17 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&
The Influence of Free Quintessence on Gravitational Frequency Shift and Deflection of Light with 4D momentum
Based on the 4D momentum, the influence of quintessence on the gravitational
frequency shift and the deflection of light are examined in modified
Schwarzschild space. We find that the frequency of photon depends on the state
parameter of quintessence : the frequency increases for and
decreases for . Meanwhile, we adopt an integral power number
() to solve the orbital equation of photon. The photon's
potentials become higher with the decrease of . The behavior of
bending light depends on the state parameter sensitively. In
particular, for the case of , there is no influence on the
deflection of light by quintessence. Else, according to the H-masers of GP-A
redshift experiment and the long-baseline interferometry, the constraints on
the quintessence field in Solar system are presented here.Comment: 12 pages, 2 figures, 4 tables. European Physical Journal C in pres
Radio Science Investigation on a Mercury Orbiter Mission
We review the results from {\it Mariner 10} regarding Mercury's gravity field
and the results from radar ranging regarding topography. We discuss the
implications of improving these results, including a determination of the polar
component, as well as the opportunity to perform relativistic gravity tests
with a future {\it Mercury Orbiter}. With a spacecraft placed in orbit with
periherm at 400 km altitude, apherm at 16,800 km, period 13.45 hr and latitude
of periherm at +30 deg, one can expect a significant improvement in our
knowledge of Mercury's gravity field and geophysical properties. The 2000 Plus
mission that evolved during the European Space Agency (ESA) {\it Mercury
Orbiter} assessment study can provide a global gravity field complete through
the 25th degree and order in spherical harmonics. If after completion of the
main mission, the periherm could be lowered to 200 km altitude, the gravity
field could be extended to 50th degree and order. We discuss the possibility
that a search for a Hermean ionosphere could be performed during the mission
phases featuring Earth occultations.
Because of its relatively large eccentricity and close proximity to the Sun,
Mercury's orbital motion provides one of the best solar-system tests of general
relativity. Consequently, we emphasize the number of feasible relativistic
gravity tests that can be performed within the context of the parameterized
post-Newtonian formalism - a useful framework for testing modern gravitational
theories. We pointed out that current results on relativistic precession of
Mercury's perihelion are uncertain by 0.5 %, and we discuss the expected
improvement using {\it Mercury Orbiter}. We discuss the importance of {\it
Mercury Orbiter} for setting limits on a possible time variation in theComment: 23 pages, LaTeX, no figure
Gravitational Lensing in Astronomy
Deflection of light by gravity was predicted by General Relativity and
observationaly confirmed in 1919. In the following decades various aspects of
the gravitational lens effect were explored theoretically, among them the
possibility of multiple or ring-like images of background sources, the use of
lensing as a gravitational telescope on very faint and distant objects, and the
possibility to determine Hubble's constant with lensing. Only relatively
recently gravitational lensing became an observational science after the
discovery of the first doubly imaged quasar in 1979. Today lensing is a booming
part of astrophysics.
In addition to multiply-imaged quasars, a number of other aspects of lensing
have been discovered since, e.g. giant luminous arcs, quasar microlensing,
Einstein rings, galactic microlensing events, arclets, or weak gravitational
lensing. By now literally hundreds of individual gravitational lens phenomena
are known.
Although still in its childhood, lensing has established itself as a very
useful astrophysical tool with some remarkable successes. It has contributed
significant new results in areas as different as the cosmological distance
scale, the large scale matter distribution in the universe, mass and mass
distribution of galaxy clusters, physics of quasars, dark matter in galaxy
halos, or galaxy structure.Comment: Review article for "Living Reviews in Relativity", see
http://www.livingreviews.org . 41 pages, latex, 22 figures (partly in GIF
format due to size constraints). High quality postscript files can be
obtained electronically at http://www.aip.de:8080/~jkw/review_figures.htm
The Confrontation between General Relativity and Experiment
The status of experimental tests of general relativity and of theoretical
frameworks for analysing them is reviewed. Einstein's equivalence principle
(EEP) is well supported by experiments such as the Eotvos experiment, tests of
special relativity, and the gravitational redshift experiment. Future tests of
EEP and of the inverse square law are searching for new interactions arising
from unification or quantum gravity. Tests of general relativity at the
post-Newtonian level have reached high precision, including the light
deflection, the Shapiro time delay, the perihelion advance of Mercury, and the
Nordtvedt effect in lunar motion. Gravitational-wave damping has been detected
in an amount that agrees with general relativity to better than half a percent
using the Hulse-Taylor binary pulsar, and other binary pulsar systems have
yielded other tests, especially of strong-field effects. When direct
observation of gravitational radiation from astrophysical sources begins, new
tests of general relativity will be possible.Comment: 89 pages, 8 figures; an update of the Living Review article
originally published in 2001; final published version incorporating referees'
suggestion
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VLBI for Gravity Probe B: the guide star, IM Pegasi
We review the radio very long baseline interferometry (VLBI) observations of the guide star, IM Peg, and three compact extragalactic reference sources, made in support of the NASA/Stanford gyroscope relativity mission, Gravity Probe B (GP-B). The main goal of the observations was the determination of the proper motion of IM Peg relative to the distant Universe. VLBI observations made between 1997 and 2005 yield a proper motion of IM Peg of -20.83 ± 0.09 mas yr-1 in α and -27.27 ± 0.09 mas yr-1 in δ in a celestial reference frame of extragalactic radio galaxies and quasars virtually identical to the International Celestial Reference Frame 2 (ICRF2). They also yield a parallax for IM Peg of 10.37 ± 0.07 mas, corresponding to a distance of 96.4 ± 0.7 pc. The uncertainties are standard errors with statistical and estimated systematic contributions added in quadrature. These results met the pre-launch requirements of the GP-B mission to not discernibly degrade the estimates of the geodetic and frame-dragging effects.Astronom
A test of general relativity using radio links with the Cassini spacecraft
According to general relativity, photons are deflected and delayed by the curvature of space-time produced by any mass(1-3). The bending and delay are proportional to gamma+1, where the parameter gamma is unity in general relativity but zero in the newtonian model of gravity. The quantity gamma-1 measures the degree to which gravity is not a purely geometric effect and is affected by other fields; such fields may have strongly influenced the early Universe, but would have now weakened so as to produce tiny-but still detectable-effects. Several experiments have confirmed to an accuracy of similar to0.1% the predictions for the deflection(4,5) and delay(6) of photons produced by the Sun. Here we report a measurement of the frequency shift of radio photons to and from the Cassini spacecraft as they passed near the Sun. Our result, gamma=1+(2.1+/-2.3)x10(-5), agrees with the predictions of standard general relativity with a sensitivity that approaches the level at which, theoretically, deviations are expected in some cosmological models(7,8)