4,223 research outputs found
Estimates for the volume of a Lorentzian manifold
We prove new estimates for the volume of a Lorentzian manifold and show
especially that cosmological spacetimes with crushing singularities have finite
volume.Comment: 8 pages, a pdf version of the preprint can also be retrieved from
http://www.math.uni-heidelberg.de/studinfo/gerhardt/LM-Volume.pdf v2: A
further estimate has been added covering the case when the mean curvature is
merely non-negative resp. non-positive (Theorem 1.1
Combining gravity with the forces of the standard model on a cosmological scale
We prove the existence of a spectral resolution of the Wheeler-DeWitt
equation when the underlying spacetime is a Friedman universe with flat spatial
slices and where the matter fields are comprised of the strong interaction,
with \SU(3) replaced by a general \SU(n), , and the electro-weak
interaction. The wave functions are maps from to a subspace of the
antisymmetric Fock space, and one noteworthy result is that, whenever the
electro-weak interaction is involved, the image of an eigenfunction is in
general not one dimensional, i.e., in general it makes no sense specifying a
fermion and looking for an eigenfunction the range of which is contained in the
one dimensional vector space spanned by the fermion.Comment: 53 pages, v6: some typos correcte
The Panchromatic Starburst Intensity Limit At Low And High Redshift
The integrated bolometric effective surface brightness S_e distributions of
starbursts are investigated for samples observed in 1. the rest frame
ultraviolet (UV), 2. the far-infrared and H-alpha, and 3. 21cm radio continuum
emission. For the UV sample we exploit a tight empirical relationship between
UV reddening and extinction to recover the bolometric flux. Parameterizing the
S_e upper limit by the 90th percentile of the distribution, we find a mean
S_{e,90} = 2.0e11 L_{sun}/kpc^2 for the three samples, with a factor of three
difference between the samples. This is consistent with what is expected from
the calibration uncertainties alone. We find little variation in S_{e,90} with
effective radii for R_e ~ 0.1 - 10 kpc, and little evolution out to redshifts z
~ 3. The lack of a strong dependence of S_{e,90} on wavelength, and its
consistency with the pressure measured in strong galactic winds, argue that it
corresponds to a global star formation intensity limit (\dot\Sigma_{e,90} ~ 45
M_{sun}/kpc^2/yr) rather than being an opacity effect. There are several
important implications of these results: 1. There is a robust physical
mechanism limiting starburst intensity. We note that starbursts have S_e
consistent with the expectations of gravitational instability models applied to
the solid body rotation portion of galaxies. 2. Elliptical galaxies and spiral
bulges can plausibly be built with maximum intensity bursts, while normal
spiral disks can not. 3. The UV extinction of high-z galaxies is significant,
implying that star formation in the early universe is moderately obscured.
After correcting for extinction, the observed metal production rate at z ~ 3
agrees well with independent estimates made for the epoch of elliptical galaxy
formation.Comment: 31 pages Latex (aas2pp4.sty,psfig.sty), 9 figures, accepted for
publication in the Astronomical Journa
Technology and politics: The regional airport experience
The findings of a comparative study of the following six regional airports were presented: Dallas/Fort Worth, Kansas City, Washington, D.C., Montreal, Tampa, and St. Louis. Each case was approached as a unique historical entity, in order to investigate common elements such as: the use of predictive models in planning, the role of symbolism to heighten dramatic effects, the roles of community and professional elites, and design flexibility. Some of the factors considered were: site selection, consolidation of airline service, accessibility, land availability and cost, safety, nuisance, and pollution constraints, economic growth, expectation of regional growth, the demand forecasting conundrum, and design decisions. The hypotheses developed include the following: the effect of political, social, and economic conflicts, the stress on large capacity and dramatic, high-technology design, projections of rapid growth to explain the need for large capital outlays
Estabelecimento de uma coleção de plasmídeos para transformação genética de eucalipto.
Editores técnicos: Marcílio José Thomazini, Elenice Fritzsons, Patrícia Raquel Silva, Guilherme Schnell e Schuhli, Denise Jeton Cardoso, Luziane Franciscon. EVINCI. Resumos
Cosmic clocks: A Tight Radius - Velocity Relationship for HI-Selected Galaxies
HI-Selected galaxies obey a linear relationship between their maximum
detected radius Rmax and rotational velocity. This result covers measurements
in the optical, ultraviolet, and HI emission in galaxies spanning a factor of
30 in size and velocity, from small dwarf irregulars to the largest spirals.
Hence, galaxies behave as clocks, rotating once a Gyr at the very outskirts of
their discs. Observations of a large optically-selected sample are consistent,
implying this relationship is generic to disc galaxies in the low redshift
Universe. A linear RV relationship is expected from simple models of galaxy
formation and evolution. The total mass within Rmax has collapsed by a factor
of 37 compared to the present mean density of the Universe. Adopting standard
assumptions we find a mean halo spin parameter lambda in the range 0.020 to
0.035. The dispersion in lambda, 0.16 dex, is smaller than expected from
simulations. This may be due to the biases in our selection of disc galaxies
rather than all halos. The estimated mass densities of stars and atomic gas at
Rmax are similar (~0.5 Msun/pc^2) indicating outer discs are highly evolved.
The gas consumption and stellar population build time-scales are hundreds of
Gyr, hence star formation is not driving the current evolution of outer discs.
The estimated ratio between Rmax and disc scale length is consistent with
long-standing predictions from monolithic collapse models. Hence, it remains
unclear whether disc extent results from continual accretion, a rapid initial
collapse, secular evolution or a combination thereof.Comment: 14 pages, 7 figures, 3 in colour. Published in MNRAS. This v2
corrects wrong journal in the references section (all instances of
"Astrophysics and Space Sciences" should have been ApJ). The Posti+2017 has
also been updated. An erratum has been submitted to MNRA
Strong extinction of a laser beam by a single molecule
We present an experiment where a single molecule strongly affects the
amplitude and phase of a laser field emerging from a subwavelength aperture. We
achieve a visibility of -6% in direct and +10% in cross-polarized detection
schemes. Our analysis shows that a close to full extinction should be possible
using near-field excitation.Comment: 5 pages, 4 figures, submitted to PR
Detecting Insect Flight Sounds in the Field: Implications for Acoustical Counting of Mosquitoes
A prototype field-deployable acoustic insect flight detector was constructed from a noise-canceling microphone coupled to an off-the-shelf digital sound recorder capable of 10 h recordings. The system was placed in an urban forest setting 25 times over the course of the summer of 2004, collecting 250 h of ambient sound recordings that were downloaded to a personal computer and used to develop detection routines. These detection routines operated on short segments of sound (0.093 s, corresponding to 4096 samples at 44100 Hz). A variety of approaches were implemented to detect insect flight tones. Simple approaches, involving sensing the fundamental frequency (1st harmonic) and 2nd harmonic, were capable of detecting insects, but generated large numbers of false positives because of other ambient sounds including human voices, birds, frogs, automobiles, aircraft, sirens, and trains. In contrast, combining information from the first four harmonics, from the interharmonic regions, and from the sound envelope, reduced false positives greatly. Specifically, in the 250 h of recordings, 726 clear insect buzzes were detected by the final algorithm, with only 52 false positives (6.5%). Running the final algorithm with all criteria liberalized by 20% increased the number of clear insect buzzes by 8%, to 784, but increased false positives to 471 (28% of total detections). The potential of using this approach for detecting mosquito activity using low-cost sensors is discussed
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