648 research outputs found
Gauge Field Preheating at the End of Inflation
Here we consider the possibility of preheating the Universe via the
parametric amplification of a massless, U(1) abelian gauge field. We assume
that the gauge field is coupled to the inflaton via a conformal factor with one
free parameter. We present the results of high-resolution three-dimensional
simulations of this model and show this mechanism efficiently preheats the
Universe to a radiation-dominated final state.Comment: 8 pages, 8 figure
Productivity of Florida Springs: first semi-annual report to Biology Division, Office of Naval Research, progress from June 1, 1952 to January 31, 1953
Work has begun on studying the factors responsible for productivity
in the Florida springs, which are nearly constant temperature, constant
chemical, steady state giant laboratories. Progress has been made on five
aspects: qualitative description, quantitative description, completion
of knowledge of chemical factors, measurement of productivity , development
of productivity theory.
Measurement of the primary productivity in Silver Springs and Green
Cove Springs by two new methods: the raising of organisms in cages, and the
measurement of night & day differences in oxygen downstream agree roughly.
Production in these springs is greater than previous production figures reported
for marine, fresh water, and land areas. Instantaneous measures of production
show large variations with season, time of day, cloud cover. Production
estimates range from 11,000 lbs per acre per year to 70,000 lbs. glucose per
acre per year during daylight hours.
Essential stability of the springs environment has been shown with
respect to temperature, phosphorus, and plant cover. A correlation of species
number with lack of stability has been shown with insects. Quantitative
studies have shown very large plant base to pyramids of mass. Correlation
of marine invasion with chlorinity has been shown. The essential aspects of
pH regulated phosphorus geochemistry in Florida have been outlined. Some
theoretical ideas on productivity have been evolved. Mapping of sessile
organisms in springs and taxonomic identification of dominants are half
completed. Plans for second six months include measurement of herbivore
and carnivore production rates and completion of food chain efficiency
determinations in Silver Springs as a preparation for subsequent
comparisons between springs. (34pp.
High resolution, low temperature photoabsorption cross-section of C2H2 with application to Saturn's atmosphere
New laboratory observations of the VUV absorption cross-section of C2H2, obtained under physical conditions approximating stratospheres of the giant planets, were combined with IUE observations of the albedo of Saturn, for which improved data reduction techniques have been used, to produce new models for that atmosphere. When the effects of C2H2 absorption are accounted for, additional absorption by other molecules is required. The best-fitting model also includes absorption by PH3, H2O, C2H6 and CH4. A small residual disagreement near 1600 A suggests that an additional trace species may be required to complete the model
The Shape and Scale of Galactic Rotation from Cepheid Kinematics
A catalog of Cepheid variables is used to probe the kinematics of the
Galactic disk. Radial velocities are measured for eight distant Cepheids toward
l = 300; these new Cepheids provide a particularly good constraint on the
distance to the Galactic center, R_0. We model the disk with both an
axisymmetric rotation curve and one with a weak elliptical component, and find
evidence for an ellipticity of 0.043 +/- 0.016 near the Sun. Using these
models, we derive R_0 = 7.66 +/- 0.32 kpc and v_circ = 237 +/- 12 km/s. The
distance to the Galactic center agrees well with recent determinations from the
distribution of RR Lyrae variables, and disfavors most models with large
ellipticities at the solar orbit.Comment: 36 pages, LaTeX, 10 figure
Squat Ground Reaction Force on a Horizontal Squat Device, Free Weights, and Smith Machine
Bed rest is an analog to spaceflight and advancement of exercise countermeasures is dependent on the development of exercise equipment that closely mimic actual upright exercise. The Horizontal Squat Device (HSD) was developed to allow a supine exerciser to perform squats that mimic upright squat exercise. PURPOSE: To compare vertical ground reaction force (GRFv) on the HSD with Free Weight (FW) or Smith Machine (SM) during squat exercise. METHODS: Subjects (3F, 3M) performed sets of squat exercise with increasing loads up to 1-repetition (rep) maximum. GRF data were collected and compared with previous GRF data for squat exercise performed with FW & SM. Loads on the HSD were adjusted to magnitudes comparable with FW & SM by subtracting the subject s body weight (BW). Peak GRFv for 45-, 55-, 64-, & 73-kg loads above BW were calculated. Percent (%) difference between HSD and the two upright conditions were computed. Effect size was calculated for the 45-kg load. RESULTS: Most subjects were unable to lift >45 kg on the HSD; however, 1 subject completed all loads. Anecdotal evidence suggested that most subjects shoulders or back failed before their legs. The mean % difference are shown. In the 45-kg condition, effect sizes were 0.37 & 0.83 (p>0.05) for HSD vs. FW and HSD vs. SM, respectively, indicating no differences between exercise modes. CONCLUSION: When BW was added to the target load, results indicated that vertical forces were similar to those in FW and SM exercise. The exercise prescription for the HSD should include a total external resistance equivalent to goal load plus subject BW. The HSD may be used as an analog to upright exercise in bed rest studies, but because most subjects were unable to lift >45 kg, it may be necessary to prescribe higher reps and lower loads to better target the leg musculatur
Gravitational-Wave Cosmology across 29 Decades in Frequency
Quantum fluctuations of the gravitational field in the early Universe, amplified by inflation, produce a primordial gravitational-wave background across a broad frequency band. We derive constraints on the spectrum of this gravitational radiation, and hence on theories of the early Universe, by combining experiments that cover 29 orders of magnitude in frequency. These include Planck observations of cosmic microwave background temperature and polarization power spectra and lensing, together with baryon acoustic oscillations and big bang nucleosynthesis measurements, as well as new pulsar timing array and ground-based interferometer limits. While individual experiments constrain the gravitational-wave energy density in specific frequency bands, the combination of experiments allows us to constrain cosmological parameters, including the inflationary spectral index nt and the tensor-to-scalar ratio r. Results from individual experiments include the most stringent nanohertz limit of the primordial background to date from the Parkes Pulsar Timing Array, ΩGW(f)\u3c2.3×10−10. Observations of the cosmic microwave background alone limit the gravitational-wave spectral index at 95% confidence to nt≲5 for a tensor-to-scalar ratio of r=0.11. However, the combination of all the above experiments limits nt\u3c0.36. Future Advanced LIGO observations are expected to further constrain nt\u3c0.34 by 2020. When cosmic microwave background experiments detect a nonzero r, our results will imply even more stringent constraints on nt and, hence, theories of the early Universe
Photoactivated release of membrane impermeant sulfonates inside cells.
Photouncaging delivers compounds with high spatial and temporal control to induce or inhibit biological processes but the released compounds may diffuse out. We here demonstrate that sulfonate anions can be photocaged so that a membrane impermeable compound can enter cells, be uncaged by photoirradiation and trapped within the cell
Scaling Solutions and reconstruction of Scalar Field Potentials
Starting from the hypothesis of scaling solutions, the general exact form of
the scalar field potential is found. In the case of two fluids, it turns out to
be a negative power of hyperbolic sine. In the case of three fluids the
analytic form is not found, but is obtained by quadratures.Comment: 5 pages, 2 figures, some changes in references and figures caption
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