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At least 1,585 records · Page 88Linked to original sources
PubMed2000

Pragmatic approach to gravitational radiation reaction in binary black holes

We study the relativistic orbit of binary black holes in systems with small mass ratio. The trajectory of the smaller object (another black hole or a neutron star), represented as a particle, is determined by the geodesic equation on the perturbed massive black hole spacetime. Here we study perturbations around a Schwarzschild black hole using Moncrief's gauge invariant formalism. We decompose the perturbations into l multipoles to show that all l-metric coefficients are C0 at the location of the particle. Summing over l, to reconstruct the full metric, gives a formally divergent result. We succeed in bringing this sum to a Riemann's zeta-function regularization scheme and numerically compute the first-order geodesics.

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PubMed2003

Chaos in a relativistic 3-body self-gravitating system.

We consider the 3-body problem in relativistic lineal [i.e., (1+1)-dimensional] gravity and obtain an exact expression for its Hamiltonian and equations of motion. While general-relativistic effects yield more tightly bound orbits of higher frequency compared to their nonrelativistic counterparts, as energy increases we find in the equal-mass case no evidence for either global chaos or a breakdown from regular to chaotic motion, despite the high degree of nonlinearity in the system. We find numerical evidence for mild chaos and a countably infinite class of nonchaotic orbits, yielding a fractal structure in the outer regions of the Poincaré plot.

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PubMed2004

Structures of a continuously fed two-dimensional viscous film under a destabilizing gravitational force.

We study the gravity induced instability of a liquid film formed below a plane grid which is used as a porous media in an original hydrodynamic experiment. The film is continuously supplied with a controlled flow rate. We give through a phase diagram the full spectrum of the different flow regimes and we investigate the dynamics of the observed structures. True secondary instabilities of a 2D periodic pattern are described. The control parameters are the flow rate and the viscosity.

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PubMed2004

Measurement of the solar gravitational deflection of radio waves using geodetic very-long-baseline interferometry data, 1979-1999.

We used very-long-baseline interferometry (VLBI) to measure the deflection by the Sun of radio waves emanating from distant compact radio sources. This bending is characterized in the parametrized post-Newtonian formalism by gamma, which is unity in general relativity. Using a large geodetic VLBI data set, we obtained gamma=0.9998(3)+/-0.0004(5) (estimated standard error). We found no systematic biases from our analysis of subgroups of data.

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PubMed2004

Experimental demonstration of a classical analog to quantum noise cancellation for use in gravitational wave detection.

We present results that are a classical analog to quantum noise cancellation. It is possible to breach the standard quantum limit in an interferometer by the use of squeezing to correlate orthogonal quadratures of quantum noise, causing their effects on the resulting sensitivity to cancel. A laser beam incident on a Fabry-Perot cavity was imprinted with classical, correlated noise in the same quadratures that cause shot noise and radiation pressure noise. Couplings between these quadratures due to a movable mirror, sensitive to radiation pressure, cause the excess classical noise to cancel. This cancellation was shown to improve the signal to noise ratio of an injected signal by approximately a factor of 10.

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PubMed2004

Measurement of gravitational spin-orbit coupling in a binary-pulsar system.

In relativistic gravity, a spinning pulsar will precess as it orbits a compact companion star. We have measured the effect of such precession on the average shape and polarization of the radiation from PSR B1534+12. We have also detected, with limited precision, special-relativistic aberration of the revolving pulsar beam due to orbital motion. Our observations fix the system geometry, including the misalignment between the spin and orbital angular momenta, and yield a measurement of the precession time scale consistent with the predictions of general relativity.

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PubMed2005

Parametric instabilities and their control in advanced interferometer gravitational-wave detectors.

A detailed simulation of Advanced LIGO test mass optical cavities shows that parametric instabilities will excite 7 acoustic modes in each fused silica test mass, with parametric gain R up to 7 and only 1 acoustic mode with R approximately 2 for alternative sapphire test masses. Fine-tuning of the test mass radii of curvature causes the instabilities to sweep through various modes with R as high as approximately 2000. Sapphire test mass cavities can be tuned to completely eliminate instabilities using thermal g-factor tuning. In the case of fused silica test mass, instabilities can be minimized but not eliminated.

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PubMed2005

Relationship between Hawking radiation and gravitational anomalies.

We show that in order to avoid a breakdown of general covariance at the quantum level the total flux in each outgoing partial wave of a quantum field in a black hole background must be equal to that of a (1+1)-dimensional blackbody at the Hawking temperature.

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PubMed2006

Hawking radiation from charged black holes via gauge and gravitational anomalies.

Extending the method of Robinson and Wolczek, we show that in order to avoid a breakdown of general covariance and gauge invariance at the quantum level the total flux of charge and energy in each outgoing partial wave of a charged quantum field in a Reissner-Nordström black hole background must be equal to that of a (1 + 1)-dimensional blackbody at the Hawking temperature with the appropriate chemical potential.

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PubMed2006

Codimension-two critical behavior in vacuum gravitational collapse.

We consider the critical behavior at the threshold of black-hole formation for the five-dimensional vacuum Einstein equations satisfying the cohomogeneity-two triaxial Bianchi type-IX ansatz. Exploiting a discrete symmetry present in this model we predict the existence of a codimension-two attractor. This prediction is confirmed numerically and the codimension-two attractor is identified as a discretely self-similar solution with two unstable modes.

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PubMed1967

Gravitational compensation and the phototropic response of oat coleoptiles.

Avena seedlings were germinated and grown while continuously rotated on the horizontal axis of a clinostat. The coleoptiles of these gravity-compensated plants were phototropically more responsive than those of plants rotated on a vertical axis. When the plants were compensated after unilateral irradiation, phototropic curvature of the shoot progressed for the next 6 hours, with the rate of curving decreasing about 3 hours after irradiation. The decrease in rate was less in the plants gravity-compensated before irradiation than in those vertically rotated. In the period 70 to 76 hours after planting, the growth rate of the compensated coleoptiles was significantly less than that of the vertically rotated seedlings. The greater phototropic curvature, the decreased growth rate, and the slower rate of straightening of the curved, compensated shoot can be correlated with several consequences of compensation: an increase in sensitivity to auxin, a lowering of auxin content in the coleoptile tip, and possibly, from an interaction between compensation and phototropic stimulation, an enhanced difference in auxin transport between the illuminated and shaded halves of the unilaterally irradiated shoot.The phototropic response of the vertically rotated seedling was significantly different from that of the vertical stationary, indicating the importance of vertically rotated controls in clinostat experiments.

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PubMed1998

Negative energy modes and gravitational instability of interpenetrating fluids.

We study the longitudinal instabilities of two interpenetrating fluids interacting only through gravity. When one of the constituents is of relatively low density, it is possible to have a band of unstable wavenumbers well separated from those involved in the usual Jeans instability. If the initial streaming is large enough, and there is no linear instability, the indefinite sign of the free energy has the possible consequence of explosive interactions between positive and negative energy modes in the nonlinear regime. The effect of dissipation on the negative energy modes is also examined.

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PubMed2001

Nonlinear gravitational growth inside and outside the standard cosmology.

We reconsider the problem of nonlinear structure formation inside and outside General Relativity (GR), both in the weakly and strongly nonlinear regime. We show how these regimes can be explored observationally through clustering of high-order cumulants and through the epoch of formation, abundance and clustering of collapse structures, using Press and Schechter (1974, Astrophys. J. 187: 425-438) formalism and its extensions.

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PubMed1998

Neptune's partial rings: action of galatea on self-gravitating arc particles

Numerical simulations of Neptune's arcs show that self-gravity between macroscopic arc particles can prevent interparticle impacts and thereby stabilize their resonant confinement by Galatea, a satellite of Neptune. Stable subkilometer arc particles provide a source for replenishing the observed dust and explain the clumpy substructure seen in arcs. A few confining kilometer-sized particles between the major arc components can account for the observed arc widths spanning several resonance sites. The modeled distribution of dust is consistent with observations and helps to explain how embedded satellites may affect the structure and evolution of planetary ring systems.

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