3,781 research outputs found
Schwinger's Propagator Is Only A Green's Function
Schwinger used an analytic continuation of the effective action to correctly
compute the particle production rate per unit volume for QED in a uniform
electric field. However, if one simply evaluates the one loop expectation value
of the current operator using his propagator, the result is zero! We analyze
this curious fact from the context of a canonical formalism of operators and
states. The explanation turns out to be that Schwinger's propagator is not
actually the expectation value of the time-ordered product of field operators
in the presence of a time-independent state, although it is of course a Green's
function. We compute the true propagator in the presence of a state which is
empty at where is the lightcone
evolution parameter. Our result can be generalized to electric fields which
depend arbitrarily on .Comment: 18 pages, LaTeX 2 epsilo
General plane wave mode functions for scalar-driven cosmology
We give a solution for plane wave scalar, vector and tensor mode functions in
the presence of any homogeneous, isotropic and spatially flat cosmology which
is driven by a single, minimally coupled scalar. The solution is obtained by
rescaling the various mode functions so that they reduce, with a suitable scale
factor and a suitable time variable, to those of a massless, minimally coupled
scalar. We then express the general solution in terms of co-moving time and the
original scale factor.Comment: 6 pages, revtex4, no figures, revised version corrects an
embarrassing mistake (in the published version) for the parameter q_C.
Affected eqns are 45 and 6
Cardiovascular magnetic resonance with an MR compatible pacemaker
Abstract Magnetic resonance imaging (MRI) within FDA guidelines for the MRI-conditional pacemaker precludes placing the heart at the center of the magnet’s bore. This in effect appears to preclude cardiovascular MR. In this manuscript, we describe a protocol for cardiovascular MR of patients with a Revo pacemaker system while operating within FDA guidelines, and the first US case of cardiovascular MR in a patient with a Revo MRI-conditional pacing system despite position constraints.</p
Fields of accelerated sources: Born in de Sitter
This paper deals thoroughly with the scalar and electromagnetic fields of
uniformly accelerated charges in de Sitter spacetime. It gives details and
makes various extensions of our Physical Review Letter from 2002. The basic
properties of the classical Born solutions representing two uniformly
accelerated charges in flat spacetime are first summarized. The worldlines of
uniformly accelerated particles in de Sitter universe are defined and described
in a number of coordinate frames, some of them being of cosmological
significance, the other are tied naturally to the particles. The scalar and
electromagnetic fields due to the accelerated charges are constructed by using
conformal relations between Minkowski and de Sitter space. The properties of
the generalized `cosmological' Born solutions are analyzed and elucidated in
various coordinate systems. In particular, a limiting procedure is demonstrated
which brings the cosmological Born fields in de Sitter space back to the
classical Born solutions in Minkowski space. In an extensive Appendix, which
can be used independently of the main text, nine families of coordinate systems
in de Sitter spacetime are described analytically and illustrated graphically
in a number of conformal diagrams.Comment: 37 pages, 23 figures, reformatted version of the paper published in
JMP; low-resolution figures due to arXiv size restrictions; for the version
with high-resolution figures see http://utf.mff.cuni.cz/~krtous/papers
A graviton propagator for inflation
We construct the scalar and graviton propagator in quasi de Sitter space up
to first order in the slow roll parameter . After
a rescaling, the propagators are similar to those in de Sitter space with an
correction to the effective mass. The limit
corresponds to the E(3) vacuum that breaks de Sitter symmetry, but does not
break spatial isotropy and homogeneity. The new propagators allow for a
self-consistent, dynamical study of quantum back-reaction effects during
inflation.Comment: 23 page
Aquifex aeolicus 3-Deoxy- d - manno -2-Octulosonic Acid 8-Phosphate Synthase: A New Class of KDO 8-P Synthase?
The relationship between 3-deoxy- d - manno 2-octulosonic acid 8-phosphate (KDO 8-P) synthase and 3-deoxy- d - arabino -2-heptulosonic acid 7-phosphate (DAH 7-P) synthase has not been adequately addressed in the literature. Based on recent reports of a metal requiring KDO 8-P synthase and the newly solved X-ray crystal structures of both Escherichia coli KDO 8-P synthase and DAH 7-P synthase, we begin to address the evolutionary kinship between these catalytically similar enzymes. Using a maximum likelihood-based grouping of 29 KDO 8-P synthase sequences, we demonstrate the existence of a new class of KDO 8-P synthase, the members of which we propose to require a metal cofactor for catalysis. Similarly, we hypothesize a class of DAH 7-P synthase that does not have the metal requirement of the heretofore model E. coli enzyme. Based on this information and a careful investigation of the reported X-ray crystal structures, we also propose that KDO 8-P synthase and DAH 7-P synthase are the product of a divergent evolutionary process from a common ancestor.Peer Reviewedhttp://deepblue.lib.umich.edu/bitstream/2027.42/42370/1/239-52-2-205_10520205.pd
The Fermion Self-Energy during Inflation
We compute the one loop fermion self-energy for massless Dirac + Einstein in
the presence of a locally de Sitter background. We employ dimensional
regularization and obtain a fully renormalized result by absorbing all
divergences with BPHZ counterterms. An interesting technical aspect of this
computation is the need for a noninvariant counterterm owing to the breaking of
de Sitter invariance by our gauge condition. Our result can be used in the
quantum-corrected Dirac equation to search for inflation-enhanced quantum
effects from gravitons, analogous to those which have been found for massless,
minimally coupled scalars.Comment: 63 pages, 3 figures (uses axodraw.sty), LaTeX 2epsilon. Revised
version (to appear in Classical and Quantum Gravity) corrects some typoes and
contains some new reference
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