696 research outputs found
Optical fiber fabrication using novel gas-phase deposition technique
We report a highly versatile chemical-in-crucible preform fabrication technique suitable for gas-phase deposition of doped optical fibers. Aluminosilicate and ytterbium-doped phosphosilicate fibers are presented demonstrating the technique and its potential for realizing complex fiber designs that are suitable for the next generation of high-power fiber devices. The results show aluminum-doped fiber with numerical aperture of 0.28 and ytterbium-doped fiber with a measured slope efficiency of 84% with respect to pump launch power
Atomic calculations and search for variation of the fine structure constant in quasar absorption spectra
A brief review of the search for variation of the fine structure constant in
quasar absorption spectra is presented. Special consideration is given to the
role of atomic calculations in the analysis of the observed data. A range of
methods which allow to perform calculations for atoms or ions with different
electron structure and which cover practically all periodic table of elements
is discussed. Critical compilation of the results of the calculations as well
as a review of the most recent results of the analysis are presented.Comment: 9 pages, 2 figures. Based on the talk at the Symposium on Atomic
Physics: A Tribute to Walter Johnson, Notre Dame, 5 April 2008. Reference 26
is correcte
Fine-structure constant variability, equivalence principle and cosmology
It has been widely believed that variability of the fine-structure constant
alpha would imply detectable violations of the weak equivalence principle. This
belief is not justified in general. It is put to rest here in the context of
the general framework for alpha variability [J. D. Bekenstein, Phys. Rev. D 25,
1527 (1982)] in which the exponent of a scalar field plays the role of the
permittivity and inverse permeability of the vacuum. The coupling of particles
to the scalar field is necessarily such that the anomalous force acting on a
charged particle by virtue of its mass's dependence on the scalar field is
cancelled by terms modifying the usual Coulomb force. As a consequence a
particle's acceleration in external fields depends only on its charge to mass
ratio, in accordance with the principle. And the center of mass acceleration of
a composite object can be proved to be independent of the object's internal
constitution, as the weak equivalence principle requires. Likewise the widely
employed assumption that the Coulomb energy of matter is the principal source
of the scalar field proves wrong; Coulomb energy effectively cancels out in the
continuum description of the scalar field's dynamics. This cancellation
resolves a cosmological conundrum: with Coulomb energy as source of the scalar
field, the framework would predict a decrease of alpha with cosmological
expansion, whereas an increase is claimed to be observed. Because of the said
cancellation, magnetic energy of cosmological baryonic matter is the main
source of the scalar field. Consequently the expansion is accompanied by an
increase in alpha; for reasonable values of the framework's sole parameter,
this occurs at a rate consistent with the observers' claims.Comment: RevTeX-4, 22 pages, no figures, added a section on caveats as well as
several new references with discussion of them in body. To appear in Phys.
Rev.
Tandem-pumped ytterbium-doped aluminosilicate fiber amplifier with low quantum defect
We show theoretically that a quantum-defect below 1% is possible in tandem-pumped Yb-doped aluminosilicate fibers operating off the gain peak. Experimentally, we reach a quantum defect of 2% and a slope efficiency of 90% or more
Isotope shift calculations for atoms with one valence electron
This work presents a method for the ab initio calculation of isotope shift in
atoms and ions with one valence electron above closed shells. As a zero
approximation we use relativistic Hartree-Fock and then calculate correlation
corrections. The main motivation for developing the method comes from the need
to analyse whether different isotope abundances in early universe can
contribute to the observed anomalies in quasar absorption spectra. The current
best explanation for these anomalies is the assumption that the fine structure
constant, alpha, was smaller at early epoch. We test the isotope shift method
by comparing the calculated and experimental isotope shift for the alkali and
alkali-like atoms Na, MgII, K, CaII and BaII. The agreement is found to be
good. We then calculate the isotope shift for some astronomically relevant
transitions in SiII and SiIV, MgII, ZnII and GeII.Comment: 11 page
A time-space varying speed of light and the Hubble Law in static Universe
We consider a hypothetical possibility of the variability of light velocity
with time and position in space which is derived from two natural postulates.
For the consistent consideration of such variability we generalize
translational transformations of the Theory of Relativity. The formulae of
transformations between two rest observers within one inertial system are
obtained. It is shown that equality of velocities of two particles is as
relative a statement as simultaneity of two events is. We obtain the expression
for the redshift of radiation of a rest source which formally reproduces the
Hubble Law. Possible experimental implications of the theory are discussed.Comment: 7 page
Limits on Cosmological Variation of Strong Interaction and Quark Masses from Big Bang Nucleosynthesis, Cosmic, Laboratory and Oklo Data
Recent data on cosmological variation of the electromagnetic fine structure
constant from distant quasar (QSO) absorption spectra have inspired a more
general discussion of possible variation of other constants. We discuss
variation of strong scale and quark masses. We derive the limits on their
relative change from (i) primordial Big-Bang Nucleosynthesis (BBN); (ii)
Oklo natural nuclear reactor, (iii) quasar absorption spectra, and (iv)
laboratory measurements of hyperfine intervals.Comment: 10 pages 2 figurs: second version have several references added and
some new comment
Variable-Speed-of-Light Cosmology from Brane World Scenario
We argue that the four-dimensional universe on the TeV brane of the
Randall-Sundrum scenario takes the bimetric structure of Clayton and Moffat,
with gravitons traveling faster than photons instead, while the radion varies
with time. We show that such brane world bimetric model can thereby solve the
flatness and the cosmological constant problems, provided the speed of a
graviton decreases to the present day value rapidly enough. The resolution of
other cosmological problems such as the horizon problem and the monopole
problem requires supplementation by inflation, which may be achieved by the
radion field provided the radion potential satisfies the slow-roll
approximation.Comment: 18 pages, LaTeX, revised version to appear in Phys. Rev.
Solar System planetary tests of \dot c/c
Analytical and numerical calculations show that a putative temporal variation
of the speed of light c, with the meaning of space-time structure constant
c_ST, assumed to be linear over timescales of about one century, would induce a
secular precession of the longitude of the pericenter \varpi of a test particle
orbiting a spherically symmetric body. By comparing such a predicted effect to
the corrections \Delta\dot\varpi to the usual Newtonian/Einsteinian perihelion
precessions of the inner planets of the Solar System, recently estimated by
E.V. Pitjeva by fitting about one century of modern astronomical observations
with the standard dynamical force models of the EPM epehemerides, we obtained
\dot c/c =(0.5 +/- 2)\times 10^-7 yr^-1. Moreover, the possibility that \dot
c/c\neq 0 over the last century is ruled out at 3-12\sigma level by taking the
ratios of the perihelia for different pairs of planets. Our results are
independent of any measurement of the variations of other fundamental constants
which may be explained by a variation of itself (with the meaning of
electromagnetic constant c_EM). It will be important to repeat such tests if
and when other teams of astronomers will estimate their own corrections to the
standard Newtonian/Einsteinian planetary perihelion precessions.Comment: Latex, 12 pages, 1 figure, 3 tables, 23 references. Minor changes. To
appear in General Relativity and Gravitation (GRG
Neutron Stars in a Varying Speed of Light Theory
We study neutron stars in a varying speed of light (VSL) theory of gravity in
which the local speed of light depends upon the value of a scalar field .
We find that the masses and radii of the stars are strongly dependent on the
strength of the coupling between and the matter field and that for
certain choices of coupling parameters, the maximum neutron star mass can be
arbitrarily small. We also discuss the phenomenon of cosmological evolution of
VSL stars (analogous to the gravitational evolution in scalar-tensor theories)
and we derive a relation showing how the fractional change in the energy of a
star is related to the change in the cosmological value of the scalar field.Comment: 15 pages, 2 figures. Added solutions with a more realistic equation
of state. To be published in PR
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