7,420 research outputs found

### Finally, results from Gravity Probe-B

Nearly fifty years after its inception, the Gravity Probe B satellite mission
delivers the first measurements of how a spinning gyroscope precesses in the
gravitational warping of spacetime.Comment: A Viewpoint article, published in Physics 4, 43 (2011), available at
http://physics.aps.org/articles/v4/43 Submitted to the arXiv by permission of
the American Physical Societ

### Constraining Lorentz-violating, Modified Dispersion Relations with Gravitational Waves

Modified gravity theories generically predict a violation of Lorentz
invariance, which may lead to a modified dispersion relation for propagating
modes of gravitational waves. We construct a parametrized dispersion relation
that can reproduce a range of known Lorentz-violating predictions and
investigate their impact on the propagation of gravitational waves. A modified
dispersion relation forces different wavelengths of the gravitational wave
train to travel at slightly different velocities, leading to a modified phase
evolution observed at a gravitational-wave detector. We show how such
corrections map to the waveform observable and to the parametrized
post-Einsteinian framework, proposed to model a range of deviations from
General Relativity. Given a gravitational-wave detection, the lack of evidence
for such corrections could then be used to place a constraint on Lorentz
violation. The constraints we obtain are tightest for dispersion relations that
scale with small power of the graviton's momentum and deteriorate for a steeper
scaling.Comment: 11 pages, 3 figures, 2 tables: title changed slightly, published
versio

### 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

### Exploring the bulk of tidal charged micro-black holes

We study the bulk corresponding to tidal charged brane-world black holes. We
employ a propagating algorithm which makes use of the three-dimensional
multipole expansion and analytically yields the metric elements as functions of
the five-dimensional coordinates and of the ADM mass, tidal charge and brane
tension. Since the projected brane equations cannot determine how the charge
depends on the mass, our main purpose is to select the combinations of these
parameters for which black holes of microscopic size possess a regular bulk.
Our results could in particular be relevant for a better understanding of
TeV-scale black holes.Comment: Latex, 15 pages, 1 table, 5 figures; Section 3.2 extended, typos
corrected, no change in conclusion

### Capture of non-relativistic particles in eccentric orbits by a Kerr black hole

We obtain approximate analytic expressions for the critical value of the
total angular momentum of a non-relativistic test particle moving in the Kerr
geometry, such that it will be captured by the black hole. The expressions
apply to arbitrary orbital inclinations, and are accurate over the entire range
of angular momentum for the Kerr black hole. The expressions can be easily
implemented in N-body simulations of the evolution of star clusters around
massive galactic black holes, where such captures play an important role.Comment: 8 pages, 1 figure, published versio

### Singularity problem in f(R) model with non-minimal coupling

We consider the non-minimal coupling between matter and the geometry in the
f(R) theory. In the new theory which we established, a new scalar $\psi$ has
been defined and we give it a certain stability condition. We intend to take a
closer look at the dark energy oscillating behavior in the de-Sitter universe
and the matter era, from which we derive the oscillating frequency, and the
oscillating condition. More importantly, we present the condition of coupling
form that the singularity can be solved. We discuss several specific coupling
forms, and find logarithmic coupling with an oscillating period $\Delta
T\sim\Delta z$ in the matter era $z>4$, can improve singularity in the early
universe. The result of numerical calculation verifies our theoretic
calculation about the oscillating frequency. Considering two toy models, we
find the cosmic evolution in the coupling model is nearly the same as that in
the normal f(R) theory when $lna>4$. We also discuss the local tests of the
non-minimal coupling f(R) model, and show the constraint on the coupling form.Comment: 13 pages, 4 figure

### The force of gravity in Schwarzschild and Gullstrand-Painlev\'e coordinates

We derive the exact equations of motion (in Newtonian, F=ma, form) for test
masses in Schwarzschild and Gullstrand-Painlev\'e coordinates. These equations
of motion are simpler than the usual geodesic equations obtained from
Christoffel tensors in that the affine parameter is eliminated. The various
terms can be compared against tests of gravity. In force form, gravity can be
interpreted as resulting from a flux of superluminal particles (gravitons). We
show that the first order relativistic correction to Newton's gravity results
from a two graviton interaction.Comment: 6 pages, Honorable mention in 2009 Gravity Essay Competition,
submitted IJMPD, added reference

### Figures of merit and constraints from testing General Relativity using the latest cosmological data sets including refined COSMOS 3D weak lensing

We use cosmological constraints from current data sets and a figure of merit
(FoM) approach to probe any deviations from general relativity (GR) at
cosmological scales. The FoM approach is used to study the constraining power
of various combinations of data sets on modified gravity (MG) parameters. We
use recently refined HST-COSMOS weak-lensing tomography data, ISW-galaxy cross
correlations from 2MASS and SDSS LRG surveys, matter power spectrum from
SDSS-DR7 (MPK), WMAP7 temperature and polarization spectra, BAO from 2DF and
SDSS-DR7, and Union2 compilation of supernovae, in addition to other bounds
from H_0 measurements and BBN. We use 3 parametrizations of MG parameters that
enter the perturbed field equations. In order to allow for variations with
redshift and scale, the first 2 parametrizations use recently suggested
functional forms while the third is based on binning methods. Using the first
parametrization, we find that CMB + ISW + WL provides the strongest constraints
on MG parameters followed by CMB+WL or CMB+MPK+ISW. Using the second
parametrization or binning methods, CMB+MPK+ISW consistently provides some of
the strongest constraints. This shows that the constraints are parametrization
dependent. We find that adding up current data sets does not improve
consistently uncertainties on MG parameters due to tensions between best-fit MG
parameters preferred by different data sets. Furthermore, some functional forms
imposed by the parametrizations can lead to an exacerbation of these tensions.
Next, unlike some studies that used the CFHTLS lensing data, we do not find any
deviation from GR using the refined HST-COSMOS data, confirming previous claims
in those studies that their result may have been due to some systematic effect.
Finally, we find in all cases that the values corresponding to GR are within
the 95% confidence level contours for all data set combinations. (abridged)Comment: 18 pages, 6 figures, matches version published in PR

### General Covariance in Quantum Gravity at a Lifshitz Point

In the minimal formulation of gravity with Lifshitz-type anisotropic scaling,
the gauge symmetries of the system are foliation-preserving diffeomorphisms of
spacetime. Consequently, compared to general relativity, the spectrum contains
an extra scalar graviton polarization. Here we investigate the possibility of
extending the gauge group by a local U(1) symmetry to "nonrelativistic general
covariance." This extended gauge symmetry eliminates the scalar graviton, and
forces the coupling constant $\lambda$ in the kinetic term of the minimal
formulation to take its relativistic value, $\lambda=1$. The resulting theory
exhibits anisotropic scaling at short distances, and reproduces many features
of general relativity at long distances.Comment: 41 pages; v2: small clarifications, references adde

### Strong field effects on binary systems in Einstein-aether theory

"Einstein-aether" theory is a generally covariant theory of gravity
containing a dynamical preferred frame. This article continues an examination
of effects on the motion of binary pulsar systems in this theory, by
incorporating effects due to strong fields in the vicinity of neutron star
pulsars. These effects are included through an effective approach, by treating
the compact bodies as point particles with nonstandard, velocity dependent
interactions parametrized by dimensionless "sensitivities". Effective
post-Newtonian equations of motion for the bodies and the radiation damping
rate are determined. More work is needed to calculate values of the
sensitivities for a given fluid source, so precise constraints on the theory's
coupling constants cannot yet be stated. It is shown, however, that strong
field effects will be negligible given current observational uncertainties if
the dimensionless couplings are less than roughly 0.01 and two conditions that
match the PPN parameters to those of pure general relativity are imposed. In
this case, weak field results suffice and imply one further condition on the
couplings. Thus, there exists a one-parameter family of Einstein-aether
theories with "small-enough" couplings that passes all current observational
tests. No conclusion can yet be reached for large couplings.Comment: 23 pages, 1 figure; v2: fixed error in Eqn. (70) and resulting bounds
on c'

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