2,634 research outputs found
Some comments on a recently derived approximated solution of the Einstein equations for a spinning body with negligible mass
Recently, an approximated solution of the Einstein equations for a rotating
body whose mass effects are negligible with respect to the rotational ones has
been derived by Tartaglia. At first sight, it seems to be interesting because
both external and internal metric tensors have been consistently found,
together an appropriate source tensor; moreover, it may suggest possible
experimental checks since the conditions of validity of the considered metric
are well satisfied at Earth laboratory scales. However, it should be pointed
out that reasonable doubts exist if it is physically meaningful because it is
not clear if the source tensor related to the adopted metric can be realized by
any real extended body. Here we derive the geodesic equations of the metric and
analyze the allowed motions in order to disclose possible unphysical features
which may help in shedding further light on the real nature of such
approximated solution of the Einstein equations.Comment: Latex2e, 17 pages, no tables, 5 figures, minor typos corrected. To
appear in general Relativity and Gravitatio
Orbital effects of Lorentz-violating Standard Model Extension gravitomagnetism around a static body: a sensitivity analysis
We analytically work out the long-term rates of change of the six osculating
Keplerian orbital elements of a test particle acted upon by the
Lorentz-violating gravitomagnetic acceleration due to a static body, as
predicted by the Standard Model Extension (SME). We neither restrict to any
specific spatial orientation for the symmetry-violating vector s nor make a
priori simplifying assumptions concerning the orbital configuration of the
perturbed test particle. Thus, our results are quite general, and can be
applied for sensitivity analyses to a variety of specific astronomical and
astrophysical scenarios. We find that, apart from the semimajor axis a, all the
other orbital elements undergo non-vanishing secular variations. By comparing
our results to the latest determinations of the supplementary advances of the
perihelia of some planets of the solar system we preliminarily obtain s_x =
(0.9 +/- 1.5) 10^-8, s_y = (-4 +/- 6) 10^-9, s_z = (0.3 +/- 1) 10^-9. Bounds
from the terrestrial LAGEOS and LAGEOS II satellites are of the order of s\sim
10^-3-10^-4.Comment: LaTex2e, 9 pages, no figures, 3 tables, 25 references. Typos fixe
The new Earth gravity models and the measurement of the Lense-Thirring effect
We examine how the new forthcoming Earth gravity models from the CHAMP and,
especially, GRACE missions could improve the measurement of the general
relativistic Lense-Thirring effect according to the various kinds of
observables which could be adopted. In a very preliminary way, we use the
recently released EIGEN2 CHAMP-only and GRACE01S GRACE-only Earth gravity
models in order to assess the impact of the mismodelling in the even zonal
harmonic coefficients of geopotential which represents one of the major sources
of systematic errors in this kind of measurement.Comment: Latex2e, 8 pages, 3 tables, no figures. Paper presented at Tenth
Marcel Grossmann Meeting on General Relativity Rio de Janeiro, July 20-26,
200
The perihelion precession of Saturn, planet X/Nemesis and MOND
We show that the retrograde perihelion precession of Saturn \Delta\dot\varpi,
recently estimated by different teams of astronomers by processing ranging data
from the Cassini spacecraft and amounting to some milliarcseconds per century,
can be explained in terms of a localized, distant body X, not yet directly
discovered. From the determination of its tidal parameter K = GM_X/r_X^3 as a
function of its ecliptic longitude \lambda_X and latitude \beta_X, we calculate
the distance at which X may exist for different values of its mass, ranging
from the size of Mars to that of the Sun. The minimum distance would occur for
X located perpendicularly to the ecliptic, while the maximum distance is for X
lying in the ecliptic. We find for rock-ice planets of the size of Mars and the
Earth that they would be at about 80-150 au, respectively, while a
Jupiter-sized gaseous giant would be at approximately 1 kau. A typical brown
dwarf would be located at about 4 kau, while an object with the mass of the Sun
would be at approximately 10 kau, so that it could not be Nemesis for which a
solar mass and a heliocentric distance of about 88 kau are predicted. If X was
directed towards a specific direction, i.e. that of the Galactic Center, it
would mimick the action of a recently proposed form of the External Field
Effect (EFE) in the framework of the MOdified Newtonian Dynamics (MOND).Comment: LaTex2e, 14 pages, no figures, no tables. Accepted by The Open
Astronomy Journal (TOAJ). Typos in eq. (17) and eq. (18) correcte
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