68 research outputs found
Current optical technologies for wireless access
The objective of this paper is to describe recent activities and investigations on free-space optics (FSO) or optical wireless and the excellent results achieved within SatNEx an EU-framework 6th programme and IC 0802 a COST action. In a first part, the FSO technology is briefly discussed. In a second part, we mention some performance evaluation criterions for the FSO. In third part, we briefly discuss some optical signal propagation experiments through the atmosphere by mentioning network architectures for FSO and then discuss the recent investigations in airborne and satellite application experiments for FSO. In part four, we mention some recent investigation results on modelling the FSO channel under fog conditions and atmospheric turbulence. Additionally, some recent major performance improvement results obtained by employing hybrid systems and using some specific modulation and coding schemes are presented
Primordial magnetic fields from inflation?
The hot plasma above the electroweak scale contains (hyper) charged scalar
particles which are coupled to Abelian gauge fields. Scalars may interact with
gravity in a non-conformally invariant way and thus their fluctuations can be
amplified during inflation. These fluctuations lead to creation of electric
currents and produce inhomogeneous distribution of charge density, resulting in
the generation of cosmological magnetic fields. We address the question whether
these fields can be coherent at large scales so that they may seed the galactic
magnetic fields. Depending upon the mass of the charged scalar and upon various
cosmological (critical fraction of energy density in matter, Hubble constant)
and particle physics parameters we found that the magnetic fields generated in
this way are much larger than vacuum fluctuations. However, their amplitude on
cosmological distances is found to be too small for seeding the galactic
magnetic fields.Comment: 32 pages in RevTex styl
Ohm's Law for Plasma in General Relativity and Cowling's Theorem
The general-relativistic Ohm's law for a two-component plasma which includes
the gravitomagnetic force terms even in the case of quasi-neutrality has been
derived. The equations that describe the electromagnetic processes in a plasma
surrounding a neutron star are obtained by using the general relativistic form
of Maxwell equations in a geometry of slow rotating gravitational object. In
addition to the general-relativistic effect first discussed by Khanna \&
Camenzind (1996) we predict a mechanism of the generation of azimuthal current
under the general relativistic effect of dragging of inertial frames on radial
current in a plasma around neutron star. The azimuthal current being
proportional to the angular velocity of the dragging of inertial
frames can give valuable contribution on the evolution of the stellar magnetic
field if exceeds (
is the number density of the charged particles, is the conductivity of
plasma). Thus in general relativity a rotating neutron star, embedded in
plasma, can in principle generate axial-symmetric magnetic fields even in
axisymmetry. However, classical Cowling's antidynamo theorem, according to
which a stationary axial-symmetric magnetic field can not be sustained against
ohmic diffusion, has to be hold in the general-relativistic case for the
typical plasma being responsible for the rotating neutron star.Comment: Accepted for publication in Astrophysics & Space Scienc
Supersymmetry Breaking through Transparent Extra Dimensions
We propose a new framework for mediating supersymmetry breaking through an
extra dimension. It predicts positive scalar masses and solves the
supersymmetric flavor problem. Supersymmetry breaks on a ``source'' brane that
is spatially separated from a parallel brane on which the standard model matter
fields and their superpartners live. The gauge and gaugino fields propagate in
the bulk, the latter receiving a supersymmetry breaking mass from direct
couplings to the source brane. Scalar masses are suppressed at the high scale
but are generated via the renormalization group. We briefly discuss the
spectrum and collider signals for a range of compactification scales.Comment: 20 page
Superadiabatic-type magnetic amplification in conventional cosmology
We consider the evolution of cosmological magnetic fields in FRW models and
outline a geometrical mechanism for their superadiabatic amplification on large
scales. The mechanism operates within standard electromagnetic theory and
applies to FRW universes with open spatial sections. We discuss the general
relativistic nature of the effect and show how it modifies the adiabatic
magnetic evolution. Assuming a universe that is only marginally open today, we
estimate the main features of the superadiabatically amplified residual field.Comment: Minor changes. Published versio
Primordial magnetic fields induced by cosmological particle creation
We study the primordial magnetic field generated by stochastic currents
produced by scalar charged particles created at the beginning of the radiation
dominated epoch. We find that for the mass range 10^(-6)GeV \leq m \leq 10^2
GeV, a field of sufficient intensity to seed different mechanisms of galactic
magnetic field generation, while still consistent with observational and
theoretical constraints, is created coherently over a galactic scale.Comment: accepted for publication in Phys. Rev.
Magnetogenesis and the dynamics of internal dimensions
The dynamical evolution of internal space-like dimensions breaks the
invariance of the Maxwell's equations under Weyl rescaling of the (conformally
flat) four-dimensional metric. Depending upon the number and upon the dynamics
of internal dimensions large scale magnetic fields can be created. The
requirements coming from magnetogenesis together with the other cosmological
constraints are examined under the assumption that the internal dimensions
either grow or shrink (in conformal time) prior to a radiation dominated epoch.
If the internal dimensions are growing the magnitude of the generated magnetic
fields can seed the galactic dynamo mechanism.Comment: 27 in RevTex style, four figure
Self consistent estimates of magnetic fields from reheating
We investigate the generation of primordial magnetic fields from stochastic
currents created by the cosmological transition from inflation to reheating. We
consider N charged scalar fields coupled to the electromagnetic field in a
curved background and derive self-consistent equations for the evolution of the
two point functions of the fields, which in the large N limit give a decoupled
set for the scalar and the electromagnetic functions. The main contribution to
the electric current comes from the infrared portion of the spectrum of created
particles, and in this limit the damping of the magnetic field is not due to
normal conductivity but to London currents in the scalar field. For a given set
of the physical parameters of the problem, we solved this equation numerically
and found that, due to the fact that the London currents are oscillating, the
field actually grows exponentially during the time interval in which our
large-N limit equations are valid. Although for the chosen parameters the
induced field is weak, the present uncertainties on their actual values leave
open the possibility for higher intensities.Comment: 25 pages, three figures. One figure added, discussion on non
adiabatic induction of the magnetic field improved. References added. Final
version accepted for publication in Phys. Rev.
Large-scale magnetic fields from inflation due to a -even Chern-Simons-like term with Kalb-Ramond and scalar fields
We investigate the generation of large-scale magnetic fields due to the
breaking of the conformal invariance in the electromagnetic field through the
-even dimension-six Chern-Simons-like effective interaction with a fermion
current by taking account of the dynamical Kalb-Ramond and scalar fields in
inflationary cosmology. It is explicitly demonstrated that the magnetic fields
on 1Mpc scale with the field strength of G at the present time
can be induced.Comment: 18 pages, 6 figures, version accepted for publication in Eur. Phys.
J.
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