23,770 research outputs found
The role of Vitamin D and Parathyroid Hormone in Cardiovascular Health
Visser, M. [Promotor]Brouwer, I.A. [Copromotor
Nutrition and depressive symptoms:a longitudinal perspective
Visser, M. [Promotor]Brouwer, I.A. [Promotor]Penninx, B.W.J.H. [Promotor
Socioeconomic differences in dietary intake of older adults
Visser, M. [Promotor]Brouwer, I.A. [Copromotor]Huisman, M. [Copromotor
Muscles growing older: inflammatory markers and sex hormones as determinants of sarcopenia and decline in physical functioning
Deeg, D.J.H. [Promotor]Visser, M. [Promotor]Pluijm, S.M.F. [Copromotor
Analogue model for quantum gravity phenomenology
So called "analogue models" use condensed matter systems (typically
hydrodynamic) to set up an "effective metric" and to model curved-space quantum
field theory in a physical system where all the microscopic degrees of freedom
are well understood. Known analogue models typically lead to massless minimally
coupled scalar fields. We present an extended "analogue space-time" programme
by investigating a condensed-matter system - in and beyond the hydrodynamic
limit - that is in principle capable of simulating the massive Klein-Gordon
equation in curved spacetime. Since many elementary particles have mass, this
is an essential step in building realistic analogue models, and an essential
first step towards simulating quantum gravity phenomenology. Specifically, we
consider the class of two-component BECs subject to laser-induced transitions
between the components, and we show that this model is an example for Lorentz
invariance violation due to ultraviolet physics. Furthermore our model suggests
constraints on quantum gravity phenomenology in terms of the "naturalness
problem" and "universality issue".Comment: Talk given at 7th Workshop on Quantum Field Theory Under the
Influence of External Conditions (QFEXT 05), Barcelona, Catalonia, Spain, 5-9
Sep 200
Construction of 3D wormhole supported by phantom energy
In this article, we have found a series solution of 3D Einstein equations
describing a wormhole for an inhomogeneous distribution of phantom energy.
Here, we assume equation of state is linear but highly anistropic.Comment: 9 papge, 4 figures. Accepted for publication in Physica Script
Back-reaction effects in acoustic black holes
Acoustic black holes are very interesting non-gravitational objects which can
be described by the geometrical formalism of General Relativity. These models
can be useful to experimentally test effects otherwise undetectable, as for
example the Hawking radiation. The back-reaction effects on the background
quantities induced by the analogue Hawking radiation could be the key to
indirectly observe it. We briefly show how this analogy works and derive the
backreaction equations for the linearized quantum fluctuations in the
background of an acoustic black hole. A first order in hbar solution is given
in the near horizon region. It indicates that acoustic black holes, unlike
Schwarzschild ones, get cooler as they radiate phonons. They show remarkable
analogies with near-extremal Reissner-Nordstrom black holes.Comment: 10 pages, 1 figure; Talk given at the conference ``Constrained
Dynamics and Quantum Gravity (QG05)", Cala Gonone (Italy), September 200
Relativistic Acoustic Geometry
Sound wave propagation in a relativistic perfect fluid with a non-homogeneous
isentropic flow is studied in terms of acoustic geometry. The sound wave
equation turns out to be equivalent to the equation of motion for a massless
scalar field propagating in a curved space-time geometry. The geometry is
described by the acoustic metric tensor that depends locally on the equation of
state and the four-velocity of the fluid. For a relativistic supersonic flow in
curved space-time the ergosphere and acoustic horizon may be defined in a way
analogous the non-relativistic case. A general-relativistic expression for the
acoustic analog of surface gravity has been found.Comment: 14 pages, LaTe
Expansion, Geometry, and Gravity
In general-relativistic cosmological models, the expansion history, matter
content, and geometry are closely intertwined. In this brief paper, we clarify
the distinction between the effects of geometry and expansion history on the
luminosity distance. We show that the cubic correction to the Hubble law,
measured recently with high-redshift supernovae, is the first cosmological
measurement, apart from the cosmic microwave background, that probes directly
the effects of spatial curvature. We illustrate the distinction between
geometry and expansion with a toy model for which the supernova results already
indicate a curvature radius larger than the Hubble distance.Comment: 4 pages, 1 color figur
Probing the stability of gravastars by dropping dust shells onto them
As a preparation for the dynamical investigations, this paper begins with a
short review of the three-layer gravastar model with distinguished attention to
the structure of the pertinent parameter space of gravastars in equilibrium.
Then the radial stability of these types of gravastars is studied by
determining their response for the totally inelastic collision of their surface
layer with a dust shell. It is assumed that the dominant energy condition holds
and the speed of sound does not exceed that of the light in the matter of the
surface layer. While in the analytic setup the equation of state is kept to be
generic, in the numerical investigations three functionally distinct classes of
equations of states are applied. In the corresponding particular cases the
maximal mass of the dust shell that may fall onto a gravastar without
converting it into a black hole is determined. For those configurations which
remain stable the excursion of their radius is assigned. It is found that even
the most compact gravastars cannot get beyond the lower limit of the size of
conventional stars, provided that the dominant energy condition holds in both
cases. It is also shown---independent of any assumption concerning the matter
interbridging the internal de Sitter and the external Schwarzschild
regions---that the better is a gravastar in mimicking a black hole the easier
is to get the system formed by a dust shell and the gravastar beyond the event
horizon of the composite system. In addition, a generic description of the
totally inelastic collision of spherical shells in spherically symmetric
spacetimes is also provided in the appendix.Comment: 29 pages, 10 figure
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