9,116 research outputs found
A New Redshift Interpretation
A nonhomogeneous universe with vacuum energy, but without spacetime
expansion, is utilized together with gravitational and Doppler redshifts as the
basis for proposing a new interpretation of the Hubble relation and the 2.7K
Cosmic Blackbody Radiation.Comment: 9 pages LaTeX, no figure
Stability analysis of coupled map lattices at locally unstable fixed points
Numerical simulations of coupled map lattices (CMLs) and other complex model
systems show an enormous phenomenological variety that is difficult to classify
and understand. It is therefore desirable to establish analytical tools for
exploring fundamental features of CMLs, such as their stability properties.
Since CMLs can be considered as graphs, we apply methods of spectral graph
theory to analyze their stability at locally unstable fixed points for
different updating rules, different coupling scenarios, and different types of
neighborhoods. Numerical studies are found to be in excellent agreement with
our theoretical results.Comment: 22 pages, 6 figures, accepted for publication in European Physical
Journal
Conditions for low-redshift positive apparent acceleration in smooth inhomogeneous models
It is known that a smooth LTB model cannot have a positive apparent central
acceleration. Using a local Taylor expansion method we study the low-redshift
conditions to obtain an apparent negative deceleration parameter
derived from the luminosity distance for a central observer in a LTB
space, confirming that central smoothness implies a positive central
deceleration. Since observational data is only available at redshift greater
than zero we find the critical values of the parameters defining a centrally
smooth LTB model which give a positive apparent acceleration at ,
providing a graphical representation of the conditions in the
plane, which are respectively the zero and first order
terms of the central Taylor expansion of . We finally derive a
coordinate independent expression for the apparent deceleration parameter based
on the expansion of the relevant functions in red-shift rather than in the
radial coordinate.
We calculate with two different methods to solve the null
geodesic equations, one based on a local central expansion of the solution in
terms of cosmic time and the other one using the exact analytical solution in
terms of generalized conformal time. %The expansion of the solution in terms of
cosmic time is quite useful also for other applications requiring foliation %of
space-time in space-like hyper-surfaces, such as spatial averaging, which is
much more difficult to study using the %analytical solution in terms of the
generalized conformal time coordinate.Comment: 18 pages, 3 figures, abstract, added section with coordinate
independent conditions, version accepted for publication in GR
The spherical symmetry Black hole collapse in expanding universe
The spherical symmetry Black holes are considered in expanding background.
The singularity line and the marginally trapped tube surface behavior are
discussed. In particular, we address the conditions whether dynamical horizon
forms for these cosmological black holes. We also discuss about the
cosmological constant effect on these black hole and the redshift of the light
which comes from the marginally trapped tube surface.Comment: 7 pages, 3 figures. Accepted for publication in International Journal
of Modern Physics D (IJMPD). arXiv admin note: text overlap with
arXiv:gr-qc/0308033 and arXiv:gr-qc/030611
Big Bang Nucleosynthesis Constraints on the Self-Gravity of Pressure
Using big bang nucleosynthesis and present, high-precision measurements of
light element abundances, we constrain the self-gravity of radiation pressure
in the early universe. The self-gravity of pressure is strictly non-Newtonian,
and thus the constraints we set provide a direct test of this prediction of
general relativity and of the standard, Robertson-Walker-Friedmann cosmology.Comment: 5 pages, 1 figure. This paper was developed from an earlier version
which was posted as arXiv:0707.358
Enhancement of the Spin Accumulation at the Interface Between a Spin-Polarized Tunnel Junction and a Semiconductor
We report on spin injection experiments at a Co/AlO/GaAs interface
with electrical detection. The application of a transverse magnetic field
induces a large voltage drop at the interface as high as 1.2mV for a
current density of 0.34 nA.. This represents a dramatic increase of
the spin accumulation signal, well above the theoretical predictions for spin
injection through a ferromagnet/semiconductor interface. Such an enhancement is
consistent with a sequential tunneling process via localized states located in
the vicinity of the AlO/GaAs interface. For spin-polarized carriers
these states act as an accumulation layer where the spin lifetime is large. A
model taking into account the spin lifetime and the escape tunneling time for
carriers travelling back into the ferromagnetic contact reproduces accurately
the experimental results
On cosmological observables in a swiss-cheese universe
Photon geodesics are calculated in a swiss-cheese model, where the cheese is
made of the usual Friedmann-Robertson-Walker solution and the holes are
constructed from a Lemaitre-Tolman-Bondi solution of Einstein's equations. The
observables on which we focus are the changes in the redshift, in the
angular-diameter--distance relation, in the luminosity-distance--redshift
relation, and in the corresponding distance modulus. We find that redshift
effects are suppressed when the hole is small because of a compensation effect
acting on the scale of half a hole resulting from the special case of spherical
symmetry. However, we find interesting effects in the calculation of the
angular distance: strong evolution of the inhomogeneities (as in the approach
to caustic formation) causes the photon path to deviate from that of the FRW
case. Therefore, the inhomogeneities are able to partly mimic the effects of a
dark-energy component. Our results also suggest that the nonlinear effects of
caustic formation in cold dark matter models may lead to interesting effects on
photon trajectories.Comment: 25 pages, 21 figures; replaced to fit the version accepted for
publication in Phys. Rev.
Formation of cosmological mass condensation within a FRW universe: exact general relativistic solutions
Within the framework of an exact general relativistic formulation of gluing
manifolds, we consider the problem of matching an inhomogeneous overdense
region to a Friedmann-Robertson-Walker background universe in the general
spherical symmetric case of pressure-free models. It is shown that, in general,
the matching is only possible through a thin shell, a fact ignored in the
literature. In addition to this, in subhorizon cases where the matching is
possible, an intermediate underdense region will necessarily arise.Comment: 6 page
Irreversible magnetization switching using surface acoustic waves
An analytical and numerical approach is developped to pinpoint the optimal
experimental conditions to irreversibly switch magnetization using surface
acoustic waves (SAWs). The layers are magnetized perpendicular to the plane and
two switching mechanisms are considered. In precessional switching, a small
in-plane field initially tilts the magnetization and the passage of the SAW
modifies the magnetic anisotropy parameters through inverse magneto-striction,
which triggers precession, and eventually reversal. Using the micromagnetic
parameters of a fully characterized layer of the magnetic semiconductor
(Ga,Mn)(As,P), we then show that there is a large window of accessible
experimental conditions (SAW amplitude/wave-vector, field
amplitude/orientation) allowing irreversible switching. As this is a resonant
process, the influence of the detuning of the SAW frequency to the magnetic
system's eigenfrequency is also explored. Finally, another - non-resonant -
switching mechanism is briefly contemplated, and found to be applicable to
(Ga,Mn)(As,P): SAW-assisted domain nucleation. In this case, a small
perpendicular field is applied opposite the initial magnetization and the
passage of the SAW lowers the domain nucleation barrier.Comment: 11 pages, 4 figure
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