4,419 research outputs found
Some notes on the Kruskal - Szekeres completion
The Kruskal - Szekeres (KS) completion of the Schwarzschild spacetime is open
to Synge's methodological criticism that the KS procedure generates "good"
coordinates from "bad". This is addressed here in two ways: First I generate
the KS coordinates from Israel coordinates, which are also "good", and then I
generate the KS coordinates directly from a streamlined integration of the
Einstein equations.Comment: One typo correcte
Signatures of the Milky Way's Dark Disk in Current and Future Experiments
In hierarchical structure formation models of disk galaxies, a dark matter
disk forms as massive satellites are preferentially dragged into the disk-plane
where they dissolve. Here, we quantify the importance of this dark disk for
direct and indirect dark matter detection. The low velocity of the dark disk
with respect to the Earth enhances detection rates in direct detection
experiments at low recoil energy. For WIMP masses M_{WIMP} >~ 50 GeV, the
detection rate increases by up to a factor of 3 in the 5 - 20 keV recoil energy
range. Comparing this with rates at higher energy is sensitive to M_{WIMP},
providing stronger mass constraints particularly for M_{WIMP}>~100 GeV. The
annual modulation signal is significantly boosted by the dark disk and the
modulation phase is shifted by ~3 weeks relative to the dark halo. The
variation of the observed phase with recoil energy determines M_{WIMP}, once
the dark disk properties are fixed by future astronomical surveys. The low
velocity of the particles in the dark disk with respect to the solar system
significantly enhances the capture rate of WIMPs in the Sun, leading to an
increased flux of neutrinos from the Sun which could be detected in current and
future neutrino telescopes. The dark disk contribution to the muon flux from
neutrino back conversion at the Earth is increased by a factor of ~5 compared
to the SHM, for rho_d/rho_h=0.5.Comment: 5 pages, 7 figures, To appear in the proceedings of Identification of
Dark Matter 2008 (IDM2008), Stockholm, 18-22 August 2008; corrected one
referenc
Visualizing Spacetime Curvature via Gradient Flows II: An Example of the Construction of a Newtonian analogue
This is the first in a series of papers in which the gradient flows of
fundamental curvature invariants are used to formulate a visualization of
curvature. We start with the construction of strict Newtonian analogues (not
limits) of solutions to Einstein's equations based on the topology of the
associated gradient flows. We do not start with any easy case. Rather, we start
with the Curzon - Chazy solution, which, as history shows, is one of the most
difficult exact solutions to Einstein's equations to interpret physically. We
show that the entire field of the Curzon - Chazy solution, up to a region very
"close" to the the intrinsic singularity, strictly represents that of a
Newtonian ring, as has long been suspected. In this regard, we consider our
approach very successful. As regrades the local structure of the singularity of
the Curzon - Chazy solution within a fully general relativistic analysis,
however, whereas we make some advances, the full structure of this singularity
remains incompletely resolved.Comment: 12 pages twocolumn revtex 4-1 9 figures. Expanded and correcte
Oscillating Fracture in Rubber
We have found an oscillating instability of fast-running cracks in thin
rubber sheets. A well-defined transition from straight to oscillating cracks
occurs as the amount of biaxial strain increases. Measurements of the amplitude
and wavelength of the oscillation near the onset of this instability indicate
that the instability is a Hopf bifurcation
An exact self-similar solution for an expanding ball of radiation
We give an exact solution of the Einstein equations which in 4D can be
interpreted as a spherically symmetric dissipative distribution of matter, with
heat flux, whose effective density and pressure are nonstatic, nonuniform, and
satisfy the equation of state of radiation. The matter satisfies the usual
energy and thermodynamic conditions. The energy density and temperature are
related by the Stefan-Boltzmann law. The solution admits a homothetic Killing
vector in , which induces the existence of self-similar symmetry in 4D,
where the line element as well as the dimensionless matter quantities are
invariant under a simple "scaling" group.Comment: New version expanded and improved. To appear in Int. J. Mod. Phys.
New Wrinkles on an Old Model: Correlation Between Liquid Drop Parameters and Curvature Term
The relationship between the volume and surface energy coefficients in the
liquid drop A^{-1/3} expansion of nuclear masses is discussed. The volume and
surface coefficients in the liquid drop expansion share the same physical
origin and their physical connection is used to extend the expansion with a
curvature term. A possible generalization of the Wigner term is also suggested.
This connection between coefficients is used to fit the experimental nuclear
masses. The excellent fit obtained with a smaller number of parameters
validates the assumed physical connection.Comment: 6 pages, 2 figure
Evolution of antiferromagnetic domains in the all-in-all-out ordered pyrochlore NdZrO
We report the observation of magnetic domains in the exotic,
antiferromagnetically ordered all-in-all-out state of NdZrO,
induced by spin canting. The all-in-all-out state can be realized by Ising-like
spins on a pyrochlore lattice and is established in NdZrO below
0.31 K for external magnetic fields up to 0.14 T. Two different spin
arrangements can fulfill this configuration which leads to the possibility of
magnetic domains. The all-in-all-out domain structure can be controlled by an
external magnetic field applied parallel to the [111] direction. This is a
result of different spin canting mechanism for the two all-in-all-out
configurations for such a direction of the magnetic field. The change of the
domain structure is observed through a hysteresis in the magnetic
susceptibility. No hysteresis occurs, however, in case the external magnetic
field is applied along [100].Comment: Accepted for publication in Phys. Rev. B, 6 pages, 6 figure
Microscopic non-equilibrium theory of quantum well solar cells
We present a microscopic theory of bipolar quantum well structures in the
photovoltaic regime, based on the non-equilibrium Green's function formalism
for a multi band tight binding Hamiltonian. The quantum kinetic equations for
the single particle Green's functions of electrons and holes are
self-consistently coupled to Poisson's equation, including inter-carrier
scattering on the Hartree level. Relaxation and broadening mechanisms are
considered by the inclusion of acoustic and optical electron-phonon interaction
in a self consistent Born approximation of the scattering self energies.
Photogeneration of carriers is described on the same level in terms of a self
energy derived from the standard dipole approximation of the electron-photon
interaction. Results from a simple two band model are shown for the local
density of states, spectral response, current spectrum, and current-voltage
characteristics for generic single quantum well systems.Comment: 10 pages, 6 figures; corrected typos, changed caption Fig. 1,
replaced Fig.
The versatility of a truss mounted mobile transporter for in-space construction
The Mobile Transporter (MT) evolution from early erectable structures assembly activities is detailed. The MT operational features which are required to support astronauts performing on-orbit structure construction or spacecraft assembly functions are presented and discussed. Use of the MT to perform a variety of assembly functions is presented. Estimated EVA assembly times for a precision segmented reflector approximately 20 m in diameter are presented. The EVA/MT technique under study for construction of the reflector (and the entire spacecraft) is illustrated. Finally, the current status of development activities and test results involving the MT and Space Station structural assembly are presented
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