binaries. Both
results are unexpected and have implications for the rate of magnetic field
decay and the isolated neutron star population.Comment: 22 pages, 50 figures; to appear in MNRA
N=1 Non-Abelian Tensor Multiplet in Four Dimensions
We carry out the N=1 supersymmetrization of a physical non-Abelian tensor
with non-trivial consistent couplings in four dimensions. Our system has three
multiplets: (i) The usual non-Abelian vector multiplet (VM) (A_\mu{}^I,
\lambda^I), (ii) A non-Abelian tensor multiplet (TM) (B_{\mu\nu}{}^I, \chi^I,
\varphi^I), and (iii) A compensator vector multiplet (CVM) (C_\mu{}^I, \rho^I).
All of these multiplets are in the adjoint representation of a non-Abelian
group G. Unlike topological theory, all of our fields are propagating with
kinetic terms. The C_\mu{}^I-field plays the role of a Stueckelberg compensator
absorbed into the longitudinal component of B_{\mu\nu}{}^I. We give not only
the component lagrangian, but also a corresponding superspace reformulation,
reconfirming the total consistency of the system. The adjoint representation of
the TM and CVM is further generalized to an arbitrary real representation of
general SO(N) gauge group. We also couple the globally N=1 supersymmetric
system to supergravity, as an additional non-trivial confirmation.Comment: 18 pages, no figur
Quantum cryptography with squeezed states
A quantum key distribution scheme based on the use of displaced squeezed
vacuum states is presented. The states are squeezed in one of two field
quadrature components, and the value of the squeezed component is used to
encode a character from an alphabet. The uncertainty relation between
quadrature components prevents an eavesdropper from determining both with
enough precision to determine the character being sent. Losses degrade the
performance of this scheme, but it is possible to use phase-sensitive
amplifiers to boost the signal and partially compensate for their effect.Comment: 15 pages, no figure
Revisiting the Rigidly Rotating Magnetosphere model for sigma Ori E. I. Observations and Data Analysis
We have obtained 18 new high-resolution spectropolarimetric observations of
the B2Vp star sigma Ori E with both the Narval and ESPaDOnS
spectropolarimeters. The aim of these observations is to test, with modern
data, the assumptions of the Rigidly Rotating Magnetosphere (RRM) model of
Townsend & Owocki (2005), applied to the specific case of sigma Ori E by
Townsend et al. (2005). This model includes a substantially offset dipole
magnetic field configuration, and approximately reproduces previous
observational variations in longitudinal field strength, photometric
brightness, and Halpha emission. We analyze new spectroscopy, including H I, He
I, C II, Si III and Fe III lines, confirming the diversity of variability in
photospheric lines, as well as the double S-wave variation of circumstellar
hydrogen. Using the multiline analysis method of Least-Squares Deconvolution
(LSD), new, more precise longitudinal magnetic field measurements reveal a
substantial variance between the shapes of the observed and RRM model
time-varying field. The phase resolved Stokes V profiles of He I 5876 A and
6678 A lines are fit poorly by synthetic profiles computed from the magnetic
topology assumed by Townsend et al. (2005). These results challenge the offset
dipole field configuration assumed in the application of the RRM model to sigma
Ori E, and indicate that future models of its magnetic field should also
include complex, higher-order components.Comment: 13 pages, 8 figures. Accepted for publication in MNRA
Surface disinfection challenges for Candida auris: an in-vitro study
The emerging pathogenic multidrug-resistant yeast Candida auris is an important source of healthcare-associated infections and of growing global clinical concern. The ability of this organism to survive on surfaces and withstand environmental stressors creates a challenge for eradicating it from hospitals. A panel of C. auris clinical isolates was evaluated on different surface environments against the standard disinfectant sodium hypochlorite and high-level disinfectant peracetic acid. C. auris was shown to selectively tolerate clinically relevant concentrations of sodium hypochlorite and peracetic acid in a surface-dependent manner, which may explain its ability to successfully persist within the hospital environment
Practical free-space quantum key distribution over 1 km
A working free-space quantum key distribution (QKD) system has been developed
and tested over an outdoor optical path of ~1 km at Los Alamos National
Laboratory under nighttime conditions. Results show that QKD can provide secure
real-time key distribution between parties who have a need to communicate
secretly. Finally, we examine the feasibility of surface to satellite QKD.Comment: 5 pages, 2 figures, 2 tables. Submitted to Physics Review Letters,
May 199
Simple generalizations of Anti-de Sitter space-time
We consider new cosmological solutions which generalize the cosmological
patch of the Anti-de Sitter (AdS) space-time, allowing for fluids with
equations of state such that . We use them to derive the associated
full manifolds. We find that these solutions can all be embedded in flat
five-dimensional space-time with signature, revealing deformed
hyperboloids. The topology and causal-structure of these spaces is therefore
unchanged, and closed time-like curves are identified, before a covering space
is considered. However the structure of Killing vector fields is entirely
different and so we may expect a different structure of Killing horizons in
these solutions.Comment: 6 Pages, 5 Figures, Corrections and additions made for publication in
Journal of Classical and Quantum Gravit
Scaling Cosmologies of N=8 Gauged Supergravity
We construct exact cosmological scaling solutions in N=8 gauged supergravity.
We restrict to solutions for which the scalar fields trace out geodesic curves
on the scalar manifold. Under these restrictions it is shown that the axionic
scalars are necessarily constant. The potential is then a sum of exponentials
and has a very specific form that allows for scaling solutions. The scaling
solutions describe eternal accelerating and decelerating power-law universes,
which are all unstable. An uplift of the solutions to 11-dimensional
supergravity is carried out and the resulting timedependent geometries are
discussed. In the discussion we briefly comment on the fact that N=2 gauged
supergravity allows stable scaling solutions.Comment: 17 pages; referenced added, reportnr changed and some corrections in
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