Michele's wiki/blog

Authors: László Á. Gergely, Zoltán Keresztes, Balázs Mikóczi

Date: Fri, 24 Mar 2006

Abstract: Binary systems subject to generic perturbations evolve on quasiperiodic orbits. We derive the most generic class of perturbations (including both the Brumberg force and gravitational radiation reaction to sufficiently high orders), which allow to evaluate secular effects via generalized complex true and eccentric anomaly parameters, by use of the residue theorem. We illustrate the power of the method by computing the self-spin contributions to the gravitational luminosity of a compact binary.

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Authors: Xavier Siemens, Jolien Creighton, Irit Maor, Saikat Ray Majumder, Kipp Cannon, Jocelyn Read

Date: Wed, 29 Mar 2006

Abstract: We discuss data analysis techniques that can be used in the search for gravitational wave bursts from cosmic strings. When data from multiple interferometers are available, we describe consistency checks that can be used to greatly reduce the false alarm rates. We construct an expression for the rate of bursts for arbitrary cosmic string loop distributions and apply it to simple known solutions. The cosmology is solved exactly and includes the effects of a late-time acceleration. We find substantially lower burst rates than previous estimates suggest and explain the disagreement. Initial LIGO is unlikely to detect field theoretic cosmic strings with the usual loop sizes, though it may detect cosmic superstrings as well as cosmic strings and superstrings with non-standard loop sizes (which may be more realistic). In the absence of a detection, we show how to set upper limits based on the loudest event. Using Initial LIGO sensitivity curves, we show that these upper limits may result in interesting constraints on the parameter space of theories that lead to the production of cosmic strings.

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Authors: Clovis Hopman, Tal Alexander (Weizmann)

Date: Mon, 13 Mar 2006

Abstract: Gravitational waves (GWs) from the inspiral of compact remnants (CRs) into massive black holes (MBHs) will be observable to cosmological distances. While a CR spirals in, 2-body scattering by field stars may cause it to fall into the MBH before reaching a short period orbit that would give an observable signal. As a result, only CRs very near (~0.01 pc) the MBH can spiral in successfully. In a multi-mass stellar population, the heaviest objects sink to the center, where they are more likely to slowly spiral into the MBH without being swallowed prematurely. We study how mass-segregation modifies the stellar distribution and the rate of GW events. We find that the inspiral rate per galaxy for white dwarfs is 30 per Gyr, for neutron stars 6 per Gyr, and for stellar black holes (SBHs) 250 per Gyr. The high rate for SBHs is due to their extremely steep density profile, n_{BH}®\propto rˆ{-2}. The GW detection rate will be dominated by SBHs.

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Authors: Christian Koenigsdoerffer, Achamveedu Gopakumar

Date: Tue, 14 Mar 2006

Abstract: We obtain an efficient description for the dynamics of nonspinning compact binaries moving in inspiralling eccentric orbits to implement the phasing of gravitational waves from such binaries at the 3.5 post-Newtonian (PN) order. Our computation heavily depends on the phasing formalism, presented in [T. Damour, A. Gopakumar, and B. R. Iyer, Phys. Rev. D \textbf{70}, 064028 (2004)], and the 3PN accurate generalized quasi-Keplerian parametric solution to the conservative dynamics of nonspinning compact binaries moving in eccentric orbits, available in [R.-M. Memmesheimer, A. Gopakumar, and G. Sch\"afer, Phys. Rev. D \textbf{70}, 104011 (2004)]. The gravitational-wave (GW) polarizations $h_{+}$ and $h_{\times}$ with 3.5PN accurate phasing should be useful for the earth-based GW interferometers, current and advanced, if they plan to search for gravitational waves from inspiralling eccentric binaries. Our results will be required to do \emph{astrophysics} with the proposed space-based GW interferometers like LISA, BBO, and DECIGO.

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

Date: Fri, 10 Mar 2006

Abstract: The recent localization of some short-hard gamma ray bursts (GRBs) in galaxies with low star formation rates has lent support to the suggestion that these events result from compact object binary mergers. We discuss how new simulations in general relativity are helping to identify the central engine of short-hard GRBs. Motivated by our latest relativistic black hole-neutron star merger calculations, we discuss a scenario in which these events may trigger short-hard GRBs, and compare this model to competing relativistic models involving binary neutron star mergers and the delayed collapse of hypermassive neutron stars. Distinguishing features of these models may help guide future GRB and gravitational wave observations to identify the nature of the sources.

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Authors: Masaru Shibata, Keisuke Taniguchi

Date: Mon, 6 Mar 2006

Abstract: Three-dimensional simulations for the merger of binary neutron stars (BNSs) are performed in the framework of full general relativity. We pay particular attention to the black hole (BH) formation case and to the resulting mass of the surrounding disk for exploring possibility for formation of the central engine of short-duration gamma-ray bursts. Hybrid equations of state (EOSs) are adopted mimicking realistic, stiff nuclear EOSs, for which the maximum allowed gravitational mass of cold and spherical neutron stars (NSs), M_sph, is larger than 2M_sun. For the simulations, we focus on BNSs of the ADM mass M>2.6M_sun. For M>M_thr, the merger results in prompt formation of a BH irrespective of the mass ratio Q_M with 0.65<Q_M<1. The value of M_thr is approximately written as 1.3-1.35M_sph for the chosen EOSs. For the BH formation case, we evolve the spacetime using a BH excision technique and determine the mass of a quasistationary disk surrounding the BH. The disk mass steeply increases with decreasing the value of Q_M for given ADM mass and EOS. For M<M_thr, the outcome is a hypermassive neutron star (HMNS) of a large ellipticity. If the HMNS collapses to a BH after the longterm angular momentum transport, the disk mass may be >0.01M_sun. Gravitational waves (GWs) are computed in terms of a gauge-invariant wave extraction technique. In the formation of the HMNS, quasiperiodic GWs of frequency (3-3.5kHz) are emitted. The effective amplitude of GWs can be >5x10ˆ{-21} at a distance of 50 Mpc. For the BH formation case, the BH excision technique enables a longterm computation and extraction of ring-down GWs associated with a BH quasinormal mode. It is found that the frequency and amplitude are 6.5-7kHz and 10ˆ{-22} at a distance of 50Mpc for M=2.7-2.9M_sun.

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Authors: Tristan L. Smith, Elena Pierpaoli, Marc Kamionkowski (Caltech)

Date: Mon, 6 Mar 2006

Abstract: Primordial gravitational waves (GWs) with frequencies > 10ˆ{-15} Hz contribute to the radiation density of the Universe at the time of decoupling of the cosmic microwave background (CMB). The effects of this GW background on the CMB and matter power spectra are identical to those due to massless neutrinos, unless the initial density-perturbation amplitude for the gravitational-wave gas is non-adiabatic, as may occur if such GWs are produced during inflation or some post-inflation phase transition. In either case, current observations provide a constraint to the GW amplitude that competes with that from big-bang nucleosynthesis (BBN), although it extends to much lower frequencies (~10ˆ{-15} Hz rather than the ~10ˆ{-10} Hz lower limit from BBN): at 95% confidence-level, Omega_gw hˆ2 < 6.9 x 10ˆ{-6} for homogeneous (i.e., non-adiabatic) initial conditions. Future CMB experiments, like Planck and CMBPol, should allow sensitivities to Omega_gw hˆ2 < 1.4 x 10ˆ{-6} and Omega_gw hˆ2 < 5 x 10ˆ{-7}, respectively.

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Authors: M. Benacquista, K. Holley-Bockelmann

Date: Sat, 4 Mar 2006

Abstract: Gravitational radiation from the Galactic population of close white dwarf binaries (CWDBs) is expected to produce a confusion-limited signal at the lower end of the sensitivity band of the Laser Interferometer Space Antenna (LISA). The canonical scale height of the disk population has been taken to be 90 pc for most studies of the nature of this confusion-limited signal. This estimate is probably too low, and the consequences of a more realistic scale height are investigated with a model of the LISA signal due to populations of close white dwarf binaries with different scale heights. If the local space density of CWDBs is held constant, increasing the scale height results in both an increase in the overall strength of the confusion-limited signal as well as in increase in the frequency at which the signals become individually resolvable. If the total number of binaries is held constant, increasing the scale height results in a reduction of the number of expected bright signals above the confusion-limited signal at low frequencies. We introduce an estimator for comparing this transition frequency that takes into account the signal spreading at higher frequencies.

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Authors: Eric Gourgoulhon (LUTH, CNRS / Observatoire de Paris)

Date: Sun, 5 Mar 2006

Abstract: This lecture provides some introduction to perfect fluid dynamics within the framework of general relativity. The presentation is based on the Carter-Lichnerowicz approach. It has the advantage over the more traditional approach of leading very straightforwardly to important conservation laws, such as the relativistic generalizations of Bernoulli's theorem or Kelvin's circulation theorem. It also permits to get easily first integrals of motion which are particularly useful for computing equilibrium configurations of relativistic stars in rotation or in binary systems. The presentation is relatively self-contained and does not require any a priori knowledge of general relativity. In particular, the three types of derivatives involved in relativistic hydrodynamics are introduced in detail: this concerns the Lie, exterior and covariant derivatives.

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Authors: Tristan L. Smith (Caltech), Marc Kamionkowski (Caltech), Asantha Cooray (Caltech and UC Irvine)

Date: Wed, 1 Mar 2006

Abstract: Inflation generically predicts a stochastic background of gravitational waves over a broad range of frequencies, from those accessible with cosmic microwave background (CMB) measurements, to those accessible directly with gravitational-wave detectors, like NASA's Big-Bang Observer (BBO) or Japan's Deci-Hertz Interferometer Gravitational-wave Observer (DECIGO), both currently under study. Here we investigate the detectability of the inflationary gravitational-wave background at BBO/DECIGO frequencies. To do so, we survey a range of slow-roll inflationary models consistent with constraints from the CMB and large-scale structure (LSS). We go beyond the usual assumption of power-law power spectra, which may break down given the 16 orders of magnitude in frequency between the CMB and direct detection, and solve instead the inflationary dynamics for four classes of inflaton potentials. Direct detection is possible in a variety of inflationary models, although probably not in any in which the gravitational-wave signal does not appear in the CMB polarization. However, direct detection by BBO/DECIGO can help discriminate between inflationary models that have the same slow-roll parameters at CMB/LSS scales.

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Authors: Jorge Pullin (Editor)

Date: Tue, 28 Feb 2006

Abstract: GGR News:
GGR program at the APS April meeting in Dallas
We hear that…, 100 years ago, by Jorge Pullin
Research Briefs:
What's new in LIGO, by David Shoemaker
LISA Pathfinder, by Paul McNamara
Recent progress in binary black hole simulations, by Thomas Baumgarte
Conference reports:
Workshop on Emergence of Spacetime, by Olaf Dreyer
Quantum gravity subprogram at the Isaac Newton Institute, by Jorma Louko
Global problems in Math Relativity at the Newton Institute, by Jim Isenberg
Loops '05, by Thomas Thiemann
Numrel 2005, by Scott Hawley and Richard Matzner
Apples With Apples Workshop in Argentina, by Sascha Husa

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