16,886 research outputs found
Similarities between action potentials and acoustic pulses in a van der Waals fluid
An action potential is typically described as a purely electrical change that
propagates along the membrane of excitable cells. However, recent experiments
have demonstrated that non-linear acoustic pulses that propagate along lipid
interfaces and traverse the melting transition, share many similar properties
with action potentials. Despite the striking experimental similarities, a
comprehensive theoretical study of acoustic pulses in lipid systems is still
lacking. Here we demonstrate that an idealized description of an interface near
phase transition captures many properties of acoustic pulses in lipid
monolayers, as well as action potentials in living cells. The possibility that
action potentials may better be described as acoustic pulses in soft interfaces
near phase transition is illustrated by the following similar properties:
correspondence of time and velocity scales, qualitative pulse shape, sigmoidal
response to stimulation amplitude (an `all-or-none' behavior), appearance in
multiple observables (particularly, an adiabatic change of temperature),
excitation by many types of stimulations, as well as annihilation upon
collision. An implication of this work is that crucial functional information
of the cell may be overlooked by focusing only on electrical measurements.Comment: 8 pages, 5 figure
Probing spatial homogeneity with LTB models: a detailed discussion
Do current observational data confirm the assumptions of the cosmological
principle, or is there statistical evidence for deviations from spatial
homogeneity on large scales? To address these questions, we developed a
flexible framework based on spherically symmetric, but radially inhomogeneous
Lemaitre-Tolman-Bondi (LTB) models with synchronous Big Bang. We expanded the
(local) matter density profile in terms of flexible interpolation schemes and
orthonormal polynomials. A Monte Carlo technique in combination with recent
observational data was used to systematically vary the shape of these profiles.
In the first part of this article, we reconsider giant LTB voids without dark
energy to investigate whether extremely fine-tuned mass profiles can reconcile
these models with current data. While the local Hubble rate and supernovae can
easily be fitted without dark energy, however, model-independent constraints
from the Planck 2013 data require an unrealistically low local Hubble rate,
which is strongly inconsistent with the observed value; this result agrees well
with previous studies. In the second part, we explain why it seems natural to
extend our framework by a non-zero cosmological constant, which then allows us
to perform general tests of the cosmological principle. Moreover, these
extended models facilitate explorating whether fluctuations in the local matter
density profile might potentially alleviate the tension between local and
global measurements of the Hubble rate, as derived from Cepheid-calibrated type
Ia supernovae and CMB experiments, respectively. We show that current data
provide no evidence for deviations from spatial homogeneity on large scales.
More accurate constraints are required to ultimately confirm the validity of
the cosmological principle, however.Comment: 18 pages, 12 figures, 2 tables; accepted for publication in A&
Accurate, Meshless Methods for Magneto-Hydrodynamics
Recently, we developed a pair of meshless finite-volume Lagrangian methods
for hydrodynamics: the 'meshless finite mass' (MFM) and 'meshless finite
volume' (MFV) methods. These capture advantages of both smoothed-particle
hydrodynamics (SPH) and adaptive mesh-refinement (AMR) schemes. Here, we extend
these to include ideal magneto-hydrodynamics (MHD). The MHD equations are
second-order consistent and conservative. We augment these with a
divergence-cleaning scheme, which maintains div*B~0 to high accuracy. We
implement these in the code GIZMO, together with a state-of-the-art
implementation of SPH MHD. In every one of a large suite of test problems, the
new methods are competitive with moving-mesh and AMR schemes using constrained
transport (CT) to ensure div*B=0. They are able to correctly capture the growth
and structure of the magneto-rotational instability (MRI), MHD turbulence, and
the launching of magnetic jets, in some cases converging more rapidly than AMR
codes. Compared to SPH, the MFM/MFV methods exhibit proper convergence at fixed
neighbor number, sharper shock capturing, and dramatically reduced noise, div*B
errors, and diffusion. Still, 'modern' SPH is able to handle most of our tests,
at the cost of much larger kernels and 'by hand' adjustment of artificial
diffusion parameters. Compared to AMR, the new meshless methods exhibit
enhanced 'grid noise' but reduced advection errors and numerical diffusion,
velocity-independent errors, and superior angular momentum conservation and
coupling to N-body gravity solvers. As a result they converge more slowly on
some problems (involving smooth, slowly-moving flows) but more rapidly on
others (involving advection or rotation). In all cases, divergence-control
beyond the popular Powell 8-wave approach is necessary, or else all methods we
consider will systematically converge to unphysical solutions.Comment: 35 pages, 39 figures. MNRAS. Updated with published version. A public
version of the GIZMO MHD code, user's guide, test problem setups, and movies
are available at http://www.tapir.caltech.edu/~phopkins/Site/GIZMO.htm
e+e- pair production from nucleon targets in the resonance region
We present consistent theoretical descriptions of the
and reactions on proton and neutron targets for total
center of mass energies ranging between 1.50 GeV and 1.75 GeV. These
reactions are complementary to study the coupling of low-lying baryon
resonances to vector meson-nucleon channels. We show in particular how the
resonant structure of the amplitudes for both processes generates specific and
large quantum interferences between - and -meson decays into
pairs. Data on the and
reactions are expected in the near future from the HADES program at GSI and
from dilepton studies with CLAS at JLab.Comment: 6 pages, 6 figures, Paper presented at Hirschegg 2004, 'Probing
nuclei and nucleons with electrons and photons', January 12th-16th, 200
Study of the threshold behavior of the N scattering amplitude through the associated photoproduction of - and -mesons
We suggest that the reaction cross section, in the
kinematics where the invariant mass in the final state lies between
the threshold value (m+m) and the N*(1535) resonance mass, is
largely determined by the scattering amplitude close to threshold. The
initial photon energy is chosen in the range GeV, in
order to reach low (absolute) values of the squared 4-momentum transfer from
the initial photon to the final -meson. In these conditions, we expect
the t-channel - and -meson exchanges to drive the dynamics
underlying the process. We show that the
-exchange is the dominating contribution to the cross section while the
-exchange is negligible. The - interference is of the order
of . The sign of this term is not known and alters significantly our
results. Data on the process would be therefore very
useful to help unravelling the behavior of the scattering amplitude
close to threshold and assessing the possibility of producing -nucleus
bound states.Comment: 8 pages, 6 figures, Invited talk at MESON 2006, June 9th-13th, 2006,
Cracow (Poland
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