arXiv subjects

Jorge Pullin

Publications and source records attributed to Jorge Pullin.

At least 163 records · Page 9Linked to original sources

arXiv1998

Nonstandard optics from quantum spacetime

We study light propagation in the picture of semi-classical space-time that emerges in canonical quantum gravity in the loop representation. In such picture, where space-time exhibits a polymer-like structure at microscales, it is natural to expect departures from the perfect non-dispersiveness of ordinary vacuum. We evaluate these departures, computing the modifications to Maxwell's equations due to quantum gravity, and showing that under certain circumstances, non-vanishing corrections appear that depend on the helicity of propagating waves. These effects could lead to observable cosmological predictions of the discrete nature of quantum spacetime. In particular, recent observations of non-dispersiveness in the spectra of gamma-ray bursts at various energies could be used to constrain the type of semi-classical state that describes the universe.

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arXiv1998

Matters of Gravity, The newsletter of the APS Topical Group on Gravitation

Contents: Editorial News: - Topical group news, by Jim Isenberg - Summer school in gravitational physics opportunity, by Jim Hartle - Bogart, Bergman and (Al)bert, by Clifford Will and Robert Riemer - New data-analysis subgroups of the LSC, by Eanna Flanagan - Marcel Bardon, a man of vision, by Richard Isaacson Research Briefs: - Status of the GEO600 project, by Harold Lueck - A nonperturbative formulation of string theory?, by Gary Horowitz - TAMA project update, by Seiji Kawamura - Neohistorical approaches to quantum gravity, by Lee Smolin - LIGO project update, by David Shoemaker - Gravitational waves from neutron stars, by Eanna Flanagan Conference reports - Perugia meeting, by Joe Kovalik - Nickel and Dime gravity meeting, by Eric Poisson - Second international LISA symposium, by Robin Stebbins - JILA meeting on seismic isolation et al., by Joe Giaime

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arXiv1998

An overview of canonical quantum gravity

This is a summary of a talk delivered at the workshop ``Quantum gravity in the Southern Cone II''. We present a very brief review of current results on canonical quantization of general relativity using Ashtekar's variables and loop quantization.

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arXiv1998

Does loop quantum gravity imply Lambda=0?

We suggest that in a recently proposed framework for quantum gravity, where Vassiliev invariants span the the space of states, the latter is dramatically reduced if one has a non-vanishing cosmological constant. This naturally suggests that the initial state of the universe should have been one with $Λ=0$.

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arXiv1998

The imposition of Cauchy data to the Teukolsky equation III: The rotating case

We solve the problem of expressing the Weyl scalars $ψ$ that describe gravitational perturbations of a Kerr black hole in terms of Cauchy data. To do so we use geometrical identities (like the Gauss-Codazzi relations) as well as Einstein equations. We are able to explicitly express $ψ$ and $\partial _tψ$ as functions only of the extrinsic curvature and the three-metric (and geometrical objects built out of it) of a generic spacelike slice of the spacetime. These results provide the link between initial data and $ψ$ to be evolved by the Teukolsky equation, and can be used to compute the gravitational radiation generated by two orbiting black holes in the close limit approximation. They can also be used to extract waveforms from spacetimes completely generated by numerical methods.

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arXiv1998

Vassiliev invariants: a new framework for quantum gravity

We show that Vassiliev invariants of knots, appropriately generalized to the spin network context, are loop differentiable in spite of being diffeomorphism invariant. This opens the possibility of defining rigorously the constraints of quantum gravity as geometrical operators acting on the space of Vassiliev invariants of spin nets. We show how to explicitly realize the diffeomorphism constraint on this space and present proposals for the construction of Hamiltonian constraints.

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arXiv1998

Colliding black holes: analytic insights

We summarize the state of the art of the ``close approximation'' to black hole collisions. We discuss results to first and second order in perturbation theory for head-on collisions of momentarily-stationary and non-stationary black holes and discuss the near-future prospect of non-axisymmetric collisions.

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arXiv1998

The collision of boosted black holes: second order close limit calculations

We study the head-on collision of black holes starting from unsymmetrized, Brill--Lindquist type data for black holes with non-vanishing initial linear momentum. Evolution of the initial data is carried out with the ``close limit approximation,'' in which small initial separation and momentum are assumed, and second-order perturbation theory is used. We find agreement that is remarkably good, and that in some ways improves with increasing momentum. This work extends a previous study in which second order perturbation calculations were used for momentarily stationary initial data, and another study in which linearized perturbation theory was used for initially moving holes. In addition to supplying answers about the collisions, the present work has revealed several subtle points about the use of higher order perturbation theory, points that did not arise in the previous studies. These points include issues of normalization, and of comparison with numerical simulations, and will be important to subsequent applications of approximation methods for collisions.

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arXiv1998

Matters of Gravity, the newsletter of the APS TG on gravitation

Contents: *News: Topical group news, by Jim Isenberg April 1997 Joint APS/AAPT Meeting, GTG program, by Abhay Ashtekar Formation of the Gravitational-Wave International Committee, by Sam Finn The 1997 Xanthopoulos award, by Abhay Asthekar We hear that..., by Jorge Pullin *Research Briefs: LIGO project update, by David Shoemaker The search for frame-dragging by NS and BH's, by Sharon Morsink Gamma-ray bursts, recent developments, by Peter Meszaros Status of the Binary Black hole Grand Challenge, by Richard Matzner *Conference Reports: Quantum gravity at GR15, by Don Marolf GR Classical, by John Friedman An Experimentalist's Idiosyncratic Report on GR15, by Peter Saulson Bangalore gravitational wave meeting, by Sharon Morsink Bangalore quantum gravity meeting, by Domenico Giulini Cleveland cosmology-topology workshop, by Neil Cornish Quantum Gravity in the Southern Cone II, by Carmen Nunez Baltimore AMS meeting, by Kirill Krasnov

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arXiv1997

Evolving the Bowen-York initial data for spinning black holes

The Bowen-York initial value data typically used in numerical relativity to represent spinning black hole are not those of a constant-time slice of the Kerr spacetime. If Bowen-York initial data are used for each black hole in a collision, the emitted radiation will be partially due to the ``relaxation'' of the individual holes to Kerr form. We compute this radiation by treating the geometry for a single hole as a perturbation of a Schwarzschild black hole, and by using second order perturbation theory. We discuss the extent to which Bowen-York data can be expected accurately to represent Kerr holes.

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arXiv1997

A slightly less grand challenge: Colliding Black Holes using perturbation techniques

Perturbation techniques can be used as an alternative to supercomputer calculations in calculating gravitational radiation emitted by colliding black holes, provided the process starts with the black holes close to each other. We give a summary of the method and of the results obtained for various initial configurations, both axisymmetric and without symmetry: Initially static, boosted towards each other, counter-rotating, or boosted at an angle (pseudo-inspiral). Where applicable, we compare the perturbation results with supercomputer calculations.

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arXiv1997

On the consistency of the constraint algebra in spin network quantum gravity

We point out several features of the quantum Hamiltonian constraints recently introduced by Thiemann for Euclidean gravity. In particular we discuss the issue of the constraint algebra and of the quantum realization of the object $q^{ab}V_b$, which is classically the Poisson Bracket of two Hamiltonians.

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arXiv1997

Matters of Gravity, the newsletter of the APS TG on gravitation

Contents: *News: April 1997 Joint APS/AAPT Meeting, by Beverly Berger The physics survey and committee on gravitational physics, by Jim Hartle *Research Briefs: Instability of rotating stars to axial perturbations, by Sharon Morsink LIGO project status, by Stan Whitcomb The Search for Frame-Dragging, by Clifford Will *Conference Reports: Conference of the Southern African Relativity Society, by Nigel Bishop II Warszaw workshop on canonical and quantum gravity, by Carlo Rovelli Alpbach summer school on fundamental physics in space, by Peter Bender MG8, an experimentalists' summary, by Riley Newman and Peter Saulson Amaldi Conference on Gravitational Waves, by M. Alessandra Papa Santa Fe workshop on simplicial quantum gravity, by Lee Smolin VII Canadian Conference on General Relativity, by David Hobill

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arXiv1997

Large quantum gravity effects: backreaction on matter

We reexamine the large quantum gravity effects discovered by Ashtekar in the context of 2+1 dimensional gravity coupled to matter. We study an alternative one-parameter family of coherent states of the theory in which the large quantum gravity effects on the metric can be diminished, at the expense of losing coherence in the matter sector. Which set of states is the one that occurs in nature will determine if the large quantum gravity effects are actually observable as wild fluctuations of the metric or rapid loss of coherence of matter fields.

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arXiv1997

Chern--Simons states in spin-network quantum gravity

In the context of canonical quantum gravity in terms of Ashtekar's new variables, it is known that there exists a state that is annihilated by all the quantum constraints and that is given by the exponential of the Chern--Simons form constructed with the Asthekar connection. We make a first exploration of the transform of this state into the spin-network representation of quantum gravity. The discussion is limited to trivalent nets with planar intersections. We adapt an invariant of tangles to construct the transform and study the action of the Hamiltonian constraint on it. We show that the first two coefficients of the expansion of the invariant in terms of the inverse cosmological constant are annihilated by the Hamiltonian constraint. We also discuss issues of framing that arise in the construction.

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arXiv1997

Matters of Gravity, the newsletter of the APS TG on gravitation

Contents: News: April 1997 Joint APS/AAPT Meeting, by Beverly Berger TGG News, by Jim Isenberg Report from NSF, by David Berley We hear that..., by Jorge Pullin Research briefs: GR in GPS, by Neil Ashby What happens near the innermost stable circular orbit? by Doug Eardley Conference reports: Journees Relativistes 96, by D. Brill, M. Heusler, G. Lavrelashvili TAMA Workshop, by Peter Saulson Midwest gravity meeting, by Comer Duncan OMNI-1 Workshop by N.S. Magalhaes, W. F. Velloso Jr and O.D. Aguiar Chandra Symposium, by Robert Wald Penn State Meeting, by Lee Smolin Aspen Winter Conference, by Syd Meshkov

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arXiv1997

Ill-posedness of a double null free evolution scheme for black hole spacetimes

We suggest that ``free evolution'' integration schemes for the Einstein equations (that do not enforce constraints) may contain exponentially growing modes that render them useless in numerical integrations of black hole spacetimes, independently of how the equations are differenced. As an example we consider the evolution of Schwarzschild and Reissner-Nordström spacetimes in double null coordinates.

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arXiv1996

Colliding black holes: how far can the close approximation go?

We study the head-on collision of two equal-mass momentarily stationary black holes, using black hole perturbation theory up to second order. Compared to first-order results, this significantly improves agreement with numerically computed waveforms and energy. Much more important, second-order results correctly indicate the range of validity of perturbation theory. This use of second-order, to provide ``error bars,'' makes perturbation theory a viable tool for providing benchmarks for numerical relativity in more generic collisions and, in some range of collision parameters, for supplying waveform templates for gravitational wave detection.

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