147 research outputs found
Commercially Available Capacitors at Cryogenic Temperatures
Commercially available capacitors are not specified for operation at 77 K or 4 K, and some devices showed a dramatic decrease of capacitance at cryogenic temperature. Furthermore, for voltage biasing of cryogenic low impedance sensors it is very important to know parasitic resistance. In this case, the parasitic Equivalent Series Resistance (ESR) of the capacitor used for the AC-biasing is a bottleneck of the voltage biasing. Involved in TES development and SQUID multiplexing, we have characterized the capacitance and the ESR values of some commercially available capacitors at 77 K and 4 K
Direction of light propagation to order G^2 in static, spherically symmetric spacetimes: a new derivation
A procedure avoiding any integration of the null geodesic equations is used
to derive the direction of light propagation in a three-parameter family of
static, spherically symmetric spacetimes within the post-post-Minkowskian
approximation. Quasi-Cartesian isotropic coordinates adapted to the symmetries
of spacetime are systematically used. It is found that the expression of the
angle formed by two light rays as measured by a static observer staying at a
given point is remarkably simple in these coordinates. The attention is mainly
focused on the null geodesic paths that we call the "quasi-Minkowskian light
rays". The vector-like functions characterizing the direction of propagation of
such light rays at their points of emission and reception are firstly obtained
in the generic case where these points are both located at a finite distance
from the centre of symmetry. The direction of propagation of the
quasi-Minkowskian light rays emitted at infinity is then straightforwardly
deduced. An intrinsic definition of the gravitational deflection angle relative
to a static observer located at a finite distance is proposed for these rays.
The expression inferred from this definition extends the formula currently used
in VLBI astrometry up to the second order in the gravitational constant G.Comment: 19 pages; revised introduction; added references for introduction;
corrected typos; published in Class. Quantum Gra
Note on Varying Speed of Light Cosmologies
The various requirements on a consistent varying speed of light (`VSL')
theory are surveyed, giving a short check-list of issues that should be
satisfactorily handled by such theories.Comment: 6 pages; to appear in the GRG Journa
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
Quelques remarques sur La Chatte de Colette
Résumé de l\u27éditeur
Quatrième de couverture
"Bien des contes, récits animaliers, romans, poèmes, encyclopédies ou histoires naturelles accordent une place de choix aux animaux. Les bestiaires, plus que tout autre genre, leur donnent un statut privilégié. Les bestiaires médiévaux, par leurs pratiques diversifiées, posent la question suivante : comment faire entrer l\u27animal dans l\u27organisation et le pouvoir de signification du livre ? Premier élément de réponse, la structure : le genre du bestiaire reflète l\u27instabilité des classifications du vivant et les incertitudes du savoir sur le monde qu\u27il soit philosophique, scientifique ou artistique. Deuxième élément : la valeur originelle de l\u27animal, à travers les codes du récit mythique et de la poésie, permet de dire l\u27origine de la littérature et de creuser la profondeur des sentiments les plus intimes. Le bestiaire individuel des écrivains renoue avec la dimension interprétative des premiers bestiaires et ouvre la voie à trois discours, sur l\u27homme, sur le monde qui l\u27entoure, comme sur la possibilité d\u27un autre monde. La complexité des liens entre animal et homme est autant affaire de relations, entre l\u27animal domestique et son maître, que de reflet, voire de caricature. Les bestiaires d\u27écrivains déploient idées et croyances, dans une perspective politique, sociale ou spirituelle qui va du grave à l\u27excentrique. Ils peuvent, à l\u27inverse, surtout à l\u27époque contemporaine, réactiver l\u27énergie de la description et traduire la fascination d\u27une rupture avec le réel, inscrite dans l\u27altérité de l\u27animal. Les contributions de ce recueil réuni en hommage à Arlette Bouloumié tentent de saisir comment le bestiaire a forgé un imaginaire inscrit tant dans son personnel animalier que dans ses codes génériques. Les écrivains, privilégiant tantôt la description des formes ou des comportements, tantôt la quête des significations, n\u27ont cessé de diversifier ce dosage.
Nonminimal isotropic cosmological model with Yang-Mills and Higgs fields
We establish a nonminimal Einstein-Yang-Mills-Higgs model, which contains six
coupling parameters. First three parameters relate to the nonminimal coupling
of non-Abelian gauge field and gravity field, two parameters describe the
so-called derivative nonminimal coupling of scalar multiplet with gravity
field, and the sixth parameter introduces the standard coupling of scalar field
with Ricci scalar. The formulated six-parameter nonminimal
Einstein-Yang-Mills-Higgs model is applied to cosmology. We show that there
exists a unique exact cosmological solution of the de Sitter type for a special
choice of the coupling parameters. The nonminimally extended Yang-Mills and
Higgs equations are satisfied for arbitrary gauge and scalar fields, when the
coupling parameters are specifically related to the curvature constant of the
isotropic spacetime. Basing on this special exact solution we discuss the
problem of a hidden anisotropy of the Yang-Mills field, and give an explicit
example, when the nonminimal coupling effectively screens the anisotropy
induced by the Yang-Mills field and thus restores the isotropy of the model.Comment: 15 pages, revised version accepted to Int. J. Mod. Phys. D, typos
correcte
Testing gravitational theories using Eccentric Eclipsing Detached Binaries
In this paper we compare the effects of different theories of gravitation on
the apsidal motion of a sample of Eccentric Eclipsing Detached Binary stars.
The comparison is performed by using the formalism of the Post-Newtonian
parametrization to calculate the theoretical advance at periastron and compare
it to the observed one, after having considered the effects of the structure
and rotation of the involved stars. A variance analysis on the results of this
comparison, shows that no significant difference can be found due to the effect
of the different theories under test with respect to the standard General
Relativity. It will be possible to observe differences, as we would expect, by
checking the observed period variation on a much larger lapse of time. It can
also be noticed from our results, that f(R) theory is the nearest to GR with
respect to the other tested theories.Comment: 15 pages, 8 figures, 5 tables; Monthly Notices of the Royal
Astronomical Society (2012) "Early View". arXiv admin note: text overlap with
arXiv:gr-qc/0603071 by other author
Gravity as Backreaction
Quadratic theory of gravity is a complicated constraint system. We
investigate some consequences of treating quadratic terms perturbatively
(higher derivative version of backreaction effects). This approach is shown to
overcome some well known problems associated with higher derivative theories,
i.e., the physical gravitational degree of freedom remains unchanged from those
of Einstein gravity.
Using such an interpretation of gravity, we investigate a
classical and Wheeler DeWitt evolution of gravity for a
particular sign of , corresponding to non- tachyon case. Matter is
described by a phenomenological . It is concluded that
both the Friedmann potential () and the
Wheeler DeWitt potential () develop repulsive barriers near for
(i.e., ). The interpretations is clear. Repulsive
barrier in implies that a contracting FRW universe ()
will bounce to an expansion phase without a total gravitational collapse.
Repulsive barrier in means that is a classically forbidden
region. Therefore, probability of finding a universe with the big bang
singularity ( ) is exponentially suppressed.Comment: Accepted for publication in Phy. Rev. D.,18 pages, 6 figures, Latex
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Application of Time Transfer Function to McVittie Spacetime: Gravitational Time Delay and Secular Increase in Astronomical Unit
We attempt to calculate the gravitational time delay in a time-dependent
gravitational field, especially in McVittie spacetime, which can be considered
as the spacetime around a gravitating body such as the Sun, embedded in the
FLRW (Friedmann-Lema\^itre-Robertson-Walker) cosmological background metric. To
this end, we adopt the time transfer function method proposed by Le
Poncin-Lafitte {\it et al.} (Class. Quant. Grav. 21:4463, 2004) and Teyssandier
and Le Poncin-Lafitte (Class. Quant. Grav. 25:145020, 2008), which is
originally related to Synge's world function and enables to
circumvent the integration of the null geodesic equation. We re-examine the
global cosmological effect on light propagation in the solar system. The
round-trip time of a light ray/signal is given by the functions of not only the
spacial coordinates but also the emission time or reception time of light
ray/signal, which characterize the time-dependency of solutions. We also apply
the obtained results to the secular increase in the astronomical unit, reported
by Krasinsky and Brumberg (Celest. Mech. Dyn. Astron. 90:267, 2004), and we
show that the leading order terms of the time-dependent component due to
cosmological expansion is 9 orders of magnitude smaller than the observed value
of , i.e., ~[m/century]. Therefore, it is not possible
to explain the secular increase in the astronomical unit in terms of
cosmological expansion.Comment: 13 pages, 2 figures, accepted for publication in General Relativity
and Gravitatio
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