9,006 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&
Tracing a relativistic Milky Way within the RAMOD measurement protocol
Advancement in astronomical observations and technical instrumentation
implies taking into account the general relativistic effects due the
gravitational fields encountered by the light while propagating from the star
to the observer. Therefore, data exploitation for Gaia-like space astrometric
mission (ESA, launch 2013) requires a fully relativistic interpretation of the
inverse ray-tracing problem, namely the development of a highly accurate
astrometric models in accordance with the geometrical environment affecting
light propagation itself and the precepts of the theory of measurement. This
could open a new rendition of the stellar distances and proper motions, or even
an alternative detection perspective of many subtle relativistic effects
suffered by light while it is propagating and subsequently recorded in the
physical measurements.Comment: Proceeding for "Relativity and Gravitation, 100 Years after Einstein
in Prague" to be published by Edition Open Access, revised versio
Generalized Galileon cosmology
We study the cosmology of a generalized Galileon field with five
covariant Lagrangians in which is replaced by general scalar functions
(i=1,...,5). For these theories, the equations of motion remain
at second-order in time derivatives. We restrict the functional forms of
from the demand to obtain de Sitter solutions responsible for
dark energy. There are two possible choices for power-law functions
, depending on whether the coupling with the Ricci
scalar is independent of or depends on . The former
corresponds to the covariant Galileon theory that respects the Galilean
symmetry in the Minkowski space-time. For generalized Galileon theories we
derive the conditions for the avoidance of ghosts and Laplacian instabilities
associated with scalar and tensor perturbations as well as the condition for
the stability of de Sitter solutions. We also carry out detailed analytic and
numerical study for the cosmological dynamics in those theories.Comment: 24 pages, 10 figures, version to appear in Physical Review
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.
Catching homologies by geometric entropy
A geometric entropy is defined as the Riemannian volume of the parameter
space of a statistical manifold associated with a given network. As such it can
be a good candidate for measuring networks complexity. Here we investigate its
ability to single out topological features of networks proceeding in a
bottom-up manner: first we consider small size networks by analytical methods
and then large size networks by numerical techniques. Two different classes of
networks, the random graphs and the scale--free networks, are investigated
computing their Betti numbers and then showing the capability of geometric
entropy of detecting homologies.Comment: 12 pages, 2 Figure
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Observations related to chronologic and gynecologic age in pregnant adolescents.
A low chronologic age (less than or equal to 15 years) and low gynecologic age (less than or equal to 2 years) have been considered factors that increase medical complications among adolescent pregnant women. Gynecologic age (GA) is defined in this study as age in years at conception minus age at menarche. Two hundred twelve consecutive pregnant teenagers were followed prospectively in the Teen OB Clinic at the University of California, San Diego Medical Center, between August 1978 and July 1981. The clinic population consisted of 37.3 percent Whites, 35.8 percent Hispanics, 20.8 percent Blacks, and 6.1 percent other (mostly Indochinese). Sixty-eight percent of the patients were funded by MediCal. The patient population was divided by chronological age (CA) at conception into those 15 years or less or 16 years or older. A low chronological age was found to be a significant risk factor for premature rupture of membranes. Teenagers with a low gynecologic age (less than or equal to 2) had a lower mean pre-pregnancy weight and body mass index (Kg/M2) than teenagers with a higher gynecologic age. In this study, we did not find that a low CA or GA was correlated with a higher frequency of pregnancy-induced hypertension, prenatal medical problems, obstetrical problems at labor or delivery, or an excessive number of low-birthweight infants
Five-Dimensional f(R) Braneworld Models
After incorporating the f(R) gravity into the general braneworld sum rules
scope, it is shown that some particular class of warped five dimensional
nonlinear braneworld models, which may be interesting for the hierarchy problem
solution, still require a negative tension brane. For other classes of warp
factors (suitable and not suitable for approaching the hierarchy problem) it is
not necessary any negative brane tension in the compactification scheme. In
this vein, it is argued that in the bulk f(R) gravity context, some types of
warp factors may be useful for approaching the hierarchy problem and for
evading the necessity of a negative brane tension in the compactification
scheme.Comment: 10 pages, references updated, small modifications. Accepted for
publication in Phys. Rev.
Screening for congenital hypothyroidism in Maltese newborns using cord blood
Routine screening for congenital hypothyroidism (CHT) has been introduced because clinical features of CHT may not be evident before the baby is a few weeks old and treatment at this stage may already be too late. Since a newborn testing programme employing liquid cord blood for other conditions had already been developed in the University of Malta and the Department of Health, we explored the possibility of implementing newborn thyroid testing using liquid cord blood. A similar programme had been implemented successfully in Finland and Philadelphia. Between September 1989 and August 1995 around 32,000 newborns were tested. This is nearly complete ascertainment. Preliminary testing was by radioimmunoassay for TSH. The sera of those with TSH levels more than 13mU/l were further tested for free T4. If the free T4 level was below 12 pmol/l, the babies were recalled for clinical evaluation and repeat testing. Other babies were recalled for technical reasons, giving a total recall rate of 3.88%. CHT was identified in seven newborns and treatment started within 3 weeks of delivery. One baby was reported normal on screening but was suspected to have CHT on clinical grounds at 3 weeks of age, confirmed biochemically. The incidence of CHT in Malta is therefore 1 in 4500.peer-reviewe
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