@article{corne_kheyfets_piasio_voegele_2011, title={Binding in Charged Spherically Symmetric Objects}, volume={50}, ISSN={["0020-7748"]}, DOI={10.1007/s10773-011-0773-3}, abstractNote={We consider the subject of self-binding in static, spherically symmetric objects consisting of a charged fluid. We have shown previously that in the case of a perfect fluid, only the localized part of the mass contributes to gravitational self-binding of such objects and that in the limiting case of objects comprised purely of electromagnetic mass, there is no gravitational binding. Here, we extend this result to the more general case of an anisotropic fluid. Our inspection of the Oppenheimer–Volkov equation allows tracking of both gravitational and non-gravitational contributions to binding of spherically symmetric objects and shows that those with pure electromagnetic mass cannot exist.}, number={9}, journal={INTERNATIONAL JOURNAL OF THEORETICAL PHYSICS}, author={Corne, Matthew and Kheyfets, Arkady and Piasio, Jennifer and Voegele, Chad}, year={2011}, month={Sep}, pages={2737–2747} }
@article{gentle_kheyfets_mcdonald_miller_2009, title={A Kirchhoff-like conservation law in Regge calculus}, volume={26}, ISSN={["0264-9381"]}, DOI={10.1088/0264-9381/26/1/015005}, abstractNote={Simplicial lattices provide an elegant framework for discrete spacetimes. The inherent orthogonality between a simplicial lattice and its circumcentric dual yields an austere representation of spacetime which provides a conceptually simple form of Einstein's geometric theory of gravitation. A sufficient understanding of simplicial spacetimes has been demonstrated in the literature for spacetimes devoid of all non-gravitational sources. However, this understanding has not been adequately extended to non-vacuum spacetime models. Consequently, a deep understanding of the diffeomorphic structure of the discrete theory is lacking. Conservation laws and symmetry properties are attractive starting points for coupling matter with the lattice. We present a simplicial form of the contracted Bianchi identity which is based on the E Cartan moment of rotation operator. This identity manifests itself in the conceptually simple form of a Kirchhoff-like conservation law. This conservation law enables one to extend Regge calculus to non-vacuum spacetimes and provides a deeper understanding of the simplicial diffeomorphism group.}, number={1}, journal={CLASSICAL AND QUANTUM GRAVITY}, author={Gentle, Adrian P. and Kheyfets, Arkady and McDonald, Jonathan R. and Miller, Warner A.}, year={2009}, month={Jan} }
@article{corne_kheyfets_miller_2007, title={Non-localizability of electric coupling and gravitational binding of charged objects}, volume={24}, ISSN={["1361-6382"]}, DOI={10.1088/0264-9381/24/23/019}, abstractNote={The energy--momentum tensor in general relativity contains only localized contributions to the total energy--momentum. Here, we consider a static, spherically symmetric object consisting of a charged perfect fluid. For this object, the total gravitational mass contains a non--localizable contribution of electric coupling (ordinarily associated with electromagnetic mass). We derive an explicit expression for the total mass which implies that the non--localizable contribution of electric coupling is not bound together by gravity, thus ruling out existence of the objects with pure Lorentz electromagnetic mass in general relativity.}, number={23}, journal={CLASSICAL AND QUANTUM GRAVITY}, author={Corne, Matthew and Kheyfets, Arkady and Miller, Warner A.}, year={2007}, month={Dec}, pages={5999–6005} }
@article{kheyfets_miller_vaulin_2006, title={Quantum geometrodynamics of the Bianchi IX cosmological model}, volume={23}, ISSN={["1361-6382"]}, DOI={10.1088/0264-9381/23/13/003}, abstractNote={The canonical quantum theory of gravity—quantum geometrodynamics (QG)—is applied to the homogeneous Bianchi type IX cosmological model. As a result, a framework for the quantum theory of homogeneous cosmologies is developed. We show that the theory is internally consistent and prove that it possesses the correct classical limit (the theory of general relativity). To emphasize the special role that the constraints play in this new theory, we compare it to the traditional ADM square-root and Wheeler–DeWitt quantization schemes. We show that, unlike traditional approaches, QG leads to a well-defined Schrödinger equation for the wavefunction of the universe that is inherently coupled to the expectation value of the constraint equations. This coupling to the constraints is responsible for the appearance of a coherent spacetime picture. Thus, the physical meaning of the constraints of the theory is quite different from Dirac's interpretation. In light of this distinctive feature of the theory, we re-address the question of the dark energy effects in the Bianchi IX cosmological model for highly non-classical quantum states. We show that, at least for this model, for any choice of the initial wavefunction, quantum corrections will not produce accelerated expansion of the universe.}, number={13}, journal={CLASSICAL AND QUANTUM GRAVITY}, author={Kheyfets, Arkady and Miller, Warner A. and Vaulin, Ruslan}, year={2006}, month={Jul}, pages={4333–4351} }
@article{gentle_george_miller_kheyfets_2004, title={Constraints in quantum geometrodynamics}, volume={19}, ISSN={["0217-751X"]}, DOI={10.1142/S0217751X04017008}, abstractNote={We compare different treatments of the constraints in canonical quantum gravity. The standard approach on the superspace of 3-geometries treats the constraints as the sole carriers of the dynamic content of the theory, thus rendering the traditional dynamical equations obsolete. Quantization of the constraints in both the Dirac and ADM square root Hamiltonian approaches leads to the well known problems of time evolution. These problems of time are of both an interpretational and technical nature. In contrast, the geometrodynamic quantization procedure on the superspace of the true dynamical variables separates the issues of quantization from the enforcement of the constraints. The resulting theory takes into account states that are off-shell with respect to the constraints, and thus avoids the problems of time. We develop, for the first time, the geometrodynamic quantization formalism in a general setting and show that it retains all essential features previously illustrated in the context of homogeneous cosmologies.}, number={10}, journal={INTERNATIONAL JOURNAL OF MODERN PHYSICS A}, author={Gentle, AP and George, ND and Miller, WA and Kheyfets, A}, year={2004}, month={Apr}, pages={1609–1638} }
@article{gentle_george_kheyfets_miller_2004, title={The constraints as evolution equations for numerical relativity}, volume={21}, ISSN={["0264-9381"]}, DOI={10.1088/0264-9381/21/1/006}, abstractNote={The Einstein equations have proved surprisingly difficult to solve numerically. A standard diagnostic of the problems which plague the field is the failure of computational schemes to satisfy the constraints, which are known to be mathematically conserved by the evolution equations. We describe a new approach to rewriting the constraints as first-order evolution equations, thereby guaranteeing that they are satisfied to a chosen accuracy by any discretization scheme. This introduces a set of four subsidiary constraints which are far simpler than the standard constraint equations and which should be more easily conserved in computational applications. We explore the manner in which the momentum constraints are already incorporated in several existing formulations of the Einstein equations, and demonstrate the ease with which our new constraint-conserving approach can be incorporated into these schemes.}, number={1}, journal={CLASSICAL AND QUANTUM GRAVITY}, author={Gentle, AP and George, ND and Kheyfets, A and Miller, WA}, year={2004}, month={Jan}, pages={83–91} }
@article{miller_george_kheyfets_mcghee_2003, title={Off-axis neutrino scattering in gamma-ray burst central engines}, volume={583}, ISSN={["1538-4357"]}, DOI={10.1086/345471}, abstractNote={The search for an understanding of an energy source great enough to explain the gamma-ray burst (GRB) phenomena has attracted much attention from the astrophysical community since its discovery. In this paper we extend the work of K. Asano and T. Fukuyama, and J. D. Salmonson and J. R. Wilson, and analyze the off-axis contributions to the energy-momentum deposition rate (MDR) from the neutrino anti-neutrino collisions above a rotating black hole/thin accretion disk system. Our calculations are performed by imaging the accretion disk at a specified observer using the full geodesic equations, and calculating the cumulative MDR from the scattering of all pairs of neutrinos and anti-neutrinos arriving at the observer. Our results shed light on the beaming efficiency of GRB models of this kind. Although we confirm Asano and Fukuyama's conjecture as to the constancy of the beaming for small angles away from the axis; nevertheless, we find the dominant contribution to the MDR comes from near the surface of the disk with a tilt of approximately \pi/4 in the direction of the disk's rotation. We find that the MDR at large radii is directed outward in a conic section centered around the symmetry axis and is larger, by a factor of 10 to 20, than the on-axis values. By including this off-axis disk source, we find a linear dependence of the MDR on the black hole angular momentum (a). In addition, we find that scattering is directed back onto the black hole in regions just above the horizon of the black hole. This gravitational ``in scatter'' may provide an observable high energy signature of the central engine, or at least another channel for accretion.}, number={2}, journal={ASTROPHYSICAL JOURNAL}, author={Miller, WA and George, ND and Kheyfets, A and McGhee, JM}, year={2003}, month={Feb}, pages={833–841} }
@article{gentle_holz_kheyfets_laguna_miller_shoemaker_2001, title={Constant crunch coordinates for black hole simulations}, volume={63}, ISSN={["2470-0029"]}, DOI={10.1103/physrevd.63.064024}, abstractNote={We reinvestigate the utility of time-independent constant mean curvature foliations for the numerical simulation of a single spherically-symmetric black hole. Each spacelike hypersurface of such a foliation is endowed with the same constant value of the trace of the extrinsic curvature tensor, $K$. Of the three families of $K$-constant surfaces possible (classified according to their asymptotic behaviors), we single out a sub-family of singularity-avoiding surfaces that may be particularly useful, and provide an analytic expression for the closest approach such surfaces make to the singularity. We then utilize a non-zero shift to yield families of $K$-constant surfaces which (1) avoid the black hole singularity, and thus the need to excise the singularity, (2) are asymptotically null, aiding in gravity wave extraction, (3) cover the physically relevant part of the spacetime, (4) are well behaved (regular) across the horizon, and (5) are static under evolution, and therefore have no ``grid stretching/sucking'' pathologies. Preliminary numerical runs demonstrate that we can stably evolve a single spherically-symmetric static black hole using this foliation. We wish to emphasize that this coordinatization produces $K$-constant surfaces for a single black hole spacetime that are regular, static and stable throughout their evolution.}, number={6}, journal={PHYSICAL REVIEW D}, author={Gentle, AP and Holz, DE and Kheyfets, A and Laguna, P and Miller, WA and Shoemaker, DM}, year={2001}, month={Mar} }
@article{kheyfets_miller_newton_2000, title={Schild's ladder parallel transport procedure for an arbitrary connection}, volume={39}, DOI={10.1023/A:1026473418439}, number={12}, journal={International Journal of Theoretical Physics}, author={Kheyfets, A. and Miller, W. A. and Newton, G. A.}, year={2000}, pages={2891–2898} }
@article{kheyfets_miller_2000, title={Time in quantum geometrodynamics}, volume={15}, DOI={10.1142/s0217751x00001361}, abstractNote={We revisit the issue of time in quantum geometrodynamics and suggest a quantization procedure on the space of true dynamic variables. This procedure separates the issue of quantization from enforcing the constraints caused by the general covariance symmetries. The resulting theory, unlike the standard approach, takes into account the states that are off shell with respect to the constraints, and thus avoids the problems of time. In this approach, quantum geometrodynamics, general covariance, and the interpretation of time emerge together as parts of the solution of the total problem of geometrodynamic evolution.}, number={26}, journal={International Journal of Modern Physics. A}, author={Kheyfets, A. and Miller, W. A.}, year={2000}, pages={4125–4140} }
@article{chakrabarti_gentle_kheyfets_miller_1999, title={Geodesic deviation in Regge calculus}, volume={16}, ISSN={["0264-9381"]}, DOI={10.1088/0264-9381/16/7/315}, abstractNote={Geodesic deviation is the most basic manifestation of the influence of gravitational fields on matter. We investigate geodesic deviation within the framework of Regge calculus, and compare the results with the continuous formulation of general relativity on two different levels. We show that the continuum and simplicial descriptions coincide when the cumulative effect of the Regge contributions over an infinitesimal element of area is considered. This comparison provides a quantitative relation between the curvature of the continuous description and the deficit angles of Regge calculus. The results presented might also be of help in developing generic ways of including matter terms in the Regge equations.}, number={7}, journal={CLASSICAL AND QUANTUM GRAVITY}, author={Chakrabarti, S and Gentle, AP and Kheyfets, A and Miller, WA}, year={1999}, month={Jul}, pages={2381–2391} }