14,203 research outputs found
Large classical universes emerging from quantum cosmology
It is generally believed that one cannot obtain a large Universe from quantum
cosmological models without an inflationary phase in the classical expanding
era because the typical size of the Universe after leaving the quantum regime
should be around the Planck length, and the standard decelerated classical
expansion after that is not sufficient to enlarge the Universe in the time
available. For instance, in many quantum minisuperspace bouncing models studied
in the literature, solutions where the Universe leave the quantum regime in the
expanding phase with appropriate size have negligible probability amplitude
with respect to solutions leaving this regime around the Planck length. In this
paper, I present a general class of moving gaussian solutions of the
Wheeler-DeWitt equation where the velocity of the wave in minisuperspace along
the scale factor axis, which is the new large parameter introduced in order to
circumvent the abovementioned problem, induces a large acceleration around the
quantum bounce, forcing the Universe to leave the quantum regime sufficiently
big to increase afterwards to the present size, without needing any classical
inflationary phase in between, and with reasonable relative probability
amplitudes with respect to models leaving the quantum regime around the Planck
scale. Furthermore, linear perturbations around this background model are free
of any transplanckian problem.Comment: 8 pages, 1 figur
Symmetry Aspects in Nonrelativistic Multi-Scalar Field Models and Application to a Coupled Two-Species Dilute Bose Gas
We discuss unusual aspects of symmetry that can happen due to entropic
effects in the context of multi-scalar field theories at finite temperature. We
present their consequences, in special, for the case of nonrelativistic models
of hard core spheres. We show that for nonrelativistic models phenomena like
inverse symmetry breaking and symmetry non-restoration cannot take place, but a
reentrant phase at high temperatures is shown to be possible for some region of
parameters. We then develop a model of interest in studies of Bose-Einstein
condensation in dilute atomic gases and discuss about its phase transition
patterns. In this application to a Bose-Einstein condensation model, however,
no reentrant phases are found.Comment: 8 pages, 1 eps figure, IOP style. Based on a talk given by R. O.
Ramos at the QFEXT05 workshop, Barcelona, Spain, September 5-9, 2005. One
reference was update
Estoque de carbono em Terra Preta do Índio no Município de Iranduba-AM.
O presente trabalho objetivou estimar o estoque de carbono até um metro de profundidade em uma área de TPI de aproximadamente seis hectares
Non-Linear Affine Embedding of the Dirac Field from the Multiplicity-Free SL(4,R) Unirreps
The correspondence between the linear multiplicity-free unirreps of SL(4, R)
studied by Ne'eman and {\~{S}}ija{\~{c}}ki and the non-linear realizations of
the affine group is worked out. The results obtained clarify the inclusion of
spinorial fields in a non-linear affine gauge theory of gravitation.Comment: 13 pages, plain TeX, macros include
Antimicrobial resistance and antimicrobial use animal monitoring policies in Europe: Where are we?
The World Health Organization has recognized antimicrobial resistance as one of the top three threats to human health. Any use of antibiotics in animals will ultimately affect humans and vice versa. Appropriate monitoring of antimicrobial use and resistance has been repeatedly emphasized along with the need for global policies. Under the auspices of the European Union research project, EFFORT, we mapped antimicrobial use and resistance monitoring programs in ten European countries. We then compared international and European guidelines and policies. In resistance monitoring, we did not find important differences between countries. Current resistance monitoring systems are focused on food animal species (using fecal samples). They ignore companion animals. The scenario is different for monitoring antibiotics use. Recently, countries have tried to harmonize methodologies, but reporting of antimicrobial use remains voluntary. We therefore identified a need for stronger policies
Recording from two neurons: second order stimulus reconstruction from spike trains and population coding
We study the reconstruction of visual stimuli from spike trains, recording
simultaneously from the two H1 neurons located in the lobula plate of the fly
Chrysomya megacephala. The fly views two types of stimuli, corresponding to
rotational and translational displacements. If the reconstructed stimulus is to
be represented by a Volterra series and correlations between spikes are to be
taken into account, first order expansions are insufficient and we have to go
to second order, at least. In this case higher order correlation functions have
to be manipulated, whose size may become prohibitively large. We therefore
develop a Gaussian-like representation for fourth order correlation functions,
which works exceedingly well in the case of the fly. The reconstructions using
this Gaussian-like representation are very similar to the reconstructions using
the experimental correlation functions. The overall contribution to rotational
stimulus reconstruction of the second order kernels - measured by a chi-squared
averaged over the whole experiment - is only about 8% of the first order
contribution. Yet if we introduce an instant-dependent chi-square to measure
the contribution of second order kernels at special events, we observe an up to
100% improvement. As may be expected, for translational stimuli the
reconstructions are rather poor. The Gaussian-like representation could be a
valuable aid in population coding with large number of neurons
Concept for a Time-of-Flight Small Angle Neutron Scattering Instrument at the European Spallation Source
A new Small Angle Neutron Scattering instrument is proposed for the European
Spallation Source. The pulsed source requires a time-of-flight analysis of the
gathered neutrons at the detector. The optimal instrument length is found to be
rather large, which allows for a polarizer and a versatile collimation. The
polarizer allows for studying magnetic samples and incoherent background
subtraction. The wide collimation will host VSANS and SESANS options that
increase the resolution of the instrument towards um and tens of um,
respectively. Two 1m2 area detectors will cover a large solid angle
simultaneously. The expected gains for this new instrument will lie in the
range between 20 and 36, depending on the assessment criteria, when compared to
up-to-date reactor based instruments. This will open new perspectives for fast
kinetics, weakly scattering samples, and multi-dimensional contrast variation
studies.Comment: 18 pages, 10 figure
Quantum Cosmology in Scalar-Tensor Theories With Non Minimal Coupling
Quantization in the minisuperspace of non minimal scalar-tensor theories
leads to a partial differential equation which is non separable. Through a
conformal transformation we can recast the Wheeler-DeWitt equation in an
integrable form, which corresponds to the minimal coupling case, whose general
solution is known. Performing the inverse conformal transformation in the
solution so found, we can construct the corresponding one in the original
frame. This procedure can also be employed with the bohmian trajectories. In
this way, we can study the classical limit of some solutions of this quantum
model. While the classical limit of these solutions occurs for small scale
factors in the Einstein's frame, it happens for small values of the scalar
field non minimally coupled to gravity in the Jordan's frame, which includes
large scale factors.Comment: latex, 18 page
QED vacuum between an unusual pair of plates
We consider the photon field between an unusual configuration of infinite
parallel plates: a perfectly conducting plate and an
infinitely permeable one . After quantizing the vector potential
in the Coulomb gauge, we obtain explicit expressions for the vacuum expectation
values of field operators of the form and . These field correlators allow us to reobtain the Casimir
effect for this set up and to discuss the light velocity shift caused by the
presence of plates (Scharnhorst effect \cite{Scharnhorst,Barton,BarScharn}) for
both scalar and spinor QED.Comment: Latex, 16 pages, no figure
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