5,516 research outputs found
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
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
A cosmic vector for dark energy
In this work we show that the presence of a vector field on cosmological
scales could explain the present phase of accelerated expansion of the
universe. The proposed theory contains no dimensional parameters nor potential
terms and does not require unnatural initial conditions in the early universe,
thus avoiding the so called cosmic coincidence problem. In addition, it fits
the data from high-redshift supernovae with excellent precision, making
definite predictions for cosmological parameters. Upcoming observations will be
able to clearly discriminate this model from standard cosmology with
cosmological constant.Comment: 5 pages, 3 figures, 1 table. New comments and references included.
Final version to appear in Phys. Rev.
Testing Alternative Theories of Gravity using LISA
We investigate the possible bounds which could be placed on alternative
theories of gravity using gravitational wave detection from inspiralling
compact binaries with the proposed LISA space interferometer. Specifically, we
estimate lower bounds on the coupling parameter \omega of scalar-tensor
theories of the Brans-Dicke type and on the Compton wavelength of the graviton
\lambda_g in hypothetical massive graviton theories. In these theories,
modifications of the gravitational radiation damping formulae or of the
propagation of the waves translate into a change in the phase evolution of the
observed gravitational waveform. We obtain the bounds through the technique of
matched filtering, employing the LISA Sensitivity Curve Generator (SCG),
available online. For a neutron star inspiralling into a 10^3 M_sun black hole
in the Virgo Cluster, in a two-year integration, we find a lower bound \omega >
3 * 10^5. For lower-mass black holes, the bound could be as large as 2 * 10^6.
The bound is independent of LISA arm length, but is inversely proportional to
the LISA position noise error. Lower bounds on the graviton Compton wavelength
ranging from 10^15 km to 5 * 10^16 km can be obtained from one-year
observations of massive binary black hole inspirals at cosmological distances
(3 Gpc), for masses ranging from 10^4 to 10^7 M_sun. For the highest-mass
systems (10^7 M_sun), the bound is proportional to (LISA arm length)^{1/2} and
to (LISA acceleration noise)^{-1/2}. For the others, the bound is independent
of these parameters because of the dominance of white-dwarf confusion noise in
the relevant part of the frequency spectrum. These bounds improve and extend
earlier work which used analytic formulae for the noise curves.Comment: 16 pages, 9 figures, submitted to Classical & Quantum Gravit
Gravity in the Randall-Sundrum Brane World
We discuss the weak gravitational field created by isolated matter sources in
the Randall-Sundrum brane-world. In the case of two branes of opposite tension,
linearized Brans-Dicke (BD) gravity is recovered on either wall, with different
BD parameters. On the wall with positive tension the BD parameter is larger
than 3000 provided that the separation between walls is larger than 4 times the
AdS radius. For the wall of negative tension, the BD parameter is always
negative but greater than -3/2. In either case, shadow matter from the other
wall gravitates upon us. For equal Newtonian mass, light deflection from shadow
matter is 25 % weaker than from ordinary matter. Hence, the effective mass of a
clustered object containing shadow dark matter would be underestimated if
naively measured through its lensing effect. For the case of a single wall of
positive tension, Einstein gravity is recovered on the wall to leading order,
and if the source is stationary the field stays localized near the wall. We
calculate the leading Kaluza-Klein corrections to the linearized gravitational
field of a non-relativistic spherical object and find that the metric is
different from the Schwarzschild solution at large distances. We believe that
our linearized solution corresponds to the field far from the horizon after
gravitational collapse of matter on the brane.Comment: 8 pages, 1 figure. Replaced with revised version to be published in
Phys. Rev. Lett. Some comments adde
Weak-field limit of Kaluza-Klein models with spherical compactification: experimental constraints
We investigate the classical gravitational tests for the six-dimensional
Kaluza-Klein model with spherical (of a radius ) compactification of the
internal space. The model contains also a bare multidimensional cosmological
constant . The matter, which corresponds to this ansatz, can be
simulated by a perfect fluid with the vacuum equation of state in the external
space and an arbitrary equation of state with the parameter in the
internal space. For example, and correspond to the
monopole two-forms and the Casimir effect, respectively. In the particular case
, the parameter is also absent: . In the
weak-field approximation, we perturb the background ansatz by a point-like
mass. We demonstrate that in the case the perturbed metric
coefficients have the Yukawa type corrections with respect to the usual
Newtonian gravitational potential. The inverse square law experiments restrict
the parameters of the model: $a/\sqrt{\omega_1}\lesssim 6\times10^{-3}\
{{cm}}\gamma\omega_1>0\omega_1=0\gamma=1/3$,
which strongly contradicts the observations.Comment: 8 pages, no figures, revised version, equations and references added,
accepted for publication in Phys. Rev. D. arXiv admin note: significant text
overlap with arXiv:1107.338
Gravitomagnetism in superconductors and compact stars
There are three experimentally observed effects in rotating superconductors
that are so far unexplained. Some authors have tried to interpret such a
phenomena as possible new gravitational properties of coherent quantum systems:
in particular, they suggest that the gravitomagnetic field of that kind of
matter may be many orders of magnitude stronger than the one expected in the
standard theory. Here I show that this interpretation would be in conflict with
the common belief that neutron stars have neutrons in superfluid state and
protons in superconductive one.Comment: 9 pages, no figur
Sense-making of consumer wellbeing in information technology-enabled services from a relational ontology position
Information technology (IT) built into products and services have become the key drivers for service innovation. How information technology-enabled services (ITESs) affect consumer wellbeing has increasingly become a concern to service scholars. In response to this, transformative service research (TSR) has emerged as a new stream in service research. This paper investigates consumer wellbeing derived from the consumption of ITESs in consumers’ daily lives. A mixed-method approach was employed in our study, including self-reflective reports, in-depth interviews and visual artistic methods. We demonstrated that a relational ontology, drawing on the ‘focal things’ concept (Borgmann, 1984) and sociomateriality (Orlikowski, 2009), could be used as a lens for us to understand consumer wellbeing in ITESs. We used four vignettes to demonstrate how relational ontology can enhance our understanding of consumer wellbeing in ITESs. Theoretically, this paper contributes to TSR by proposing and demonstrating the need to shift or at least extend the extant predominant technology ontology in marketing literature to make sense of consumer experiences and wellbeing in ITESs. In practice, this research encourages ITESs designers to emphasise the relational entanglement of technology with consumer routine practices in their service innovations for the purposes of consumer wellbeing
A new PPN parameter to test Chern-Simons gravity
We study Chern-Simons (CS) gravity in the parameterized post-Newtonian (PPN)
framework through a weak-field solution of the modified field equations. We
find that CS gravity possesses the same PPN parameters as general relativity,
except for the inclusion of a new term, proportional to the CS coupling and the
curl of the PPN vector potential. This new term leads to a modification of
frame dragging and gyroscopic precession and we provide an estimate of its
size. This correction might be used in experiments, such as Gravity Probe B, to
bound CS gravity and test string theory.Comment: 4 pages, replaced with version accepted for publication in Phys. Rev.
Letters (December, 2007
Description and simulation of an integrated power and attitude control system concept for space-vehicle application
An Integrated Power and Attitude Control System (IPACS) concept with potential application to a broad class of space missions is discussed. A description is given of the basic concept of combining the onboard energy storage and attitude control functions by storing energy in spinning flywheels which are used to provide control torques. A shuttle-launched Research and Applications Module (RAM) A303B solar-observatory mission having stringent pointing requirements (1.0 arc second) is selected to investigate possible interactions between energy storage and attitude control. A simulation of this spacecraft involving actual laboratory-model control-system hardware is presented. Simulation results are discussed which indicate that the IPACS concept, even in a failure-mode configuration, can readily meet the RAM A303B pointing requirements
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