179 research outputs found
Large-distance behaviour of the graviton two-point function in de Sitter spacetime
It is known that the graviton two-point function for the de Sitter invariant
"Euclidean" vacuum in a physical gauge grows logarithmically with distance in
spatially-flat de Sitter spacetime. We show that this logarithmic behaviour is
a gauge artifact by explicitly demonstrating that the same behaviour can be
reproduced by a pure-gauge two-point function.Comment: 19 pages, no figures, misprints and minor errors correcte
On the scalar sector of the covariant graviton two-point function in de Sitter spacetime
We examine the scalar sector of the covariant graviton two-point function in
de Sitter spacetime. This sector consists of the pure-trace part and another
part described by a scalar field. We show that it does not contribute to
two-point functions of gauge-invariant quantities. We also demonstrate that the
long-distance growth present in some gauges is absent in this sector for a wide
range of gauge parameters.Comment: 15 pages, no figures, LaTeX, considerably shortene
Atomistic calculations of interface elastic properties in noncoherent metallic bilayers
The paper describes theoretical and computational studies associated with the interface elastic properties of noncoherent metallic bicrystals. Analytical forms of interface energy, interface stresses, and interface elastic constants are derived in terms of interatomic potential functions. Embedded-atom method potentials are then incorporated into the model to compute these excess thermodynamics variables, using energy minimization in a parallel computing environment. The proposed model is validated by calculating surface thermodynamic variables and comparing them with preexisting data. Next, the interface elastic properties of several fcc-fcc bicrystals are computed. The excess energies and stresses of interfaces are smaller than those on free surfaces of the same crystal orientations. In addition, no negative values of interface stresses are observed. Current results can be applied to various heterogeneous materials where interfaces assume a prominent role in the systems' mechanical behavior.open322
Further oblique-incidence ionospheric soundings over Central Europe to test nowcasting and long term prediction models
After a first oblique-incidence ionospheric sounding campaign over Central Europe performed during the period 2003-2004 over the radio links between Inskip (UK, 53.5° N, 2.5° W) and Rome (Italy, 41.8 N, 12.5E) and between Inskip and Chania (Crete, 35.7° N, 24.0° E), new and more extensive analysis of systematic MUF measurements from January 2005 to December 2006 have been performed. MUF measurements collected during moderately disturbed days (17 ≤ Ap ≤ 32), disturbed days (32 50), have been used to test the long term prediction models (ASAPS, ICEPAC and SIRM&LKW), and the now casting models (SIRMUP&LKW and ISWIRM&LKW). The performances of the different prediction methods in terms of r.m.s are shown for selected range of geomagnetic activity and for each season.Submitted3.9. Fisica della magnetosfera, ionosfera e meteorologia spazialeN/A or not JCRope
Stress tensor fluctuations in de Sitter spacetime
The two-point function of the stress tensor operator of a quantum field in de
Sitter spacetime is calculated for an arbitrary number of dimensions. We assume
the field to be in the Bunch-Davies vacuum, and formulate our calculation in
terms of de Sitter-invariant bitensors. Explicit results for free minimally
coupled scalar fields with arbitrary mass are provided. We find long-range
stress tensor correlations for sufficiently light fields (with mass m much
smaller than the Hubble scale H), namely, the two-point function decays at
large separations like an inverse power of the physical distance with an
exponent proportional to m^2/H^2. In contrast, we show that for the massless
case it decays at large separations like the fourth power of the physical
distance. There is thus a discontinuity in the massless limit. As a byproduct
of our work, we present a novel and simple geometric interpretation of de
Sitter-invariant bitensors for pairs of points which cannot be connected by
geodesics.Comment: 35 pages, 4 figure
SPINN: Synergistic Progressive Inference of Neural Networks over Device and Cloud
Despite the soaring use of convolutional neural networks (CNNs) in mobile
applications, uniformly sustaining high-performance inference on mobile has
been elusive due to the excessive computational demands of modern CNNs and the
increasing diversity of deployed devices. A popular alternative comprises
offloading CNN processing to powerful cloud-based servers. Nevertheless, by
relying on the cloud to produce outputs, emerging mission-critical and
high-mobility applications, such as drone obstacle avoidance or interactive
applications, can suffer from the dynamic connectivity conditions and the
uncertain availability of the cloud. In this paper, we propose SPINN, a
distributed inference system that employs synergistic device-cloud computation
together with a progressive inference method to deliver fast and robust CNN
inference across diverse settings. The proposed system introduces a novel
scheduler that co-optimises the early-exit policy and the CNN splitting at run
time, in order to adapt to dynamic conditions and meet user-defined
service-level requirements. Quantitative evaluation illustrates that SPINN
outperforms its state-of-the-art collaborative inference counterparts by up to
2x in achieved throughput under varying network conditions, reduces the server
cost by up to 6.8x and improves accuracy by 20.7% under latency constraints,
while providing robust operation under uncertain connectivity conditions and
significant energy savings compared to cloud-centric execution.Comment: Accepted at the 26th Annual International Conference on Mobile
Computing and Networking (MobiCom), 202
Near-Earth space plasma modelling and forecasting
In the frame of the European COST 296 project (Mitigation of Ionospheric Effects on Radio Systems, MIERS)in the Working Package 1.3, new ionospheric models, prediction and forecasting methods and programs as well as ionospheric imaging techniques have been developed. They include (i) topside ionosphere and meso-scale irregularity models, (ii) improved forecasting methods for real time forecasting and for prediction of foF2,
M(3000)F2, MUF and TECs, including the use of new techniques such as Neurofuzzy, Nearest Neighbour, Cascade Modelling and Genetic Programming and (iii) improved dynamic high latitude ionosphere models through tomographic imaging and model validation. The success of the prediction algorithms and their improvement over
existing methods has been demonstrated by comparing predictions with later real data. The collaboration between different European partners (including interchange of data) has played a significant part in the development and validation of these new prediction and forecasting methods, programs and algorithms which can be applied to a variety of practical applications leading to improved mitigation of ionosphereic and space weather effects.Published255-2713.9. Fisica della magnetosfera, ionosfera e meteorologia spazialeJCR Journalope
Near-Earth space plasma modelling and forecasting
In the frame of the European COST 296 project (Mitigation of Ionospheric Effects on Radio Systems, MIERS)in the Working Package 1.3, new ionospheric models, prediction and forecasting methods and programs as well as ionospheric imaging techniques have been developed. They include (i) topside ionosphere and meso-scale irregularity models, (ii) improved forecasting methods for real time forecasting and for prediction of foF2,
M(3000)F2, MUF and TECs, including the use of new techniques such as Neurofuzzy, Nearest Neighbour, Cascade Modelling and Genetic Programming and (iii) improved dynamic high latitude ionosphere models through tomographic imaging and model validation. The success of the prediction algorithms and their improvement over
existing methods has been demonstrated by comparing predictions with later real data. The collaboration between different European partners (including interchange of data) has played a significant part in the development and validation of these new prediction and forecasting methods, programs and algorithms which can be applied to a variety of practical applications leading to improved mitigation of ionosphereic and space weather effects
de Sitter invariance of the dS graviton vacuum
The two-point function of linearized gravitons on de Sitter space is infrared
divergent in the standard transverse traceless synchronous gauge defined by
cosmological coordinates (also called conformal or Poincare coordinates).
We show that this divergence can be removed by adding a linearized
diffeomorphism to each mode function; i.e., by an explicit change of gauge. It
follows that the graviton vacuum state is well-defined and de Sitter invariant
in agreement with various earlier arguments.Comment: 14 pages, 1 figur
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