Michele Vallisneri's BlogSpace

[1003.0882] Ultra-relativistic grazing collisions of black holes

Authors: U.Sperhake, V.Cardoso, F.Pretorius, E.Berti, T.Hinderer, N.Yunes

Date: 3 Mar 2010

Abstract: We study gravitational wave emission, zoom-whirl behavior and the resulting spin of the remnant black hole in highly boosted collisions of equal-mass, non spinning black-hole binaries with generic impact parameter.

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[1003.0485] Binary black hole merger gravitational waves and recoil in the large mass ratio limit

Authors: Pranesh A. Sundararajan, Gaurav Khanna, Scott A. Hughes

Date: 2 Mar 2010

Abstract: Spectacular breakthroughs in numerical relativity now make it possible to compute spacetime dynamics in almost complete generality, allowing us to model the coalescence and merger of binary black holes with essentially no approximations. The primary limitation of these calculations is now computational. In particular, it is difficult to model systems with large mass ratio and large spins, since one must accurately resolve the multiple lengthscales which play a role in such systems. Perturbation theory can play an important role in extending the reach of computational modeling for binary systems. In this paper, we present first results of a code which allows us to model the gravitational waves generated by the inspiral, merger, and ringdown of a binary system in which one member of the binary is much more massive than the other. This allows us to accurately calibrate binary dynamics in the large mass ratio regime. We focus in this analysis on the recoil imparted to the merged remnant by these waves. We closely examine the "antikick", an anti-phase cancellation of the recoil arising from the plunge and ringdown waves, described in detail by Schnittman et al. We find that, for orbits aligned with the black hole spin, the antikick grows as a function of spin. The total recoil is smallest for prograde coalescence into a rapidly rotating black hole, and largest for retrograde coalescence. Amusingly, this completely reverses the predicted trend for kick versus spin from analyses that only include inspiral information.

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[1003.0390] Binary spinning black hole Hamiltonian in canonical center-of-mass and rest-frame coordinates through higher post-Newtonian order

Authors: Tilman J. Rothe, Gerhard Schäfer

Date: 1 Mar 2010

Abstract: The recently constructed Hamiltonians for spinless binary black holes through third post-Newtonian order and for spinning ones through formal second post-Newtonian order, where the spins are counted of zero post-Newtonian order, are transformed into fully canonical center-of-mass and rest-frame variables. The mixture terms in the Hamiltonians between center-of-mass and rest-frame variables are in accordance with the relation between the total linear momentum and the center-of-mass velocity as demanded by global Lorentz invariance. The various generating functions for the center-of-mass and rest-frame canonical variables are explicitly given in terms of the single-particle canonical variables. The no-interaction theorem does not apply because the world-line condition of Lorentz covariant position variables is not imposed.

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[1003.0597] Binary black hole merger in the extreme-mass-ratio limit: a multipolar analysis

Authors: Sebastiano Bernuzzi, Alessandro Nagar

Date: 2 Mar 2010

Abstract: Building up on previous work, we present a new calculation of the gravitational wave (GW) emission generated during the transition from quasi-circular inspiral to plunge, merger and ringdown by a binary system of nonspinning black holes, of masses $m_1$ and $m_2$, in the extreme mass ratio limit, $m_1 m_2\ll(m_1+m_2)ˆ2$. The relative dynamics of the system is computed {\it without making any adiabatic approximation} by using an effective one body (EOB) description, namely by representing the binary by an effective particle of mass $\mu=m_1 m_2/(m_1+m_2)$ moving in a (quasi-)Schwarzschild background of mass $M=m_1+m_2$ and submitted to an $\O(\nu)$ 5PN-resummed analytical radiation reaction force, with $\nu=\mu/M$. The gravitational wave emission is calculated via a multipolar Regge-Wheeler-Zerilli type perturbative approach (valid in the limit $\nu\ll 1$). We consider three mass ratios, $\nu={10ˆ{-2},10ˆ{-3},10ˆ{-4}}$,and we compute the multipolar waveform up to $\ell=8$. We estimate energy and angular momentum losses during the quasi-universal and quasi-geodesic part of the plunge phase and we analyze the structure of the ringdown. We calculate the gravitational recoil, or "kick", imparted to the merger remnant by the gravitational wave emission and we emphasize the importance of higher multipoles to get a final value of the recoil $v/(c\nuˆ2)=0.0446$. We finally show that there is an {\it excellent fractional agreement} ($\sim 10ˆ{-3}$) (even during the plunge) between the 5PN EOB analytically-resummed radiation reaction flux and the numerically computed gravitational wave angular momentum flux. This is a further confirmation of the aptitude of the EOB formalism to accurately model extreme-mass-ratio inspirals, as needed for the future space-based LISA gravitational wave detector.

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[1002.4552] A 4PN-exact approximation to General Relativity

Authors: David Brizuela, Gerhard Schaefer

Date: 24 Feb 2010

Abstract: An approximation to General Relativity is presented which agrees with the Einstein field equations up to and including the fourth post-Newtonian (PN) order. This approximation is formulated in a fully constrained scheme: all involved equations are explicitly elliptic except the wave equation that describes the two independent degrees of freedom of the gravitational field. The formalism covers naturally the conformal-flat-condition (CFC) approach by Isenberg, Wilson, and Mathews and the improved second PN-order exact approach CFC+. For stationary configurations, like Kerr black holes, agreement with General Relativity is achieved even through 5PN order. In addition, we analyze in detail a particularly interesting 2PN-exact waveless approximation which results from imposing more restrictive conditions. The proposed scheme can be considered as a further development on the waveless approach suggested by Schaefer and Gopakumar [Phys. Rev. D {\bf 69}, 021501 (2004)].

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[1002.3153] Astrophysical Measurement of the Equation of State of Neutron Star Matter

Authors: Feryal Ozel, Gordon Baym, Tolga Guver

Date: 16 Feb 2010

Abstract: We present the first astrophysical measurement of the pressure of cold matter above nuclear saturation density, based on recently determined masses and radii of three neutron stars. The pressure at higher densities are below the predictions of equations of state that account only for nucleonic degrees of freedom, and thus present a challenge to the microscopic theory of neutron star matter.

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[1002.3515] IGEC2: A 17-month search for gravitational wave bursts in 2005-2007

Authors: P. Astone, L. Baggio, M. Bassan, M. Bignotto, M. Bonaldi, P. Bonifazi, M. Cerdonio, E. Coccia, L. Conti, S. D'Antonio, M. di Paolo Emilio, M. Drago, V. Fafone, P. Falferi, S. Foffa, P. Fortini, S. Frasca, G. Giordano, W.O. Hamilton, J. Hanson, W.W. Johnson, N. Liguori, S. Longo, M. Maggiore, F. Marin, A. Marini, M. P. McHugh, R. Mezzena, P. Miller, Y. Minenkov, A. Mion, G. Modestino, A. Moleti, D. Nettles, A. Ortolan, G.V. Pallottino, G. Pizzella, S. Poggi, G.A. Prodi, V. Re, A. Rocchi, F. Ronga, F. Salemi, R. Sturani, L. Taffarello, R. Terenzi, G. Vedovato, A. Vinante, M. Visco, S. Vitale, J. Weaver, J.P. Zendri, P. Zhang

Date: 18 Feb 2010

Abstract: We present here the results of a 515 days long run of the IGEC2 observatory, consisting of the four resonant mass detectors ALLEGRO, AURIGA, EXPLORER and NAUTILUS. The reported results are related to the fourfold observation time from Nov. 6 2005 until Apr. 14 2007, when Allegro ceased its operation. This period overlapped with the first long term observations performed by the LIGO interferometric detectors. The IGEC observations aim at the identification of gravitational wave candidates with high confidence, keeping the false alarm rate at the level of 1 per century, and high duty cycle, namely 57% with all four sites and 94% with at least three sites in simultaneous observation. The network data analysis is based on time coincidence searches over at least three detectors: the four 3-fold searches and the 4-fold one are combined in a logical OR. We exchanged data with the usual blind procedure, by applying a unique confidential time offset to the events in each set of data. The accidental background was investigated by performing sets of 10ˆ8 coincidence analyses per each detector configuration on off-source data, obtained by shifting the time series of each detector. The thresholds of the five searches were tuned so as to control the overall false alarm rate to 1/century. When the confidential time shifts was disclosed, no gravitational wave candidate was found in the on-source data. As an additional output of this search, we make available to other observatories the list of triple coincidence found below search thresholds, corresponding to a false alarm rate of 1/month.

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[1002.4185] EM counterparts of recoiling black holes: general relativistic simulations of non-Keplerian discs

Authors: Olindo Zanotti, Luciano Rezzolla, Luca Del Zanna, Carlos Palenzuela

Date: 22 Feb 2010

Abstract: We investigate the dynamics of a circumbinary disc that responds to the loss of mass and to the recoil velocity of the black hole produced by the merger of a binary system of supermassive black holes. More specifically, we perform the first two-dimensional general relativistic hydrodynamics simulations of \textit{extended} non-Keplerian discs and employ a new technique to construct a "shock detector", thus determining the precise location of the shocks produced in the accreting disc by the recoiling black hole. In this way we can study how the properties of the system, such as the spin, mass and recoil velocity of the black hole, affect the mass accretion rate and are imprinted on the electromagnetic emission from these sources. In contrast with what done in similar works, we here question the estimates of the bremsstrahlung luminosity when computed without properly taking into account the radiation transfer, thus yielding cooling times that are unrealistically short. At the same time we show, through an approximation based on the relativistic analogue of the isothermal evolution of \citet{Corrales2009}, that the luminosity produced can reach a peak value above $L \simeq 10ˆ{43} {\rm erg/s} $ at about $\sim 20 {\rm d}$ after the merger of a binary with total mass $M\simeq 10ˆ6 M_\odot$ and persist for several days at values which are a factor of a few smaller. If confirmed by more sophisticated calculations such a signal could indeed lead to an electromagnetic counterpart of the merger of binary black-hole system.

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[1002.2386] Gravitational self-force on a particle in eccentric orbit around a Schwarzschild black hole

Authors: Leor Barack, Norichika Sago

Date: 11 Feb 2010

Abstract: We present a numerical code for calculating the local gravitational self-force acting on a pointlike particle in a generic (bound) geodesic orbit around a Schwarzschild black hole. The calculation is carried out in the Lorenz gauge: For a given geodesic orbit, we decompose the Lorenz-gauge metric perturbation equations (sourced by the delta-function particle) into tensorial harmonics, and solve for each harmonic using numerical evolution in the time domain (in 1+1 dimensions). The physical self-force along the orbit is then obtained via mode-sum regularization. The total self-force contains a dissipative piece as well as a conservative piece, and we describe a simple method for disentangling these two pieces in a time-domain framework. The dissipative component is responsible for the loss of orbital energy and angular momentum through gravitational radiation; as a test of our code we demonstrate that the work done by the dissipative component of the computed force is precisely balanced by the asymptotic fluxes of energy and angular momentum, which we extract independently from the wave-zone numerical solutions. The conservative piece of the self force does not affect the time-averaged rate of energy and angular-momentum loss, but it influences the evolution of the orbital phases; this piece is calculated here for the first time in eccentric strong-field orbits. As a first concrete application of our code we recently reported the value of the shift in the location and frequency of the innermost stable circular orbit due to the conservative self-force [Phys. Rev. Lett.\ {\bf 102}, 191101 (2009)]. Here we provide full details of this analysis, and discuss future applications.

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[1002.1962] Testing General Relativity with Current Cosmological Data

Authors: Scott F. Daniel, Eric V. Linder, Tristan L. Smith, Robert R. Caldwell, Asantha Cooray, Alexie Leauthaud, Lucas Lombriser

Date: 10 Feb 2010

Abstract: Deviations from general relativity, such as could be responsible for the cosmic acceleration, would influence the growth of large scale structure and the deflection of light by that structure. We clarify the relations between several different model independent approaches to deviations from general relativity appearing in the literature, devising a translation table. We examine current constraints on such deviations, using weak gravitational lensing data of the CFHTLS and COSMOS surveys, cosmic microwave background radiation data of WMAP5, and supernova distance data of Union2. Markov Chain Monte Carlo likelihood analysis of the parameters over various redshift ranges yields consistency with general relativity at the 95% confidence level.

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[1002.2591] Probing strong-field gravity and black holes with gravitational waves

Authors: Scott A. Hughes

Date: 12 Feb 2010

Abstract: Gravitational wave observations will be excellent tools for making precise measurements of processes that occur in very strong-field regions of spacetime. Extreme mass ratio systems, formed by the capture of a stellar mass body compact by a massive black hole, will be targets for planned space-based interferometers such as LISA and DECIGO. These systems will be especially powerful tools for testing the spacetime nature of black hole candidates. In this writeup of the talk I gave at JGRG19, I describe how the properties of black holes are imprinted on their waveforms, and how measurements can be used to study these properties and thereby learn about the astrophysics of black holes and about strong-field gravity.

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[1002.2643] Final spins from the merger of precessing binary black holes

Authors: Michael Kesden, Ulrich Sperhake, Emanuele Berti

Date: 13 Feb 2010

Abstract: The inspiral of binary black holes is governed by gravitational radiation reaction at binary separations r < 1000 M, yet it is too computationally expensive to begin numerical-relativity simulations with initial separations r > 10 M. Fortunately, binary evolution between these separations is well described by post-Newtonian equations of motion. We examine how this post-Newtonian evolution affects the distribution of spin orientations at separations r near 10 M where numerical-relativity simulations typically begin. Although isotropic spin distributions at r =1000 M remain isotropic at r = 10 M, distributions that are initially partially aligned with the orbital angular momentum can be significantly distorted during the post-Newtonian inspiral. Spin-orbit resonances tend to align (anti-align) the binary black hole spins with each other if the spins were initially partially aligned (anti-aligned) with respect to the orbital angular momentum, thus increasing (decreasing) the average final spin. Resonant effects are stronger for comparable-mass binaries, and they could produce significant spin alignment in massive black hole mergers at high redshifts and in stellar-mass black hole binaries. We also point out that precession induces an intrinsic accuracy limitation of 0.03 in the dimensionless spin magnitude, and about 20 degrees in the direction in predicting the final spin resulting from widely separated binary configurations.

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[1002.1876] Searching for gravitational waves emitted by binaries with spinning components

Authors: Gareth Jones

Date: 9 Feb 2010

Abstract: In this thesis we consider the data analysis problem of detecting gravitational waves emitted by inspiraling binary systems. Detection of gravitational waves will open a new window on the Universe enabling direct detection of systems such as binary black holes for the first time. In the first Chapter we show how gravitational waves are derived from Einstein's General theory of Relativity and discuss the emission of gravitational waves from inspiraling binaries and how this radiation may be detected using laser interferometers. Around two thirds of stars inhabit binary systems. As they orbit each other they will emit both energy and angular momentum in the form of gravitational waves which will inevitably lead to their inspiral and eventual merger. To date, searches for gravitational waves emitted during the inspiral of binary systems have concentrated on systems with non-spinning components. In Chapter 2 we detail the first dedicated search for binaries consisting of spinning stellar mass compact objects. We analysed 788 hours of data collected during the third science run (S3) of the LIGO detectors, no detection of gravitational waves was made and we set an upper limit on the rate of coalescences of stellar mass binaries. The inspiral of stellar mass compact objects into super massive black holes will radiate gravitational waves at frequencies detectable by the planned space-based LISA mission. In Chapter 3 we describe the development and testing of a computationally cheap method to detect the loudest few extreme mass ratio inspiral events that LISA will be sensitive to.

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[gr-qc/0207052] On the frequency shift of gravitational waves

Authors: Claudio M. G. de Sousa

Date: 13 Jul 2002

Abstract: Considering plane gravitational waves propagating through flat spacetime, it is shown that curvatures experienced both in the starting point and during their arrival at the earth can cause a considerable shift in the frequencies as measured by earth and space-based detectors. Particularly for the case of resonant bar detectors this shift can cause noise-filters to smother the signal.

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[1002.1555] Gravitational Waves from Collapsing Domain Walls

Authors: Takashi Hiramatsu, Masahiro Kawasaki, Ken'ichi Saikawa

Date: 8 Feb 2010

Abstract: We study the production of gravitational waves from cosmic domain walls created during phase transition in the early universe. We investigate the process of formation and evolution of domain walls by running three dimensional lattice simulations. If we introduce an approximate discrete symmetry, walls become metastable and finally disappear. We calculate the spectrum of gravitational waves produced by collapsing metastable domain walls. Extrapolating the numerical results, we find the signal of gravitational waves produced by domain walls whose energy scale is around 10ˆ10-10ˆ12GeV will be observable in the next generation gravitational wave interferometers.

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[1002.2093] Reduced Hamiltonian for next-to-leading order Spin-Squared Dynamics of General Compact Binaries

Authors: Steven Hergt, Jan Steinhoff, Gerhard Schaefer

Date: 10 Feb 2010

Abstract: Within the post Newtonian framework the fully reduced Hamiltonian (i.e., with eliminated spin supplementary condition) for the next-to-leading order spin-squared dynamics of general compact binaries is presented. The Hamiltonian is applicable to the spin dynamics of all kinds of binaries with self-gravitating components like black holes and/or neutron stars taking into account spin-induced quadrupolar deformation effects in second post-Newtonian order perturbation theory of Einstein's field equations. The corresponding equations of motion for spin, position and momentum variables are given in terms of canonical Poisson brackets. Comparison with a nonreduced potential calculated within the Effective Field Theory approach is made.

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[1002.1291] Discriminating between a Stochastic Gravitational Wave Background and Instrument Noise

Authors: Matthew R. Adams, Neil J. Cornish

Date: 5 Feb 2010

Abstract: The detection of a stochastic background of gravitational waves could significantly impact our understanding of the physical processes that shaped the early Universe. The challenge lies in separating the cosmological signal from other stochastic processes such as instrument noise and astrophysical foregrounds. One approach is to build two or more detectors and cross correlate their output, thereby enhancing the common gravitational wave signal relative to the uncorrelated instrument noise. When only one detector is available, as will likely be the case with the Laser Interferometer Space Antenna (LISA), alternative analysis techniques must be developed. Here we show that models of the noise and signal transfer functions can be used to tease apart the gravitational and instrument noise contributions. We discuss the role of gravitational wave insensitive "null channels" formed from particular combinations of the time delay interferometry, and derive a new combination that maintains this insensitivity for unequal arm length detectors. We show that, in the absence of astrophysical foregrounds, LISA could detect signals with energy densities as low as $\Omega_{\rm gw} = 6 \times 10ˆ{-13}$ with just one month of data. We describe an end-to-end Bayesian analysis pipeline that is able to search for, characterize and assign confidence levels for the detection of a stochastic gravitational wave background, and demonstrate the effectiveness of this approach using simulated data from the third round of Mock LISA Data Challenges.

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[1002.1036] All-sky search for gravitational-wave bursts in the first joint LIGO-GEO-Virgo run

Authors: The LIGO Scientific Collaboration, Virgo Collaboration

Date: 4 Feb 2010

Abstract: We present results from an all-sky search for unmodeled gravitational-wave bursts in the data collected by the LIGO, GEO 600 and Virgo detectors between November 2006 and October 2007. The search is performed by three different analysis algorithms over the frequency band 50-6000,Hz. Data are analyzed for times with at least two of the four LIGO-Virgo detectors in coincident operation, with a total live time of 266 days. No events produced by the search algorithms survive the selection cuts. We set a frequentist upper limit on the rate of gravitational-wave bursts impinging on our network of detectors. When combined with the previous LIGO search of the data collected between November 2005 and November 2006, the upper limit on the rate of detectable gravitational-wave bursts in the 64-2048 Hz band is 2.0 events per year at 90% confidence. We also present event rate versus strength exclusion plots for several types of plausible burst waveforms. The sensitivity of the combined search is expressed in terms of the root-sum-squared strain amplitude for a variety of simulated waveforms and lies in the range 6x10-22 1/sqrt(Hz) to 2x10-20 1/sqrt(Hz. This is the first untriggered burst search to use data from the LIGO and Virgo detectors together, and the most sensitive untriggered burst search performed so far.

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[1001.3686] The Pioneer Anomaly

Authors: Slava G. Turyshev, Viktor T. Toth

Date: 20 Jan 2010

Abstract: Radio-metric Doppler tracking data received from the Pioneer 10 and 11 spacecraft from heliocentric distances of 20-70 AU has consistently indicated the presence of a small, anomalous, blue-shifted frequency drift uniformly changing with a rate of ~6 x 10ˆ{-9} Hz/s. Ultimately, the drift was interpreted as a constant sunward deceleration of each particular spacecraft at the level of a_P = (8.74 +/- 1.33) x 10ˆ{-10} m/sˆ2. This apparent violation of the Newton's gravitational inverse-square law has become known as the Pioneer anomaly; the nature of this anomaly remains unexplained. In this review, we summarize the current knowledge of the physical properties of the discovered effect and the conditions that led to its detection and characterization. We review various mechanisms proposed to explain the anomaly and discuss the current state of efforts to determine its nature. A comprehensive new investigation of the anomalous behavior of the two Pioneers has begun recently. The new efforts rely on the much-extended set of radio-metric Doppler data for both spacecraft in conjunction with the newly available complete record of their telemetry files and a large archive of original project documentation. As the new study is yet to report its findings, this review provides the necessary background for the new results to appear in the near future. In particular, we provide a significant amount of information on the design, operations and behavior of the two Pioneers during their entire missions, including descriptions of various data formats and techniques used for their navigation and radio-science data analysis. As most of this information was recovered relatively recently, it was not used in the previous studies of the Pioneer anomaly, but it is critical for the new investigation.

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[1001.4704] Precision timing of PSR J1012 5307 and strong-field GR tests

Authors: Kosmas Lazaridis, Norbert Wex, Axel Jessner, Michael Kramer, J. Anton Zensus, Ben W. Stappers, Gemma H. Janssen, Mark B. Purver, Andrew G. Lyne, Christine A. Jordan, Gregory Desvignes, Ismael Cognard, Gilles Theureau

Date: 26 Jan 2010

Abstract: We report on the high precision timing analysis of the pulsar-white dwarf binary PSR J1012+5307. Using 15 years of multi-telescope data from the European Pulsar Timing Array (EPTA) network, a significant measurement of the variation of the orbital period is obtained. Using this ideal strong-field gravity laboratory we derive theory independent limits for both the dipole radiation and the variation of the gravitational constant.

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