419 research outputs found
Time-dependent gravitating solitons in five dimensional warped space-times
Time-dependent soliton solutions are explicitly derived in a five-dimensional
theory endowed with one (warped) extra-dimension. Some of the obtained
geometries, everywhere well defined and technically regular, smoothly
interpolate between two five-dimensional anti-de Sitter space-times for fixed
value of the conformal time coordinate. Time dependent solutions containing
both topological and non-topological sectors are also obtained. Supplementary
degrees of freedom can be also included and, in this case, the resulting
multi-soliton solutions may describe time-dependent kink-antikink systems.Comment: 19 pages, 10 figure
Gravitating multidefects from higher dimensions
Warped configurations admitting pairs of gravitating defects are analyzed.
After devising a general method for the construction of multidefects, specific
examples are presented in the case of higher-dimensional Einstein-Hilbert
gravity. The obtained profiles describe diverse physical situations such as
(topological) kink-antikink systems, pairs of non-topological solitons and
bound configurations of a kink and of a non-topological soliton. In all the
mentioned cases the geometry is always well behaved (all relevant curvature
invariants are regular) and tends to five-dimensional anti-de Sitter space-time
for large asymptotic values of the bulk coordinate. Particular classes of
solutions can be generalized to the framework where the gravity part of the
action includes, as a correction, the Euler-Gauss-Bonnet combination. After
scrutinizing the structure of the zero modes, the obtained results are compared
with conventional gravitating configurations containing a single topological
defect.Comment: 27 pages, 5 included figure
Is there a âpandemic effectâ on individuals' willingness to take genetic tests?
In this cross-sectional, semi-longitudinal and quasi-experimental study, our goal was to determine the effect of data storage conditions on willingness to take a genetic test. We compared individualsâ preferences regarding how they want to store health data collected from genetic tests through two survey experiments fielded in Switzerland in March 2020 and January 2022. We tested for differences whether genetic data are presented as private goods or public goods. Results confirm our initial research expectation: more control over storage increases willingness, so does framing genetic data as private good. However, they also show that the willingness to take a genetic test has noticeably increased between 2020 and 2022. Our results point toward a âpandemic effectâ which would have increased willingness take a genetic test, nevertheless, more data are needed to understand this putative effect
Lorentz-violating vs ghost gravitons: the example of Weyl gravity
We show that the ghost degrees of freedom of Einstein gravity with a Weyl
term can be eliminated by a simple mechanism that invokes local Lorentz
symmetry breaking. We demonstrate how the mechanism works in a cosmological
setting. The presence of the Weyl term forces a redefinition of the quantum
vacuum state of the tensor perturbations. As a consequence the amplitude of
their spectrum blows up when the Lorentz-violating scale becomes comparable to
the Hubble radius. Such a behaviour is in sharp contrast to what happens in
standard Weyl gravity where the gravitational ghosts smoothly damp out the
spectrum of primordial gravitational waves.Comment: 14 pages, 3 figures, REVTeX 4.
ASYMPTOTIC BEHAVIOR OF COMPLEX SCALAR FIELDS IN A FRIEDMAN-LEMAITRE UNIVERSE
We study the coupled Einstein-Klein-Gordon equations for a complex scalar
field with and without a quartic self-interaction in a curvatureless
Friedman-Lema\^{\i}\-tre Universe. The equations can be written as a set of
four coupled first order non-linear differential equations, for which we
establish the phase portrait for the time evolution of the scalar field. To
that purpose we find the singular points of the differential equations lying in
the finite region and at infinity of the phase space and study the
corresponding asymptotic behavior of the solutions. This knowledge is of
relevance, since it provides the initial conditions which are needed to solve
numerically the differential equations. For some singular points lying at
infinity we recover the expected emergence of an inflationary stage.Comment: uuencoded, compressed tarfile containing a 15 pages Latex file and 2
postscipt figures. Accepted for publication on Phys. Rev.
A Rigorous Treatment of Energy Extraction from a Rotating Black Hole
The Cauchy problem is considered for the scalar wave equation in the Kerr
geometry. We prove that by choosing a suitable wave packet as initial data, one
can extract energy from the black hole, thereby putting supperradiance, the
wave analogue of the Penrose process, into a rigorous mathematical framework.
We quantify the maximal energy gain. We also compute the infinitesimal change
of mass and angular momentum of the black hole, in agreement with
Christodoulou's result for the Penrose process. The main mathematical tool is
our previously derived integral representation of the wave propagator.Comment: 19 pages, LaTeX, proof of Propositions 2.3 and 3.1 given in more
detai
New features of flat (4+1)-dimensional cosmological model with a perfect fluid in Gauss-Bonnet gravity
We investigated a flat multidimensional cosmological model in Gauss-Bonnet
gravity in presence of a matter in form of perfect fluid. We found analytically
new stationary regimes (these results are valid for arbitrary number of spatial
dimensions) and studied their stability by means of numerical recipes in
4+1-dimensional case. In the vicinity of the stationary regime we discovered
numerically another non-singular regime which appears to be periodical.
Finally, we demonstrated that the presence of matter in form of a perfect fluid
lifts some constraints on the dynamics of the 4+1-dimensional model which have
been found earlier.Comment: 14 pages, 5 figures, 1 table; v2 minor corrections, conclusions
unchange
Long-wavelength iteration scheme and scalar-tensor gravity
Inhomogeneous and anisotropic cosmologies are modeled withing the framework
of scalar-tensor gravity theories. The inhomogeneities are calculated to
third-order in the so-called long-wavelength iteration scheme. We write the
solutions for general scalar coupling and discuss what happens to the
third-order terms when the scalar-tensor solution approaches at first-order the
general relativistic one. We work out in some detail the case of Brans-Dicke
coupling and determine the conditions for which the anisotropy and
inhomogeneity decay as time increases. The matter is taken to be that of
perfect fluid with a barotropic equation of state.Comment: 13 pages, requires REVTeX, submitted to Phys. Rev.
The COVID-19 crisis and the rise of the European Centre for Disease Prevention and Control (ECDC)
This is the final version. Available on open access from Routledge via the DOI in this record.European institutionalisation of public health policy has never been more topical than in the COVID-19 era. One European agency has come to the fore: the European Centre for Disease Prevention and Control (ECDC). Historically, the ECDCâs mandate has expanded only gradually and the management of transboundary health crises has remained ultimately in the hands of Member States. The unprecedented severity of COVID-19 has led the European Commission to propose an extension of the ECDCâs mandate. This study assesses the expansion of the formal and informal mandates of the ECDC over 15âyears to contextualise the catalytic impact of COVID-19. It is found that while institutional change occurs in the aftermath of a transboundary health crisis, it builds on a long-term process of gradual institutionalisation that is accelerated by the crisis acting as a catalyst but not fully determined by it
On linearized gravity in the Randall-Sundrum scenario
In the literature about the Randall-Sundrum scenario one finds on one hand
that there exist (small) corrections to Newton's law of gravity on the brane,
and on another that the exact (and henceforth linearized) Einstein equations
can be recovered on the brane. The explanation for these seemingly
contradictory results is that the behaviour of the bulk far from the brane is
different in both models. We show that explicitely in this paper.Comment: 12 pages, plain TeX, no figure
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