92 research outputs found
New limits on the violation of local position invariance of gravity
Within the parameterized post-Newtonian (PPN) formalism, there could be an
anisotropy of local gravity induced by an external matter distribution, even
for a fully conservative metric theory of gravity. It reflects the breakdown of
the local position invariance of gravity and, within the PPN formalism, is
characterized by the Whitehead parameter . We present three different
kinds of observation, from the Solar system and radio pulsars, to constrain it.
The most stringent limit comes from recent results on the extremely stable
pulse profiles of solitary millisecond pulsars, that gives (95% CL), where the hat denotes the strong-field generalization
of . This limit is six orders of magnitude more constraining than the
current best limit from superconducting gravimeter experiments. It can be
converted into an upper limit of on the spatial
anisotropy of the gravitational constant.Comment: 13 pages, 4 figures; accepted by Classical and Quantum Gravit
New tests of local Lorentz invariance of gravity with small-eccentricity binary pulsars
In the post-Newtonian parametrization of semi-conservative gravity theories,
local Lorentz invariance (LLI) violation is characterized by two parameters,
alpha_1 and alpha_2. In binary pulsars the isotropic violation of LLI in the
gravitational sector leads to characteristic preferred frame effects (PFEs) in
the orbital dynamics, if the barycenter of the binary is moving relative to the
preferred frame with a velocity w. For small-eccentricity binaries, the effects
induced by alpha_1 and alpha_2 decouple, and can therefore be tested
independently. We use recent timing results of two compact pulsar-white dwarf
binaries with known 3D velocity, PSRs J1012+5307 and J1738+0333, to constrain
PFEs for strongly self-gravitating bodies. We derive a limit |alpha_2| < 1.8e-4
(95% CL), which is the most constraining limit for strongly self-gravitating
systems up to now. Concerning alpha_1, we propose a new, robust method to
constrain this parameter. Our most conservative result, alpha_1 =
-0.4^{+3.7}_{-3.1} e-5 (95% CL) from PSR J1738+0333, constitutes a significant
improvement compared to current most stringent limits obtained both in Solar
system and binary pulsar tests. We also derive corresponding limits for alpha_1
and alpha_2 for a preferred frame that is at rest with respect to our Galaxy,
and preferred frames that locally co-move with the rotation of our Galaxy.
(Abridged)Comment: 34 pages, 8 figures, 2 tables; accepted by Classical and Quantum
Gravit
Testing Relativistic Gravity with Radio Pulsars
Before the 1970s, precision tests for gravity theories were constrained to
the weak gravitational fields of the Solar system. Hence, only the weak-field
slow-motion aspects of relativistic celestial mechanics could be investigated.
Testing gravity beyond the first post-Newtonian contributions was for a long
time out of reach.
The discovery of the first binary pulsar by Russell Hulse and Joseph Taylor
in the summer of 1974 initiated a completely new field for testing the
relativistic dynamics of gravitationally interacting bodies. For the first time
the back reaction of gravitational wave emission on the binary motion could be
studied. Furthermore, the Hulse-Taylor pulsar provided the first test bed for
the orbital dynamics of strongly self-gravitating bodies.
To date there are a number of pulsars known, which can be utilized for
precision test of gravity. Depending on their orbital properties and their
companion, these pulsars provide tests for various different aspects of
relativistic dynamics. Besides tests of specific gravity theories, like general
relativity or scalar-tensor gravity, there are pulsars that allow for generic
constraints on potential deviations of gravity from general relativity in the
quasi-stationary strong-field and the radiative regime.
This article presents a brief overview of this modern field of relativistic
celestial mechanics, reviews some of the highlights of gravity tests with radio
pulsars, and discusses their implications for gravitational physics and
astronomy, including the upcoming gravitational wave astronomy.Comment: 80 pages, 22 figures, to appear in the Brumberg Festschrift, edited
by S. M. Kopeikein, and to be published by de Gruyter, Berlin, 2014.
Submitted to arXiv after final formatting for de Gruyte
Constraining nonperturbative strong-field effects in scalar-tensor gravity by combining pulsar timing and laser-interferometer gravitational-wave detectors
Pulsar timing and gravitational-wave (GW) detectors are superb laboratories
to study gravity theories in the strong-field regime. Here we combine those
tools to test the mono-scalar-tensor theory of Damour and Esposito-Far{\`e}se
(DEF), which predicts nonperturbative scalarization phenomena for neutron stars
(NSs). First, applying Markov-chain Monte Carlo techniques, we use the absence
of dipolar radiation in the pulsar-timing observations of five binary systems
composed of a NS and a white dwarf, and eleven equations of state (EOSs) for
NSs, to derive the most stringent constraints on the two free parameters of the
DEF scalar-tensor theory. Since the binary-pulsar bounds depend on the NS mass
and the EOS, we find that current pulsar-timing observations leave
scalarization windows, i.e., regions of parameter space where scalarization can
still be prominent. Then, we investigate if these scalarization windows could
be closed and if pulsar-timing constraints could be improved by
laser-interferometer GW detectors, when spontaneous (or dynamical)
scalarization sets in during the early (or late) stages of a binary NS (BNS)
evolution. For the early inspiral of a BNS carrying constant scalar charge, we
employ a Fisher matrix analysis to show that Advanced LIGO can improve
pulsar-timing constraints for some EOSs, and next-generation detectors, such as
the Cosmic Explorer and Einstein Telescope, will be able to improve those
bounds for all eleven EOSs. Using the late inspiral of a BNS, we estimate that
for some of the EOSs under consideration the onset of dynamical scalarization
can happen early enough to improve the constraints on the DEF parameters
obtained by combining the five binary pulsars. Thus, in the near future the
complementarity of pulsar timing and direct observations of GWs on the ground
will be extremely valuable in probing gravity theories in the strong-field
regime.Comment: 19 pages, 11 figures; accepted by Physical Review
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