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At least 253 records · Page 14Linked to original sources
PubMed2003

Gravitational self-force on a particle orbiting a Kerr black hole.

We present a practical method for calculating the gravitational self-force, as well as the electromagnetic and scalar self-forces, for a particle in a generic orbit around a Kerr black hole. In particular, we provide the values of all the regularization parameters needed for implementing the (previously introduced) mode-sum regularization method. We also address the gauge-regularization problem, as well as a few other issues involved in the calculation of gravitational radiation reaction in Kerr spacetime.

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PubMed2003

Long-term evolution of stellar self-gravitating systems away from thermal equilibrium: connection with nonextensive statistics.

With particular attention to the recently postulated introduction of a nonextensive generalization of Boltzmann-Gibbs statistics, we study the long-term stellar dynamical evolution of self-gravitating systems on time scales much longer than the two-body relaxation time. In a self-gravitating N-body system confined in an adiabatic wall, we show that the quasiequilibrium sequence arising from the Tsallis entropy, so-called stellar polytropes, plays an important role in characterizing the transient states away from the Boltzmann-Gibbs equilibrium state.

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PubMed2003

Model for gravitational interaction between dark matter and baryons.

We propose a phenomenological model where the gravitational interaction between dark matter and baryons is suppressed on small, subgalactic scales. We describe the gravitational force by adding a Yukawa contribution to the standard Newtonian potential and show that this interaction scheme is effectively suggested by the available observations of the inner rotation curves of small mass galaxies. Besides helping in interpreting the cuspy profile of dark matter halos observed in N-body simulations, this potential regulates the quantity of baryons within halos of different masses.

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PubMed2004

Regularization of the linearized gravitational self-force for branes.

We discuss the linearized, gravitational self-interaction of a brane of arbitrary codimension in a spacetime of arbitrary dimension. We find that in the codimension two case the gravitational self-force is exactly zero for a Nambu-Goto equation of state, generalizing a previous result for a string in four dimensions. For the case of a 3-brane, this picks out the case of a six-dimensional brane-world model as having special properties that we discuss. In particular, we see that bare tension on the brane has no effect locally, suppressing the cosmological constant problem.

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PubMed2004

Gravitational radiation from inspiralling compact binaries completed at the third post-Newtonian order.

The gravitational radiation from point particle binaries is computed at the third post-Newtonian (3PN) approximation of general relativity. Three previously introduced ambiguity parameters, coming from the Hadamard self-field regularization of the 3PN source-type mass quadrupole moment, are consistently determined by means of dimensional regularization, and proved to have the values xi=-9871/9240, kappa=0, and zeta=-7/33. These results complete the derivation of the general relativistic prediction for compact binary inspiral up to 3.5PN order, and should be of use for searching and deciphering the signals in the current network of gravitational wave detectors.

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PubMed2004

Is the quantum Hall effect influenced by the gravitational field?

Most of the experiments on the quantum Hall effect (QHE) were made at approximately the same height above sea level. A future international comparison will determine whether the gravitational field g(x) influences the QHE. In the realm of (1+2)-dimensional phenomenological macroscopic electrodynamics, the Ohm-Hall law is metric independent ("topological"). This suggests that it does not couple to g(x). We corroborate this result by a microscopic calculation of the Hall conductance in the presence of a post-Newtonian gravitational field.

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PubMed2005

Limits on gravitational-wave emission from selected pulsars using LIGO data.

We place direct upper limits on the amplitude of gravitational waves from 28 isolated radio pulsars by a coherent multidetector analysis of the data collected during the second science run of the LIGO interferometric detectors. These are the first direct upper limits for 26 of the 28 pulsars. We use coordinated radio observations for the first time to build radio-guided phase templates for the expected gravitational-wave signals. The unprecedented sensitivity of the detectors allows us to set strain upper limits as low as a few times 10(-24). These strain limits translate into limits on the equatorial ellipticities of the pulsars, which are smaller than 10(-5) for the four closest pulsars.

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PubMed2005

Constraining nuclear equations of state using gravitational waves from hypermassive neutron stars.

Latest general relativistic simulations for the merger of binary neutron stars with realistic equations of states (EOSs) show that a hypermassive neutron star of an ellipsoidal figure is formed after the merger if the total mass is smaller than a threshold value which depends on the EOSs. The effective amplitude of quasiperiodic gravitational waves from such hypermassive neutron stars is approximately 6-7 x 10(-21) at a distance of 50 Mpc, which may be large enough for detection by advanced laser interferometric gravitational wave detectors although the frequency is high, approximately 3 kHz. We point out that the detection of such signals may lead to constraining the EOSs for neutron stars.

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PubMed2005

Critical behavior in vacuum gravitational collapse in 4 + 1 dimensions.

We show that the (4 + 1)-dimensional vacuum Einstein equations admit gravitational waves with radial symmetry. The dynamical degrees of freedom correspond to deformations of the three-sphere orthogonal to the (t,r) plane. Gravitational collapse of such waves is studied numerically and shown to exhibit discretely self-similar type II critical behavior at the threshold of black hole formation.

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PubMed2005

Radio-quiet neutron star 1E 1207.4-5209: a possible strong gravitational-wave source.

There are four puzzles on 1E 1207.4-5209: (1) The characteristic age of the pulsar is much higher than the estimated age of the supernova remnant; (2) the magnetic field inferred from spin down is significantly different from the value obtained from the cyclotron absorption lines; (3) the spinning down of the pulsar is nonmonotonic; (4) the magnitude of the frequency's first derivative varies significantly and its sign is also variable. The third puzzle can be explained by a wide binary system, with orbital period from 0.2 to 6 yr. This Letter proposes that all four puzzles can be explained naturally by an ultracompact binary with an orbital period between 0.5 and 3.3 min. With the shortest orbital period and a close distance of 2 kpc, the characteristic amplitude of gravitational waves is h approximately 3 x 10(-21). It would be an excellent source for gravitational-wave detectors such as the Laser Interferometer Space Antenna.

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PubMed2006

Gravitational-wave extraction from an inspiraling configuration of merging black holes.

We present new ideas for evolving black holes through a computational grid without excision, which enable accurate and stable evolutions of binary black hole systems with the accurate determination of gravitational waveforms directly from the wave zone region. Rather than excising the black hole interiors, our approach follows the "puncture" treatment of black holes, but utilizing a new gauge condition which allows the black holes to move successfully through the computational domain. We apply these techniques to an inspiraling binary, modeling the radiation generated during the final plunge and ringdown. We demonstrate convergence of the waveforms and good conservation of mass-energy, with just over 3% of the system's mass converted to gravitational radiation.

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PubMed2006

Gravitational instantons from gauge theory.

A gauge theory can be formulated on a noncommutative (NC) spacetime. This NC gauge theory has anequivalent dual description through the so-called Seiberg-Witten (SW) map in terms of an ordinary gauge theory on a commutative spacetime. We show that all NC U(1) instantons of Nekrasov-Schwarz type are mapped to asymptotically locally Euclidean (ALE) gravitational instantons by the exact SW map and thatthe NC gauge theory of U(1) instantons is equivalent to the theory of hyper-Kähler geometries. It implies the remarkable consequence that ALE gravitational instantons can emerge from local condensates of purely NC photons.

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PubMed2006

Prospects for direct detection of the circular polarization of the gravitational-wave background.

We discuss the prospects for directly detecting a circular polarization signal of the gravitational-wave background. We find it is generally difficult to probe the monopole mode of the signal due to the broad directivity of the gravitational-wave detectors. But the dipole (l=1) and octupole (l=3) modes of the signal can be measured in a simple manner by combining outputs of two unaligned detectors, and we can dig them deeply under confusion and detector noises. Around f approximately 0.1 mHz the Laser Interferometer Space Antenna will provide ideal data streams to detect these patterns whose magnitudes are as small as approximately 1 percent of the detector noise level in terms of the nondimensional energy density OmegaGW(f).

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PubMed2006

Interferometers for displacement-noise-free gravitational-wave detection.

We propose a class of displacement- and laser-noise-free gravitational-wave-interferometer configurations, which does not sense nongeodesic mirror motion and laser noise, but provides a nonvanishing gravitational-wave signal. Our interferometers consist of four mirrors and two beam splitters, which form four Mach-Zehnder interferometers. By contrast to previous works, no composite mirrors with multiple reflective surfaces are required. Each mirror in our configuration is sensed redundantly, by at least two pairs of incident and reflected beams. Displacement- and laser-noise-free detection is achieved when output signals from these four interferometers are combined appropriately. Our 3-dimensional interferometer configuration has a low-frequency response proportional to f2, which is better than the f3 achievable by previous 2-dimensional configurations.

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PubMed1989

Changes in endothelial cell mass, luminal volume and capillary number in the gravitational syndrome.

The histopathological changes in the skin of 31 patients with the gravitational syndrome and 10 normal control subjects were studied. To quantify the clinical severity of the syndrome, a new index of severity was used. Histometric evaluation of the cutaneous vasculature was performed on biopsies taken from the edge of the ulcers in the 31 patients and from the medical aspect of the lower legs in control subjects. Endothelial cell mass did not correlate with the clinical index of disease severity, suggesting that angiogenesis did not necessarily signal healing. The mean luminal volume appeared to be of greater prognostic value as there was a significant negative correlation of this parameter with the index of clinical severity. The degree of epidermal hyperplasia was found to be related to both the amount of inflammatory cell infiltrate present and the endothelial cell mass per unit tissue volume. The results also showed that the clinical index of disease severity was positively correlated to the mean epidermal thickness. The quantitative evaluation of biopsy material can be used to provide prognostic information in the gravitational syndrome.

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PubMed2001

A new angle on gravitational clustering.

A new approach to gravitational instability in large-scale structure is described, where the equations of motion are written and solved as in field theory in terms of Feynman diagrams. The basic objects of interest are the propagator (which propagates solutions forward in time), the vertex (which describes nonlinear interactions between waves) and a source with prescribed statistics which describes the effect of initial conditions. We show that loop corrections renormalize these quantities, and discuss applications of this formalism to a better understanding of gravitational instability and to improving nonlinear perturbation theory in the transition to the nonlinear regime. We also consider the role of vorticity creation due to shell-crossing and show using N-body simulations for which at small (virialized) scales the velocity field reaches equipartition, that is, the vorticity power spectrum is about twice the divergence power spectrum.

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PubMed2001

Collisions and gravitational reaccumulation: forming asteroid families and satellites.

Numerical simulations of the collisional disruption of large asteroids show that although the parent body is totally shattered, subsequent gravitational reaccumulation leads to the formation of an entire family of large and small objects with dynamical properties similar to those of the parent body. Simulations were performed in two different collisional regimes representative of asteroid families such as Eunomia and Koronis. Our results indicate that all large family members must be made of gravitationally reaccumulated fragments; that the post-collision member size distribution and the orbital dispersion are steeper and smaller, respectively, than for the evolved families observed today; and that satellites form frequently around family members.

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PubMed1997

The Origin of Gravitational Lensing: A Postscript to Einstein's 1936 Science Paper

Gravitational lensing, now taken as an important astrophysical consequence of the general theory of relativity, was found even before this theory was formulated but was discarded as a speculative idea without any chance of empirical confirmation. Reconstruction of some of Einstein's research notes dating back to 1912 reveals that he explored the possibility of gravitational lensing 3 years before completing his general theory of relativity. On the basis of preliminary insights into this theory, Einstein had already derived the basic features of the lensing effect. When he finally published the very same results 24 years later, it was only in response to prodding by an amateur scientist.

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