2,943 research outputs found
Myopic PPPs: Risk allocation and hidden liabilities for taxpayers and users
Drawing on evidence from three case studies, we show how the State's Financial Liability has worked in assigning risk in large PPP contracts in Spain. Project failure and the concessionaires' bankruptcy have resulted in the government having to assume heavy financial obligations, which have ultimately been absorbed by taxpayers and users. In contrast, Spain's leading construction companies, which were also major investors in the concessionaires, have been able to minimize their risk. Myopic PPPs have been entered into based on the transference of liabilities to taxpayers and users, and the, consequent, minimization of risks for the main private investors
Ergodicity Breaking in a Deterministic Dynamical System
The concept of weak ergodicity breaking is defined and studied in the context
of deterministic dynamics. We show that weak ergodicity breaking describes a
weakly chaotic dynamical system: a nonlinear map which generates subdiffusion
deterministically. In the non-ergodic phase non-trivial distribution of the
fraction of occupation times is obtained. The visitation fraction remains
uniform even in the non-ergodic phase. In this sense the non-ergodicity is
quantified, leading to a statistical mechanical description of the system even
though it is not ergodic.Comment: 11 pages, 4 figure
Grid-scale Fluctuations and Forecast Error in Wind Power
The fluctuations in wind power entering an electrical grid (Irish grid) were
analyzed and found to exhibit correlated fluctuations with a self-similar
structure, a signature of large-scale correlations in atmospheric turbulence.
The statistical structure of temporal correlations for fluctuations in
generated and forecast time series was used to quantify two types of forecast
error: a timescale error () that quantifies the deviations between
the high frequency components of the forecast and the generated time series,
and a scaling error () that quantifies the degree to which the
models fail to predict temporal correlations in the fluctuations of the
generated power. With no knowledge of the forecast models, we
suggest a simple memory kernel that reduces both the timescale error
() and the scaling error ()
Electromagnetic radiation produces frame dragging
It is shown that for a generic electrovacuum spacetime, electromagnetic
radiation produces vorticity of worldlines of observers in a Bondi--Sachs
frame. Such an effect (and the ensuing gyroscope precession with respect to the
lattice) which is a reminiscence of generation of vorticity by gravitational
radiation, may be linked to the nonvanishing of components of the Poynting and
the super--Poynting vectors on the planes othogonal to the vorticity vector.
The possible observational relevance of such an effect is commented.Comment: 8 pages RevTex 4-1; updated version to appear in Physical Review
Reference frames and rigid motions in relativity: Applications
The concept of rigid reference frame and of constricted spatial metric, given
in the previous work [\emph{Class. Quantum Grav.} {\bf 21}, 3067,(2004)] are
here applied to some specific space-times: In particular, the rigid rotating
disc with constant angular velocity in Minkowski space-time is analyzed, a new
approach to the Ehrenfest paradox is given as well as a new explanation of the
Sagnac effect. Finally the anisotropy of the speed of light and its measurable
consequences in a reference frame co-moving with the Earth are discussed.Comment: 13 pages, 1 figur
Conserved superenergy currents
We exploit once again the analogy between the energy-momentum tensor and the
so-called ``superenergy'' tensors in order to build conserved currents in the
presence of Killing vectors. First of all, we derive the divergence-free
property of the gravitational superenergy currents under very general
circumstances, even if the superenergy tensor is not divergence-free itself.
The associated conserved quantities are explicitly computed for the
Reissner-Nordstrom and Schwarzschild solutions. The remaining cases, when the
above currents are not conserved, lead to the possibility of an interchange of
some superenergy quantities between the gravitational and other physical fields
in such a manner that the total, mixed, current may be conserved. Actually,
this possibility has been recently proved to hold for the Einstein-Klein-Gordon
system of field equations. By using an adequate family of known exact
solutions, we present explicit and completely non-obvious examples of such
mixed conserved currents.Comment: LaTeX, 19 pages; improved version adding new content to the second
section and some minor correction
Spectral state dependence of the 0.4-2 MeV polarized emission in Cygnus X-1 seen with INTEGRAL/IBIS, and links with the AMI radio data
Polarization of the >~400 keV hard tail of the microquasar Cygnus X-1 has
been independently reported by INTEGRAL/IBIS, and INTEGRAL/SPI and interpreted
as emission from a compact jet. These conclusions were, however, based on the
accumulation of all INTEGRAL data regardless of the spectral state. We utilize
additional INTEGRAL exposure accumulated until December 2012, and include the
AMI/Ryle (15 GHz) radio data in our study. We separate the observations into
hard, soft, and intermediate/transitional states and detect radio emission from
a compact jet in hard and intermediate states, but not in the soft. The 10-400
keV INTEGRAL (JEM-X and IBIS) state resolved spectra are well modeled with
thermal Comptonization and reflection components. We detect a hard tail in the
0.4-2 MeV range for the hard state only. We extract the state dependent
polarigrams of Cyg X-1, which all are compatible to no or undetectable level of
polarization except in 400-2000 keV range in the hard state where the
polarization fraction is 7532 % and the polarization angle 40.0 +-14 deg.
An upper limit on the 0.4-2 MeV soft state polarization fraction is 70%. Due to
the short exposure, we obtain no meaningful constraint for the intermediate
state. The likely detection of a >400 keV polarized tail in the hard state,
together with the simultaneous presence of a radio jet, reinforce the notion of
a compact jet origin of the 400 keV emission.Comment: 13 pages, 5 figures, accepted for publication in Ap
Integration of quality of service in avionics architecture
International audienceTraditionally, avionics systems have followed a federated approach - separate software functions al- located to dedicated (often physically disjoint) com-puting ”black-boxes”
The principle of equivalence and projective structure in space-times
This paper discusses the extent to which one can determine the space-time
metric from a knowledge of a certain subset of the (unparametrised) geodesics
of its Levi-Civita connection, that is, from the experimental evidence of the
equivalence principle. It is shown that, if the space-time concerned is known
to be vacuum, then the Levi-Civita connection is uniquely determined and its
associated metric is uniquely determined up to a choice of units of
measurement, by the specification of these geodesics. It is further
demonstrated that if two space-times share the same unparametrised geodesics
and only one is assumed vacuum then their Levi-Civita connections are again
equal (and so the other metric is also a vacuum metric) and the first result
above is recovered.Comment: 23 pages, submitted to Classical and Quantum Gravit
On the Energy-Momentum Density of Gravitational Plane Waves
By embedding Einstein's original formulation of GR into a broader context we
show that a dynamic covariant description of gravitational stress-energy
emerges naturally from a variational principle. A tensor is constructed
from a contraction of the Bel tensor with a symmetric covariant second degree
tensor field and has a form analogous to the stress-energy tensor of the
Maxwell field in an arbitrary space-time. For plane-fronted gravitational waves
helicity-2 polarised (graviton) states can be identified carrying non-zero
energy and momentum.Comment: 10 pages, no figure
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