21,274 research outputs found
Hypothesis of path integral duality: Applications to QED
We use the modified propagator for quantum field based on a ``principle of
path integral duality" proposed earlier in a paper by Padmanabhan to
investigate several results in QED. This procedure modifies the Feynman
propagator by the introduction of a fundamental length scale. We use this
modified propagator for the Dirac particles to evaluate the first order
radiative corrections in QED. We find that the extra factor of the modified
propagator acts like a regulator at the Planck scales thereby removing the
divergences that otherwise appear in the conventional radiative correction
calculations of QED. We find that:(i) all the three renormalisation factors
, , and pick up finite corrections and (ii) the modified
propagator breaks the gauge invariance at a very small level of
. The implications of this result to generation of the
primordial seed magnetic fields are discussed.Comment: 15 pages, LaTeX2e (uses ijmpd.sty); To appear in IJMP-D; References
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Reddenings of FGK supergiants and classical Cepheids from spectroscopic data
Accurate and homogeneous atmospheric parameters (Teff, log (g), Vt, [Fe/H])
are derived for 74 FGK non-variable supergiants from high-resolution, high
signal-to-noise ratio, echelle spectra. Extremely high precision for the
inferred effective temperatures (10-40 K) is achieved by using the line-depth
ratio method. The new data are combined with atmospheric values for 164
classical Cepheids, observed at 675 different pulsation phases, taken from our
previously published studies. The derived values are correlated with unreddened
B-V colours compiled from the literature for the investigated stars in order to
obtain an empirical relationship of the form: (B-V)o = 57.984 - 10.3587(log
Teff)^2 + 1.67572(log Teff)^3 - 3.356(log (g)) + 0.0321(Vt) + 0.2615[Fe/H] +
0.8833((log (g))(log Teff)). The expression is used to estimate colour excesses
E(B-V) for individual supergiants and classical Cepheids, with a precision of
+-0.05 mag. for supergiants and Cepheids with n=1-2 spectra, reaching +-0.025
mag. for Cepheids with n>2 spectra, matching uncertainties for the most
sophisticated photometric techniques. The reddening scale is also a close match
to the system of space reddenings for Cepheids. The application range is for
spectral types F0--K0 and luminosity classes I and II.Comment: accepted for publication (MNRAS
Recommended from our members
Pyrolysis-GC×GC-TOFMS to characterize carbonaceous chondrites
Using pyrolysis-GCxGC-TOFMS to analyze organic carbon in carbonaceous chondrites gives a massive increase in both sensitivity and structural information from samples when compared to traditional Py-GC-MS
Gravitational lensing constraint on the cosmic equation of state
Recent redshift-distance measurements of Type Ia supernovae (SNe Ia) at
cosmological distances suggest that two-third of the energy density of the
universe is dominated by dark energy component with an effective negative
pressure. This dark energy component is described by the equation of state
. We use gravitational lensing statistics to
constrain the equation of state of this dark energy. We use ,
image separation distribution function of lensed quasars, as a tool to probe
. We find that for the observed range of ,
should lie between in order to have five lensed quasars
in a sample of 867 optical quasars. This limit is highly sensitive to lens and
Schechter parameters and evolution of galaxies.Comment: Modified results and inclusion of calculations with new set of
parameter
Inferring DNA sequences from mechanical unzipping: an ideal-case study
We introduce and test a method to predict the sequence of DNA molecules from
in silico unzipping experiments. The method is based on Bayesian inference and
on the Viterbi decoding algorithm. The probability of misprediction decreases
exponentially with the number of unzippings, with a decay rate depending on the
applied force and the sequence content.Comment: Source as TeX file with ps figure
Autocatalytic plume pinch-off
A localized source of buoyancy flux in a non-reactive fluid medium creates a
plume. The flux can be provided by either heat, a compositional difference
between the fluid comprising the plume and its surroundings, or a combination
of both. For autocatalytic plumes produced by the iodate-arsenous acid
reaction, however, buoyancy is produced along the entire reacting interface
between the plume and its surroundings. Buoyancy production at the moving
interface drives fluid motion, which in turn generates flow that advects the
reaction front. As a consequence of this interplay between fluid flow and
chemical reaction, autocatalytic plumes exhibit a rich dynamics during their
ascent through the reactant medium. One of the more interesting dynamical
features is the production of an accelerating vortical plume head that in
certain cases pinches-off and detaches from the upwelling conduit. After
pinch-off, a new plume head forms in the conduit below, and this can lead to
multiple generations of plume heads for a single plume initiation. We
investigated the pinch-off process using both experimentation and simulation.
Experiments were performed using various concentrations of glycerol, in which
it was found that repeated pinch-off occurs exclusively in a specific
concentration range. Autocatalytic plume simulations revealed that pinch-off is
triggered by the appearance of accelerating flow in the plume conduit.Comment: 10 figures. Accepted for publication in Phys Rev E. See also
http://www.physics.utoronto.ca/nonlinear/papers_chemwave.htm
Could the cosmic acceleration be transient? A cosmographic evaluation
A possible slowing down of the cosmic expansion is investigated through a
cosmographic approach. By expanding the luminosity distance to fourth order and
fitting the SN Ia data from the most recent compilations (Union, Constitution
and Union 2), the marginal likelihood distributions for the deceleration
parameter today suggest a recent reduction of the cosmic acceleration and
indicate that there is a considerable probability for . Also in contrast
to the prediction of the CDM model, the cosmographic
reconstruction permits a cosmic expansion history where the cosmic acceleration
could already have peaked and be presently slowing down, which would imply that
the recent accelerated expansion of the Universe is a transient phenomenon. It
is also shown that to describe a transient acceleration the luminosity distance
needs to be expanded at least to fourth order. The present cosmographic results
depend neither on the validity of general relativity nor on the matter-energy
contents of the Universe.Comment: 11 pages, 4 figures. Accepted for publication in Classical and
Quantum Gravit
Spicules and the effect of rigid rods on enclosing membrane tubes
Membrane tubes (spicules) arise in cells, or artificial membranes, in the
nonlinear deformation regime due to, e.g. the growth of microtubules, actin
filaments or sickle hemoglobin fibers towards a membrane. We calculate the
axial force exerted by the cylindrical membrane tube, and its average radius,
by taking into account steric interactions between the fluctuating membrane and
the enclosed rod. The force required to confine a fluctuating membrane near the
surface of the enclosed rod diverges as the separation approaches zero. This
results in a smooth crossover of the axial force between a square root and a
linear dependence on the membrane tension as the tension increases and the tube
radius shrinks. This crossover can occur at the most physiologically relevant
membrane tensions. Our work may be important in (i) interpreting experiments in
which axial force is related to the tube radius or membrane tension (ii)
dynamical theories for biopolymer growth in narrow tubes where these
fluctuation effects control the tube radius.Comment: 10 pages, 1 figur
Inferring DNA sequences from mechanical unzipping data: the large-bandwidth case
The complementary strands of DNA molecules can be separated when stretched
apart by a force; the unzipping signal is correlated to the base content of the
sequence but is affected by thermal and instrumental noise. We consider here
the ideal case where opening events are known to a very good time resolution
(very large bandwidth), and study how the sequence can be reconstructed from
the unzipping data. Our approach relies on the use of statistical Bayesian
inference and of Viterbi decoding algorithm. Performances are studied
numerically on Monte Carlo generated data, and analytically. We show how
multiple unzippings of the same molecule may be exploited to improve the
quality of the prediction, and calculate analytically the number of required
unzippings as a function of the bandwidth, the sequence content, the elasticity
parameters of the unzipped strands
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