Michele's wiki/blog

Authors: P. Jaranowski, K. G. Arun, L. Barack, L. Blanchet, A. Buonanno, M. F. De Laurentis, S. Detweiler, H. Dittus, M. Favata, G. Faye, J. L. Friedman, K. Ganz, W. Hikida, B. R. Iyer, T. S. Keidl, Dong-Hoon Kim, K. D. Kokkotas, B. Kol, A. S. Kubeka, C. Lämmerzahl, J. Majár, E. Messaritaki, A. Nagar, H. Nakano, L. R. Price, M. S. S. Qusailah, N. Radicella, N. Sago, D. Singh, H. Sotani, T. Tanaka, A. Tartaglia, M. Vasúth, I. Vega, B. F. Whiting, A. G. Wiseman, S. Yoshida

Date: 30 Oct 2007

Abstract: The paper summarizes the parallel session B3 {\em Analytic approximations, perturbation methods, and their applications} of the GR18 conference. The talks in the session reported notably recent advances in black hole perturbations and post-Newtonian approximations as applied to sources of gravitational waves.

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Authors: Giovanni Amelino-Camelia, John Stachel

Date: 30 Oct 2007

Abstract: Several recent studies have been devoted to investigating the limitations that ordinary quantum mechanics and/or quantum gravity might impose on the measurability of space-time observables. These analyses are often confined to the simplified context of two-dimensional flat space-time and rely on a simple procedure for the measurement of space-like distances based on the exchange of light signals. We present a generalization of this measurement procedure applicable to all three types of space-time intervals between two events in space-times of any number of dimensions. We also present some preliminary observations on an alternative measurement procedure that can be applied taking into account the gravitational field of the measuring apparatus, and briefly discuss quantum limitations of measurability in this context.

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Authors: Carlos O. Lousto, Hiroyuki Nakano (RIT)

Date: 29 Oct 2007

Abstract: There are periodic solutions to the equal-mass three-body (and N-body) problem in Newtonian gravity. The figure-eight solution is one of them. In this paper, we discuss its solution in the first and second post-Newtonian approximations to General Relativity. To do so we derive the canonical equations of motion in the ADM gauge from the three-body Hamiltonian. We then integrate those equations numerically, showing that quantities such as the energy, linear and angular momenta are conserved down to numerical error. We also study the scaling of the initial parameters with the physical size of the triple system. In this way we can assess when general relativistic results are important and we determine that this occur for distances of the order of 100M, with M the total mass of the system. For distances much closer than those, presumably the system would completely collapse due to gravitational radiation. This sets up a natural cut-off to Newtonian N-body simulations. The method can also be used to dynamically provide initial parameters for subsequent full nonlinear numerical simulations.

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Authors: Keisuke Taniguchi, Thomas W. Baumgarte, Joshua A. Faber, Stuart L. Shapiro

Date: 26 Oct 2007

Abstract: We construct new models of black hole-neutron star binaries in quasiequilibrium circular orbits by solving Einstein's constraint equations in the conformal thin-sandwich decomposition together with the relativistic equations of hydrostationary equilibrium. We adopt maximal slicing, assume spatial conformal flatness, and impose equilibrium boundary conditions on an excision surface (i.e., the apparent horizon) to model the black hole. In our previous treatment we adopted a "leading-order" approximation for a parameter related to the black-hole spin in these boundary conditions to construct approximately nonspinning black holes. Here we improve on the models by computing the black hole's quasilocal spin angular momentum and setting it to zero. As before, we adopt a polytropic equation of state with adiabatic index Gamma=2 and assume the neutron star to be irrotational. In addition to recomputing several sequences for comparison with our earlier results, we study a wider range of neutron star masses and binary mass ratios. To locate the innermost stable circular orbit we search for turning points along both the binding energy and total angular momentum curves for these sequences. Unlike for our previous approximate boundary condition, these two minima now coincide. We also identify the formation of cusps on the neutron star surface, indicating the onset of tidal disruption. Comparing these two critical binary separations for different mass ratios and neutron star compactions we distinguish those regions that will lead to a tidal disruption of the neutron star from those that will result in the plunge into the black hole of a neutron star more or less intact, albeit distorted by tidal forces.

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Authors: Jonathan R Gair, Ilya Mandel, Linqing Wen

Date: 27 Oct 2007

Abstract: Extreme-mass-ratio inspirals (EMRIs) of ~ 1-10 solar-mass compact objects into ~ million solar-mass massive black holes can serve as excellent probes of strong-field general relativity. The Laser Interferometer Space Antenna (LISA) is expected to detect gravitational wave signals from apprxomiately one hundred EMRIs per year, but the data analysis of EMRI signals poses a unique set of challenges due to their long duration and the extensive parameter space of possible signals. One possible approach is to carry out a search for EMRI tracks in the time-frequency domain. We have applied a time-frequency search to the data from the Mock LISA Data Challenge (MLDC) with promising results. Our analysis used the Hierarchical Algorithm for Clusters and Ridges to identify tracks in the time-frequency spectrogram corresponding to EMRI sources. We then estimated the EMRI source parameters from these tracks. In these proceedings, we discuss the results of this analysis of the MLDC round 1.3 data.

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Authors: Alicia M. Sintes, for the LIGO Scientific Collaboration

Date: 25 Oct 2007

Abstract: We report on an all-sky search with the LIGO detectors for periodic gravitational waves in the frequency range 50-1000 Hz and having a negative frequency time derivative with magnitude between zero and $10ˆ{-8}$ Hz/s. Data from the fourth LIGO science run have been used in this search. Three different semi-coherent methods of summing strain power were applied. Observing no evidence for periodic gravitational radiation, we report upper limits on strain amplitude and interpret these limits to constrain radiation from rotating neutron stars.

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Authors: Francisco S. N. Lobo

Date: 24 Oct 2007

Abstract: The General Theory of Relativity has been an extremely successful theory, with a well established experimental footing, at least for weak gravitational fields. Its predictions range from the existence of black holes, gravitational radiation to the cosmological models, predicting a primordial beginning, namely the big-bang. All these solutions have been obtained by first considering a plausible distribution of matter, and through the Einstein field equation, the spacetime metric of the geometry is determined. However, one may solve the Einstein field equation in the reverse direction, namely, one first considers an interesting and exotic spacetime metric, then finds the matter source responsible for the respective geometry. In this manner, it was found that some of these solutions possess a peculiar property, namely 'exotic matter,' involving a stress-energy tensor that violates the null energy condition. These geometries also allow closed timelike curves, with the respective causality violations. These solutions are primarily useful as 'gedanken-experiments' and as a theoretician's probe of the foundations of general relativity, and include traversable wormholes and superluminal 'warp drive' spacetimes. Thus, one may be tempted to denote these geometries as 'exotic' solutions of the Einstein field equation, as they violate the energy conditions and generate closed timelike curves. In this article, in addition to extensively exploring interesting features, in particular, the physical properties and characteristics of these 'exotic spacetimes,' we also analyze other non-trivial general relativistic geometries which generate closed timelike curves.

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Authors: Gregory Mendell, Karl Wette

Date: 23 Oct 2007

Abstract: We investigate methods to estimate the parameters of the gravitational-wave signal from a spinning neutron star using Fourier transformed segments of the strain response from an interferometric detector. Estimating the parameters from the power, we find generalizations of the PowerFlux method. Using simulated elliptically polarized signals injected into Gaussian noise, we apply the generalized methods to estimate the squared amplitudes of the plus and cross polarizations (and, in the most general case, the polarization angle), and test the relative detection efficiencies of the various methods.

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Authors: Ryan N. Lang, Scott A. Hughes

Date: 19 Oct 2007

Abstract: Massive black hole binary coalescences are prime targets for space-based gravitational-wave (GW) observatories. GW measurements can localize the position of a coalescing binary on the sky to an ellipse with major axis ${a few} \times 10$ arcminutes to a few degrees, depending on source redshift, and a minor axis which is $2 - 4$ times smaller. Neglecting weak gravitational lensing, GWs would also determine the source's luminosity distance to better than percent accuracy for close sources, degrading to several percent for more distant sources. Assuming a well-measured cosmology, the source's redshift could be inferred with similar accuracy. GWs alone can thus pinpoint a binary to a 3-dimensional ‘pixel,’ guiding searches for the hosts of these events. We examine the time evolution of this pixel, studying it at merger and at several intervals before merger. One day before merger, the major axis of the error ellipse is typically larger than its final value by a factor $\sim 1.5-6$. The minor axis is larger by $\sim 2-9$, and the error in the luminosity distance is larger by a factor $\sim 1.5-7$. This large change over short time is due to spin-induced precession, which is strongest in the final days before merger. The evolution is slower as we go back further in time. For $z = 1$, we find that GWs will localize a binary to within $\sim 10$ square degrees as early as a month prior to merger and determine distance (and hence redshift) to several percent. [Abridged]

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Authors: Kjell Rosquist, Tomas Bylund, Lars Samuelsson

Date: 23 Oct 2007

Abstract: A new formulation of Carter's constant for geodesic motion in Kerr black holes is given. It is shown that Carter's constant corresponds to the total angular momentum plus a precisely defined part which is quadratic in the linear momenta. The characterization is exact in the weak field limit obtained by letting the gravitational constant go to zero. It is suggested that the new form can be useful in current studies of the dynamics of extreme mass ratio inspiral (EMRI) systems emitting gravitational radiation.

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Authors: U. Sperhake, E. Berti, V. Cardoso, J. A. Gonzalez, B. Bruegmann, M. Ansorg

Date: 21 Oct 2007

Abstract: We study the transition from inspiral to plunge in general relativity by computing gravitational waveforms of non-spinning, equal-mass black-hole binaries. We consider two sequences of simulations. The longer (shorter) sequence starts with a quasi-circular inspiral completing about 2.3 (1.5) orbits prior to coalescence of the holes. For each sequence, the binding energy of the system is kept constant and the orbital angular momentum is progressively reduced to zero, producing orbits of increasing eccentricity and eventually a head-on collision. We analyze in detail the radiation of energy and angular momentum in gravitational waves, the contribution of different multipolar components and the final spin of the remnant. We find that the motion transitions from inspiral to plunge when the orbital angular momentum L=L_crit is about 0.8M. For L<L_crit the radiated energy drops very rapidly. Orbits with L of about L_crit produce the largest dimensionless Kerr parameter for the remnant, j=J/Mˆ2=0.705. Generalizing a model recently proposed by Buonanno, Kidder and Lehner to eccentric binaries, we conjecture that (1) j=0.705 is the maximal Kerr parameter that can be obtained by any merger of non-spinning holes, and (2) no binary merger (even if the binary members are extremal Kerr black holes with spins aligned to the orbital angular momentum, and the inspiral is highly eccentric) can violate the cosmic censorship conjecture.

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Authors: Auriga-Explorer-Nautilus-Virgo collaborations

Date: 19 Oct 2007

Abstract: We present results of the search for coincident burst excitations over a 24 hours long data set collected by AURIGA, EXPLORER, NAUTILUS and Virgo detectors during September 2005. The search of candidate triggers was performed independently on each of the data sets from single detectors. We looked for two-fold time coincidences between these candidates using an algorithm optimized for a given population of sources and we calculated the efficiency of detection through injections of templated signal waveforms into the streams of data. To this purpose we have considered the case of signals shaped as damped sinusoids coming from the galactic center direction. In this framework our method targets an optimal balance between high efficiency and low false alarm rate, aiming at setting confidence intervals as stringent as possible in terms of the rate of the selected source models.

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Authors: Brandon Carter

Date: 18 Oct 2007

Abstract: This is the first part of a survey whose ultimate purpose is to clarify the significance of the famous coincidence between the Hubble age of the universe and a certain combination of microphysical parameters. In this part the way is prepared by a discussion of the manner in which familiar local phenomena depend qualitatively, and in order of magnitude, quantitatively on the fundamental parameters of microphysics. In order to keep the account concise while remaining self contained, only the barest essentials of the standard nuclear physical and astrophysical calculations involved are given. Only six of the fundamental parameters play a dominant part, namely the coupling constants of the strong, electromagnetic, and gravitational forces, and the mass ratios of the proton, neutron, electron and pi-meson. Attention is drawn to the important consequences of three coincidental relationships between these parameters. It is shown that most of the principle limiting masses of astrophysics arise (in fundamental units) simply as the reciprocal of the gravitational fine structure constant, with relatively small adjustment factors. The dividing point between red dwarf and blue giant stars turns out to be an exception: this division occurs within the range of the main sequence stars only as a consequence of the rather exotic coincidence that the ninth power of the electromagnetic fine structure constant is roughly equal to the square root of the gravitational fine structure constant.

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Authors: Andrew Moylan

Date: 18 Oct 2007

Abstract: The software tool GRworkbench is an ongoing project in visual, numerical General Relativity at The Australian National University. This year, GRworkbench has been significantly extended to facilitate numerical experimentation. The numerical differential geometric engine has been rewritten using functional programming techniques, enabling fundamental concepts to be directly represented as variables in the C++ code of GRworkbench. Sophisticated general numerical methods have replaced simpler specialised algorithms. Various tools for numerical experimentation have been implemented, allowing for the simulation of complex physical situations.
A recent claim, that the mass of the Milky Way can be measured using a small interferometer located on the surface of the Earth, has been investigated, and found to be an artifact of the approximations employed in the analysis. This difficulty is symptomatic of the limitations of traditional pen-and-paper analysis in General Relativity, which was the motivation behind the original development of GRworkbench. The physical situation pertaining to the claim has been modelled in a numerical experiment in GRworkbench, without the necessity of making any simplifying assumptions, and an accurate estimate of the effect has been obtained.

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Authors: Luciano Rezzolla, Peter Diener, Ernst Nils Dorband, Denis Pollney, Christian Reisswig, Erik Schnetter, Jennifer Seiler

Date: 17 Oct 2007

Abstract: Determining the final spin of a black-hole binary is a question of key importance in astrophysics, and it has attracted a renewed interest since numerical-relativity simulations have allowed the first quantitative measures. Modelling this quantity in general is made difficult by the fact that it depends on the 7-dimensional space of parameters characterizing the two initial black holes. However, in special cases, when symmetries can be exploited, the description can become considerably simpler. For black-hole binaries with unequal masses but with equal spins which are aligned with the orbital angular momentum, we show that the use of recent numerical-relativity simulations and basic but exact constraints inspired from the extreme mass-ratio limit allow to model this quantity with a simple analytic expression. Despite the simple dependence, the fitting expression models very accurately all of the available estimates, with errors of a couple of percent at most. We also discuss how to use the fitting function to predict when a Schwarzschild black hole is produced by the merger of two spinning black holes, when the spin of the spacetime ‘flips’ sign, or under what conditions the final black hole is ‘spun-up’ by the merger. Last but not least, we show that the fit obtained here can be trivially extended to consider unequal-spin binaries, thus providing a complete description of the final spin from the coalescence of generic black-hole binaries with spins aligned to the orbital angular momentum.

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Authors: Andrew J. Moylan, Susan M. Scott, Antony C. Searle

Date: 16 Oct 2007

Abstract: The software tool GRworkbench is an ongoing project in visual, numerical General Relativity at The Australian National University. Recently, the numerical differential geometric engine of GRworkbench has been rewritten using functional programming techniques. By allowing functions to be directly represented as program variables in C++ code, the functional framework enables the mathematical formalism of Differential Geometry to be more closely reflected in GRworkbench . The powerful technique of ‘automatic differentiation’ has replaced numerical differentiation of the metric components, resulting in more accurate derivatives and an order-of-magnitude performance increase for operations relying on differentiation.

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Authors: Viktor T. Toth, Slava G. Turyshev

Date: 14 Oct 2007

Abstract: The Pioneer 10/11 spacecraft yielded the most precise navigation in deep space to date. However, their radio-metric tracking data received from the distances between 20--70 astronomical units from the Sun consistently indicated the presence of a small, anomalous, Doppler frequency drift. The drift is a blue frequency shift that can be interpreted as a sunward acceleration of a_P = (8.74 +/- 1.33) x 10ˆ(-10) m/sˆ2 for each particular spacecraft. This signal has become known as the Pioneer anomaly; the nature of this anomaly remains unexplained.
New Pioneer 10 and 11 radio-metric Doppler data recently became available. The much extended set of Pioneer Doppler data is the primary source for new upcoming investigation of the anomaly. We also have almost entire records of flight telemetry files received from the the Pioneers. Together with original project documentation and newly developed software tools, this additional information is now used to reconstruct the engineering history of both spacecraft. To that extent, a thermal model of the Pioneer vehicles is being developed to study possible contribution of thermal recoil force acting on the two spacecraft. In addition, to improve the accuracy of orbital reconstruction, we developed a new approach that uses actual flight telemetry data during trajectory analysis of radio-metric Doppler files. The ultimate goal of these efforts is to investigate possible contributions of the thermal recoil force to the detected anomalous acceleration.

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Authors: P. Ajith, S. Babak, Y. Chen, M. Hewitson, B. Krishnan, A. M. Sintes, J. T. Whelan, B. Bruegmann, P. Diener, N. Dorband, J. Gonzalez, M. Hannam, S. Husa, D. Pollney, L. Rezzolla, L. Santamaria, U. Sperhake, J. Thornburg

Date: 11 Oct 2007

Abstract: Gravitational waveforms from the inspiral and ring-down stages of the binary black hole coalescences can be modelled accurately by approximation/perturbation techniques in general relativity. Recent progress in numerical relativity has enabled us to model also the non-perturbative merger phase of the binary black-hole coalescence problem. This enables us to \emph{coherently} search for all three stages of the coalescence of non-spinning binary black holes using a single template bank. Taking our motivation from these results, we propose a family of template waveforms which can model the inspiral, merger, and ring-down stages of the coalescence of non-spinning binary black holes that follow quasi-circular inspiral. This two-dimensional template family is explicitly parametrized by the physical parameters of the binary. We show that the template family is not only \emph{effectual} in detecting the signals from black hole coalescences, but also \emph{faithful} in estimating the parameters of the binary. We compare the sensitivity of a search (in the context of different ground-based interferometers) using all three stages of the black hole coalescence with other template-based searches which look for individual stages separately. We find that the proposed search is significantly more sensitive than other template-based searches for a substantial mass-range, potentially bringing about remarkable improvement in the event-rate of ground-based interferometers. As part of this work, we also prescribe a general procedure to construct interpolated template banks using non-spinning black hole waveforms produced by numerical relativity.

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Authors: M.V. van der Sluys, C. Roever, A. Stroeer, N. Christensen, V. Kalogera, R. Meyer, A. Vecchio

Date: 10 Oct 2007

Abstract: Inspiral signals from binary compact objects (black holes and neutron stars) are primary targets of the ongoing searches by a number of ground-based gravitational-wave interferometers (LIGO, Virgo, GEO-600 and TAMA-300). Detection of such inspirals and ensuing mergers is expected to provide us with important physical information about the properties of the sources, bearing on outstanding issues in compact-object astrophysics, including the progenitors of short gamma-ray bursts. Compact-object spin effects add to the challenges associated with searches and anticipated detections, but on the other hand they provide some interesting possibilities for extracting astrophysical information. We present parameter-estimation simulations for inspirals of black-hole binaries with neutron-star companions using Markov-Chain Monte-Carlo methods. We specifically highlight the potential for measurements of masses, spins, source sky location and distance of such objects with just one or two gravitational-wave detectors.

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Authors: Atsushi Nishizawa, Seiji Kawamura, Tomotada Akutsu, Koji Arai, Kazuhiro Yamamoto, Daisuke Tatsumi, Erina Nishida, Masa-aki Sakagami, Takeshi Chiba, Ryuichi Takahashi, Naoshi Sugiyama

Date: 10 Oct 2007

Abstract: Recently, observational searches for gravitational wave background (GWB) have developed and given direct and indirect constraints on the energy density of GWB in a broad range of frequencies. These constraints have already rejected some theoretical models of large GWB spectra. However, at 100 MHz, there is no strict upper limit from direct observation, though the indirect limit by He4 abundance due to big-bang nucleosynthesis exists. In this paper, we propose an experiment with laser interferometers searching GWB at 100 MHz. We considered three detector designs and evaluated the GW response functions of a single detector. As a result, we found that, at 100 MHz, the most sensitive detector is the design, a so-called synchronous recycling interferometer, which has better sensitivity than an ordinary Fabry-Perot Michelson interferometer by a factor of 3.3 at 100 MHz. We also give the best sensitivity achievable at 100 MHz with realistic experimental parameters.

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Authors: Ariel Caticha

Date: 4 Oct 2007

Abstract: What is information? Is it physical? We argue that in a Bayesian theory the notion of information must be defined in terms of its effects on the beliefs of rational agents. Information is whatever constrains rational beliefs and therefore it is the force that induces us to change our minds. This problem of updating from a prior to a posterior probability distribution is tackled through an eliminative induction process that singles out the logarithmic relative entropy as the unique tool for inference. The resulting method of Maximum relative Entropy (ME), which is designed for updating from arbitrary priors given information in the form of arbitrary constraints, includes as special cases both MaxEnt (which allows arbitrary constraints) and Bayes' rule (which allows arbitrary priors). Thus, ME unifies the two themes of these workshops -- the Maximum Entropy and the Bayesian methods -- into a single general inference scheme that allows us to handle problems that lie beyond the reach of either of the two methods separately. I conclude with a couple of simple illustrative examples.

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Authors: Frans Pretorius

Date: 6 Oct 2007

Abstract: The two-body problem in general relativity is reviewed, focusing on the final stages of the coalescence of the black holes as uncovered by recent successes in numerical solution of the field equations.

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Authors: J.-F. Pascual-Sanchez (University of Valladolid)

Date: 5 Oct 2007

Abstract: Emission relativistic coordinates are a class of spacetime coordinates defined and generated by four emitters (satellites, pulsars) broadcasting their proper time by radio signals. They are the main ingredient of the simplest conceivable relativistic positioning system. The emission coordinates are independent of any observer. Receiving directly the proper time at emission of four satellites, any user or observer can measure the values of the emission coordinates, from which he/she can obtain his trajectory and hence, in particular, his position. Moreover, if and only if the four satellites also broadcast to the users the proper times they are receiving by cross-link autonavigation from the other emitters, the positioning system is called autolocated or autonomous. In an autolocated positioning system the trajectories of the satellites of the constellation can also be known by the users and they can also obtain the metric of the spacetime (the gravitational field) on the constellation.
The study of autolocated relativistic positioning systems has been initiated by Coll and coll. several years ago and it has been aimed for developing an exact fully relativistic theory of positioning systems and gravimetry, based on the framework and concepts of General Relativity. This exact relativistic framework is the alternative to considering post-newtonian relativistic corrections in a classical Newtonian framework, which is the customary approach yet now used in GPS and GLONASS.

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Authors: T.W. Murphy, Jr., E.G. Adelberger, J.B.R. Battat, L.N. Carey, C.D. Hoyle, P. LeBlanc, E.L. Michelsen, K. Nordtvedt, A.E. Orin, J.D. Strasburg, C.W. Stubbs, H.E. Swanson, E. Williams

Date: 3 Oct 2007

Abstract: A next-generation lunar laser ranging apparatus using the 3.5 m telescope at the Apache Point Observatory in southern New Mexico has begun science operation. APOLLO (the Apache Point Observatory Lunar Laser-ranging Operation) has achieved one-millimeter range precision to the moon which should lead to approximately one-order-of-magnitude improvements in the precision of several tests of fundamental properties of gravity. We briefly motivate the scientific goals, and then give a detailed discussion of the APOLLO instrumentation.

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

Date: 3 Oct 2007

Abstract: We present the first fully relativistic longterm numerical evolutions of three equal-mass black holes in a hierarchical system consisting of a third black hole in orbit about a black-hole binary at twice the binaries separation. We find that these close-three-black-hole systems can have very different merger dynamics than black-hole binaries. In particular, we see distinctive waveforms, a suppression of the emitted gravitational radiation, and a redistribution of the energy of the system that can impart substantial kicks to one of the members of the binary. We evolve two such configurations and find very different behaviors. In one configuration the binary is quickly disrupted and the individual holes follow complicated trajectories and merge with the third hole in rapid succession, while in the other, the binary completes a half-orbit before the initial merger of one of the members with the third black hole, and the resulting two-black-hole system forms a highly elliptical, well separated binary that shows no significant inspiral for (at least) the first t~1000M of evolution.

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Authors: Lawrence E. Kidder

Date: 2 Oct 2007

Abstract: The increasing sophistication and accuracy of numerical simulations of compact binaries (especially binary black holes) presents the opportunity to test the regime in which post-Newtonian (PN) predictions for the emitted gravitational waves are accurate. In order to confront numerical results with those of post-Newtonian theory, it is convenient to compare multipolar decompositions of the two waveforms. It is pointed out here that the individual modes can be computed to higher post-Newtonian order by examining the radiative multipole moments of the system, rather than by decomposing the 2.5PN polarization waveforms. In particular, the dominant (l = 2, m = 2) mode can be computed to 3PN order. Individual modes are computed to as high a post-Newtonian order as possible given previous post-Newtonian results.

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Authors: Hideki Asada

Date: 2 Oct 2007

Abstract: We reexamined the gravitational time delay for static spherically symmetric spacetimes, allowing for various models of modified gravity. There exists a degeneracy among theoretical models when the time delay along a single light path is measured. In order to break this degeneracy, multiple light paths are required. Measuring the time delays along two different light paths can probe a deviation from general relativity (or the parameterized post-Newtonian gravity). In order to distinguish modified gravity models, three (or more) paths are needed. Experiments by radio signal from a spacecraft such as New Horizons and future space-borne laser interferometric detectors such as LISA and DECIGO could be a probe of modified gravity in the solar system.

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Authors: Rahul Biswas, Patrick R. Brady, Jolien D. E. Creighton, Stephen Fairhurst

Date: 2 Oct 2007

Abstract: The use of the loudest observed event to generate statistical statements about rate and strength has become standard in searches for gravitational waves from compact binaries and pulsars. The Bayesian formulation of the method is generalized in this paper to allow for uncertainties both in the background estimate and in the properties of the population being constrained. The method is also extended to allow rate interval construction. Finally, it is shown how to combine the results from multiple experiments and a comparison is drawn between the upper limit obtained in a single search and the upper limit obtained by combining the results of two experiments each of half the original duration. To illustrate this, we look at an example case, motivated by the search for gravitational waves from binary inspiral.

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Authors: Michael Boyle, Duncan A. Brown, Lawrence E. Kidder, Abdul H. Mroue, Harald P. Pfeiffer, Mark A. Scheel, Gregory B. Cook, Saul A. Teukolsky

Date: 30 Sep 2007

Abstract: Numerical simulations of 15 orbits of an equal-mass binary black hole system are presented. Gravitational waveforms from these simulations, covering more than 30 cycles and ending about 1.5 cycles before merger, are compared with those from quasi-circular zero-spin post-Newtonian (PN) formulae. The cumulative phase uncertainty of these comparisons is about 0.05 radians, dominated by effects arising from the small residual spins of the black holes and the small residual orbital eccentricity in the simulations. Matching numerical results to PN waveforms early in the run yields excellent agreement (within 0.05 radians) over the first $\sim 15$ cycles, thus validating the numerical simulation and establishing a regime where PN theory is accurate. In the last 15 cycles to merger, however, {\em generic} time-domain Taylor approximants build up phase differences of several radians. But, apparently by coincidence, one specific post-Newtonian approximant, TaylorT4 at 3.5PN order, agrees much better with the numerical simulations, with accumulated phase differences of less than 0.05 radians over the 30-cycle waveform. Gravitational-wave amplitude comparisons are also done between numerical simulations and post-Newtonian, and the agreement depends on the post-Newtonian order of the amplitude expansion: the amplitude difference is about 6--7% for zeroth order and becomes smaller for increasing order. A newly derived 3.0PN amplitude correction improves agreement significantly ($<1%$ amplitude difference throughout most of the run, increasing to 4% near merger) over the previously known 2.5PN amplitude terms.

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Authors: Alessandra Buonanno

Date: 28 Sep 2007

Abstract: These lectures are envisioned to be an introductory, basic course in gravitational-wave physics.

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Authors: Craig J. Hogan

Date: 5 Sep 2007

Abstract: A brief survey is presented of new science that will emerge during the decades ahead from direct detection of gravitational radiation. Interferometers on earth and in space will probe the universe in an entirely new way by directly sensing motions of distant matter over a range of more than a million in frequency. The most powerful sources of gravitational (or indeed any form of) energy in the universe are inspiralling and merging binary black holes; with LISA data, they will become the most distant, most completely and precisely modeled, and most accurately measured systems in astronomy outside the solar system. Other sources range from already known and named nearby Galactic binary stars, to compact objects being swallowed by massive black holes, to possible effects of new physics: phase transitions and superstrings from the early universe, or holographic noise from quantum fluctuations of local spacetime.

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Authors: Max Tegmark (MIT)

Date: 25 Sep 2007

Abstract: I advocate an extreme "shut-up-and-calculate" approach to physics, where our external physical reality is assumed to be purely mathematical. This brief essay motivates this "it's all just equations" assumption and discusses its implications.

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