## [0904.4481] Enhanced tidal disruption rates from massive black hole binaries

Authors: X. Chen, P. Madau, A. Sesana, F. K. Liu

Date: 28 Apr 2009

Abstract: "Hard" massive black hole (MBH) binaries embedded in steep stellar cusps can shrink via three-body slingshot interactions. We show that this process will inevitably be accompanied by a burst of stellar tidal disruptions, at a rate that can be several orders of magnitude larger than that appropriate for a single MBH. Our numerical scattering experiments reveal that: 1) a significant fraction of stars initially bound to the primary hole are scattered into its tidal disruption loss cone by gravitational interactions with the secondary hole, an enhancement effect that is more pronounced for very unequal-mass binaries; 2) about 25% (40%) of all strongly interacting stars are tidally disrupted by a MBH binary of mass ratio q=1/81 (q=1/243) and eccentricity 0.1; and 3) two mechanisms dominate the fueling of the tidal disruption loss cone, a Kozai non-resonant interaction that causes the secular evolution of the stellar angular momentum in the field of the binary, and the effect of close encounters with the secondary hole that change the stellar orbital parameters in a chaotic way. For a hard MBH binary of 10ˆ7 solar masses and mass ratio 0.01, embedded in an isothermal stellar cusp of velocity dispersion sigma*=100 km/s, the tidal disruption rate can be as large as 1/yr. This is 4 orders of magnitude higher than estimated for a single MBH fed by two-body relaxation. When applied to the case of a putative intermediate-mass black hole inspiraling onto Sgr A*, our results predict tidal disruption rates ~0.05-0.1/yr.

#### Apr 30, 2009

0904.4481 (/preprints)
2009-04-30, 09:06 

## [0904.3810] An Efficient Numerical Method for Computing Gravitational Waves Induced by a Particle Moving on Eccentric Inclined Orbits around a Kerr Black Hole

Authors: Ryuichi Fujita, Wataru Hikida, Hideyuki Tagoshi

Date: 24 Apr 2009

Abstract: We develop a numerical code to compute gravitational waves induced by a particle moving on eccentric inclined orbits around a Kerr black hole. For such systems, the black hole perturbation method is applicable. The gravitational waves can be evaluated by solving the Teukolsky equation with a point like source term, which is computed from the stress-energy tensor of a test particle moving on generic bound geodesic orbits. In our previous papers, we computed the homogeneous solutions of the Teukolsky equation using a formalism developed by Mano, Suzuki and Takasugi and showed that we could compute gravitational waves efficiently and very accurately in the case of circular orbits on the equatorial plane. Here, we apply this method to eccentric inclined orbits. The geodesics around a Kerr black hole have three constants of motion: energy, angular momentum and the Carter constant. We compute the rates of change of the Carter constant as well as those of energy and angular momentum. This is the first time that the rate of change of the Carter constant has been evaluated accurately. We also treat the case of highly eccentric orbits with $e=0.9$. To confirm the accuracy of our codes, several tests are performed. We find that the accuracy is only limited by the truncation of $\ell$-, $k$- and $n$-modes, where $\ell$ is the index of the spin-weighted spheroidal harmonics, and $n$ and $k$ are the harmonics of the radial and polar motion, respectively. When we set the maximum of $\ell$ to 20, we obtain a relative accuracy of $10ˆ{-5}$ even in the highly eccentric case of $e=0.9$. The accuracy is better for lower eccentricity. Our numerical code is expected to be useful for computing templates of the extreme mass ratio inspirals, which is one of the main targets of the Laser Interferometer Space Antenna (LISA).

#### Apr 30, 2009

0904.3810 (/preprints)
2009-04-30, 09:05 

## [0904.4501] Extreme- and Intermediate-Mass Ratio Inspirals in Dynamical Chern-Simons Modified Gravity

Authors: Carlos F. Sopuerta (ICE, CSIC-IEEC), Nicolas Yunes (Princeton)

Date: 29 Apr 2009

Abstract: [abridged] Chern-Simons (CS) modified gravity is a 4D effective theory that descends both from string theory and loop quantum gravity, and that corrects the Einstein-Hilbert action by adding the product of a scalar field and the parity-violating, Pontryagin density. In this theory, the gravitational field of spinning black holes is described by a modified Kerr geometry whose multipole moments deviate from the Kerr ones only at the fourth multipole, l = 4. We investigate possible signatures of this theory in the gravitational wave emission produced in the inspiral of stellar compact objects into massive black holes, both for intermediate- and extreme-mass ratios. We use the semi-relativistic approximation, where the trajectories are geodesics of the massive black hole geometry and the gravitational waveforms are obtained from a multipolar decomposition of the radiative field. The main CS corrections to the waveforms arise from modifications to the geodesic trajectories, due to changes to the massive black hole geometry, and manifest themselves as an accumulating dephasing relative to the general relativistic case. We also explore the propagation and the stress-energy tensor of gravitational waves in this theory. We find that, although this tensor has the same form as in General Relativity, the energy and angular momentum balance laws are indeed modified through the stress-energy tensor of the CS scalar field. These balance laws could be used to describe the inspiral through adiabatic changes in the orbital parameters, which in turn would enhance the dephasing effect. Gravitational-wave observations of intermediate- or extreme-mass ratio inspirals with advanced ground detectors or with LISA could use such dephasing to test the dynamical theory to unprecedented levels.

#### Apr 30, 2009

0904.4501 (/preprints)
2009-04-30, 09:04 

## [0904.4418] Schwarzschild black hole as moving puncture in isotropic coordinates

Authors: Bernd Bruegmann

Date: 28 Apr 2009

Abstract: The success of the moving puncture method for the numerical simulation of black hole systems can be partially explained by the properties of stationary solutions of the 1+log coordinate condition. We compute stationary 1+log slices of the Schwarzschild spacetime in isotropic coordinates in order to investigate the coordinate singularity that the numerical methods have to handle at the puncture. We present an alternative integration method to obtain isotropic coordinates that simplifies numerical integration and that gives direct access to a local expansion in the isotropic radius near the puncture. Numerical results have shown that certain quantities are well approximated by a function linear in the isotropic radius near the puncture, while here we show that in some cases the isotropic radius appears with an exponent that is close to but unequal to one.

#### Apr 30, 2009

0904.4418 (/preprints)
2009-04-30, 09:04 

## [0904.4551] Longterm general relativistic simulation of binary neutron stars collapsing to a black hole

Authors: Kenta Kiuchi, Yuichiro Sekiguchi, Masaru Shibata, Keisuke Taniguchi

Date: 29 Apr 2009

Abstract: General relativistic simulations for the merger of binary neutron stars are performed as an extension of a previous work\cite{Shibata:2006nm}. We prepare binary neutron stars with a large initial orbital separation and employ the moving-puncture formulation, which enables to follow merger and ringdown phases for a long time, even after black hole formation. For modeling inspiraling neutron stars, which should be composed of cold neutron stars, the Akmal-Pandhalipande-Ravenhall (APR) equation of state (EOS) is adopted. After the onset of the merger, the hybrid-type EOS is used; i.e., the cold and thermal parts are given by the APR and $\Gamma$-law EOSs, respectively. Three equal-mass binaries each with mass $1.4M_\odot,1.45M_\odot,1.5M_\odot$ and two unequal-mass binaries with mass 1.3--$1.6M_\odot$, 1.35--$1.65M_\odot$ are prepared. We focus primarily on the black hole formation case, and explore mass and spin of the black hole, mass of disks which surround the black hole, and gravitational waves emitted during the black hole formation. We find that (i) for the systems of $m_0=2.9$--$3.0M_\odot$ and of mass ratio $\approx 0.8$, the mass of disks which surround the formed black hole is 0.006--$0.02M_{\odot}$; (ii) the spin of the formed black hole is $0.78 \pm 0.02$ when a black hole is formed after the merger in the dynamical time scale. This value depends weakly on the total mass and mass ratio, and is about 0.1 larger than that of a black hole formed from nonspinning binary black holes; (iii) for the black-hole formation case, Fourier spectrum shape of gravitational waves emitted in the merger and ringdown phases has a universal qualitative feature irrespective of the total mass and mass ratio, but quantitatively, the spectrum reflects the parameters of the binary neutron stars.

#### Apr 30, 2009

0904.4551 (/preprints)
2009-04-30, 09:04 

## [0904.3541] Remnant Masses, Spins and Recoils from the Merger of Generic Black-Hole Binaries

Authors: Carlos O. Lousto, Manuela Campanelli, Yosef Zlochower (RIT)

Date: 22 Apr 2009

Abstract: We propose simple empirical formulae to describe the final remnant mass, spin, and recoil velocity from the merger of quasi-circular black-hole binaries with arbitrary mass ratios and spins. Our formulae are based on the post-Newtonian scaling with constant parameters chosen by a least-squares fit of the available data from recent fully nonlinear numerical simulations and is relevant to statistical studies of N-body simulations of galaxy cores and clusters, and the cosmological growth of supermassive black holes.

#### Apr 25, 2009

0904.3541 (/preprints)
2009-04-25, 16:02 

## [0904.2207] Delayed rejection schemes for efficient Markov-Chain Monte-Carlo sampling of multimodal distributions

Authors: Miquel Trias, Alberto Vecchio, John Veitch

Date: 14 Apr 2009

Abstract: A number of problems in a variety of fields are characterised by target distributions with a multimodal structure in which the presence of several isolated local maxima dramatically reduces the efficiency of Markov Chain Monte Carlo sampling algorithms. Several solutions, such as simulated tempering or the use of parallel chains, have been proposed to facilitate the exploration of the relevant parameter space. They provide effective strategies in the cases in which the dimension of the parameter space is small and/or the computational costs are not a limiting factor. These approaches fail however in the case of high-dimensional spaces where the multimodal structure is induced by degeneracies between regions of the parameter space. In this paper we present a fully Markovian way to efficiently sample this kind of distribution based on the general Delayed Rejection scheme with an arbitrary number of steps, and provide details for an efficient numerical implementation of the algorithm.

#### Apr 25, 2009

0904.2207 (/preprints)
2009-04-25, 16:02 

## [0904.2577] Predicting the direction of the final spin from the coalescence of two black holes

Authors: Enrico Barausse (UMD), Luciano Rezzolla (AEI)

Date: 16 Apr 2009

Abstract: The knowledge of the spin of the black hole resulting from the merger of a generic binary system of black holes is of great importance to study the cosmological evolution of supermassive black holes. Several attempts have been recently made to model the spin via simple expressions exploiting the results of numerical-relativity simulations. While these expressions are in good agreement with the simulations, they are intrinsically imprecise when predicting the final spin direction, especially if applied to binaries with separations of hundred or thousands of gravitational radii. This is due to neglecting the precession of the orbital plane of the binary, and is a clear drawback if the formulas are employed in cosmological merger-trees or N-body simulations, which provide the spins and angular momentum of the two black holes when their separation is of thousands of gravitational radii. We remove this problem by proposing an expression which is built on improved assumptions and that gives, for any separation, a very accurate prediction both for the norm of the final spin and for its direction. By comparing with the numerical data, we also show that the final spin direction is very accurately aligned with the total angular momentum of the binary at large separation. Hence, observations of the final spin direction (e.g. via a jet) can provide information on the orbital plane of the binary at large separations and could be relevant, for instance, to study X-shaped radio sources.

#### Apr 25, 2009

0904.2577 (/preprints)
2009-04-25, 16:02 

## [0903.4877] Gravitational waves from merging compact binaries

Authors: Scott A. Hughes

Date: 27 Mar 2009

Abstract: Largely motivated by the development of highly sensitive gravitational-wave detectors, our understanding of merging compact binaries and the gravitational waves they generate has improved dramatically in recent years. Breakthroughs in numerical relativity now allow us to model the coalescence of two black holes with no approximations or simplifications. There has also been outstanding progress in our analytical understanding of binaries. We review these developments, examining merging binaries using black hole perturbation theory, post-Newtonian expansions, and direct numerical integration of the field equations. We summarize these approaches and what they have taught us about gravitational waves from compact binaries. We place these results in the context of gravitational-wave generating systems, analyzing the impact gravitational wave emission has on their sources, as well as what we can learn about them from direct gravitational-wave measurements.

#### Apr 15, 2009

0903.4877 (/preprints)
2009-04-15, 16:15 

## [0904.1943] Response of a spaceborne gravitational wave antenna to solar oscillations

Authors: A. G. Polnarev, I. W. Roxburgh, D. Baskaran

Date: 13 Apr 2009

Abstract: We investigate the possibility of observing very small amplitude low frequency solar oscillations with the proposed laser interferometer space antenna (LISA). For frequencies $\nu$ below $3\times 10ˆ{-4} {\rm Hz}$ the dominant contribution is from the near zone time dependent gravitational quadrupole moments associated with the normal modes of oscillation. For frequencies $\nu$ above $3\times 10ˆ{-4} {\rm Hz}$ the dominant contribution is from gravitational radiation generated by the quadrupole oscillations which is larger than the Newtonian signal by a factor of the order $(2 \pi r \nu/ c)ˆ4$, where $r$ is the distance to the Sun, and $c$ is the velocity of light.
The low order solar quadrupole pressure and gravity oscillation modes have not yet been detected above the solar background by helioseismic velocity and intensity measurements. We show that for frequencies $\nu \lesssim 2\times 10ˆ{-4} {\rm Hz}$, the signal due to solar oscillations will have a higher signal to noise ratio in a LISA type space interferometer than in helioseismology measurements. Our estimates of the amplitudes needed to give a detectable signal on a LISA type space laser interferometer imply surface velocity amplitudes on the sun of the order of 1-10 mm/sec in the frequency range $1\times 10ˆ{-4} -5\times 10ˆ{-4} {\rm Hz}$. If such modes exist with frequencies and amplitudes in this range they could be detected with a LISA type laser interferometer.

#### Apr 15, 2009

0904.1943 (/preprints)
2009-04-15, 16:13 

## [0904.1017] Exploring short gamma-ray bursts as gravitational-wave standard sirens

Authors: Samaya Nissanke, Scott A. Hughes, Daniel E. Holz, Neal Dalal, Jonathan L. Sievers

Date: 7 Apr 2009

Abstract: Recent observations support the hypothesis that a large fraction of "short-hard" gamma-ray bursts (SHBs) are associated with compact binary inspiral. Since gravitational-wave (GW) measurements of well-localized inspiraling binaries can measure absolute source distances, simultaneous observation of a binary's GWs and SHB would allow us to independently determine both its luminosity distance and redshift. Such a "standard siren" (the GW analog of a standard candle) would provide an excellent probe of the relatively nearby universe's expansion, complementing other standard candles. In this paper, we examine binary measurement using a Markov Chain Monte Carlo technique to build the probability distributions describing measured parameters. We assume that each SHB observation gives both sky position and the time of coalescence, and we take both binary neutron stars and black hole-neutron star coalescences as plausible SHB progenitors. We examine how well parameters (particularly luminosity distance) can be measured from GW observations of these sources by a range of ground-based detector networks. We find that earlier estimates overstate how well distances can be measured, even at fairly large signal-to-noise ratio. The fundamental limitation to determining distance to these sources is the gravitational waveform's degeneracy between luminosity distance and source inclination. Despite this, we find that excellent results can be achieved by measuring a large number of coalescing binaries, especially if the worldwide network consists of many widely separated detectors. Advanced GW detectors will be able to determine the absolute luminosity distance to an accuracy of 10-30% for NS-NS (out to 600 Mpc) and NS-BH binaries (out to 1400 Mpc). (Abridged)

#### Apr 15, 2009

0904.1017 (/preprints)
2009-04-15, 16:13 

## [0904.1715] Template banks to search for compact binaries with spinning components in gravitational wave data

Authors: Chris Van Den Broeck, Duncan A. Brown, Thomas Cokelaer, Ian Harry, Gareth Jones, B.S. Sathyaprakash, Hideyuki Tagoshi, Hirotaka Takahashi

Date: 10 Apr 2009

Abstract: Gravitational waves from coalescing compact binaries are one of the most promising sources for detectors such as LIGO, Virgo and GEO600. If the components of the binary posess significant angular momentum (spin), as is likely to be the case if one component is a black hole, spin-induced precession of a binary's orbital plane causes modulation of the gravitational-wave amplitude and phase. If the templates used in a matched-filter search do not accurately model these effects then the sensitivity, and hence the detection rate, will be reduced. We investigate the ability of several search pipelines to detect gravitational waves from compact binaries with spin. We use the post-Newtonian approximation to model the inspiral phase of the signal and construct two new template banks using the phenomenological waveforms of Buonanno, Chen and Vallisneri. We compare the performance of these template banks to that of banks constructed using the stationary phase approximation to the non-spinning post-Newtonian inspiral waveform currently used by LIGO and Virgo in the search for compact binary coalescence. We find that, at the same false alarm rate, a search pipeline using phenomenological templates is no more effective than a pipeline which uses non-spinning templates. We recommend the continued use of the non-spinning stationary phase template bank until the false alarm rate associated with templates which include spin effects can be substantially reduced.

#### Apr 15, 2009

0904.1715 (/preprints)
2009-04-15, 16:13 

## [0904.1986] The Breaking Strain of Neutron Star Crust and Gravitational Waves

Authors: C. J. Horowitz (Indiana), Kai Kadau (LANL)

Date: 13 Apr 2009

Abstract: Mountains on rapidly rotating neutron stars efficiently radiate gravitational waves. The maximum possible size of these mountains depends on the breaking strain of neutron star crust. With multi-million ion molecular dynamics simulations of Coulomb solids representing the crust, we show that the breaking strain of pure single crystals is very large and that impurities, defects, and grain boundaries only modestly reduce the breaking strain to around 0.1. Due to the collective behavior of the ions during failure found in our simulations, the neutron star crust is likely very strong and can support mountains large enough so that their gravitational wave radiation could limit the spin periods of some stars and might be detectable in large scale interferometers. Furthermore, our microscopic modeling of neutron star crust material can help analyze mechanisms relevant in magnetar giant and micro flares.

#### Apr 15, 2009

0904.1986 (/preprints)
2009-04-15, 16:12 

## [0904.1826] An exploration of CUDA and CBEA for a gravitational wave data-analysis application (Einstein@Home)

Authors: Jens Breitbart, Gaurav Khanna

Date: 12 Apr 2009

Abstract: We present a detailed approach for making use of two new computer hardware architectures -- CBEA and CUDA -- for accelerating a scientific data-analysis application (Einstein@Home). Our results suggest that both the architectures suit the application quite well and the achievable performance in the same software developmental time-frame, is nearly identical.

#### Apr 15, 2009

0904.1826 (/preprints)
2009-04-15, 16:12 

## [0904.1544] Use of the MultiNest algorithm for gravitational wave data analysis

Authors: Farhan Feroz, Jonathan R Gair, Michael P Hobson, Edward K Porter

Date: 9 Apr 2009

Abstract: We describe an application of the MultiNest algorithm to gravitational wave data analysis. MultiNest is a multimodal nested sampling algorithm designed to efficiently evaluate the Bayesian evidence and return posterior probability densities for likelihood surfaces containing multiple secondary modes. The algorithm employs a set of live points which are updated by partitioning the set into multiple overlapping ellipsoids and sampling uniformly from within them. This set of live points climbs up the likelihood surface through nested iso-likelihood contours and the evidence and posterior distributions can be recovered from the point set evolution. The algorithm is model-independent in the sense that the specific problem being tackled enters only through the likelihood computation, and does not change how the live point set is updated. In this paper, we consider the use of the algorithm for gravitational wave data analysis by searching a simulated LISA data set containing two non-spinning supermassive black hole binary signals. The algorithm is able to rapidly identify all the modes of the solution and recover the true parameters of the sources to high precision.

#### Apr 10, 2009

0904.1544 (/preprints)
2009-04-10, 19:45 

## [0904.1190] Bounding the mass of the graviton with gravitational waves: Effect of higher harmonics in gravitational waveform templates

Authors: K G Arun, Clifford M Will

Date: 7 Apr 2009

Abstract: Observations by laser interferometric detectors of gravitational waves from inspiraling compact binary systems can be used to search for a dependence of the waves' propagation speed on wavelength, and thereby to bound the mass or Compton wavelength of a putative graviton. We study the effect of including higher harmonics, as well as their post-Newtonian amplitude corrections, in the template gravitational waveforms employed in the process of parameter estimation using matched filtering. We consider the bounds that could be achieved using advanced LIGO, a proposed third generation instrument called Einstein Telescope, and the proposed space interferometer LISA. We find that in all cases, the bounds on the graviton Compton wavelength are improved by almost an order of magnitude for higher masses when amplitude corrections are included.

#### Apr 08, 2009

0904.1190 (/preprints)
2009-04-08, 09:49 

## [0903.4177] Observing gravitational waves from the first generation of black holes

Authors: Alberto Sesana, Jonathan Gair, Ilya Mandel, Alberto Vecchio

Date: 24 Mar 2009

Abstract: The properties of the first generation of black-hole seeds trace and distinguish different models of formation of cosmic structures in the high-redshift universe. The observational challenge lies in identifying black holes in the mass range ~100-1000 solar masses at redshift z~10. Here we show that future ground-based laser interferometers could observe gravitational waves produced by the coalescence of the first generation of light seed black-hole binaries and provide, possibly unique, information on the evolution of structures in this era. Using galaxy merger trees and four different models of black hole accretion we find that future detectors could study a few to a few tens of seed black-hole merger events in three years. We show further that a network of detectors will be able to measure the luminosity distance to typical sources to a precision of ~30%, allowing us to be confident of the high-redshift nature of the sources.

#### Apr 07, 2009

0903.4177 (/preprints)
2009-04-07, 12:31 

## [0904.0493] Orbiting binary black hole evolutions with a multipatch high order finite-difference approach

Authors: Enrique Pazos, Manuel Tiglio, Matthew D. Duez, Lawrence E. Kidder, Saul A. Teukolsky

Date: 3 Apr 2009

Abstract: We present numerical simulations of orbiting black holes for around twelve cycles, using a high-order multipatch approach. Unlike some other approaches, the computational speed scales almost perfectly for thousands of processors. Multipatch methods are an alternative to AMR (adaptive mesh refinement), with benefits of simplicity and better scaling for improving the resolution in the wave zone. The results presented here pave the way for multipatch evolutions of black hole-neutron star and neutron star-neutron star binaries, where high resolution grids are needed to resolve details of the matter flow.

#### Apr 06, 2009

0904.0493 (/preprints)
2009-04-06, 08:46 

## [0904.0347] Chance Estimations for Detecting Gravitational Waves with LIGO/Virgo Associated with Gamma Ray Bursts

Authors: Alexander Dietz

Date: 2 Apr 2009

Abstract: Short Gamma Ray Bursts (SGRB) are believed to originate from the merger of two compact objects. If this scenario is correct, SGRB will be accompanied by the emission of strong gravitational waves, detectable by current or planned GW detectors, such as LIGO and Virgo. No detection of a gravitational wave has been made up to date. In this paper I will use a set of SGRB with observed redshifts to fit a model describing the cumulative number of SGRB as a function of redshift, to determine the rate of such merger events in the nearby universe. These estimations will be used to make probability statements about detecting a gravitational wave associated with a short gamma ray burst during the latest science run of LIGO/Virgo. Chance estimations for the enhanced and advanced detectors will also be made, and a comparison between the rates deduced from this work will be compared to the existing literature.

#### Apr 06, 2009

0904.0347 (/preprints)
2009-04-06, 08:46 

## [0903.4686] Gravitational Waves from Broken Cosmic Strings: The Bursts and the Beads

Authors: Louis Leblond, Benjamin Shlaer, Xavier Siemens

Date: 27 Mar 2009

Abstract: We analyze the gravitational wave signatures of a network of metastable cosmic strings. We consider the case of cosmic string instability to breakage, with no primordial population of monopoles. This scenario is well motivated from GUT and string theoretic models with an inflationary phase below the GUT/string scale. The network initially evolves according to a scaling solution, but with breakage events resulting from confined monopoles (beads) being pair produced and accelerated apart. We find these ultra-relativistic beads to be a potent source of gravitational waves bursts, detectable by Initial LIGO, Advanced LIGO, and LISA. Indeed, Advanced LIGO could observe bursts from strings with tensions as low as $G\mu \sim 10ˆ{-12}$. In addition, we find that ultra-relativistic beads produce a scale-invariant stochastic background detectable by LIGO, LISA, and pulsar timing experiments. The stochastic background is scale invariant up to Planckian frequencies. This phenomenology provides new constraints and signatures of cosmic strings that disappear long before the present day.

#### Apr 03, 2009

0903.4686 (/preprints)
2009-04-03, 14:22 

## [0904.0024] Approaches to Understanding Cosmic Acceleration

Authors: Alessandra Silvestri, Mark Trodden

Date: 1 Apr 2009

Abstract: Theoretical approaches to explaining the observed acceleration of the universe are reviewed. We briefly discuss the evidence for cosmic acceleration, and the implications for standard General Relativity coupled to conventional sources of energy-momentum. We then address three broad methods of addressing an accelerating universe: the introduction of a cosmological constant, its problems and origins; the possibility of dark energy, and the associated challenges for fundamental physics; and the option that an infrared modification of general relativity may be responsible for the large-scale behavior of the universe.

#### Apr 03, 2009

0904.0024 (/preprints)
2009-04-03, 14:21 

## [0904.0385] Massive black hole binary evolution in gas-rich mergers

Authors: M. Colpi, S. Callegari, M. Dotti, L. Mayer

Date: 2 Apr 2009

Abstract: We report on key studies on the dynamics of black holes (BHs) in gas-rich galaxy mergers that underscore the vital role played by gas dissipation in promoting BH inspiral down to the smallest scales ever probed with use of high-resolution numerical simulations. In major mergers, the BHs sink rapidly under the action of gas-dynamical friction while orbiting inside the massive nuclear disc resulting from the merger. The BHs then bind and form a Keplerian binary on a scale of 5 pc. In minor mergers, BH pairing proceeds down to the minimum scale explored of 10-100 pc only when the gas fraction in the less massive galaxy is comparatively large to avoid its tidal and/or ram pressure disruption and the wandering of the light BH in the periphery of the main halo. Binary BHs enter the gravitational wave dominated inspiral only when their relative distance is typically of 0.001 pc. If the gas preserves the degree of dissipation expected in a star-burst environment, binary decay continues down to 0.1 pc, the smallest length-scale ever attained. Stalling versus hardening below 0.1 pc is still matter of deep investigations.

#### Apr 03, 2009

0904.0385 (/preprints)
2009-04-03, 14:21