512 research outputs found
On the efficient determination of optimal Bayesian experimental designs using ABC: a case study in optimal observation of epidemics
We present a new method for determining optimal Bayesian experimental designs, which we refer to as ABCdE. ABCdE uses Approximate Bayesian Computation to calculate the utility of possible designs. For problems with a low-dimensional design space, it evaluates the designsâ utility in less computation time compared to existing methods. We apply ABCdE to stochastic epidemic models. Optimal designs evaluated using ABCdE are compared to those evaluated using existing methods for the stochastic death and susceptibleâinfectious (SI) models. We present the Bayesian optimal experimental designs for the susceptibleâinfectiousâsusceptible (SIS) model using ABCdE.David J. Price, Nigel G. Bean, Joshua V. Ross, Jonathan Tuk
Kaluza-Klein Cosmology With Modified Holographic Dark Energy
We investigate the compact Kaluza-Klein cosmology in which modified
holographic dark energy is interacting with dark matter. Using this scenario,
we evaluate equation of state parameter as well as equation of evolution of the
modified holographic dark energy. Further, it is shown that the generalized
second law of thermodynamics holds without any constraint.Comment: 13 pages, accepted for publication in Gen. Relativ. Gravi
On the growth of perturbations in interacting dark energy and dark matter fluids
The covariant generalizations of the background dark sector coupling
suggested in G. Mangano, G. Miele and V. Pettorino, Mod. Phys. Lett. A 18, 831
(2003) are considered. The evolution of perturbations is studied with detailed
attention to interaction rate that is proportional to the product of dark
matter and dark energy densities. It is shown that some classes of models with
coupling of this type do not suffer from early time instabilities in strong
coupling regime.Comment: 11 pages, 2 figures. v3: minor changes, typos fixe
A Statistical Approach to Multifield Inflation: Many-field Perturbations Beyond Slow Roll
We study multifield contributions to the scalar power spectrum in an ensemble
of six-field inflationary models obtained in string theory. We identify
examples in which inflation occurs by chance, near an approximate inflection
point, and we compute the primordial perturbations numerically, both exactly
and using an array of truncated models. The scalar mass spectrum and the number
of fluctuating fields are accurately described by a simple random matrix model.
During the approach to the inflection point, bending trajectories and
violations of slow roll are commonplace, and 'many-field' effects, in which
three or more fields influence the perturbations, are often important. However,
in a large fraction of models consistent with constraints on the tilt the
signatures of multifield evolution occur on unobservably large scales. Our
scenario is a concrete microphysical realization of quasi-single-field
inflation, with scalar masses of order , but the cubic and quartic couplings
are typically too small to produce detectable non-Gaussianity. We argue that
our results are characteristic of a broader class of models arising from
multifield potentials that are natural in the Wilsonian sense.Comment: 39 pages, 17 figures. References added. Matches version published in
JCA
Inflation with a constant ratio of scalar and tensor perturbation amplitudes
The single scalar field inflationary models that lead to scalar and tensor
perturbation spectra with amplitudes varying in direct proportion to one
another are reconstructed by solving the Stewart-Lyth inverse problem to
next-to-leading order in the slow-roll approximation.
The potentials asymptote at high energies to an exponential form,
corresponding to power law inflation, but diverge from this model at low
energies, indicating that power law inflation is a repellor in this case. This
feature implies that a fine-tuning of initial conditions is required if such
models are to reproduce the observations. The required initial conditions might
be set through the eternal inflation mechanism.
If this is the case, it will imply that the spectral indices must be nearly
constant, making the underlying model observationally indistinguishable from
power law inflation.Comment: 20 pages, 7 figures. Major changes to the Introduction following
referee's comments. One figure added. Some other minor changes. No conclusion
was modifie
Magnetic flux jumps in textured Bi2Sr2CaCu2O(8+d)
Magnetic flux jumps in textured Bi2Sr2CaCu2O(8+d) have been studied by means
of magnetization measurements in the temperature range between 1.95 K and Tc,
in an external magnetic field up to 9 T. Flux jumps were found in the
temperature range 1.95 K - 6 K, with the external magnetic field parallel to
the c axis of the investigated sample. The effect of sample history on magnetic
flux jumping was studied and it was found to be well accounted for by the
available theoretical models. The magnetic field sweep rate strongly influences
the flux jumping and this effect was interpreted in terms of the influence of
both flux creep and the thermal environment of the sample. Strong flux creep
was found in the temperature and magnetic field range where flux jumps occur
suggesting a relationship between the two. The heat exchange conditions between
the sample and the experimental environment also influence the flux jumping
behavior. Both these effects stabilize the sample against flux instabilities,
and this stabilizing effect increases with decreasing magnetic field sweep
rate. Demagnetizing effects are also shown to have a significant influence on
flux jumping.Comment: 10 pages, 6 figures, RevTeX4, submitted to Phys. Rev.
Combined constraints on modified Chaplygin gas model from cosmological observed data: Markov Chain Monte Carlo approach
We use the Markov Chain Monte Carlo method to investigate a global
constraints on the modified Chaplygin gas (MCG) model as the unification of
dark matter and dark energy from the latest observational data: the Union2
dataset of type supernovae Ia (SNIa), the observational Hubble data (OHD), the
cluster X-ray gas mass fraction, the baryon acoustic oscillation (BAO), and the
cosmic microwave background (CMB) data. In a flat universe, the constraint
results for MCG model are,
()
,
()
,
()
,
()
, and ()
.Comment: 12 pages, 1figur
Constraints on coupling constant between dark energy and dark matter
We have investigated constraints on the coupling between dark matter and the
interacting Chaplygin gas. Our results indicate that the coupling constant
between these two entities can take arbitrary values, which can be either
positive or negative, thus giving arbitrary freedom to the inter-conversion
between Chaplygin gas and dark matter. Thus our results indicate that the
restriction on the coupling constant occurs as a very special case. Our
analysis also supports the existence of phantom energy under certain conditions
on the coupling constant.Comment: 16 Pages, 3 figure
Observational constraint on generalized Chaplygin gas model
We investigate observational constraints on the generalized Chaplygin gas
(GCG) model as the unification of dark matter and dark energy from the latest
observational data: the Union SNe Ia data, the observational Hubble data, the
SDSS baryon acoustic peak and the five-year WMAP shift parameter. It is
obtained that the best fit values of the GCG model parameters with their
confidence level are ()
, ()
. Furthermore in this model, we can see that the
evolution of equation of state (EOS) for dark energy is similar to quiessence,
and its current best-fit value is with the confidence
level .Comment: 9 pages, 5 figure
Models of quintessence coupled to the electromagnetic field and the cosmological evolution of alpha
We study the change of the effective fine structure constant in the
cosmological models of a scalar field with a non-vanishing coupling to the
electromagnetic field. Combining cosmological data and terrestrial observations
we place empirical constraints on the size of the possible coupling and explore
a large class of models that exhibit tracking behavior. The change of the fine
structure constant implied by the quasar absorption spectra together with the
requirement of tracking behavior impose a lower bound of the size of this
coupling. Furthermore, the transition to the quintessence regime implies a
narrow window for this coupling around in units of the inverse Planck
mass. We also propose a non-minimal coupling between electromagnetism and
quintessence which has the effect of leading only to changes of alpha
determined from atomic physics phenomena, but leaving no observable
consequences through nuclear physics effects. In doing so we are able to
reconcile the claimed cosmological evidence for a changing fine structure
constant with the tight constraints emerging from the Oklo natural nuclear
reactor.Comment: 13 pages, 10 figures, RevTex, new references adde
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