68 research outputs found
FRW Universe Models in Conformally Flat Spacetime Coordinates. III: Universe models with positive spatial curvature
We deduce general expressions for the line element of universe models with
positive spatial curvature described by conformally flat spacetime coordinates.
Models with dust, radiation and vacuum energy are exhibited. Discussing the
existence of particle horizons we show that there is continual annihilation of
space, matter and energy in a dust and radiation dominated universe, and
continual creation in a LIVE domined universe when conformal time is used in
Friedmann-Robertson-Walker models with positive spatial curvature. A general
procedure is given for finding coordinates to be used in Penrose diagrams. We
also calculate the age and the redshift of some universe models using conformal
time.Comment: 22 pages, 9 figure
FRW Universe Models in Conformally Flat Spacetime Coordinates. II: Universe models with negative and vanishing spatial curvature
We deduce general expressions for the line element of universe models with
negative and vanishing spatial curvature described by conformally flat
spacetime coordinates. The empty Milne universe model and models with dust,
radiation and vacuum energy are exhibited. Discussing the existence of particle
horizons we show that there is continual creation of space, matter and energy
when conformal time is used in Friedmann-Robertson-Walker models with negative
spatial curvature.Comment: 25 pages, 12 figure
FRW Universe Models in Conformally Flat Spacetime Coordinates. I: General Formalism
The 3-space of a universe model is defined at a certain simultaneity. Hence
space depends on which time is used. We find a general formula generating all
known and also some new transformations to conformally flat spacetime
coordinates. A general formula for the recession velocity is deduced.Comment: 12 page
Energy efficiency studies for dual-grating dielectric laser-driven accelerators
Dielectric laser-driven accelerators (DLAs) can provide high accelerating gradients in the GV/m range due to their having higher breakdown thresholds than metals, which opens the way for the miniaturization of the next generation of particle accelerator facilities. Two kinds of scheme, the addition of a Bragg reflector and the use of pulse-front-tilted (PFT) laser illumination, have been studied separately to improve the energy efficiency for dual-grating DLAs. The Bragg reflector enhances the accelerating gradient of the structure, while the PFT increases the effective interaction length. In this paper, we investigate numerically the advantages of using the two schemes in conjunction. Our calculations show that, for a 100-period structure with a period of 2 micrometer, such a design effectively increases the energy gain by more than 100 % when compared to employing the Bragg reflector with a normal laser, and by about 50 % when using standard structures with a PFT laser. A total energy gain of as much as 2.6 MeV can be obtained for a PFT laser beam when illuminating a 2000-period dual-grating structure with a Bragg reflector
Conformally flat spacetimes and Weyl frames
We discuss the concepts of Weyl and Riemann frames in the context of metric
theories of gravity and state the fact that they are completely equivalent as
far as geodesic motion is concerned. We apply this result to conformally flat
spacetimes and show that a new picture arises when a Riemannian spacetime is
taken by means of geometrical gauge transformations into a Minkowskian flat
spacetime. We find out that in the Weyl frame gravity is described by a scalar
field. We give some examples of how conformally flat spacetime configurations
look when viewed from the standpoint of a Weyl frame. We show that in the
non-relativistic and weak field regime the Weyl scalar field may be identified
with the Newtonian gravitational potential. We suggest an equation for the
scalar field by varying the Einstein-Hilbert action restricted to the class of
conformally-flat spacetimes. We revisit Einstein and Fokker's interpretation of
Nordstr\"om scalar gravity theory and draw an analogy between this approach and
the Weyl gauge formalism. We briefly take a look at two-dimensional gravity as
viewed in the Weyl frame and address the question of quantizing a conformally
flat spacetime by going to the Weyl frame.Comment: LATEX - 18 page
The emergence of gravity as a retro-causal post-inflation macro-quantum-coherent holographic vacuum Higgs-Goldstone field
We present a model for the origin of gravity, dark energy and dark matter:
Dark energy and dark matter are residual pre-inflation false vacuum random zero
point energy (w=-1) of large-scale negative, and short-scale positive pressure,
respectively, corresponding to the "zero point" (incoherent) component of a
superfluid (supersolid) ground state. Gravity, in contrast, arises from the 2nd
order topological defects in the post-inflation virtual "condensate" (coherent)
component. We predict, as a consequence, that the LHC will never detect exotic
real on-mass-shell particles that can explain dark matter. We also point out
that the future holographic dark energy de Sitter horizon is a total absorber
(in the sense of retro-causal Wheeler-Feynman action-at-a-distance
electrodynamics) because it is an infinite redshift surface for static
detectors. Therefore, the advanced Hawking-Unruh thermal radiation from the
future de Sitter horizon is a candidate for the negative pressure dark vacuum
energy.Comment: 8 pages, no figures. To appear in Proc. DICE2008 From Quantum
Mechanics through Complexity to Spacetime: the role of emergent dynamical
structures. Castello Pasquini/Castiglioncello (Tuscany), September 22-26,
200
Exponential Metric Fields
The Laser Interferometer Space Antenna (LISA) mission will use advanced
technologies to achieve its science goals: the direct detection of
gravitational waves, the observation of signals from compact (small and dense)
stars as they spiral into black holes, the study of the role of massive black
holes in galaxy evolution, the search for gravitational wave emission from the
early Universe. The gravitational red-shift, the advance of the perihelion of
Mercury, deflection of light and the time delay of radar signals are the
classical tests in the first order of General Relativity (GR). However, LISA
can possibly test Einstein's theories in the second order and perhaps, it will
show some particular feature of non-linearity of gravitational interaction. In
the present work we are seeking a method to construct theoretical templates
that limit in the first order the tensorial structure of some metric fields,
thus the non-linear terms are given by exponential functions of gravitational
strength. The Newtonian limit obtained here, in the first order, is equivalent
to GR.Comment: Accepted for publication in Astrophysics and Space Science, 17 page
Ultraweak excitations of the quantum vacuum as physical models of gravity
It has been argued by several authors that the space-time curvature observed
in gravitational fields, and the same idea of forms of physical equivalence
different from the Lorentz group, might emerge from the dynamical properties of
the physical flat-space vacuum in a suitable hydrodynamic limit. To explore
this idea, one could start by representing the physical vacuum as a Bose
condensate of elementary quanta and look for vacuum excitations that, on a
coarse grained scale, resemble the Newtonian potential. In this way, it is
relatively easy to match the weak-field limit of classical General Relativity
or of some of its possible variants. The idea that Bose condensates can provide
various forms of gravitational dynamics is not new. Here, I want to emphasize
some genuine quantum field theoretical aspects that can help to understand i)
why infinitesimally weak, 1/r interactions can indeed arise from the same
physical vacuum of electroweak and strong interactions and ii) why, on a
coarse-grained scale, their dynamical effects can be re-absorbed into an
effective curved metric structure.Comment: 30 pages, no figures, accepted by Classical and Quantum Gravit
- …