5,115 research outputs found
Testing general relativity by micro-arcsecond global astrometry
The global astrometric observations of a GAIA-like satellite were modeled
within the PPN formulation of Post-Newtonian gravitation. An extensive
experimental campaign based on realistic end-to-end simulations was conducted
to establish the sensitivity of global astrometry to the PPN parameter \gamma,
which measures the amount of space curvature produced by unit rest mass. The
results show that, with just a few thousands of relatively bright,
photometrically stable, and astrometrically well behaved single stars, among
the ~10^9 objects that will be observed by GAIA, \gamma can be estimated after
1 year of continuous observations with an accuracy of ~10^{-5} at the 3\sigma
level. Extrapolation to the full 5-year mission of these results based on the
scaling properties of the adjustment procedure utilized suggests that the
accuracy of \simeq 2x10^{-7}, at the same 3\sigma level, can be reached with
\~10^6 single stars, again chosen as the most astrometrically stable among the
millions available in the magnitude range V=12-13. These accuracies compare
quite favorably with recent findings of scalar-tensor cosmological models,
which predict for \gamma a present-time deviation, |1-\gamma|, from the General
Relativity value between 10^{-5} and 10^{-7}.Comment: 7 pages, 2 figures, to be published in A&
A general relativistic model for the light propagation in the gravitational field of the Solar System: the dynamical case
Modern astrometry is based on angular measurements at the micro-arcsecond
level. At this accuracy a fully general relativistic treatment of the data
reduction is required. This paper concludes a series of articles dedicated to
the problem of relativistic light propagation, presenting the final
microarcsecond version of a relativistic astrometric model which enable us to
trace back the light path to its emitting source throughout the non-stationary
gravity field of the moving bodies in the Solar System. The previous model is
used as test-bed for numerical comparisons to the present one. Here we also
test different versions of the computer code implementing the model at
different levels of complexity to start exploring the best trade-off between
numerical efficiency and the micro-arcsecond accuracy needed to be reached.Comment: 40 pages, 5 figures. Accepted for publication on The Astrophysical
Journal. Manuscript prepared with AASLaTeX macros v.5.
Il nuovo allestimento della collezione archeologica della Fondazione Sicilia fra tecnologie e creativitĂ
In May 2012 in the historic center of Palermo, Sicily, thanks to the Fondazione Sicilia the Palazzo Branciforte was opened to the public after a long and costly restoration based on a project by the Italian architect Gae Aulenti (http://www.palazzobranciforte.it/). For the creation of this innovative cultural center, the University of Foggia was responsible for the execution of the archaeological museum project and the multimedia system, which is presented in this paper. The activities were conducted by a group of young experts from different fields with the assistance of researchers and technicians from the Laboratory of Digital Archaeology. The paper describes the production process of the multimedia system and the method used to create digital content for the cultural heritage that effectively uses a wide spectrum of normally underused skills and proposes an innovative approach to technologies, creativity and knowledge
Foreword: Safety and preservation of cultural heritage: to do or not to do?
Biographical notes: Gianmarco de Felice is a Professor and Head of the PhD School in Civil Engineering at Roma Tr e University, he was chairmen of the Rilem TC-250, member of the drafting Committee of the Charter of Rome on the Resilience of Art Cities to Natural Catastrophes, f ounder of the Summer School on Historic Masonry Structures and currently Weinberg Fellow at Columbia University. He designed engi neering projects on cultural heritage, such as: restoration of S. Clemente in Casauria Abbey supported by World Monuments Fund and awarded by 2016 Domus International Prize, restauration of Farnese Palace in Ischia di Castro by Antonio da Sangallo awarded by 2017 Mastrodicasa Prize
Conditions for the cosmological viability of the most general scalar-tensor theories and their applications to extended Galileon dark energy models
In the Horndeski's most general scalar-tensor theories with second-order
field equations, we derive the conditions for the avoidance of ghosts and
Laplacian instabilities associated with scalar, tensor, and vector
perturbations in the presence of two perfect fluids on the flat
Friedmann-Lemaitre-Robertson-Walker (FLRW) background. Our general results are
useful for the construction of theoretically consistent models of dark energy.
We apply our formulas to extended Galileon models in which a tracker solution
with an equation of state smaller than -1 is present. We clarify the allowed
parameter space in which the ghosts and Laplacian instabilities are absent and
we numerically confirm that such models are indeed cosmologically viable.Comment: 18 pages, 6 figure
A note on the factorization conjecture
We give partial results on the factorization conjecture on codes proposed by
Schutzenberger. We consider finite maximal codes C over the alphabet A = {a, b}
with C \cap a^* = a^p, for a prime number p. Let P, S in Z , with S = S_0 +
S_1, supp(S_0) \subset a^* and supp(S_1) \subset a^*b supp(S_0). We prove that
if (P,S) is a factorization for C then (P,S) is positive, that is P,S have
coefficients 0,1, and we characterize the structure of these codes. As a
consequence, we prove that if C is a finite maximal code such that each word in
C has at most 4 occurrences of b's and a^p is in C, then each factorization for
C is a positive factorization. We also discuss the structure of these codes.
The obtained results show once again relations between (positive)
factorizations and factorizations of cyclic groups
The Formation of non-Keplerian Rings of Matter about Compact Stars
The formation of energetic rings of matter in a Kerr spacetime with an
outward pointing acceleration field does not appear to have previously been
noted as a relativistic effect. In this paper we show that such rings are a
gravimagneto effect with no Newtonian analog, and that they do not occur in the
static limit. The energy efficiency of these rings can, depending of the
strength of the acceleration field, be much greater than that of Keplerian
disks. The rings rotate in a direction opposite to that of compact star about
which they form. The size and energy efficiency of the rings depend on the
fundamental parameters of the spacetime as well as the strength the
acceleration field.Comment: 19 pages, 7 figures, 1 diagram. Figures are included in the text
using the "graphicx" package. If you do not have this package you can use
epsfig, or another package as long as you alter the tex file appropriately.
Alternatively you could print the figures out seperatel
Cosmological dynamics of fourth order gravity with a Gauss-Bonnet term
We consider cosmological dynamics in fourth order gravity with both
and correction to the Einstein gravity ( is
the Gauss-Bonnet term). The particular case for which both terms are equally
important on power-law solutions is described. These solutions and their
stability are studied using the dynamical system approach. We also discuss
condition of existence and stability of de Sitter solution in a more general
situation of power-law and .Comment: published version, references update
Cosmological constraints on extended Galileon models
The extended Galileon models possess tracker solutions with de Sitter
attractors along which the dark energy equation of state is constant during the
matter-dominated epoch, i.e. w_DE = -1-s, where s is a positive constant. Even
with this phantom equation of state there are viable parameter spaces in which
the ghosts and Laplacian instabilities are absent. Using the observational data
of the supernovae type Ia, the cosmic microwave background (CMB), and baryon
acoustic oscillations, we place constraints on the tracker solutions at the
background level and find that the parameter s is constrained to be s=0.034
(-0.034,+0.327) (95% CL) in the flat Universe. In order to break the degeneracy
between the models we also study the evolution of cosmological density
perturbations relevant to the large-scale structure (LSS) and the
Integrated-Sachs-Wolfe (ISW) effect in CMB. We show that, depending on the
model parameters, the LSS and the ISW effect is either positively or negatively
correlated. It is then possible to constrain viable parameter spaces further
from the observational data of the ISW-LSS cross-correlation as well as from
the matter power spectrum.Comment: 17 pages, 9 figures, uses RevTeX4-
Social interaction is a catalyst for adult human learning in online contexts
Human learning is highly social. Advances in technology have increasingly moved learning online, and the recent coronavirus disease 2019 (COVID-19) pandemic has accelerated this trend. Online learning can vary in terms of how “socially” the material is presented (e.g., live or recorded), but there are limited data on which is most effective, with the majority of studies conducted on children and inconclusive results on adults. Here, we examine how young adults (aged 18–35) learn information about unknown objects, systematically varying the social contingency (live versus recorded lecture) and social richness (viewing the teacher’s face, hands, or slides) of the learning episodes. Recall was tested immediately and after 1 week. Experiment 1 (n = 24) showed better learning for live presentation and a full view of the teacher (hands and face). Experiment 2 (n = 27; pre-registered) replicated the live-presentation advantage. Both experiments showed an interaction between social contingency and social richness: the presence of social cues affected learning differently depending on whether teaching was interactive or not. Live social interaction with a full view of the teacher’s face provided the optimal setting for learning new factual information. However, during observational learning, social cues may be more cognitively demanding and/or distracting,resulting in less learning from rich social information if there is no interactivity. We suggest that being part of a genuine social interaction catalyzes learning, possibly via mechanisms of joint attention, common ground, or (inter-)active discussion, and as such, interactive learning benefits from rich social setting
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