6,711 research outputs found
Weyl Geometry as Characterization of Space-Time
Motivated by an axiomatic approach to characterize space-time it is
investigated a reformulation of Einstein's gravity where the pseudo-riemannian
geometry is substituted by a Weyl one. It is presented the main properties of
the Weyl geometry and it is shown that it gives extra contributions to the
trajectories of test particles, serving as one more motivation to study general
relativity in Weyl geometry. It is introduced its variational formalism and it
is established the coupling with other physical fields in such a way that the
theory acquires a gauge symmetry for the geometrical fields. It is shown that
this symmetry is still present for the red-shift and it is concluded that for
cosmological models it opens the possibility that observations can be fully
described by the new geometrical scalar field. It is concluded then that this
reformulation, although representing a theoretical advance, still needs a
complete description of their objects.Comment: 12 page
Effects of Low pH on Lactate Dehydrogenase Kinetics of Diving and Nondiving Reptiles
The properties of lactate dehydrogenase were examined in two snake species, Nerodia rhombifera and Elaphe obsoleta, and a turtle species, Pseudemys scripta. Our purpose was to compare the LDH activity of reptiles with limited anaerobic capabilities with that of the well established diver Pseudemys. The Michaelis-Menten kinetics of LDH and its susceptibility to inhibition by elevated pyruvate concentrations were investigated in the brain and heart of the three species. All tissue incubations and enzyme activity determinations were done at a pH of 7.0 in order to stimulate a diving blood pH in the three species. In both tissues the LDH activity of the snakes was higher than that of Pseudemys at pyruvate concentrations ranging between .03 mM and .50 mM. The Km values of the snakes were lower than those of Pseudemys, suggesting a greater enzyme-substrate affinity in the snake tissues. The Vmax values were higher in the snake tissues indicating a faster conversion of substrate to product. Heart LDH activity was reduced to an equal extent by high pyruvate concentrations in each of the three species. Elaphe brain LDH was most susceptible to pyruvate inhibition, but Nerodia and Pseudemys brain LDH were inhibited to an equal extent. The results indicate that the kinetic behavior of brain and heart LDH of the three species is similar at a pH of 7.4 and a pH of 7.0. The results also suggest that the LDH of Pseudemys is no better adapted to withstand anaerobic conditions than that of Nerodia or Elaphe at a pH of 7.0
Disjoining Pressure of an Electrolyte Film Confined between Semipermeable Membranes
We consider an electrolyte solution confined by semipermeable membranes in
contact with a salt-free solvent. Membranes are uncharged, but since small
counter-ions leak-out into infinite salt-free reservoirs, we observe a
distance-dependent membrane potential, which generates a repulsive
electrostatic disjoining pressure. We obtain the distribution of the potential
and of ions, and derive explicit formulas for the disjoining pressure, which
are validated by computer simulations. We predict a strong short-range
power-law repulsion, and a weaker long-range exponential decay. Our results
also demonstrate that an interaction between membranes does strongly depend on
the screening lengths, valency of an electrolyte solution, and an
inter-membrane film thickness. Finally, our analysis can be directly extended
to the study of more complex situations and some biological problems.Comment: 9 pages, 8 figure
Model Independent Bound on the Unitarity Triangle from CP Violation in B-> pi+ pi- and B-> psi K_S
We derive model independent lower bounds on the CKM parameters (1-rhobar) and
etabar as functions of the mixing-induced CP asymmetry S in B-> pi+ pi- and
sin(2 beta) from B->psi K_S. The bounds do not depend on specific results of
theoretical calculations for the penguin contribution to B-> pi+ pi-. They
require only the very conservative condition that a hadronic phase, which
vanishes in the heavy-quark limit, does not exceed 90 degrees in magnitude. The
bounds are effective if -sin(2 beta) < S < 1. Dynamical calculations indicate
that the limits on rhobar and etabar are close to their actual values.Comment: 5 pages, 2 figure
Electrostatic Interaction of Heterogeneously Charged Surfaces with Semipermeable Membranes
In this paper we study the electrostatic interaction of a heterogeneously
charged wall with a neutral semipermeable membrane. The wall consists of
periodic stripes, where the charge density varies in one direction. The
membrane is in a contact with a bulk reservoir of an electrolyte solution and
separated from the wall by a thin film of salt-free liquid. One type of ions
(small counterions) permeates into the gap and gives rise to a
distance-dependent membrane potential, which translates into a repulsive
electrostatic disjoining pressure due to an overlap of counterion clouds in the
gap. To quantify it we use two complementary approaches. First, we propose a
mean-field theory based on a linearized Poisson-Boltzmann equation and Fourier
analysis. These calculations allow us to estimate the effect of a heterogeneous
charge pattern at the wall on the induced heterogeneous membrane potential, and
the value of the disjoining pressure as a function of the gap. Second, we
perform Langevin dynamics simulations of the same system with explicit ions.
The results of the two approaches are in good agreement with each other at low
surface charge and small gap, but differ due to nonlinearity at the higher
charge. These results demonstrate that a heterogeneity of the wall charge can
lead to a huge reduction in the electrostatic repulsion, which could
dramatically facilitate a self-assembly in complex synthetic and biological
systems.Comment: 14 pages, 6 figure
Precision Feeding in Dairy Ration Cost Minimization Under Producer's Risk Management
The biophysical simulation data from Cornell Net Carbohydrate and Protein System were used in non-linear programming model for least cost ration incorporating ingredient nutrient and price variations. Precision feeding practice indicated to have lower mean cost ration than whole herd feeding in terms of ration cost.Non-linear programming, nutrient variation, price variation, precision feeding, environmental pollution, phosphorus, nitrogen, mean cost ration, whole herd feeding, Livestock Production/Industries, Risk and Uncertainty,
Star polymers as unit cells for coarse-graining cross-linked networks
Reducing the complexity of cross-linked polymer networks by preserving their
main macroscale properties, is key to understanding them, and a crucial issue
is to relate individual properties of the polymer constituents to those of the
reduced network. Here we study polymer networks in a good solvent, by
considering star polymers as their unit elements, and first quantify the
interaction between their centers of masses. We then reduce the complexity of a
network by replacing sets of its bridged star polymers by equivalent effective
soft particles with dense cores. Our coarse graining allows us to approximate
complex polymer networks by much simpler ones, keeping their relevant
mechanical properties, as illustrated in computer experiments on an isotropic
compression.Comment: 5 pages, 8 figure
Sgr A* ``Visual Binaries'': A Direct Measurement of the Galactocentric Distance
We present a new geometrical method for measuring the distance to the
Galactic center (R_0) by solving for the Keplerian orbit of individual stars
bound to the black hole associated with the Sgr A* from radial velocity and
proper motion measurements. We identify three stars to which the method may be
applied, and show that 1-5 % accuracy of R_0 can be expected after 15 years of
observing, and 0.5-2 % after 30 years of observing, depending on what the
orbital parameters of these three stars turn out to be. Combining the
measurements of the three stars with favorable orbital parameters leads to even
more precise values. In the example that we present, such combined solution
yields 4 % accuracy already by the year 2002. All these estimates assume that
annual position measurements will continue to be made with the 2 mas precision
recently reported by Ghez et al. The precision of the distance measurement is
relatively insensitive to the radial velocity errors, provided that the latter
are less than 50 km/s. Besides potentially giving an estimate of R_0 that is
better than any currently in use, the greatest advantage of this method is that
it is free from systematic errors.Comment: Submitted to ApJ, 14 pages, 8 figure
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