4,000 research outputs found
Testing the LCDM model (and more) with the time evolution of the redshift
With the many ambitious proposals afoot for new generations of very large
telescopes, along with spectrographs of unprecedented resolution, there arises
the real possibility that the time evolution of the cosmological redshift may,
in the not too distant future, prove to be a useful tool rather than merely a
theoretical curiosity. Here I contrast this approach with the standard
cosmological procedure based on the luminosity (or any other well-defined)
distance. I then show that such observations would not only provide a direct
measure of all the associated cosmological parameters of the LCDM model, but
would also provide wide-ranging internal consistency checks. Further, in a more
general context, I show that without introducing further time derivatives of
the redshift one could in fact map out the dark energy equation of state should
the LCDM model fail. A consideration of brane-world scenarios and interacting
dark energy models serves to emphasize the fact that the usefulness of such
observations would not be restricted to high redshifts.Comment: In final form as to appear in Physical Review D. 12 pages 6 figure
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
Neutral buoyancy test evaluation of hardware and extravehicular activity procedures for on-orbit assembly of a 14 meter precision reflector
A procedure that enables astronauts in extravehicular activity (EVA) to perform efficient on-orbit assembly of large paraboloidal precision reflectors is presented. The procedure and associated hardware are verified in simulated Og (neutral buoyancy) assembly tests of a 14 m diameter precision reflector mockup. The test article represents a precision reflector having a reflective surface which is segmented into 37 individual panels. The panels are supported on a doubly curved tetrahedral truss consisting of 315 struts. The entire truss and seven reflector panels were assembled in three hours and seven minutes by two pressure-suited test subjects. The average time to attach a panel was two minutes and three seconds. These efficient assembly times were achieved because all hardware and assembly procedures were designed to be compatible with EVA assembly capabilities
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.
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
Optical Spectroscopic Survey of High-latitude WISE-selected Sources
We report on the results of an optical spectroscopic survey at high Galactic latitude (|b| ≥ 30°) of a sample of WISE-selected targets, grouped by WISE W1 (λ_eff = 3.4 μm) flux, which we use to characterize the sources WISE detected. We observed 762 targets in 10 disjoint fields centered on ultraluminous infrared galaxy candidates using DEIMOS on Keck II. We find 0.30 ± 0.02 galaxies arcmin–2 with a median redshift of z = 0.33 ± 0.01 for the sample with W1 ≥ 120 μJy. The foreground stellar densities in our survey range from 0.23 ± 0.07 arcmin–2 to 1.1 ± 0.1 arcmin–2 for the same sample. We obtained spectra that produced science grade redshifts for ≥90% of our targets for sources with W1 flux ≥120 μJy that also had an i-band flux gsim 18 μJy. We used this for targeting very preliminary data reductions available to the team in 2010 August. Our results therefore present a conservative estimate of what is possible to achieve using WISE's Preliminary Data Release for the study of field galaxies
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