Michele's wiki/blog

Authors: Naoki Seto, Asantha Cooray

Date: Wed, 21 Feb 2007

Abstract: We discuss the possibility of detecting the presence of primordial black holes (PBHs), such as those that might account for galactic dark matter, using modification of pulsar timing residuals when PBHs pass within ~1000 AU and impart impulse accelerations to the Earth. With this technique, PBHs with masses around 10ˆ{25} g (~0.1 lunar mass) can be detected. Currently, the constraints on the abundance of such dark matter candidates are weak. A 30 year-long monitoring campaign with the proposed Square Kilometer Array (SKA) can rule out a PBH fraction more than ~1/10 in the solar neighborhood in the form of dark matter with mass ~10ˆ{25} g.

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Authors: Enrico Barausse, Luciano Rezzolla, David Petroff, Marcus Ansorg

Date: Mon, 26 Feb 2007

Abstract: To investigate the imprint on the gravitational-wave emission from extreme mass-ratio inspirals in non-pure Kerr spacetimes, we have studied the ‘kludge’ waveforms generated in highly-accurate, numerically-generated spacetimes containing a black hole and a self-gravitating, homogeneous torus with comparable mass and spin. In order to maximize their impact on the produced waveforms, we have considered tori that are compact, massive and close to the central black hole, investigating under what conditions the LISA experiment could detect their presence. Our results show that for a large portion of the space of parameters the waveforms produced by EMRIs in these black hole-torus systems are indistinguishable from pure-Kerr waveforms. Hence, a ‘confusion problem’ will be present for observations carried out over a timescale below or comparable to the dephasing time.

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Authors: D. C. Salisbury

Date: Thu, 25 Jan 2007

Abstract: In a paper appearing in Annalen der Physik in 1930 Leon Rosenfeld invented the first procedure for producing Hamiltonian constraints. He displayed and correctly distinguished the vanishing Hamiltonian generator of time evolution, and the vanishing generator of gauge transformations for general relativity with Dirac electron and electrodynamic field sources. Though he did not do so, had he chosen one of his tetrad fields to be normal to his spacetime foliation, he would have anticipated by almost thirty years the general relativisitic Hamiltonian first published by Paul Dirac.

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Authors: Manuela Campanelli, Carlos O. Lousto, Yosef Zlochower, David Merritt

Date: Sun, 25 Feb 2007

Abstract: Recent calculations of gravitational radiation recoil generated during black-hole binary mergers have reopened the possibility that a merged binary can be ejected even from the nucleus of a massive host galaxy. Here we report the first systematic study of gravitational recoil of equal-mass binaries with equal, but anti-aligned, spins parallel to the orbital plane. Such an orientation of the spins is expected to maximize the recoil. We find that recoil velocity (which is perpendicular to the orbital plane) varies sinusoidally with the angle that the initial spin directions make with the initial linear momenta of each hole and scales up to a maximum of ~4000 km/s for maximally-rotating holes. Our results show that the amplitude of the recoil velocity can depend sensitively on spin orientations of the black holes prior to merger.

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Authors: Matthew D. Duez, Lawrence E. Kidder, Saul A. Teukolsky

Date: Sun, 25 Feb 2007

Abstract: We present a new code for solving the coupled Einstein-hydrodynamics equations to evolve relativistic, self-gravitating fluids. The Einstein field equations are solved on one grid using pseudospectral methods, while the fluids are evolved on another grid by finite differencing. We discuss implementation details, such as the communication between the grids and the treatment of stellar surfaces, and present code tests.

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Authors: Jeremy D. Schnittman, Alessandra Buonanno (U. Maryland)

Date: Fri, 23 Feb 2007

Abstract: We calculate the linear momentum flux from merging black holes (BHs) with arbitrary masses and spin orientations, using the effective-one-body (EOB) model. This model includes an analytic description of the inspiral phase, a short merger, and a superposition of exponentially damped quasi-normal ringdown modes of a Kerr BH. By varying the matching point between inspiral and ringdown, we can estimate the systematic errors generated with this method. Within these confidence limits, we find close agreement with previously reported results from numerical relativity. Using a Monte Carlo implementation of the EOB model, we are able to sample a large volume of BH parameter space and estimate the distribution of recoil velocities. For a range of mass ratios 1 <= m_1/m_2 <= 10, spin magnitudes of a_{1,2}=0.9, and uniform random spin orientations, we find that a fraction f_{500}=0.12ˆ{+0.06}_{-0.05} of binaries have recoil velocities greater than 500 km/s and f_{1000}=0.027ˆ{+0.021}_{-0.014} have kicks greater than 1000 km/s. These velocities likely are capable of ejecting the final BH from its host galaxy. Limiting the sample to comparable-mass binaries with m_1/m_2 <= 4, the typical kicks are even larger, with f_{500}=0.31_{-0.12}ˆ{+0.13} and f_{1000}=0.079ˆ{+0.062}_{-0.042}.

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Authors: Eirini Messaritaki

Date: Fri, 23 Feb 2007

Abstract: The singular field of a point charge has recently been described in terms of a new Green's function of curved spacetime. This singular field plays an important role in the calculation of the self-force acting upon the particle. We provide a method for calculating the singular field and a catalog of expansions of the singular field associated with the geodesic motion of monopole and dipole sources for scalar, electromagnetic and gravitational fields. These results can be used, for example, to calculate the effects of the self-force acting on a particle as it moves through spacetime.

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Authors: Harald P. Pfeiffer, Duncan A. Brown, Lawrence E. Kidder, Lee Lindblom, Geoffrey Lovelace, Mark A. Scheel

Date: Tue, 20 Feb 2007

Abstract: Binary black hole simulations starting from quasi-circular (i.e., zero radial velocity) initial data have orbits with small but non-zero orbital eccentricities. In this paper the quasi-equilibrium initial-data method is extended to allow non-zero radial velocities to be specified in binary black hole initial data. New low-eccentricity initial data are obtained by adjusting the orbital frequency and radial velocities to minimize the orbital eccentricity, and the resulting ($\sim 5$ orbit) evolutions are compared with those of quasi-circular initial data. Evolutions of the quasi-circular data clearly show eccentric orbits, with eccentricity that decays over time. The precise decay rate depends on the definition of eccentricity; if defined in terms of variations in the orbital frequency, the decay rate agrees well with the prediction of Peters (1964). The gravitational waveforms, which contain $\sim 8$ cycles in the dominant l=m=2 mode, are largely unaffected by the eccentricity of the quasi-circular initial data. The overlap between the dominant mode in the quasi-circular evolution and the same mode in the low-eccentricity evolution is about 0.99.

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Authors: Takamasa Chiba, Tatsunori Imai, Hideki Asada

Date: Thu, 8 Feb 2007

Abstract: None of N-body gravitating systems have been considered to emit periodic gravitational waves because of their chaotic orbits when N=3 (or more). We employ a figure-eight orbit as a specific model for a 3-body system in order to illustrate that some of triple stars are capable of generating periodic waves. This illustration would imply that a certain class of N-body gravitating systems may be relevant to the gravitational waves generation. We show also that the total angular momentum of this 3-body system is not carried away by gravitational waves. A waveform generated by this system is volcano-shaped and thus different from that of a binary system. Finally, by evaluating the radiation reaction time scale, we give an order-of-magnitude estimate of merging event rates. The estimate suggests that figure-eight sources, which require carefully prepared initial states, may be too rare to detect.

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Authors: Andrea Passamonti, Nikolaos Stergioulas, Alessandro Nagar

Date: Sat, 17 Feb 2007

Abstract: The post-bounce oscillations of newly-born relativistic stars are expected to lead to gravitational-wave emission through the excitation of nonradial oscillation modes. At the same time, the star is oscillating in its radial modes, with a central density variation that can reach several percent. Nonlinear couplings between radial oscillations and polar nonradial modes lead to the appearance of combination frequencies (sums and differences of the linear mode frequencies). We study such combination frequencies using a gauge-invariant perturbative formalism, which includes bilinear coupling terms between different oscillation modes. For typical values of the energy stored in each mode we find that gravitational waves emitted at combination frequencies could become detectable in galactic core-collapse supernovae with advanced interferometric or wide-band resonant detectors.

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Authors: Linqing Wen

Date: Fri, 16 Feb 2007

Abstract: Several large-scale gravitational wave interferometers have achieved long term operation at design sensitivity. Questions arise on how to best combine all available data from detectors of different sensitivities for detection, consistency check or veto, localization and waveform extraction. We show that these problems can be formulated using the singular value decomposition (SVD) method. We present techniques based on the SVD method for (1) detection statistic, (2) stable solutions to waveforms, (3) null-stream construction for an arbitrary number of detectors, and (4) source localization for GWs of unknown waveforms.

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Authors: John G. Baker, William D. Boggs, Joan Centrella, Bernard J. Kelly, Sean T. McWilliams, M. Coleman Miller, James R. van Meter

Date: Wed, 14 Feb 2007

Abstract: Several groups have recently computed the gravitational radiation recoil produced by the merger of two spinning black holes. The results suggest that spin can be the dominant contributor to the kick, with reported recoil speeds of hundreds to even thousands of kilometers per second. The parameter space of spin kicks is large, however, and it is ultimately desirable to have a simple formula that gives the approximate magnitude of the kick given a mass ratio, spin magnitudes, and spin orientations. As a step toward this goal, we perform a systematic study of the recoil speeds from mergers of black holes with mass ratio $q\equiv m_1/m_2=2/3$ and dimensionless spin parameters of $a_1/m_1$ and $a_2/m_2$ equal to 0 or 0.2, with directions aligned or anti-aligned with the orbital angular momentum. We also run an equal-mass $a_1/m_1=-a_2/m_2=0.2$ case, and find good agreement with previous results. We find that, for currently reported kicks from aligned or anti-aligned spins, a simple kick formula inspired by post-Newtonian analyses can reproduce the numerical results to better than $\sim$10%.

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Authors: Frans Pretorius, Deepak Khurana

Date: Thu, 15 Feb 2007

Abstract: We describe recent numerical simulations of the merger of a class of equal mass, non-spinning, eccentric binary black hole systems in general relativity. We show that with appropriate fine-tuning of the initial conditions to a region of parameter space we denote the threshold of immediate merger, the binary enters a phase of close interaction in a near-circular orbit, stays there for an amount of time proportional to logarithmic distance from the threshold in parameter space, then either separates or merges to form a single Kerr black hole. To gain a better understanding of this phenomena we study an analogous problem in the evolution of equatorial geodesics about a central Kerr black hole. A similar threshold of capture exists for appropriate classes of initial conditions, and tuning to threshold the geodesics approach one of the unstable circular geodesics of the Kerr spacetime. Remarkably, with a natural mapping of the parameters of the geodesic to that of the equal mass system, the scaling exponent describing the whirl phase of each system turns out to be quite similar. Armed with this lone piece of evidence that an approximate correspondence might exist between near-threshold evolution of geodesics and generic binary mergers, we illustrate how this information can be used to estimate the cross section and energy emitted in the ultra relativistic black hole scattering problem. This could eventually be of use in providing estimates for the related problem of parton collisions at the Large Hadron Collider in extra dimension scenarios where black holes are produced.

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Authors: David Garfinkle

Date: Wed, 14 Feb 2007

Abstract: GGR News:
- The View from the NSF, by Beverly Berger
- GGR Program at the APS meeting in Jacksonville we hear that…, by David Garfinkle
- 100 years ago, by David Garfinkle
Research Briefs:
- The Double Pulsar, by Michael Kramer
- Theoretical Approaches to Cosmic Acceleration, by Mark Trodden
Conference Reports:
- Numerical Relativity-Data Analysis, by Patrick Brady
- Note on Numerical Relativity-Data Analysis, by Peter Saulson
- Unruh and Wald Fest, by Carsten Gundlach and David Garfinkle
- Will Fest, by Eric Poisson
- Brane-World Gravity, by Andrew Mennim
- Gravity and Theoretical Physics, by Marco Cavaglia

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Authors: Harald Dimmelmeier, Christian D. Ott, Hans-Thomas Janka, Andreas Marek, Ewald Mueller

Date: Mon, 12 Feb 2007

Abstract: We perform general relativistic simulations of stellar core collapse to a proto-neutron star, using a microphysical equation of state as well as an approximate description of deleptonization. We show that for a wide variety of rotation rates and profiles the gravitational wave burst signals from the core bounce are of a generic type, known as Type I in the literature. In our systematic study, using both general relativity and Newtonian gravity, we identify and individually quantify the micro- and macrophysical mechanisms leading to this result, i.e. the effects of rotation, the equation of state, and deleptonization. Such a generic type of signal templates will likely facilitate a more efficient search in current and future gravitational wave detectors of both interferometric and resonant type.

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Authors: Reinhard Prix

Date: Mon, 12 Feb 2007

Abstract: We derive a simple algebraic criterion to select the optimal detector network for a coherent wide parameter-space (all-sky) search for continuous gravitational waves. Optimality in this context is defined as providing the highest (average) sensitivity per computing cost. This criterion is a direct consequence of the properties of the multi-detector F-statistic metric, which has been derived recently. Interestingly, the choice of the optimal network only depends on the noise-levels and duty-cycles of the respective detectors, and not on the available computing power.

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Authors: R. Oechslin, H.-T. Janka (MPI for Astrophysics, Garching, Germany)

Date: Thu, 8 Feb 2007

Abstract: We analyze the gravitational wave (GW) emission from our recently published set of relativistic neutron star (NS) merger simulations and determine characteristic signal features that allow one to link GW measurements to the properties of the merging binary stars. We find that the distinct peak in the GW energy spectrum that is associated with the formation of a hypermassive merger remnant has a frequency that depends strongly on the properties of the nuclear equation of state (EoS) and on the total mass of the binary system, whereas the mass ratio and the NS spins have a weak influence. If the total mass can be determined from the inspiral chirp signal, the peak frequency of the postmerger signal is a sensitive indicator of the EoS.

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Authors: J. A. Gonzalez, M. D. Hannam, U. Sperhake, B. Brugmann, S. Husa

Date: Fri, 9 Feb 2007

Abstract: Recent calculations of the recoil velocity in binary black hole mergers have found the kick velocity to be of the order of a few hundred km/s in the case of non-spinning binaries and about $500 $km/s in the case of spinning configurations, and have lead to predictions of a maximum kick of up to $1300 $km/s. We test these predictions and demonstrate that kick velocities of at least $2500 $km/s are possible for equal-mass binaries with appropriate alignments of the spins. Kicks of that magnitude are likely to have significant repercussions for models of black-hole formation, the population of intergalactic black holes and the structure of host galaxies.

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Authors: Hideyuki Tagoshi, Himan Mukhopadhyay, Sanjeev Dhurandhar, Norichika Sago, Hirotaka Takahashi, Nobuyuki Kanda

Date: Sat, 3 Feb 2007

Abstract: We discuss the coherent search strategy to detect gravitational waves from inspiraling compact binaries by a network of correlated laser interferometric detectors. From the maximum likelihood ratio statistic, we obtain a coherent statistic which is slightly different from and generally better than what we obtained in our previous work. In the special case when the cross spectrum of two detectors normalized by the power spectrum density is constant, the new statistic agrees with the old one. The quantitative difference of the detection probability for a given false alarm rate is also evaluated in a simple case.

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Authors: Dae-Il Choi, Bernard J. Kelly, William D. Boggs, John G. Baker, Joan Centrella, James van Meter

Date: Fri, 2 Feb 2007

Abstract: Recoil ‘kicks’ induced by gravitational radiation are expected in the inspiral and merger of black holes. Recently the numerical relativity community has begun to measure the significant kicks found when both unequal masses and spins are considered. Because understanding the cause and magnitude of each component of this kick may be complicated in inspiral simulations, we consider these effects in the context of a simple test problem. We study recoils from collisions of binaries with initially head-on trajectories, starting with the simplest case of equal masses with no spin; adding spin and varying the mass ratio, both separately and jointly. We find spin-induced recoils to be significant even in head-on configurations. Additionally, it appears that the scaling of transverse kicks with spins is consistent with post-Newtonian (PN) theory, even though the kick is generated in the nonlinear merger interaction, where PN theory should not apply. This suggests that a simple heuristic description might be effective in the estimation of spin-kicks.

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