5,827 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
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
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
Temporal Relational Reasoning in Videos
Temporal relational reasoning, the ability to link meaningful transformations
of objects or entities over time, is a fundamental property of intelligent
species. In this paper, we introduce an effective and interpretable network
module, the Temporal Relation Network (TRN), designed to learn and reason about
temporal dependencies between video frames at multiple time scales. We evaluate
TRN-equipped networks on activity recognition tasks using three recent video
datasets - Something-Something, Jester, and Charades - which fundamentally
depend on temporal relational reasoning. Our results demonstrate that the
proposed TRN gives convolutional neural networks a remarkable capacity to
discover temporal relations in videos. Through only sparsely sampled video
frames, TRN-equipped networks can accurately predict human-object interactions
in the Something-Something dataset and identify various human gestures on the
Jester dataset with very competitive performance. TRN-equipped networks also
outperform two-stream networks and 3D convolution networks in recognizing daily
activities in the Charades dataset. Further analyses show that the models learn
intuitive and interpretable visual common sense knowledge in videos.Comment: camera-ready version for ECCV'1
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