159 research outputs found
Improved Action Functionals in Non-Perturbative Quantum Gravity
Models of gravity with variable G and Lambda have acquired greater relevance
after the recent evidence in favour of the Einstein theory being
non-perturbatively renormalizable in the Weinberg sense. The present paper
builds a modified Arnowitt-Deser-Misner (ADM) action functional for such models
which leads to a power-law growth of the scale factor for pure gravity and for
a massless phi**4 theory in a Universe with Robertson-Walker symmetry, in
agreement with the recently developed fixed-point cosmology. Interestingly, the
renormalization-group flow at the fixed point is found to be compatible with a
Lagrangian description of the running quantities G and Lambda.Comment: Latex file. Record without file already exists on SLAC-SPIRES, and
hence that record and the one for the present arxiv submission should become
one record onl
Noether symmetry approach to scalar-field-dominated cosmology with dynamically evolving G and Lambda
This paper studies the cosmological equations for a scalar field Phi in the
framework of a quantum gravity modified Einstein--Hilbert Lagrangian where G
and Lambda are dynamical variables. It is possible to show that there exists a
Noether symmetry for the point Lagrangian describing this scheme in a FRW
universe. Our main result is that the Noether Symmetry Approach fixes both
Lambda = Lambda(G) and the potential V = V(Phi) of the scalar field. The method
does not lead, however, to easily solvable equations, by virtue of the higher
dimensionality of the reduced configuration space involved, the additional
variable being the running Newton coupling.Comment: 10 pages, Revtex
Time-resolved photoluminescence of n-doped SrTiO_3
Following the recent surge of interest in n-doped strontium titanate as a
possible blue light emitter, a time-resolved photoluminescence analysis was
performed on nominally pure, Nb-doped and oxygen-deficient single-crystal
SrTiO3 samples. The doping-effects on both the electronic states involved in
the transition and the decay mechanism are respectively analyzed by comparing
the spectral and dynamic features and the yields of the emission. Our
time-resolved analysis, besides shedding some light on the basic recombination
mechanisms acting in these materials, sets the intrinsic bandwidth limit of the
proposed blue light emitting optoelectronic devices made of Ti-based
perovskites heterostructures in the GHz range
Quintessence duality
We join quintessence cosmological scenarios with the duality simmetry
existing in string dilaton cosmologies. Actually, we consider the tracker
potential type and show that duality is only
established if .Comment: 6 LaTex Pages, submitted to Physics Letters A; completely revised
version: majior changes in the last par
Noether symmetry approach in phantom quintessence cosmology
In the framework of phantom quintessence cosmology, we use the Noether
Symmetry Approach to obtain general exact solutions for the cosmological
equations. This result is achieved by the quintessential (phantom) potential
determined by the existence of the symmetry itself. A comparison between the
theoretical model and observations is worked out. In particular, we use type Ia
supernovae and large scale structure parameters determined from the 2-degree
Field Galaxy Redshift Survey (2dFGRS)and from the Wide part of the VIMOS-VLT
Deep Survey (VVDS). It turns out that the model is compatible with the
presently available observational data. Moreover we extend the approach to
include radiation. We show that it is compatible with data derived from
recombination and it seems that quintessence do not affect nucleosynthesis
results.Comment: 26 pages, 13 figure
Slow rolling, inflation and quintessence
We comment on the choice of the quintessence potential, examining the
slow-roll approximation in a minimally coupled theory of gravity. We make some
considerations on the potential behaviors, the related \gamma parameter, and
their relationships to phantom cosmology.Comment: 7 page
Coupling parameters and the form of the potential via Noether symmetry
We explore the conditions for the existence of Noether symmetries in the
dynamics of FRW metric, non minimally coupled with a scalar field, in the most
general situation, and with nonzero spatial curvature. When such symmetries are
present we find general exact solution for the Einstein equations. We also show
that non Noether symmetries can be found.
Finally,we present an extension of the procedure to the Kantowski- Sachs
metric which is particularly interesting in the case of degenerate Lagrangian.Comment: 13 pages, no figure
Enhanced spin orbit interaction of light in highly confining optical fibers for mode division multiplexing
Light carries both orbital angular momentum (OAM) and spin angular momentum (SAM), related to wavefront rotation and polarization, respectively. These are usually approximately independent quantities, but they become coupled by light's spin-orbit interaction (SOI) in certain exotic geometries and at the nanoscale. Here we reveal a manifestation of strong SOI in fibers engineered at the micro-scale and supporting the only known example of propagating light modes with non-integer mean OAM. This enables propagation of a record number (24) of states in a single optical fiber with low cross-talk (purity > 93%), even as tens-of-meters long fibers are bent, twisted or otherwise handled, as fibers are practically deployed. In addition to enabling the investigation of novel SOI effects, these light states represent the first ensemble with which mode count can be potentially arbitrarily scaled to satisfy the exponentially growing demands of high-performance data centers and supercomputers, or telecommunications network nodes
Q-plates for Switchable Excitation of Fiber OAM Modes
We demonstrate that a |q|=1/2 plate plus polarization optics can tunably excite all linear combinations of |l|=1 fiber OAM modes with up to ~30 dB purity, enabling switch fabrics in fiber-OAM networks and disentangling of degenerate mode mixing effects in long fibers
On exact solutions for quintessential (inflationary) cosmological models with exponential potentials
We first study dark energy models with a minimally-coupled scalar field and
exponential potentials, admitting exact solutions for the cosmological
equations: actually, it turns out that for this class of potentials the
Einstein field equations exhibit alternative Lagrangians, and are completely
integrable and separable (i.e. it is possible to integrate the system
analytically, at least by quadratures). We analyze such solutions, especially
discussing when they are compatible with a late time quintessential expansion
of the universe. As a further issue, we discuss how such quintessential scalar
fields can be connected to the inflationary phase, building up, for this class
of potentials, a quintessential inflationary scenario: actually, it turns out
that the transition from inflation toward late-time exponential quintessential
tail admits a kination period, which is an indispensable ingredient of this
kind of theoretical models. All such considerations have also been done by
including radiation into the model.Comment: Revtex4, 10 figure
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