4,996 research outputs found
Revisiting the double-binary-pulsar probe of non-dynamical Chern-Simons gravity
One of the popular modifications to the theory of general relativity is
non-dynamical Chern-Simons (CS) gravity, in which the metric is coupled to an
externally prescribed scalar field. Setting accurate constraints to the
parameters of the theory is important owing to their implications for the
scalar field and/or the underlying fundamental theory. The current best
constraints rely on measurements of the periastron precession rate in the
double-binary-pulsar system and place a very tight bound on the characteristic
CS lengthscale k_cs^{-1} <~ 3*10^{-9} km. This paper considers several effects
that were not accounted for when deriving this bound and lead to a substantial
suppression of the predicted rate of periastron precession. It is shown, in
particular, that the point mass approximation for extended test bodies does not
apply in this case. The constraint to the characteristic CS lengthscale is
revised to k_cs^{-1} <~ 0.4 km, eight orders of magnitude weaker than what was
previously found.Comment: 12 pages, 4 figures, to be submitted to PRD. Comments are welcom
Post-Newtonian gravitational radiation and equations of motion via direct integration of the relaxed Einstein equations. V. Evidence for the strong equivalence principle to second post-Newtonian order
Using post-Newtonian equations of motion for fluid bodies valid to the second
post-Newtonian order, we derive the equations of motion for binary systems with
finite-sized, non-spinning but arbitrarily shaped bodies. In particular we
study the contributions of the internal structure of the bodies (such as
self-gravity) that would diverge if the size of the bodies were to shrink to
zero. Using a set of virial relations accurate to the first post-Newtonian
order that reflect the stationarity of each body, and redefining the masses to
include 1PN and 2PN self-gravity terms, we demonstrate the complete
cancellation of a class of potentially divergent, structure-dependent terms
that scale as s^{-1} and s^{-5/2}, where s is the characteristic size of the
bodies. This is further evidence of the Strong Equivalence Principle, and
supports the use of post-Newtonian approximations to derive equations of motion
for strong-field bodies such as neutron stars and black holes. This extends
earlier work done by Kopeikin.Comment: 14 pages, submitted to Phys. Rev. D; small changes to coincide with
published versio
Testing Scalar-Tensor Gravity Using Space Gravitational-Wave Interferometers
We calculate the bounds which could be placed on scalar-tensor theories of
gravity of the Jordan, Fierz, Brans and Dicke type by measurements of
gravitational waveforms from neutron stars (NS) spiralling into massive black
holes (MBH) using LISA, the proposed space laser interferometric observatory.
Such observations may yield significantly more stringent bounds on the
Brans-Dicke coupling parameter \omega than are achievable from solar system or
binary pulsar measurements. For NS-MBH inspirals, dipole gravitational
radiation modifies the inspiral and generates an additional contribution to the
phase evolution of the emitted gravitational waveform. Bounds on \omega can
therefore be found by using the technique of matched filtering. We compute the
Fisher information matrix for a waveform accurate to second post-Newtonian
order, including the effect of dipole radiation, filtered using a currently
modeled noise curve for LISA, and determine the bounds on \omega for several
different NS-MBH canonical systems. For example, observations of a 1.4 solar
mass NS inspiralling to a 1000 solar mass MBH with a signal-to-noise ratio of
10 could yield a bound of \omega > 240,000, substantially greater than the
current experimental bound of \omega > 3000.Comment: 18 pages, 4 figures, 1 table; to be submitted to Phys. Rev.
Cerenkov's Effect and Neutrino Oscillations in Loop Quantum Gravity
Bounds on the scale parameter {\cal L} arising in loop quantum gravity theory
are derived in the framework of Cerenkov's effect and neutrino oscillations.
Assuming that {\cal L} is an universal constant, we infer {\cal L}>
10^{-18}eV^{-1}, a bound compatible with ones inferred in different physical
context.Comment: 6 pages, no figures, in print on MPL
Limit to General Relativity in f(R) theories of gravity
We discuss two aspects of f(R) theories of gravity in metric formalism. We
first study the reasons to introduce a scalar-tensor representation for these
theories and the behavior of this representation in the limit to General
Relativity, f(R)--> R. We find that the scalar-tensor representation is well
behaved even in this limit. Then we work out the exact equations for
spherically symmetric sources using the original f(R) representation, solve the
linearized equations, and compare our results with recent calculations of the
literature. We observe that the linearized solutions are strongly affected by
the cosmic evolution, which makes very unlikely that the cosmic speedup be due
to f(R) models with correcting terms relevant at low curvatures.Comment: 8 pages; small changes to match published version (some comments,
references added, title corrected); to appear in Phys.Rev.
Solar irradiance models and measurements: a comparison in the 220 nm to 240 nm wavelength band
Solar irradiance models that assume solar irradiance variations to be due to
changes in the solar surface magnetic flux have been successfully used to
reconstruct total solar irradiance on rotational as well as cyclical and
secular time scales. Modelling spectral solar irradiance is not yet as
advanced, and also suffers from a lack of comparison data, in particular on
solar-cycle time scales. Here we compare solar irradiance in the 220 nm to 240
nm band as modelled with SATIRE-S and measured by different instruments on the
UARS and SORCE satellites.
We find good agreement between the model and measurements on rotational time
scales. The long-term trends, however, show significant differences. Both SORCE
instruments, in particular, show a much steeper gradient over the decaying part
of cycle 23 than the modelled irradiance or that measured by UARS/SUSIM.Comment: 8 pages, 2 figures, conference proceedings to appear in Surveys in
Geophysic
Probing the Brans-Dicke Gravitational Field by Cerenkov Radiation
The possibility that a charged particle propagating in a gravitational field
described by Brans-Dicke theory of gravity could emit Cerenkov radiation is
explored. This process is kinematically allowed depending on parameters
occurring in the theory. The Cerenkov effect disappears as the BD parameter
omega tends to inftinity, i.e. in the limit in which the Einstein theory is
recovered, giving a signature to probe the validity of the Brans-Dicke theory.Comment: 8 pages, no figure
K-Chameleon and the Coincidence Problem
In this paper we present a hybrid model of k-essence and chameleon, named as
k-chameleon. In this model, due to the chameleon mechanism, the directly strong
coupling between the k-chameleon field and matters (cold dark matters and
baryons) is allowed. In the radiation dominated epoch, the interaction between
the k-chameleon field and background matters can be neglected, the behavior of
the k-chameleon therefore is the same as that of the ordinary k-essence. After
the onset of matter domination, the strong coupling between the k-chameleon and
matters dramatically changes the result of the ordinary k-essence. We find that
during the matter-dominated epoch, only two kinds of attractors may exist: one
is the familiar {\bf K} attractor and the other is a completely {\em new},
dubbed {\bf C} attractor. Once the universe is attracted into the {\bf C}
attractor, the fraction energy densities of the k-chameleon and
dust matter are fixed and comparable, and the universe will undergo
a power-law accelerated expansion. One can adjust the model so that the {\bf K}
attractor do not appear. Thus, the k-chameleon model provides a natural
solution to the cosmological coincidence problem.Comment: Revtex, 17 pages; v2: 18 pages, two figures, more comments and
references added, to appear in PRD, v3: published versio
Probing Strong-Field Scalar-Tensor Gravity with Gravitational Wave Asteroseismology
We present an alternative way of tracing the existence of a scalar field
based on the analysis of the gravitational wave spectrum of a vibrating neutron
star. Scalar-tensor theories in strong-field gravity can potentially introduce
much greater differences in the parameters of a neutron star than the
uncertainties introduced by the various equations of state. The detection of
gravitational waves from neutron stars can set constraints on the existence and
the strength of scalar fields. We show that the oscillation spectrum is
dramatically affected by the presence of a scalar field, and can provide unique
confirmation of its existence.Comment: 14 pages, 7 figure
Post-Newtonian gravitational radiation and equations of motion via direct integration of the relaxed Einstein equations. IV. Radiation reaction for binary systems with spin-spin coupling
Using post-Newtonian equations of motion for fluid bodies that include
radiation-reaction terms at 2.5 and 3.5 post-Newtonian (PN) order O[(v/c)^5]
and O[(v/c)^7] beyond Newtonian order), we derive the equations of motion for
binary systems with spinning bodies, including spin-spin effects. In particular
we determine the effects of radiation-reaction coupled to spin-spin effects on
the two-body equations of motion, and on the evolution of the spins. We find
that radiation damping causes a 3.5PN order, spin-spin induced precession of
the individual spins. This contrasts with the case of spin-orbit coupling,
where there is no effect on the spins at 3.5PN order. Employing the equations
of motion and of spin precession, we verify that the loss of total energy and
total angular momentum induced by spin-spin effects precisely balances the
radiative flux of those quantities calculated by Kidder et al.Comment: 10 pages, coincides with published versio
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