254 research outputs found
Hair cortisol concentrations are associated with hippocampal subregional volumes in children
Irradiated asymmetric Friedmann branes
We consider a Friedmann brane moving in a bulk impregnated by radiation. The
setup is strongly asymmetric, with only one black hole in the bulk. The
radiation emitted by this bulk black hole can be reflected, absorbed or
transmitted through the brane. Radiation pressure accelerates the brane,
behaving as dark energy. Absorption however generates a competing effect: the
brane becomes heavier and gravitational attraction increases. We analyse the
model numerically, assuming a total absorbtion on the brane for k=1. We
conclude that due to the two competing effects, in this asymmetric scenario the
Hawking radiation from the bulk black hole is not able to change the
recollapsing fate of this brane-world universe. We show that for light branes
and early times the radiation pressure is the dominant effect. In contrast, for
heavy branes the self-gravity of the absorbed radiation is a much stronger
effect. We find the critical value of the initial energy density for which
these two effects roughly cancel each other.Comment: 27 pages, 12 figure
Friedmann branes with variable tension
We introduce brane-worlds with non-constant tension, strenghtening the
analogy with fluid membranes, which exhibit a temperature-dependence according
to the empirical law established by E\"otv\"os. This new degree of freedom
allows for evolving gravitational and cosmological constants, the latter being
a natural candidate for dark energy. We establish the covariant dynamics on a
brane with variable tension in full generality, by considering asymmetrically
embedded branes and allowing for non-standard model fields in the 5-dimensional
space-time. Then we apply the formalism for a perfect fluid on a Friedmann
brane, which is embedded in a 5-dimensional charged Vaidya-Anti de Sitter
space-time.Comment: 12 pages, to appear in Phys. Rev.
Semi-transparent brane-worlds
We study the evolution of a closed Friedmann brane perturbed by the Hawking
radiation escaping a bulk black hole. The semi-transparent brane absorbes some
of the infalling radiation, the rest being transmitted across the brane to the
other bulk region. We characterize the cosmological evolution in terms of the
transmission rate . For small values of a critical-like
behaviour could be observed, when the acceleration due to radiation pressure
and the deceleration induced by the increasing self-gravity of the brane
roughly compensate each other, and cosmological evolution is approximately the
same as without radiation. Lighter (heavier) branes than those with the
critical energy density will recollapse slower (faster). This feature is
obstructed at high values of , where the overall effect of the
radiation is to speed-up the recollapse. We determine the maximal value of the
transmission rate for which the critical-like behaviour is observed. We also
study the effect of transmission on the evolution of different source terms of
the Friedmann equation. We conclude that among all semi-transparent branes the
slowest recollapse occurs for light branes with total absorption.Comment: 15 pages, 8 figure
The Kepler equation for inspiralling compact binaries
Compact binaries consisting of neutron stars / black holes on eccentric orbit
undergo a perturbed Keplerian motion. The perturbations are either of
relativistic origin or are related to the spin, mass quadrupole and magnetic
dipole moments of the binary components. The post-Newtonian motion of such
systems decouples into radial and angular parts. We present here for the first
time the radial motion of such a binary encoded in a generalized Kepler
equation, with the inclusion of all above-mentioned contributions, up to linear
order in the perturbations. Together with suitably introduced parametrizations,
the radial motion is solved completely
The paradox of soft singularity crossing and its resolution by distributional cosmological quantitities
A cosmological model of a flat Friedmann universe filled with a mixture of
anti-Chaplygin gas and dust-like matter exhibits a future soft singularity,
where the pressure of the anti-Chaplygin gas diverges (while its energy density
is finite). Despite infinite tidal forces the geodesics pass through the
singularity. Due to the dust component, the Hubble parameter has a non-zero
value at the encounter with the singularity, therefore the dust implies further
expansion. With continued expansion however, the energy density and the
pressure of the anti-Chaplygin gas would become ill-defined, hence from the
point of view of the anti-Chaplygin gas only a contraction is allowed.
Paradoxically, the universe in this cosmological model would have to expand and
contract simultaneously. This obviosly could not happen. We solve the paradox
by redefining the anti-Chaplygin gas in a distributional sense. Then a
contraction could follow the expansion phase at the singularity at the price of
a jump in the Hubble parameter. Although such an abrupt change is not common in
any cosmological evolution, we explicitly show that the set of Friedmann,
Raychaudhuri and continuity equations are all obeyed both at the singularity
and in its vicinity. We also prove that the Israel junction conditions are
obeyed through the singular spatial hypersurface. In particular we enounce and
prove a more general form of the Lanczos equation.Comment: 12 pages; to be published in Phys.Rev.
MixitĂ© en Education Physique et Sportive: Evolution de la motivation des Ă©lĂšves dans le cadre dâun enseignement mixte au cours dâun cycle de danse
Dans le cadre de notre formation en EPS, nous nous sommes intĂ©ressĂ©s Ă lâengagement des Ă©lĂšves dans les classes mixtes. Cette question de lâengagement dans un contexte mixte semblait important Ă nos yeux, dans un contexte dâenseignement de plus en plus en classes mixtes dans le Canton de Vaud. Il nous semblait judicieux de porter notre regard sur les variations de la motivation des Ă©lĂšves dans ce cas de figure, notamment dans les activitĂ©s fortement connotĂ©es par genre (football, danse, âŠ). La motivation en EPS a Ă©tĂ© investiguĂ©e par de nombreux auteurs. Nous avons choisi de porter la focale sur les effets Ă©ventuels du guidage de lâenseignant, du travail par petits groupes, de la crĂ©ation de sens par lâengagement des Ă©lĂšves dans un projet et de lâutilisation de la tablette numĂ©rique dans le processus dâapprentissage des Ă©lĂšves. Nous avons rĂ©alisĂ© une Ă©tude avec deux classes de 9H sur 17 leçons, dans une activitĂ© connotĂ©e fĂ©minine (danse). Nous avons procĂ©dĂ© Ă des observations, soutenues par trois focus groups et deux questionnaires (MIXEPS et ISEP). Des indicateurs ont Ă©tĂ© dĂ©finis pour mesurer les variations de motivation des Ă©lĂšves, permettant de repĂ©rer les effets potentiels des diffĂ©rentes variables : guidage de lâenseignant, travail par petits groupes, projet et utilisation de la tablette numĂ©rique. Les rĂ©sultats montrent que le guidage de lâenseignant et la mise en projet ont un impact positif sur lâĂ©volution de la motivation au cours du cycle. Il est plus difficile de conclure avec certitude que lâenseignement par petits groupes et lâutilisation de la tablette gĂ©nĂšrent une motivation plus importante. Dâun point de vue professionnel, cette Ă©tude souligne lâimportance du sens des apprentissages par le biais de la mise en projet ainsi que lâattention Ă porter sur lâĂ©volution des Ă©lĂšves afin dâadapter les formes de guidage au plus prĂšs de cette Ă©volution
Inflating wormholes in the braneworld models
The braneworld model, in which our Universe is a three-brane embedded in a
five-dimensional bulk, allows the existence of wormholes, without any violation
of the energy conditions. A fundamental ingredient of traversable wormholes is
the violation of the null energy condition (NEC). However, in the brane world
models, the stress energy tensor confined on the brane, threading the wormhole,
satisfies the NEC. In conventional general relativity, wormholes existing
before inflation can be significantly enlarged by the expanding spacetime. We
investigate the evolution of an inflating wormhole in the brane world scenario,
in which the wormhole is supported by the nonlocal brane world effects. As a
first step in our study we consider the possibility of embedding a
four-dimensional brane world wormhole into a five dimensional bulk. The
conditions for the embedding are obtained by studying the junction conditions
for the wormhole geometry, as well as the full set of the five dimensional bulk
field equations. For the description of the inflation we adopt the chaotic
inflation model. We study the dynamics of the brane world wormholes during the
exponential inflation stage, and in the stage of the oscillating scalar field.
A particular exact solution corresponding to a zero redshift wormhole is also
obtained. The resulting evolution shows that while the physical and geometrical
parameters of a zero redshift wormhole decay naturally, a wormhole satisfying
some very general initial conditions could turn into a black hole, and exist
forever.Comment: 30 pages, no figures, accepted for publication in CQ
Gravitational radiation reaction in compact binary systems: Contribution of the quadrupole-monopole interaction
The radiation reaction in compact spinning binaries on eccentric orbits due
to the quadrupole-monopole interaction is studied. This contribution is of
second post-Newtonian order. As result of the precession of spins the magnitude
of the orbital angular momentum is not conserved. Therefore a proper
characterization of the perturbed radial motion is provided by the energy
and angular average . As powerful computing tools, the generalized
true and eccentric anomaly parametrizations are introduced. Then the secular
losses in energy and magnitude of orbital angular momentum together with the
secular evolution of the relative orientations of the orbital angular momentum
and spins are found for eccentric orbits by use of the residue theorem. The
circular orbit limit of the energy loss agrees with Poisson's earlier result.Comment: accepted for publication in Phys. Rev.
Covariant gravitational dynamics in 3+1+1 dimensions
We develop a 3+1+1 covariant formalism with cosmological and astrophysical
applications. First we give the evolution and constraint equations both on the
brane and off-brane in terms of 3-space covariant kinematical,
gravito-electro-magnetic (Weyl) and matter variables. We discuss the junction
conditions across the brane in terms of the new variables. Then we establish a
closure condition for the equations on the brane. We also establish the
connection of this formalism with isotropic and anisotropic cosmological
brane-worlds. Finally we derive a new brane solution in the framework of our
formalism: the tidal charged Taub-NUT-(A)dS brane, which obeys the closure
condition.Comment: 35 pages 1 fig; significantly expanded with (a) new closure condition
on the brane, (b) discussion of anisotropic brane-worlds, (c) stationary
vacuum space-times with local rotational symmetry including (d) a new tidal
charged Taub-NUT-(A)dS brane solution; published version
- âŠ