1,129 research outputs found
Time scales in LISA
The LISA mission is a space interferometer aiming at the detection of
gravitational waves in the [,] Hz frequency band. In order to
reach the gravitational wave detection level, a Time Delay Interferometry (TDI)
method must be applied to get rid of (most of) the laser frequency noise and
optical bench noise. This TDI analysis is carried out in terms of the
coordinate time corresponding to the Barycentric Coordinate Reference System
(BCRS), TCB, whereas the data at each of the three LISA stations is recorded in
terms of each station proper time. We provide here the required proper time
versus BCRS time transformation. We show that the difference in rate of station
proper time versus TCB is of the order of . The difference between
station proper times and TCB exhibits an oscillatory trend with a maximum
amplitude of about s.Comment: 14 pages, 5 eps figures, 0 table, accepted in Classical and Quantum
Gravit
Light deflection in Weyl gravity: constraints on the linear parameter
Light deflection offers an unbiased test of Weyl's gravity since no
assumption on the conformal factor needs to be made. In this second paper of
our series ``Light deflection in Weyl gravity'', we analyze the constraints
imposed by light deflection experiments on the linear parameter of Weyl's
theory. Regarding solar system experiments, the recent CASSINI Doppler
measurements are used to infer an upper bound, m, on the
absolute value of the above Weyl parameter. In non-solar system experiments, a
condition for unbound orbits together with gravitational mirage observations
enable us to further constrain the allowed negative range of the Weyl parameter
to m. We show that the characteristics of the light
curve in microlensing or gravitational mirages, deduced from the lens equation,
cannot be recast into the General Relativistic predictions by a simple
rescaling of the deflector mass or of the ring radius. However, the corrective
factor, which depends on the Weyl parameter value and on the lensing
configuration, is small, even perhaps negligible, owing to the upper bound
inferred on the absolute value of a negative Weyl parameter. A statistical
study on observed lensing systems is required to settle the question.
Our Weyl parameter range is more reliable than the single value derived by
Mannheim and Kazanas from fits to galactic rotation curves, $\sim +10^{-26}\
^{-1}$. Indeed, the latter, although consistent with our bounds, is biased
by the choice of a specific conformal factor.Comment: 20 pages, 2 figures (see published version for a better resolution,
or online at stacks.iop/CQG/21/1). To be published in Classical and Quantum
Gravit
Light deflection in Weyl gravity: critical distances for photon paths
The Weyl gravity appears to be a very peculiar theory. The contribution of
the Weyl linear parameter to the effective geodesic potential is opposite for
massive and nonmassive geodesics. However, photon geodesics do not depend on
the unknown conformal factor, unlike massive geodesics. Hence light deflection
offers an interesting test of the Weyl theory.
In order to investigate light deflection in the setting of Weyl gravity, we
first distinguish between a weak field and a strong field approximation.
Indeed, the Weyl gravity does not turn off asymptotically and becomes even
stronger at larger distances.
We then take full advantage of the conformal invariance of the photon
effective potential to provide the key radial distances in Weyl gravity.
According to those, we analyze the weak and strong field regime for light
deflection. We further show some amazing features of the Weyl theory in the
strong regime.Comment: 20 pages, 9 figures (see published version for a better resolution,
or online version at stacks.iop.org/CQG/21/1897
The observable light deflection angle
The physical deflection angle of a light ray propagating in a space-time
supplied with an asymptotically flat metric has to be expressed in terms of the
impact parameter.Comment: 11 pages, 1 figur
(SC)RMI: A (S)emi-(C)lassical (R)elativistic (M)otion (I)integrator, to model the orbits of space probes around the Earth and other planets
Today, the motion of spacecrafts is still described according to the
classical Newtonian equations plus the so-called "relativistic corrections",
computed with the required precision using the Post-(Post-)Newtonian formalism.
The current approach, with the increase of tracking precision (Ka-Band Doppler,
interplanetary lasers) and clock stabilities (atomic fountains) is reaching its
limits in terms of complexity, and is furthermore error prone. In the
appropriate framework of General Relativity, we study a method to numerically
integrate the native relativistic equations of motion for a weak gravitational
field, also taking into account small non-gravitational forces. The latter are
treated as perturbations, in the sense that we assume that both the local
structure of space-time is not modified by these forces, and that the
unperturbed satellite motion follows the geodesics of the local space-time. The
use of a symplectic integrator to compute the unperturbed geodesic motion
insures the constancy of the norm of the proper velocity quadrivector. We
further show how this general relativistic framework relates to the classical
one.Comment: 13 pages, 5 eps figures, 1 table, accepted in Acta Astronautica,
presented at the International Astronautical Congress, Vancouver 2004,
reference IAC-04-A.7.0
The Sun Asphericities: Astrophysical Relevance
Of all the fundamental parameters of the Sun (diameter, mass,
temperature...), the gravitational multipole moments (of degree l and order m)
that determine the solar moments of inertia, are still poorly known. However,
at the first order (l=2), the quadrupole moment is relevant to many
astrophysical applications. It indeed contributes to the relativistic
perihelion advance of planets, together with the post-Newtonian (PN)
parameters; or to the precession of the orbital plane about the Sun polar axis,
the latter being unaffected by the purely relativistic PN contribution. Hence,
a precise knowledge of the quadrupole moment is necessary for accurate orbit
determination, and alternatively, to obtain constraints on the PN parameters.
Moreover, the successive gravitational multipole moments have a physical
meaning: they describe deviations from a purely spherical mass distribution.
Thus, their precise determination gives indications on the solar internal
structure. Here, we explain why it is difficult to compute these parameters,
how to derive the best values, and how they will be determined in a near future
by means of space experiments.Comment: 14 pages, 9 figures (see published version for a better resolution),
submited to Proceedings of the Royal Society: Mathematical, Physical and
Engineering Science
Solar gravitational energy and luminosity variations
Due to non-homogeneous mass distribution and non-uniform velocity rate inside
the Sun, the solar outer shape is distorted in latitude. In this paper, we
analyze the consequences of a temporal change in this figure on the luminosity.
To do so, we use the Total Solar Irradiance (TSI) as an indicator of
luminosity. Considering that most of the authors have explained the largest
part of the TSI modulation with magnetic network (spots and faculae) but not
the whole, we could set constraints on radius and effective temperature
variations (dR, dT). However computations show that the amplitude of solar
irradiance modulation is very sensitive to photospheric temperature variations.
In order to understand discrepancies between our best fit and recent
observations of Livingston et al. (2005), showing no effective surface
temperature variation during the solar cycle, we investigated small effective
temperature variation in irradiance modeling. We emphasized a phase-shift
(correlated or anticorrelated radius and irradiance variations) in the (dR,
dT)-parameter plane. We further obtained an upper limit on the amplitude of
cyclic solar radius variations, deduced from the gravitational energy
variations. Our estimate is consistent with both observations of the
helioseismic radius through the analysis of f-mode frequencies and observations
of the basal photospheric temperature at Kitt Peak. Finally, we suggest a
mechanism to explain faint changes in the solar shape due to variation of
magnetic pressure which modifies the granules size. This mechanism is supported
by our estimate of the asphericity-luminosity parameter, which implies an
effectiveness of convective heat transfer only in very outer layers of the Sun.Comment: 17 pages, 2 figure, 1 table, published in New Astronom
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