Jeandrew Brink

Visualizing spacetime curvature via frame-drag vortexes and tidal tendexes: General theory and weak-gravity applications

When one splits spacetime into space plus time, the Weyl curvature tensor (vacuum Riemann tensor) gets split into two spatial, symmetric, and trace-free tensors: (i) the Weyl tensor’s so-called electric part or tidal field Ɛ_(jk), which raises tides on the Earth’s oceans and drives geodesic deviation (the relative acceleration of two freely falling test particles separated by a spatial vector ξ^k is Δa_j=-Ɛ_(jk)ξ^k), and (ii) the Weyl tensor’s so-called magnetic part or (as we call it) frame-drag field B_(jk), which drives differential frame dragging (the precessional angular velocity of a gyroscope at the tip of ξ^k, as measured using a local inertial frame at the tail of ξ^k, is ΔΩ_j=B_(jk)ξ^k). Being symmetric and trace-free, Ɛ_(jk) and B_(jk) each have three orthogonal eigenvector fields which can be depicted by their integral curves. We call the integral curves of Ɛ_(jk)’s eigenvectors tidal tendex lines or simply tendex lines, we call each tendex line’s eigenvalue its tendicity, and we give the name tendex to a collection of tendex lines with large tendicity. The analogous quantities for B_(jk) are frame-drag vortex lines or simply vortex lines, their vorticities, and their vortexes. These concepts are powerful tools for visualizing spacetime curvature. We build up physical intuition into them by applying them to a variety of weak-gravity phenomena: a spinning, gravitating point particle, two such particles side-by-side, a plane gravitational wave, a point particle with a dynamical current-quadrupole moment or dynamical mass-quadrupole moment, and a slow-motion binary system made of nonspinning point particles. We show that a rotating current quadrupole has four rotating vortexes that sweep outward and backward like water streams from a rotating sprinkler. As they sweep, the vortexes acquire accompanying tendexes and thereby become outgoing current-quadrupole gravitational waves. We show similarly that a rotating mass quadrupole has four rotating, outward-and-backward sweeping tendexes that acquire accompanying vortexes as they sweep, and become outgoing mass-quadrupole gravitational waves. We show, further, that an oscillating current quadrupole ejects sequences of vortex loops that acquire accompanying tendex loops as they travel, and become current-quadrupole gravitational waves; and similarly for an oscillating mass quadrupole. And we show how a binary’s tendex lines transition, as one moves radially, from those of two static point particles in the deep near zone, to those of a single spherical body in the outer part of the near zone and inner part of the wave zone (where the binary’s mass monopole moment dominates), to those of a rotating quadrupole in the far wave zone (where the quadrupolar gravitational waves dominate). In Paper II we will use these vortex and tendex concepts to gain insight into the quasinormal modes of black holes, and in subsequent papers, by combining these concepts with numerical simulations, we will explore the nonlinear dynamics of curved spacetime around colliding black holes. We have published a brief overview of these applications in R. Owen et al. Phys. Rev. Lett. 106 151101 (2011). We expect these vortex and tendex concepts to become powerful tools for general relativity research in a variety of topics.

Spacetime encodings. I. A spacetime reconstruction problem

This paper explores features of an idealized mathematical machine (algorithm) that would be capable of reconstructing the gravitational nature (the multipolar structure or spacetime metric) of a compact object, by observing gravitational radiation emitted by a small object that orbits and spirals into it. An outline is given of the mathematical developments that must be carried out in order to construct such a machine.

Spacetime encodings. III. Second order Killing tensors

This paper explores the Petrov type D, stationary axisymmetric vacuum (SAV) spacetimes that were found by Carter to have separable Hamilton-Jacobi equations, and thus admit a second-order Killing tensor. The derivation of the spacetimes presented in this paper borrows from ideas about dynamical systems, and illustrates concepts that can be generalized to higher-order Killing tensors. The relationship between the components of the Killing equations and metric functions are given explicitly. The origin of the four separable coordinate systems found by Carter is explained and classified in terms of the analytic structure associated with the Killing equations. A geometric picture of what the orbital invariants may represent is built. Requiring that a SAV spacetime admits a second-order Killing tensor is very restrictive, selecting very few candidates from the group of all possible SAV spacetimes. This restriction arises due to the fact that the consistency conditions associated with the Killing equations require that the field variables obey a second-order differential equation, as opposed to a fourth-order differential equation that imposes the weaker condition that the spacetime be SAV. This paper introduces ideas that could lead to the explicit computation of more general orbital invariants in the form of higher-order Killing tensors.

Formal Solution of the Fourth Order Killing equations for Stationary Axisymmetric Vacuum Spacetimes

An analytic understanding of the geodesic structure around non-Kerr spacetimes will result in a powerful tool that could make the mapping of spacetime around massive quiescent compact objects possible. To this end, I present an analytic closed form expression for the components of a the fourth order Killing tensor for stationary axisymmetric vacuum (SAV) spacetimes. It is as yet unclear what subset of SAV spacetimes admit this solution. The solution is written in terms of an integral expression involving the metric functions and two specific Green’s functions. A second integral expression has to vanish in order for the solution to be exact. In the event that the second integral does not vanish it is likely that the best fourth order approximation to the invariant has been found. This solution can be viewed as a generalized Carter constant providing an explicit expression for the fourth invariant, in addition to the energy, azimuthal angular momentum and rest mass, associated with geodesic motion in SAV spacetimes, be it exact or approximate. I further comment on the application of this result for the founding of a general algorithm for mapping the spacetime around compact objects using gravitational wave observatories.

Gravitational radiation from plunging orbits: Perturbative study

Numerical relativity has recently yielded a plethora of results about kicks from spinning mergers which has, in turn, vastly increased our knowledge about the spin interactions of black hole systems. In this work we use black hole perturbation theory to calculate accurately the gravitational waves emanating from the end of the plunging stage of an extreme mass ratio merger in order to further understand this phenomenon. This study focuses primarily on spin induced effects with emphasis on the maximally spinning limit and the identification of possible causes of generic behavior. We find that gravitational waves emitted during the plunging phase exhibit damped oscillatory behavior, corresponding to a coherent excitation of quasinormal modes by the test particle. This feature is universal in the sense that the frequencies and damping time do not depend on the orbital parameters of the plunging particle. Furthermore, the observed frequencies are distinct from those associated with the usual free quasinormal ringing. Our calculation suggests that a maximum in radiated energy and momentum occurs at spin parameters equal to a/M=0.86 and a/M=0.81, respectively, for the plunge stage of a polar orbit. The dependence of linear momentum emission on the angle at which a polar orbit impacts the horizon is quantified. One of the advantages of the perturbation approach adopted here is that insight into the actual mechanism of radiation emission and its relationship to black hole ringing is obtained by carefully identifying the dominant terms in the expansions used.

Orbital resonances around black holes.

We compute the length and time scales associated with resonant orbits around Kerr black holes for all orbital and spin parameters. Resonance-induced effects are potentially observable when the Event Horizon Telescope resolves the inner structure of Sgr A*, when space-based gravitational wave detectors record phase shifts in the waveform during the resonant passage of a compact object spiraling into the black hole, or in the frequencies of quasiperiodic oscillations for accreting black holes. The onset of geodesic chaos for non-Kerr spacetimes should occur at the resonance locations quantified here.

Geometrically motivated coordinate system for exploring spacetime dynamics in numerical-relativity simulations using a quasi-Kinnersley tetrad

We investigate the suitability and properties of a quasi-Kinnersley tetrad and a geometrically motivated coordinate system as tools for quantifying both strong-field and wave-zone effects in numerical relativity (NR) simulations. We fix two of the coordinate degrees of freedom of the metric, namely, the radial and latitudinal coordinates, using the Coulomb potential associated with the quasi-Kinnersley transverse frame. These coordinates are invariants of the spacetime and can be used to unambiguously fix the outstanding spin-boost freedom associated with the quasi-Kinnersley frame (and thus can be used to choose a preferred quasi-Kinnersley tetrad). In the limit of small perturbations about a Kerr spacetime, these geometrically motivated coordinates and quasi-Kinnersley tetrad reduce to Boyer-Lindquist coordinates and the Kinnersley tetrad, irrespective of the simulation gauge choice. We explore the properties of this construction both analytically and numerically, and we gain insights regarding the propagation of radiation described by a super-Poynting vector, further motivating the use of this construction in NR simulations. We also quantify in detail the peeling properties of the chosen tetrad and gauge. We argue that these choices are particularly well-suited for a rapidly converging wave-extraction algorithm as the extraction location approaches infinity, and we explore numerically the extent to which this property remains applicable on the interior of a computational domain. Using a number of additional tests, we verify numerically that the prescription behaves as required in the appropriate limits regardless of simulation gauge; these tests could also serve to benchmark other wave extraction methods. We explore the behavior of the geometrically motivated coordinate system in dynamical binary-black-hole NR mergers; while we obtain no unexpected results, we do find that these coordinates turn out to be useful for visualizing NR simulations (for example, for vividly illustrating effects such as the initial burst of spurious junk radiation passing through the computational domain). Finally, we carefully scrutinize the head-on collision of two black holes and, for example, the way in which the extracted waveform changes as it moves through the computational domain.

The Astrophysics of Resonant Orbits in the Kerr Metric

This paper gives a complete characterization of the location of resonant orbits in a Kerr spacetime for all possible black hole spins and orbital parameter values. A resonant orbit in this work is defined as a geodesic for which the longitudinal and radial orbital frequencies are commensurate. Our analysis is based on expressing the resonance condition in its most transparent form using Carlson’s elliptic integrals, which enable us to provide exact results together with a number of concise formulas characterizing the explicit dependence on the system parameters. The locations of resonant orbits identify regions where intriguing observable phenomena could occur in astrophysical situations where various sources of perturbation act on the binary system . Resonant effects may have observable implications for the in-spirals of compact objects into a super-massive black hole. During a generic in-spiral the slowly evolving orbital frequencies will pass through a series of loworder resonances where the ratio of orbital frequencies is equal to the ratio of two small integers. At these locations rapid changes in the orbital parameters could produce a measurable phase shift in the emitted gravitational and electromagnetic radiation. Resonant orbits may also capture gas or larger objects leading to further observable characteristic electromagnetic emission. According to the KAM theorem, low order resonant orbits demarcate the regions where the onset of geodesic chaos could occur when the Kerr Hamiltonian is perturbed. Perturbations are induced for example if the spacetime of the central object is non-Kerr, if gravity is modified, if the orbiting particle has large multipole moments, or if additional masses are nearby. We find that the 1/2 and 2/3 resonances occur at approximately 4 and 5.4 Schwarzschild radii (Rs) from the black hole’s event horizon. For compact object in-spirals into super-massive black holes (� 10 6 M⊙) this region lies within the sensitivity band of space-based gravitational wave detectors such as eLISA. When interpreted within the context of the super-massive black hole at the galactic center, Sgr A*, this implies that characteristic length scales of 41µas and 55µas and timescales of 50min and 79min respectively should be associated with resonant effects if Sgr A* is non-spinning, while spin decreases these values by up to � 32% and � 28%. These length-scales are potentially resolvable with radio VLBI measurements using the Event Horizon Telescope. We find that all low-order resonances are localized to the strong field region. In particular, for distances r > 50Rs from the black hole, the order of the resonances is sufficiently large that resonant effects of generic perturbations are not expected to lead to drastic changes in the dynamics. This fact guarantees the validity of using approximations based on averaging to model the orbital trajectory and frequency evolution of a test object in this region. Observing orbital motion in the intermediate region 50Rs < r < 1000Rs is thus a “sweet spot” for systematically extracting the multipole moments of the central object by observing the orbit of a pulsar – since the object is close enough to be sensitive to the quadruple moment of the central object but far enough away not to be subjected to resonant effects.

Avenues for analytic exploration in axisymmetric spacetimes: Foundations and the triad formalism

Axially symmetric spacetimes are the only vacuum models for isolated systems with continuous symmetries that also include dynamics. For such systems, we review the reduction of the vacuum Einstein field equations to their most concise form by dimensionally reducing to the three-dimensional space of orbits of the Killing vector, followed by a conformal rescaling. The resulting field equations can be written as a problem in three-dimensional gravity with a complex scalar field as source. This scalar field, the Ernst potential, is constructed from the norm and twist of the spacelike Killing field. In the case where the axial Killing vector is twist-free, we discuss the properties of the axis and simplify the field equations using a triad formalism. We study two physically motivated triad choices that further reduce the complexity of the equations and exhibit their hierarchical structure. The first choice is adapted to a harmonic coordinate that asymptotes to a cylindrical radius and leads to a simplification of the three-dimensional Ricci tensor and the boundary conditions on the axis. We illustrate its properties by explicitly solving the field equations in the case of static axisymmetric spacetimes. The other choice of triad is based on geodesic null coordinates adapted to null infinity as in the Bondi formalism. We then explore the solution space of the twist-free axisymmetric vacuum field equations, identifying the known (unphysical) solutions together with the assumptions made in each case. This singles out the necessary conditions for obtaining physical solutions to the equations.