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When Black Holes Collide Frans Pretorius Princeton University STScI Colloquium February 18, 2009 STScI Colloquium February 18, 2009

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Outline I Motivation: why explore black hole collisions?Motivation: why explore black hole collisions? –gravitational wave astronomy, studying dynamical strong-field general relativity The conventional picture of generic non-extreme-mass-ratio mergersThe conventional picture of generic non-extreme-mass-ratio mergers –phases of the merger “Newtonian”“Newtonian” inspiral quasi-circular inspiralinspiral quasi-circular inspiral plunge/mergerplunge/merger ringdownringdown –highlights from the recent explosion of numerical results on the late- inspiral/merger phase how well post-Newtonian techniques model the late stages of inspiralhow well post-Newtonian techniques model the late stages of inspiral large re-coil velocitieslarge re-coil velocities

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Outline II Is this the complete story?Is this the complete story? –theoretical studies suggests modest to high eccentricity mergers may be as likely, if not the dominant source of detectable signals if this is the case, search strategies will need to be updatedif this is the case, search strategies will need to be updated –orbital dynamics becomes much more interesting, exhibiting zoom- whirl behavior –unstable circular orbits are the key to understanding zoom-whirl behavior for elliptic orbits in general relativity consequences for binary black hole mergersconsequences for binary black hole mergers more speculative implications for black hole/neutron star mergersmore speculative implications for black hole/neutron star mergers ConclusionsConclusions

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Motivation: why explore black hole collisions? gravitational wave astronomygravitational wave astronomy –almost overwhelming circumstantial evidence that black holes exist in our universe –to obtain conclusive evidence, we need to “see” the black holes in the “light” they emit … gravitational waves. However, isolated single black holes do not radiate, so we need to look for binary mergers for the cleanest direct signature of the existence of black holes –understanding the nature of the waves emitted in the process is important for detecting such events, and moreover will be crucial in deciphering the signals extracting the parameters of the binaryextracting the parameters of the binary obtain clues about the environment of the binaryobtain clues about the environment of the binary how accurately does Einstein’s theory describe the event?how accurately does Einstein’s theory describe the event?

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The network of gravitational wave detectors LIGO Hanford LIGO Livingston ground based laser interferometers LIGO/VIRGO/GEO/TAMA space-based laser interferometer (hopefully with get funded for a 20?? Launch) LISA ALLEGRO/NAUTILUS/AURIGA/… resonant bar detectors ALLEGRO AURIGA Pulsar timing network, CMB anisotropy The Crab nebula … a supernovae remnant harboring a pulsar Segment of the CMB from WMAP

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source frequency (Hz) source frequency (Hz) source “strength” source “strength” 10 4 10 -12 10 -8 10 -4 1 relics from the big bang, inflation relics from the big bang, inflation exotic physics in the early universe: phase transitions, cosmic strings, domain walls, … exotic physics in the early universe: phase transitions, cosmic strings, domain walls, … 1-10 M ๏ BH/BH mergers 1-10 M ๏ BH/BH mergers NS/BH mergers NS/NS mergers pulsars, supernovae EMR inspiral EMR inspiral NS binaries WD binaries 10 2 -10 6 M ๏ BH/BH mergers 10 2 -10 6 M ๏ BH/BH mergers >10 6 M ๏ BH/BH mergers >10 6 M ๏ BH/BH mergers CMB anisotropy Pulsar timing LISALIGO/… Bar detectors Overview of expected gravitational wave sources

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Anatomy of a Merger In the conventional scenario of a black hole merger in the universe, one can break down the evolution into 4 stages: Newtonian, inspiral, plunge/merger and ringdownIn the conventional scenario of a black hole merger in the universe, one can break down the evolution into 4 stages: Newtonian, inspiral, plunge/merger and ringdown NewtonianNewtonian –in isolation, radiation reaction will cause two black holes of mass M in a circular orbit with initial separation R to merge within a time t m relative to the Hubble time t H –label the phase of the orbit Newtonian when the separation is such that the binary will take longer than the age of the universe to merge, for then to be of relevance to gravitational wave detection, other “Newtonian” processes need to operate, e.g. dynamical friction, n-body encounters, gas-drag, etc. For e.g., two solar mass black holes need to be within 1 million Schwarzschild radii ~ 3 million kmtwo solar mass black holes need to be within 1 million Schwarzschild radii ~ 3 million km two 10 9 solar mass black holes need to be within 6 thousand Schwarzschild radii ~ 1 parsectwo 10 9 solar mass black holes need to be within 6 thousand Schwarzschild radii ~ 1 parsec

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Anatomy of a Merger inspiral quasi-circular inspiral (QSI)inspiral quasi-circular inspiral (QSI) –In the inspiral phase, energy loss through gravitational wave emission is the dominate mechanism forcing the black holes closer together –to get an idea for the dominant timescale during inspiral, for equal mass, circular binaries the Keplarian orbital frequency offers a good approximation until very close to merger the dominant gravitational wave frequency is twice thisthe dominant gravitational wave frequency is twice this –Post-Newtonian techniques provide an accurate description of certain aspects of the process until remarkably close to merger –if the initial pericenter of the orbit is sufficiently large, the orbit will loose its eccentricity long before merger [Peters & Matthews, Phys.Rev. 131 (1963)] and become quasi-circular

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Anatomy of a Merger plunge/mergerplunge/merger –this is the time in the merger when the two event horizons coalesce into one we know the two black holes must merge into one if cosmic censorship holds (and no indications of a failure yet in any merger simulations)we know the two black holes must merge into one if cosmic censorship holds (and no indications of a failure yet in any merger simulations) –full numerical solution of the field equations are required to solve for the geometry of spacetime in this stage –full numerical solution of the field equations are required to solve for the geometry of spacetime in this stage Only within the last 3 years, following a couple of breakthroughs, has numerical relativity been able to complete the picture by filling in the details of the final, non-perturbative phase of the mergerOnly within the last 3 years, following a couple of breakthroughs, has numerical relativity been able to complete the picture by filling in the details of the final, non-perturbative phase of the merger At present two known stable formulations of the field equations, generalized harmonic [FP, PRL 95, 121101 (2005) ], and BSSN with moving punctures [M. Campanelli, C. O. Lousto, P. Marronetti, Y. Zlochower PRL 96, 111101, (2006); J. G. Baker, J. Centrella, D. Choi, M. Koppitz, J. van Meter PRL 96, 111102, (2006)]At present two known stable formulations of the field equations, generalized harmonic [FP, PRL 95, 121101 (2005) ], and BSSN with moving punctures [M. Campanelli, C. O. Lousto, P. Marronetti, Y. Zlochower PRL 96, 111101, (2006); J. G. Baker, J. Centrella, D. Choi, M. Koppitz, J. van Meter PRL 96, 111102, (2006)] –in all cases studied to date, this stage is exceedingly short, leaving its imprint in on the order of 1-2 gravitational wave cycles, at roughly twice the final orbital frequency

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Anatomy of a Merger ringdownringdown –in the final phase of the merger, the remnant black hole “looses all its hair”, settling down to a Kerr black hole –one possible definition for when plunge/merger ends and ringdown begins, is when the spacetime can adequately be described as a Kerr black hole perturbed by a set of quasi-normal modes (QNM) –the ringdown portion of the waveform will be dominated by the fundamental harmonic of the quadrupole QNM, with characteristic frequency and decay time [Echeverria, PRD 34, 384 (1986)]: j=a/M f, the Kerr spin parameter of the black hole

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Sample evolution --- Cook-Pfeiffer Quasi-circular initial data This animation shows the lapse function in the orbital plane. The lapse function represents the relative time dilation between a hypothetical observer at the given location on the grid, and an observer situated very far from the system --- the redder the color, the slower local clocks are running relative to clocks at infinity If this were in “real-time” it would correspond to the merger of two ~5000 solar mass black holes Initial black holes are close to non-spinning Schwarzschild black holes; final black hole is a Kerr a black hole with spin parameter ~0.7, and ~4% of the total initial rest-mass of the system is emitted in gravitational waves A. Buonanno, G.B. Cook and F.P.; Phys.Rev.D75:124018,2007

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Gravitational waves from the simulation A depiction of the gravitational waves emitted in the orbital plane of the binary. Shown is the real component of the Newman Penrose scalar , which in the wave zone is proportional to the second time derivative of the usual plus- polarization The plus-component of the wave from the same simulation, measured on the axis normal to the orbital plane

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What does the merger wave represent? Scale the system to two 10 solar mass (~ 2x10 31 kg) BHsScale the system to two 10 solar mass (~ 2x10 31 kg) BHs –radius of each black hole in the binary is ~ 30km –radius of final black hole is ~ 60km –distance from the final black hole where the wave was measured ~ 1500km –frequency of the wave ~ 200Hz (early inspiral) - 800Hz (ring-down)

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What does the merger wave represent? fractional oscillatory “distortion” in space induced by the wave transverse to the direction of propagation has a maximum amplitude L/L ~ 3x10 -3fractional oscillatory “distortion” in space induced by the wave transverse to the direction of propagation has a maximum amplitude L/L ~ 3x10 -3 a 2m tall person will get stretched/squeezed by ~ 6 mm as the wave passesa 2m tall person will get stretched/squeezed by ~ 6 mm as the wave passes LIGO’s arm length would change by ~ 12m. Wave amplitude decays like 1/distance from source; e.g. at 10Mpc the change in arms ~ 5x10 -17 m (1/20 the radius of a proton, which is well within the ballpark of what LIGO is trying to measure!!)LIGO’s arm length would change by ~ 12m. Wave amplitude decays like 1/distance from source; e.g. at 10Mpc the change in arms ~ 5x10 -17 m (1/20 the radius of a proton, which is well within the ballpark of what LIGO is trying to measure!!) despite the seemingly small amplitude for the wave, the energy it carries is enormous — around 10 30 kg c 2 ~ 10 47 J ~ 10 54 ergsdespite the seemingly small amplitude for the wave, the energy it carries is enormous — around 10 30 kg c 2 ~ 10 47 J ~ 10 54 ergs peak luminosity is about 1/100 th the Planck luminosity of 10 59 ergs/s !!peak luminosity is about 1/100 th the Planck luminosity of 10 59 ergs/s !! luminosity of the sun ~ 10 33 ergs/s, a bright supernova or milky-way type galaxy ~ 10 42 ergs/sluminosity of the sun ~ 10 33 ergs/s, a bright supernova or milky-way type galaxy ~ 10 42 ergs/s if all the energy reaching LIGO from the 10Mpc event could directly be converted to sound waves, it would have an intensity level of ~ 80dBif all the energy reaching LIGO from the 10Mpc event could directly be converted to sound waves, it would have an intensity level of ~ 80dB

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Highlights of recent results: simplicity of merger waveform the “non-linear” phase of the merger is surprisingly shortthe “non-linear” phase of the merger is surprisingly short great boon for data analysis, as this suggests an efficient LIGO template bank could be compiled by stitching together quick-to-calculate perturbative waveforms, guided by a handful of numerical waveformsgreat boon for data analysis, as this suggests an efficient LIGO template bank could be compiled by stitching together quick-to-calculate perturbative waveforms, guided by a handful of numerical waveforms to-date, the furthest the idea has been pushed is for quasi-circular inspiral of non-spinning BH’s [A. Buonanno et al., Phys.Rev.D76:104049,2007]to-date, the furthest the idea has been pushed is for quasi-circular inspiral of non-spinning BH’s [A. Buonanno et al., Phys.Rev.D76:104049,2007] –effective-one-body (EOB) PN inspiral connected to the 3 dominant quasi normal modes (QNMs) –added “pseudo” 4PN term to EOB model, with coefficient determined by a best-fit match to a set of numerical results –used simulation results for final spin and black hole mass to fix the QNM frequencies and decay constants 4:1 mass ratio example

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Highlights of recent results : large recoil velocities significant recoil can be imparted to the remnant black hole due to asymmetric beaming of radiation during the merger, up to 4000km/s in some casessignificant recoil can be imparted to the remnant black hole due to asymmetric beaming of radiation during the merger, up to 4000km/s in some cases –Herrmann et al., gr-qc/0701143; Koppitz et al., gr-qc/0701163; Campanelli et al. gr- qc/0701164 & gr-qc/0702133, Gonzalez et al, arXiv:gr-qc/0702052, Tichy & Marronetti, arXiv:gr-qc/0703075v1 there are far reaching consequences to this, some that could be detected via electromagnetic observations, in particular for supermassive black hole mergersthere are far reaching consequences to this, some that could be detected via electromagnetic observations, in particular for supermassive black hole mergers –offset or double galactic nuclei, displaced active galactic nuclei, wiggling jets, enlarged cores, lopsided cores, x-ray afterglows, feedback trails, off- center flares from tidally disrupted stars, hypervelocity stars, a population of galaxies without supermassive black holes, etc. Merritt et al., ApJ. 607 (2004) L9-L12; Milosavljevic & Phinney, ApJ 622, L93(2005); Gualandris & Merrit arXiv:0708.0771 &, arXiv:0708.3083; Lippai et al. arXiv:0801.0739; Kornreich & Lovelace, arXiv:0802.2058; Devecchi et al. arXiv:0805.2609; Komossa & Merrit arXiv:0807.0223 & arXiv:0811.1037; Fujita arXiv:0808.1726 & arXiv:0810.1520 –a 2650km/s recoiling black hole could explain the emission line spectra from quasar SDSSJ092712.65+294344. 0 [S. Komossa et al., ApJ.678:L81,2008]

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What is “wrong” with the QCI picture If we want LIGO be anything more than a simple detector, i.e. be able to identify the gravitational waves, we need templates for all plausible sources; howeverIf we want LIGO be anything more than a simple detector, i.e. be able to identify the gravitational waves, we need templates for all plausible sources; however –limited computational power for template searches –having too many templates increases the probability of false detections LIGO does need to be selective in what it looks forLIGO does need to be selective in what it looks for Recent studies suggest QCI may not be the dominant LIGO (or LISA) sourceRecent studies suggest QCI may not be the dominant LIGO (or LISA) source –Eccentric mergers, potentially transitioning from inspiral to merger through a series of zoom-whirl orbits, may be more likely

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Merging with eccentricity Binary stars are unlikely progenitors for BH binaries that could merge within the Hubble time, as such close binaries will likely evolve through a common envelope phase, causing a stellar merger before BH formation [Belczynski et al., ApJ 662, 2007].Binary stars are unlikely progenitors for BH binaries that could merge within the Hubble time, as such close binaries will likely evolve through a common envelope phase, causing a stellar merger before BH formation [Belczynski et al., ApJ 662, 2007]. –this cuts off the most promising channel for QCI A promising source for stellar mass BH binaries is then n-body interactions involving BHs in dense stellar environments [e.g. Sigurdsson & Hernquist, Nature 364 (1993), Portegies Zwart & McMilla, ApJ 528 L17 (2000), Sadowski et al., arXiv:0710.0878]A promising source for stellar mass BH binaries is then n-body interactions involving BHs in dense stellar environments [e.g. Sigurdsson & Hernquist, Nature 364 (1993), Portegies Zwart & McMilla, ApJ 528 L17 (2000), Sadowski et al., arXiv:0710.0878] –these often lead to binaries with large eccentricities that are sufficiently tight that they do not have enough time to circularize before merging O’Leary et al. (arXiv:0807.2638) estimate 90% will have e>0.9 when entering the LIGO band, with Advanced LIGO rates of ~ 1-10 3 /year from mergers in galactic nuclei aloneO’Leary et al. (arXiv:0807.2638) estimate 90% will have e>0.9 when entering the LIGO band, with Advanced LIGO rates of ~ 1-10 3 /year from mergers in galactic nuclei alone For supermassive BH mergers, studies also suggest mergers may occur with non-negligible eccentricity, e.g. Berentzen et al., arXiv:0812.2756For supermassive BH mergers, studies also suggest mergers may occur with non-negligible eccentricity, e.g. Berentzen et al., arXiv:0812.2756

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Why may eccentric mergers be a problem? 1 extra parameter … not too much of a issue for template searches1 extra parameter … not too much of a issue for template searches However, how the transition from inspiral to plunge happens could affect the feasability of present PN techniquesHowever, how the transition from inspiral to plunge happens could affect the feasability of present PN techniques –the expansions are accurate for adiabatic evolution of the orbital parameters, and as long as v/c < 1 For QCI of comparable mass binaries, v/c ~ 0.2-0.3 before common horizon formation, which is one reason why PN-techniques work so well modeling the entire inspiralFor QCI of comparable mass binaries, v/c ~ 0.2-0.3 before common horizon formation, which is one reason why PN-techniques work so well modeling the entire inspiral For eccentric orbits v/c will become much largerFor eccentric orbits v/c will become much larger

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Why may eccentric mergers be a problem? a second argument given why PN matches numerical simulations so well, is the regime where it breaks down is very short, and happens well within the effective potential barrier of the spacetime and so can not leave a significant imprint on the waveforma second argument given why PN matches numerical simulations so well, is the regime where it breaks down is very short, and happens well within the effective potential barrier of the spacetime and so can not leave a significant imprint on the waveform –this argument will fail if zoom-whirl behavior sets in at the transition from inspiral to merger, and this may generically happen with high-eccentricity inspirals

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Zoom-whirl orbits At a first glance might look like “extreme’’ pericenter precessionAt a first glance might look like “extreme’’ pericenter precession –examples: geodesics about a Schwarzschild BH, apoapsis 30M, inner circle is event horizon (2M), outer circle ISCO (6M) usual (but large) pericenter precession zoom-whirl orbit

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Zoom-whirl orbits However, what’s different about zoom-whirl behavior is once the pericenter distance crosses the isco, you can find orbits which are essentially indistinguishable at apoapsis but that exhibit an arbitrary number of zooms per whirl at the same pericenter distanceHowever, what’s different about zoom-whirl behavior is once the pericenter distance crosses the isco, you can find orbits which are essentially indistinguishable at apoapsis but that exhibit an arbitrary number of zooms per whirl at the same pericenter distance 300 zoom-whirl orbits, initial tangential velocity ranging from 0.1207600 to 0.12076290 150 “regular” orbits, initial tangential velocity ranging from 0.128 to 0.132

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Zoom-whirl orbits Zoom-whirl orbits are perturbations of unstable circular orbits that exists within the ISCOZoom-whirl orbits are perturbations of unstable circular orbits that exists within the ISCO –In Schwarzschild, radial perturbations of circular orbits in the range 4M to 6M lead to elliptic zoom-whirl orbits4M to 6M lead to elliptic zoom-whirl orbits 3M (the “light ring”) to 4M lead to a hyperbolic orbit with one whirl episode3M (the “light ring”) to 4M lead to a hyperbolic orbit with one whirl episode –depend on the sign of the perturbation, the geodesic will fall into the black hole or not after a whirl phase The number of whirls n is related to the magnitude of the perturbation r and the instability exponent of the orbit viaThe number of whirls n is related to the magnitude of the perturbation r and the instability exponent of the orbit via

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Beyond geodesics Levin, Grossman & Perez-Giz [arXiv:0811.3815, arXiv:0811.3814, arXiv:0811.3798, arXiv:0809.3838, arXiv:0802.0459] have shown the behavior persists in the conservative dynamics of the PN expansion up to 3 rd order, including spin-orbit interactionsLevin, Grossman & Perez-Giz [arXiv:0811.3815, arXiv:0811.3814, arXiv:0811.3798, arXiv:0809.3838, arXiv:0802.0459] have shown the behavior persists in the conservative dynamics of the PN expansion up to 3 rd order, including spin-orbit interactions –with spin (even for geodesics), the orbital plane precesses, and so the unstable orbits are “spherical” rather than circular –they introduce an interesting taxonomy of the subset of exactly periodic orbits, where each orbit is classified by a rational number q where w is the number of whirls, z the number of leaves that make up the zooms, and v describes the sequence in which the leaves are traced out (v/z <1) –any non-closed orbit is arbitrarily close to some periodic orbit q=1 + 753/1000 q=1 + 3/4

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Beyond geodesics Numerical simulations of equal mass non-QCI binaries also show zoom- whirl behavior [FP & Khurana, CQG 24 (2007); Washik et al. PRL 101 (2008)]Numerical simulations of equal mass non-QCI binaries also show zoom- whirl behavior [FP & Khurana, CQG 24 (2007); Washik et al. PRL 101 (2008)] Will argue that this must generically be present in the GR two-body problem because of the possibility of two distinct end-states — one or two black holes — and this is intimately related to the existence of unstable orbitsWill argue that this must generically be present in the GR two-body problem because of the possibility of two distinct end-states — one or two black holes — and this is intimately related to the existence of unstable orbits –simulations are beginning to confirm this However, unlike with geodesics, there must be a limit to the number of whirls because of radiation-reactionHowever, unlike with geodesics, there must be a limit to the number of whirls because of radiation-reaction –eccentric orbits have more energy than a QC orbit with radius ~ the pericenter radius; the whirling could in principle persist until this excess energy is radiated away

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The threshold of immediate merger Consider the black hole scattering problemConsider the black hole scattering problem in general two, distinct end-states possiblein general two, distinct end-states possible one black hole, after a collisionone black hole, after a collision two isolated black holes, after a deflectiontwo isolated black holes, after a deflection because there are two distinct end-states, there must be some kind of threshold behavior approaching a critical impact parameter b*because there are two distinct end-states, there must be some kind of threshold behavior approaching a critical impact parameter b* b m 1,v 1 m 2,v 2

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The threshold of immediate merger The following illustrates what could happen as one tunes to threshold, assuming smooth dependence of the trajectories as a function of bThe following illustrates what could happen as one tunes to threshold, assuming smooth dependence of the trajectories as a function of b non-spinning case (so we have evolution in a plane)non-spinning case (so we have evolution in a plane) only showing one of the BH trajectories for clarityonly showing one of the BH trajectories for clarity solid blue (black) – merger (escape)solid blue (black) – merger (escape) dashed blue (black) – merger ( escape ) for values of b closer to thresholddashed blue (black) – merger ( escape ) for values of b closer to threshold

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The threshold of immediate merger : geodesics We know the previous argument works for geodesics … a couple more examples below repeating the “scattering” experiment for hyperbolic and elliptic geodesics, tuned to the threshold to within ~ 1 part in 10 16 (giving ~ 8 whirl orbits here)We know the previous argument works for geodesics … a couple more examples below repeating the “scattering” experiment for hyperbolic and elliptic geodesics, tuned to the threshold to within ~ 1 part in 10 16 (giving ~ 8 whirl orbits here) unbound orbits (green scatter, blue capture) bound orbits [the non-capture case is not a two-leaf orbit … integration just stopped after the second zoom]

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The threshold of immediate merger : equal mass binaries The figures below are from full numerical simulations of the field equations for equal mass orbits, showing qualitatively the same behavior as the geodesic problemThe figures below are from full numerical simulations of the field equations for equal mass orbits, showing qualitatively the same behavior as the geodesic problem –however, the binary in the whirl phase is emitting copious amounts of gravitational radiation; on the order of 1-1.5% of the total mass of the system per orbit two cases tuned close to threshold (only 1 BH trajectory shown) dominant component of emitted gravitational waves as measure by NP scalars

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Animations from merger case … Lapse function , orbital plane Real component of the Newman-Penrose scalar 4 ( times rM), orbital plane

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Key Open Question The scattering problem highlights a potential show-stopper for the relevance of zoom-whirl behavior for generic astrophysical binaries — excessive fine tuning of initial conditionsThe scattering problem highlights a potential show-stopper for the relevance of zoom-whirl behavior for generic astrophysical binaries — excessive fine tuning of initial conditions However, accessing the probability of a single near-critical encounter is not the relevant question. Rather, in a binary inspiral scenario where the pericenter distance reaches an effective ISCO while the binary still possesses a large eccentricity, does the transition from inspiral to merger cross a separatrix of unstable quasi-circular orbits?However, accessing the probability of a single near-critical encounter is not the relevant question. Rather, in a binary inspiral scenario where the pericenter distance reaches an effective ISCO while the binary still possesses a large eccentricity, does the transition from inspiral to merger cross a separatrix of unstable quasi-circular orbits? –we know the answer is “yes” for extreme-mass-ratio inspirals (an important source for LISA) –Levin et al. claim generically it will be “yes”, given in their taxonomy the QCIs are in a sense a set of measure zero of all possible orbits –energy arguments suggest it should be “yes” for some sufficiently large eccentricity that will depend on the mass-ratio of the binary

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Consequences if “yes” Eccentric orbits have a larger GW luminosity near periapsis in general; if whirliness occurs the luminosity will be even higherEccentric orbits have a larger GW luminosity near periapsis in general; if whirliness occurs the luminosity will be even higher –so even if these events are rarer, the can be seen to a much larger distance than QCI, hence could be an important LIGO source [ O’Leary et al. (arXiv:0807.2638)] but need appropriate templates to see it! Case-and-point results of NINJA (Numerical INJection Analysis) [arXiv:0901.4399], where a zoom- whirl merger was regularly missed in the simulation detectionsbut need appropriate templates to see it! Case-and-point results of NINJA (Numerical INJection Analysis) [arXiv:0901.4399], where a zoom- whirl merger was regularly missed in the simulation detections Such events will offer exquisite tests of general relativity, as more of the GW signal comes from the strong-field regionSuch events will offer exquisite tests of general relativity, as more of the GW signal comes from the strong-field region –Some interesting alternative theories/extensions of GR, such as Chern-Simons modified gravity, are consistent will all existing weak field tests, but have quite different strong field solutions

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More speculative consequence if “yes” arguments for a high eccentricity BH binary population may apply to black hole/neutron star binaries.arguments for a high eccentricity BH binary population may apply to black hole/neutron star binaries. a back-of-the-envelope calculation shows that a 1.5 M๏ neutron star will reach it’s Roche-limit within the range of unstable circular orbits (3-6M) for black holes with masses ~ 5-14 M๏a back-of-the-envelope calculation shows that a 1.5 M๏ neutron star will reach it’s Roche-limit within the range of unstable circular orbits (3-6M) for black holes with masses ~ 5-14 M๏ –if the whirl phase sets in, the black hole could “peel” the outer layers of the neutron star, with a sizeable amount of the material flung back out into an accretion disk this is in contrast to tidal disruption in a QCI, where within less that an orbit almost all of the material fallsthis is in contrast to tidal disruption in a QCI, where within less that an orbit almost all of the material falls –would also be a significant source of E&M activity (a flavor of GRB?) The highest luminosity GW burst could even come after the GRB, if the peeled NS zooms out again before merging in a subsequent whirl.The highest luminosity GW burst could even come after the GRB, if the peeled NS zooms out again before merging in a subsequent whirl.

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Conclusions It is not too much of a stretch of the imagination to state that we are on the verge of a new era in observational astronomy with gravitational wave detectorsIt is not too much of a stretch of the imagination to state that we are on the verge of a new era in observational astronomy with gravitational wave detectors To realize the full potential of this generation of detectors requires that we understand the theory of expected sources … both the astrophysical populations and the nature of the gravitational wave emissionTo realize the full potential of this generation of detectors requires that we understand the theory of expected sources … both the astrophysical populations and the nature of the gravitational wave emission However, given the many open questions in the theoretical models, the complexity of the plausible scenarios, and the infeasibility of simulating them all, one can anticipate that the most exciting discoveries and advances will come through a synergistic interplay between observation and theoryHowever, given the many open questions in the theoretical models, the complexity of the plausible scenarios, and the infeasibility of simulating them all, one can anticipate that the most exciting discoveries and advances will come through a synergistic interplay between observation and theory For more details, check out the Observational Signatures of Black Hole Mergers conference at the STSCI March 30-April 1For more details, check out the Observational Signatures of Black Hole Mergers conference at the STSCI March 30-April 1

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