## [1106.4984] Nonlocal metric formulations of MOND with sufficient lensing

Authors: Cedric Deffayet, Gilles Esposito-Farese, Richard P. Woodard

Date: 24 Jun 2011

Abstract: We demonstrate how to construct purely metric modifications of gravity which agree with general relativity in the weak field regime appropriate to the solar system, but which possess an ultra-weak field regime when the gravitational acceleration becomes comparable to $a_0 \sim 10ˆ{-10} {\rm m/s}ˆ2$. In this ultra-weak field regime, the models reproduce the MOND force without dark matter and also give enough gravitational lensing to be consistent with existing data. Our models are nonlocal and might conceivably derive from quantum corrections to the effective field equations.

#### Jun 30, 2011

1106.4984 (/preprints)
2011-06-30, 11:54 

## [1106.5184] Final State of Gregory-Laflamme Instability

Authors: Luis Lehner, Frans Pretorius

Date: 26 Jun 2011

Abstract: We describe the behavior of a perturbed 5-dimensional black string subject to the Gregory-Laflamme instability. We show that the horizon evolves in a self-similar manner, where at any moment in the late-time development of the instability the horizon can be described as a sequence of 3-dimensional spherical black holes of varying size, joined by black string segments of similar radius. As with the initial black string, each local string segment is itself unstable, and this fuels the self-similar cascade to (classically) arbitrarily small scales; in the process the horizon develops a fractal structure. In finite asymptotic time, the remaining string segments shrink to zero-size, yielding a naked singularity. Since no fine-tuning is required to excite the instability, this constitutes a generic violation of cosmic censorship. We further discuss how this behavior is related to satellite formation in low-viscosity fluid streams subject to the Rayleigh-Plateau instability, and estimate the fractal dimension of the horizon prior to formation of the naked singularity.

#### Jun 30, 2011

1106.5184 (/preprints)
2011-06-30, 11:52 

## [1106.5152] Scalar-tensor theory with enhanced gravitational effects

Authors: F. O. Minotti

Date: 25 Jun 2011

Abstract: It is shown that a Brans-Dicke scalar-tensor gravitational theory, which also includes Bekerstein's kind of interaction between the Maxwell and scalar fields, has a particular kind of solutions with highly enhanced gravitational effects as compared with General Relativity, prone to laboratory tests.

#### Jun 30, 2011

1106.5152 (/preprints)
2011-06-30, 11:52 

## [1106.5582] Gravitational Waves in Viable f(R) Models

Authors: Louis Yang, Chung-Chi Lee, Chao-Qiang Geng

Date: 28 Jun 2011

Abstract: We study gravitational waves in viable $f(R)$ theories under a non-zero background curvature. In general, an $f(R)$ theory contains an extra scalar degree of freedom corresponding to a massive scalar mode of gravitational wave. For viable $f(R)$ models, since there always exits a de-Sitter point where the background curvature in vacuum is non-zero, the mass squared of the scalar mode of gravitational wave is about the de-Sitter point curvature $R_{d}\sim10ˆ{-66}eVˆ{2}$. We illustrate our results in two types of viable $f(R)$ models: the exponential gravity and Starobinsky models. In both cases, the mass will be in the order of $10ˆ{-33}eV$ when it propagates in vacuum. However, in the presence of matter density in galaxy, the scalar mode can be heavy. Explicitly, in the exponential gravity model, the mass becomes almost infinity, implying the disappearance of the scalar mode of gravitational wave, while the Starobinsky model gives the lowest mass around $10ˆ{-24}eV$, corresponding to the lowest frequency of $10ˆ{-9}$ Hz, which may be detected by the current and future gravitational wave probes, such as LISA and ASTROD-GW.

#### Jun 30, 2011

1106.5582 (/preprints)
2011-06-30, 11:51 

## [1106.5555] Anisotropies in the Gravitational-Wave Stochastic Background

Authors: S. Olmez, V. Mandic, X. Siemens

Date: 28 Jun 2011

Abstract: We consider anisotropies in the stochastic background of gravitational-waves (SBGW) arising from random fluctuations in the number of gravitational-wave sources. We first develop the general formalism which can be applied to different cosmological or astrophysical scenarios. We then apply this formalism to calculate the anisotropies of SBGW associated with the fluctuations in the number of cosmic string loops, considering both cosmic string cusps and kinks. We calculate the anisotropies as a function of angle and frequency.

#### Jun 30, 2011

1106.5555 (/preprints)
2011-06-30, 11:51 

## [1106.6036] Mach's Principle and Higher-Dimensional Dynamics

Authors: B. Mashhoon, P.S. Wesson

Date: 29 Jun 2011

Abstract: We briefly discuss the current status of Mach's principle in general relativity and point out that its last vestige, namely, the gravitomagnetic field associated with rotation, has recently been measured for the earth in the GP-B experiment. Furthermore, in his analysis of the foundations of Newtonian mechanics, Mach provided an operational definition for inertial mass and pointed out that time and space are conceptually distinct from their operational definitions by means of masses. Mach recognized that this circumstance is due to the lack of any a priori connection between the inertial mass of a body and its Newtonian state in space and time. One possible way to improve upon this situation in classical physics is to associate mass with an extra dimension. Indeed, Einstein's theory of gravitation can be locally embedded in a Ricci-flat 5D manifold such that the 4D energy-momentum tensor appears to originate from the existence of the extra dimension. An outline of such a 5D Machian extension of Einstein's general relativity is presented.

#### Jun 30, 2011

1106.6036 (/preprints)
2011-06-30, 11:48 

## [1106.5795] Gravitational wave background from binary systems

Date: 28 Jun 2011

Abstract: Basic aspects of the background of gravitational waves and its mathematical characterization are reviewed. The spectral energy density parameter $\Omega(f)$, commonly used as a quantifier of the background, is derived for an ensemble of many identical sources emitting at different times and locations. For such an ensemble, $\Omega(f)$ is generalized to account for the duration of the signals and of the observation, so that one can distinguish the resolvable and unresolvable parts of the background. The unresolvable part, often called confusion noise or stochastic background, is made by signals that cannot be either individually identified or subtracted out of the data. To account for the resolvability of the background, the overlap function is introduced. This function is a generalization of the duty cycle, which has been commonly used in the literature, in some cases leading to incorrect results. The spectra produced by binary systems (stellar binaries and massive black hole binaries) are presented over the frequencies of all existing and planned detectors. A semi-analytical formula for $\Omega(f)$ is derived in the case of stellar binaries (containing white dwarfs, neutron stars or stellar-mass black holes). Besides a realistic expectation of the level of background, upper and lower limits are given, to account for the uncertainties in some astrophysical parameters such as binary coalescence rates. One interesting result concerns all current and planned ground-based detectors (including the Einstein Telescope). In their frequency range, the background of binaries is resolvable and only sporadically present. In other words, there is no stochastic background of binaries for ground-based detectors.

#### Jun 30, 2011

1106.5795 (/preprints)
2011-06-30, 11:47 

## [1106.5787] Geometry of fractional spaces

Authors: Gianluca Calcagni

Date: 28 Jun 2011

Abstract: We introduce fractional flat space, described by a continuous geometry with constant non-integer Hausdorff and spectral dimensions. This is the analogue of Euclidean space, but with anomalous scaling and diffusion properties. The basic tool is fractional calculus, which is cast in a way convenient for the definition of the differential structure, distances, volumes, and symmetries. By an extensive use of concepts and techniques of fractal geometry, we clarify the relation between fractional calculus and fractals, showing that fractional spaces can be regarded as fractals when the ratio of their Hausdorff and spectral dimension is greater than one. All the results are analytic and constitute the foundation for field theories living on multi-fractal spacetimes, which will be presented in a companion paper.

#### Jun 30, 2011

1106.5787 (/preprints)
2011-06-30, 11:47 

## [1106.4349] Effective one body Hamiltonian of two spinning black-holes with next-to-next-to-leading order spin-orbit coupling

Authors: Alessandro Nagar

Date: 21 Jun 2011

Abstract: Building on the recently computed next-to-next-to-leading order (NNLO) post-Newtonian (PN) spin-orbit Hamiltonian for spinning binaries \cite{Hartung:2011te} we extend the effective-one-body (EOB) description of the dynamics of two spinning black-holes to NNLO in the spin-orbit interaction. The calculation that is presented extends to NNLO the next-to-leading order (NLO) spin-orbit Hamiltonian computed in Ref. \cite{Damour:2008qf}. The present EOB Hamiltonian reproduces the spin-orbit coupling through NNLO in the test-particle limit case. In addition, in the case of spins parallel or antiparallel to the orbital angular momentum, when circular orbits exist, we find that the inclusion of NNLO spin-orbit terms moderates the effect of the NLO spin-orbit coupling.

#### Jun 22, 2011

1106.4349 (/preprints)
2011-06-22, 18:49 

## [1106.4047] Minimum Requirements for Detecting a Stochastic Gravitational Wave Background Using Pulsars

Authors: J. M. Cordes, R. M. Shannon

Date: 20 Jun 2011

Abstract: We assess the detectability of a nanohertz gravitational wave (GW) background with respect to additive red and white noise in the timing of millisecond pulsars. We develop detection criteria based on the cross-correlation function summed over pulsar pairs in a pulsar timing array. The distribution of correlation amplitudes is found to be non-Gaussian and highly skewed, which significantly influences detection and false-alarm probabilities. When only white noise and GWs contribute, our detection results are consistent with those found by others. Red noise, however, drastically alters the results. We discuss methods to meet the challenge of GW detection ("climbing mount significance") by distinguishing between GW-dominated and red or white-noise limited regimes. We characterize detection regimes by evaluating the number of millisecond pulsars that must be monitored in a high-cadence, 5-year timing program for a GW background spectrum $h_c(f) = A fˆ{-2/3}$ with $A = 10ˆ{-15}$ yr$ˆ{-2/3}$. Unless a sample of 20 super-stable millisecond pulsars can be found — those with timing residuals from red-noise contributions $\sigma_r \lesssim 20$ ns — a much larger timing program on $\gtrsim 50 - 100$ MSPs will be needed. For other values of $A$, the constraint is $\sigma_r \lesssim 20 {\rm ns} (A/10ˆ{-15} {\rm yr}ˆ{-2/3})$. Identification of suitable MSPs itself requires an aggressive survey campaign followed by characterization of the level of spin noise in the timing residuals of each object. The search and timing programs will likely require substantial fractions of time on new array telescopes in the southern hemisphere as well as on existing ones.

#### Jun 22, 2011

1106.4047 (/preprints)
2011-06-22, 14:51 

## [1106.3278] Periastron Advance in Black Hole Binaries

Authors: Alexandre Le Tiec, Abdul H. Mrou&#xe9;, Leor Barack, Alessandra Buonanno, Harald P. Pfeiffer, Norichika Sago, Andrea Taracchini

Date: 16 Jun 2011

Abstract: The general relativistic periastron advance of non-spinning black hole binaries on quasi-circular orbits has been computed using black hole perturbation theory, post-Newtonian expansions, and the effective-one-body formalism. We compare these approximations with accurate numerical relativity simulations of mass ratios 1/8 < m1/m2 < 1. We find the leading-order self-force prediction to be remarkably accurate, even for equal mass binaries, if one substitutes m1/m2 --> m1m2/(m1+m2)ˆ2. The effective-one-body prediction also agrees very well over the entire mass-ratio range considered.

#### Jun 21, 2011

1106.3278 (/preprints)
2011-06-21, 16:19 

## [1106.3412] Testing the Gravitational Redshift with Atomic Gravimeters?

Authors: Peter Wolf, Luc Blanchet, Christian J. Bord&#xe9;, Serge Reynaud, Christophe Salomon, Claude Cohen-Tannoudji

Date: 17 Jun 2011

Abstract: Atom interferometers allow the measurement of the acceleration of freely falling atoms with respect to an experimental platform at rest on Earth's surface. Such experiments have been used to test the universality of free fall by comparing the acceleration of the atoms to that of a classical freely falling object. In a recent paper, M\"uller, Peters and Chu [Nature {\bf 463}, 926-929 (2010)] argued that atom interferometers also provide a very accurate test of the gravitational redshift (or universality of clock rates). Considering the atom as a clock operating at the Compton frequency associated with the rest mass, they claimed that the interferometer measures the gravitational redshift between the atom-clocks in the two paths of the interferometer at different values of gravitational potentials. In the present paper we analyze this claim in the frame of general relativity and of different alternative theories, and conclude that the interpretation of atom interferometers as testing the gravitational redshift at the Compton frequency is unsound. The present work is a summary of our extensive paper [Wolf et al., arXiv:1012.1194, Class. Quant. Grav. 28, 145017, (2011)], to which the reader is referred for more details.

#### Jun 21, 2011

1106.3412 (/preprints)
2011-06-21, 16:18 

## [1106.2767] Comparison of LISA and Atom Interferometry for Gravitational Wave Astronomy in Space

Authors: Peter L. Bender

Date: 14 Jun 2011

Abstract: One of the atom interferometer gravitational wave missions proposed by Dimopoulos et al.1 in 2008 was called AGIS-Sat. 2. It had a suggested gravitational wave sensitivity set by the atom state detection shot noise level that started at 1 mHz, was comparable to LISA sensitivity from 1 to about 20 mHz, and had better sensitivity from 20 to 500 mHz. The separation between the spacecraft was 1,000 km, with atom interferometers 200 m long and shades from sunlight used at each end. A careful analysis of many error sources was included, but requirements on the time-stability of both the laser wavefront aberrations and the atom temperatures in the atom clouds were not investigated. After including these considerations, the laser wavefront aberration stability requirement to meet the quoted sensitivity level is about 1\times10-8 wavelengths, and is far tighter than for LISA. Also, the temperature fluctuations between atom clouds have to be less than 1 pK. An alternate atom interferometer GW mission in Earth orbit called AGIS-LEO with 30 km satellite separation has been suggested recently. The reduction of wavefront aberration noise by sending the laser beam through a high-finesse mode-scrubbing optical cavity is discussed briefly, but the requirements on such a cavity are not given. Unfortunately, such an Earth-orbiting mission seems to be considerably more difficult to design than a non-geocentric mission and does not appear to have comparably attractive scientific goals.

#### Jun 15, 2011

1106.2767 (/preprints)
2011-06-15, 10:27 

## [1106.2547] Estimating parameters of coalescing compact binaries with a detector network including LIGO Australia

Authors: Benjamin Aylott, Benjamin Farr, Vassiliki Kalogera, Ilya Mandel, Vivien Raymond, Carl Rodriguez, Marc van der Sluys, Alberto Vecchio, John Veitch

Date: 13 Jun 2011

Abstract: One of the goals of gravitational-wave astronomy is simultaneous detection of gravitational-wave signals from merging compact-object binaries and the electromagnetic transients from these mergers. With the next generation of advanced ground-based gravitational wave detectors under construction, we examine the benefits of the proposed extension of the detector network to include a fourth site in Australia in addition to the network of Hanford, Livingston and Cascina sites. Using Bayesian parameter-estimation analyses of simulated gravitational-wave signals from a range of coalescing-binary locations and orientations, we study the improvement in parameter estimation. We find that an Australian detector can break degeneracies in several parameters; in particular, the localization of the source on the sky is improved by a factor of ~4, with more modest improvements in distance and binary inclination estimates. This enhanced ability to localize sources on the sky will be crucial in any search for electromagnetic counterparts to detected gravitational-wave signals.

#### Jun 15, 2011

1106.2547 (/preprints)
2011-06-15, 10:27 

## [1106.1021] Inspiral-merger-ringdown multipolar waveforms of nonspinning black-hole binaries using the effective-one-body formalism

Authors: Yi Pan, Alessandra Buonanno, Michael Boyle, Luisa T. Buchman, Lawrence E. Kidder, Harald P. Pfeiffer, Mark A. Scheel

Date: 6 Jun 2011

Abstract: We calibrate an effective-one-body (EOB) model to numerical-relativity simulations of mass ratios 1, 2, 3, 4, and 6, by maximizing phase and amplitude agreement of the leading (2,2) mode and of the subleading modes (2,1), (3,3), (4,4) and (5,5). Aligning the calibrated EOB waveforms and the numerical waveforms at low frequency, the phase difference of the (2,2) mode between model and numerical simulation remains below 0.1 rad throughout the evolution for all mass ratios considered. The fractional amplitude difference at peak amplitude of the (2,2) mode is 2% and grows to 12% during the ringdown. Using the Advanced LIGO noise curve we study the effectualness and measurement accuracy of the EOB model, and stress the relevance of modeling the higher-order modes for parameter estimation. We find that the effectualness, measured by the mismatch, between the EOB and numerical-relativity polarizations which include only the (2,2) mode is smaller than 0.2% for binaries with total mass 20-200 Msun and mass ratios 1, 2, 3, 4, and 6. When numerical-relativity polarizations contain the strongest seven modes, and stellar-mass black holes with masses less than 50Msun are considered, the mismatch for mass ratio 6 (1) can be as high as 5% (0.2%) when only the EOB (2,2) mode is included, and an upper bound of the mismatch is 0.5% (0.07%) when all the four subleading EOB modes calibrated in this paper are taken into account. For binaries with intermediate-mass black holes with masses greater than 50Msun the mismatches are larger. We also determine for which signal-to-noise ratios the EOB model developed here can be used to measure binary parameters with systematic biases smaller than statistical errors due to detector noise.

#### Jun 13, 2011

1106.1021 (/preprints)
2011-06-13, 09:27 

## [1106.1264] Note on the Post-Newtonian limit of massive Brans-Dicke theory

Authors: Mahmood Roshan, Fatimah Shojai

Date: 7 Jun 2011

Abstract: We consider the Post-Newtonian limit of massive Brans-Dicke theory and we make some notes about the Post-Newtonian limit of the case $\omega=0$. This case is dynamically equivalent to the metric $f(R)$ theory. It is known that this theory can be compatible with the solar system tests if Chameleon mechanism occurs. Also, it is known that this mechanism is because of the non-linearity in the field equations produced by the largeness of the local curvature relative to the background curvature. Thus, the linearization of the field equations breaks down. On the other hand we know that Chameleon mechanism exists when a coupling between the matter and the scalar field exists. In the Jordan frame of Brans-Dicke theory, we have not such a coupling. But in the Einstein frame this theory behaves like a Chameleon scalar field. By confining ourselves to the case $\omega=0$, we show that "Chameleon-like" behavior can exist also in the Jordan frame but it has an important difference compared with the Chameleon mechanism. Also we show that the conditions which lead to the existence of "Chameleon-like" mechanism are consistent with the conditions in the Post-Newtonian limit which correspond to a heavy scalar filed at the cosmological scale and a small effective cosmological constant. Thus, one can linearize field equations to the Post-Newtonian order and this linearization has not any contradiction with the existence of "Chameleon-like" behavior.

#### Jun 13, 2011

1106.1264 (/preprints)
2011-06-13, 09:17 

## [1106.1568] When effective theories predict: the inevitability of Mercury's anomalous perihelion precession

Authors: James D. Wells

Date: 8 Jun 2011

Abstract: If the concepts underlying Effective Theory were appreciated from the earliest days of Newtonian gravity, Le Verrier's announcement in 1845 of the anomalous perihelion precession of Mercury would have been no surprise. Furthermore, the size of the effect could have been anticipated through "naturalness" arguments well before the definitive computation in General Relativity. Thus, we have an illustration of how Effective Theory concepts can guide us in extending our knowledge to "new physics", and not just in how to reduce larger theories to restricted (e.g., lower energy) domains.

#### Jun 13, 2011

1106.1568 (/preprints)
2011-06-13, 09:16 

## [1106.1429] Tidal disruptions of separated binaries in galactic nuclei

Authors: Pau Amaro-Seoane, M. Coleman Miller, Gareth F. Kennedy

Date: 7 Jun 2011

Abstract: Several galaxies have exhibited X-ray flares that are consistent with the tidal disruption of a star by a central supermassive black hole. In theoretical treatments of this process it is usually assumed that the star was initially on a nearly parabolic orbit relative to the black hole. Such an assumption leads in the simplest approximation to a $tˆ{-5/3}$ decay of the bolometric luminosity and this is indeed consistent with the relatively poorly sampled light curves of such flares. We point out that there is another regime in which the decay would be different: if a binary is tidally separated and the star that remains close to the hole is eventually tidally disrupted from a moderate eccentricity orbit, the decay is slower, typically $\sim tˆ{-1.2}$. As a result, careful sampling of the light curves of such flares could distinguish between these processes and yield insight into the dynamics of binaries as well as single stars in galactic centres. We explore this process using three-body simulations and analytic treatments and discuss the consequences for present-day X-ray detections and future gravitational wave observations.

#### Jun 13, 2011

1106.1429 (/preprints)
2011-06-13, 09:16 

## [1106.1864] Super-Extremal Spinning Black Holes via Accretion

Authors: Tanja Bode, Pablo Laguna, Richard A. Matzner

Date: 9 Jun 2011

Abstract: A Kerr black hole with mass $M$ and angular momentum $J$ satisfies the extremality inequality $|J| \le Mˆ2$. In the presence of matter and/or gravitational radiation, this bound needs to be reformulated in terms of local measurements of the mass and the angular momentum directly associated with the black hole. The isolated and dynamical horizon framework provides such quasi-local characterization of black hole mass and angular momentum. With this framework, it is possible in axisymmetry to reformulate the extremality limit as $|J| \le 2\,M_Hˆ2$, with $M_H$ the irreducible mass of the black hole computed from its apparent horizon area and $J$ obtained using approximate rotational Killing vectors on the apparent horizon. The $|J| \le 2\,M_Hˆ2$ condition is also equivalent to requiring a non-negative black hole surface gravity. We present numerical experiments of an accreting black hole that temporarily violates this extremality inequality. The initial configuration consists of a single, rotating black hole surrounded by a thick, shell cloud of negative energy density. For these numerical experiments, we introduce a new matter-without-matter evolution method.

#### Jun 13, 2011

1106.1864 (/preprints)
2011-06-13, 09:15 

## [1106.1692] Testing the Cosmic Censorship Conjecture with point particles: the effect of radiation reaction and the self-force

Authors: Enrico Barausse, Vitor Cardoso, Gaurav Khanna

Date: 9 Jun 2011

Abstract: A classical thought-experiment to destroy black holes was envisaged by Wald in 1974: it consists of throwing particles with large angular momentum into an extremal black hole, checking whether their capture can over-spin the black hole past the extremal limit and create a naked singularity. Wald showed that in the test-particle limit, particles capable of producing naked singularities are simply scattered. Recently Jacobson and Sotiriou showed that if one considers instead a black hole that is almost, but not exactly extremal, naked singularities can be formed by particle capture, thus violating the Cosmic Censorship Conjecture in four-dimensional, asymptotically flat spacetimes. However, Jacobson and Sotiriou followed Wald in adopting the test-particle approximation, which neglects radiative and self-force effects. Here we analyze these effects and show that for some of the trajectories giving rise to naked singularities, radiative effects can be neglected. However, for these orbits the conservative self-force is important, and seems to have the right sign to prevent the formation of naked singularities.

#### Jun 13, 2011

1106.1692 (/preprints)
2011-06-13, 09:14 

## [1106.0328] R^n gravity and the chameleon

Authors: Valerio Faraoni (Bishop&#x27;s University)

Date: 1 Jun 2011

Abstract: The Solar System bounds on Rˆn gravity are often ignored in the literature by invoking the chameleon mechanism. We show that in order for the latter to work, the exponent n must be ridicolously close to unity and, therefore, these theories are severely constrained.

#### Jun 03, 2011

1106.0328 (/preprints)
2011-06-03, 21:50 

## [1106.0397] The coalescence rates of double black holes

Authors: Krzysztof Belczynski, Tomasz Bulik, Michal Dominik, Andrea Prestwich

Date: 2 Jun 2011

Abstract: We present the summary of the recent investigations of double black hole binaries in context of their formation and merger rates. In particular we discuss the spectrum of black hole masses, the formation scenarios in the local Universe and the estimates of detection rates for gravitational radiation detectors like LIGO and VIRGO. Our study is based on observed properties of known Galactic and extra-galactic stellar mass black holes and evolutionary predictions. We argue that the binary black holes are the most promising source of gravitational radiation.

#### Jun 03, 2011

1106.0397 (/preprints)
2011-06-03, 21:50 

## [1105.5391] Extreme black hole simulations: collisions of unequal mass black holes and the point particle limit

Authors: Ulrich Sperhake, Vitor Cardoso, Christian D. Ott, Erik Schnetter, Helvi Witek

Date: 26 May 2011

Abstract: Numerical relativity has seen incredible progress in the last years, and is being applied with success to a variety of physical phenomena, from gravitational-wave research and relativistic astrophysics to cosmology and high-energy physics. Here we probe the limits of current numerical setups, by studying collisions of unequal mass, non-rotating black holes of mass-ratios up to 1:100 and making contact with a classical calculation in General Relativity: the infall of a point-like particle into a massive black hole.
Our results agree well with the predictions coming from linearized calculations of the infall of point-like particles into non-rotating black holes. In particular, in the limit that one hole is much smaller than the other, and the infall starts from an infinite initial separation, we recover the point-particle limit. Thus, numerical relativity is able to bridge the gap between fully non-linear dynamics and linearized approximations, which may have important applications. Finally, we also comment on the "spurious" radiation content in the initial data and the linearized predictions.

#### Jun 01, 2011

1105.5391 (/preprints)
2011-06-01, 11:57 

## [1105.5562] Testing Gravity Theories In The Radiative Regime Using Pulsar Timing Arrays

Authors: K.J.Lee

Date: 27 May 2011

Abstract: General relativity has predicted the existence of gravitational waves (GW), which are waves of the distortions of space-time with two degrees of polarization and the propagation speed of light. Alternative theories predict more polarizations, up to a maximum of six, and possible deviation of propagation speed from the light speed. The present paper reviews recent proposals to test the gravity theories in the radiation regime by observing GWs using pulsar timing arrays.

#### Jun 01, 2011

1105.5562 (/preprints)
2011-06-01, 11:57 

## [1105.5927] Radioscience simulations in General Relativity and in alternative theories of gravity

Authors: Aurelien Hees, Peter Wolf, Brahim Lamine, Serge Reynaud, Marc-Thierry Jaekel, Christophe Le Poncin-Lafitte, Valery Lainey, Andre Fuzfa, Veronique Dehant

Date: 30 May 2011

Abstract: In this communication, we focus on the possibility to test GR with radioscience experiments. We present a new software that in a first step simulates the Range/Doppler signals directly from the space time metric (thus in GR and in alternative theories of gravity). In a second step, a least-squares fit of the involved parameters is performed in GR. This software allows one to get the order of magnitude and the signature of the modifications induced by an alternative theory of gravity on radioscience signals. As examples, we present some simulations for the Cassini mission in Post-Einsteinian gravity and with the MOND External Field Effect.

#### Jun 01, 2011

1105.5927 (/preprints)
2011-06-01, 11:55 

## [1105.5842] The cross-correlation search for a hot spot of gravitational waves

Date: 30 May 2011

Abstract: The cross-correlation search has been previously applied to map the gravitational wave (GW) stochastic background in the sky and also to target GW from rotating neutron stars/pulsars. Here we investigate how the cross-correlation method can be used to target a small region in the sky spanning at most a few pixels, where a pixel in the sky is determined by the diffraction limit which depends on the (i) baseline joining a pair of detectors and (ii) detector bandwidth. Here as one of the promising targets, we consider the Virgo cluster - a "hot spot" spanning few pixels - which could contain, as estimates suggest $\sim 10ˆ{11}$ neutron stars, of which a small fraction would continuously emit GW in the bandwidth of the detectors. For the detector baselines, we consider advanced detector pairs among LCGT, LIGO, Virgo, ET etc. Our results show that sufficient signal to noise can be accumulated with integration times of the order of a year. The results improve for the multibaseline search. This analysis could as well be applied to other likely hot spots in the sky and other possible pairs of detectors.

#### Jun 01, 2011

1105.5842 (/preprints)
2011-06-01, 11:54 

## [1105.5843] Multimessenger Astronomy

Authors: N.L. Christensen, for the LIGO Scientific Collaboration, the Virgo Collaboration

Date: 30 May 2011

Abstract: Multimessenger astronomy incorporating gravitational radiation is a new and exciting field that will potentially provide significant results and exciting challenges in the near future. With advanced interferometric gravitational wave detectors (LCGT, LIGO, Virgo) we will have the opportunity to investigate sources of gravitational waves that are also expected to be observable through other messengers, such as electromagnetic (gamma-rays, x-rays, optical, radio) and/or neutrino emission. The LIGO-Virgo interferometer network has already been used for multimessenger searches for gravitational radiation that have produced insights on cosmic events. The simultaneous observation of electromagnetic and/or neutrino emission could be important evidence in the first direct detection of gravitational radiation. Knowledge of event time, source sky location, and the expected frequency range of the signal enhances our ability to search for the gravitational radiation signatures with an amplitude closer to the noise floor of the detector. Presented here is a summary of the status of LIGO-Virgo multimessenger detection efforts, along with a discussion of questions that might be resolved using the data from advanced or third generation gravitational wave detector networks.

#### Jun 01, 2011

1105.5843 (/preprints)
2011-06-01, 11:54 

## [1105.5811] The harmonic structure of generic Kerr orbits

Authors: Rebecca Grossman, Janna Levin, Gabe Perez-Giz

Date: 29 May 2011

Abstract: Generic Kerr orbits exhibit intricate three-dimensional motion. We offer a classification scheme for these intricate orbits in terms of periodic orbits. The crucial insight is that for a given effective angular momentum $L$ and angle of inclination $\iota$, there exists a discrete set of orbits that are geometrically $n$-leaf clovers in a precessing {\it orbital plane}. When viewed in the full three dimensions, these orbits are periodic in $r-\theta$. Each $n$-leaf clover is associated with a rational number, $1+q_{r\theta}=\omega_\theta/\omega_r$, that measures the degree of perihelion precession in the precessing orbital plane. The rational number $q_{r\theta}$ varies monotonically with the orbital energy and with the orbital eccentricity. Since any bound orbit can be approximated as near one of these periodic $n$-leaf clovers, this special set offers a skeleton that illuminates the structure of all bound Kerr orbits, in or out of the equatorial plane.

#### Jun 01, 2011

1105.5811 (/preprints)
2011-06-01, 11:54 

## [1105.6166] The European Space Agency {\Gaia} mission: exploring the Galaxy

Authors: C. Jordi

Date: 31 May 2011

Abstract: The {\Gaia} astrometric mission was approved by the European Space Agency in 2000 and the construction of the spacecraft and payload is on-going for a launch in late 2012. {\Gaia} will continuously scan the entire sky for 5 years, yielding positional and velocity measurements with the accuracies needed to produce a stereoscopic and kinematic census of about one billion stars throughout our Galaxy and beyond. The main scientific goal is to quantify early formation and the subsequent dynamic and chemical evolution of the Milky way. The stellar survey will have a completeness to $V = 20$ mag, with a precision of about 25 $\mu$as at 15 mag. The astrometric information will be combined with astrophysical data acquired through on-board spectrophotometry and spectroscopy, allowing the chemical composition and age of the stars to be derived. Data acquired and processed as a result of the {\Gaia} mission are estimated to amount to about 1 petabyte. One of the challenging problems is the close relationship between astrometric and astrophysical data, which involves a global iterative solution that updates instruments parameters, the attitude of the satellite, and the properties of the observed objects. The European community is organized to deal with {\Gaia} products: (a) the Data Processing and Analysis Consortium is a joint European effort in charge of preparation and execution of data processing, (b) the GREAT network is a platform for collaboration on the preparation of scientific exploitation.

#### Jun 01, 2011

1105.6166 (/preprints)
2011-06-01, 11:53 

## [1105.5645] Testing the No-Hair Theorem with Sgr A*

Authors: Tim Johannsen (Arizona)

Date: 27 May 2011

Abstract: The no-hair theorem characterizes the fundamental nature of black holes in general relativity. This theorem can be tested observationally by measuring the mass and spin of a black hole as well as its quadrupole moment, which may deviate from the expected Kerr value. Sgr A*, the supermassive black hole at the center of the Milky Way, is a prime candidate for such tests thanks to its large angular size, high brightness, and rich population of nearby stars. In this review I discuss a new theoretical framework for a test of the no-hair theorem that is ideal for imaging observations of Sgr A* with very-long baseline interferometry (VLBI). The approach is formulated in terms of a Kerr-like spacetime that depends on a free parameter and is regular everywhere outside of the event horizon. Together with the results from astrometric and timing observations, VLBI imaging of Sgr A* may lead to a secure test of the no-hair theorem.

#### Jun 01, 2011

1105.5645 (/preprints)
2011-06-01, 11:53 

## [1105.5815] Testing MOND in the Solar System

Authors: Luc Blanchet (IAP), Jerome Novak (LUTH)

Date: 29 May 2011

Abstract: The Modified Newtonian Dynamics (MOND) generically predicts a violation of the strong version of the equivalence principle. As a result the gravitational dynamics of a system depends on the external gravitational field in which the system is embedded. This so-called external field effect is shown to imply the existence of an anomalous quadrupolar correction, along the direction of the external galactic field, in the gravitational potential felt by planets in the Solar System. We compute this effect by a numerical integration of the MOND equation in the presence of an external field, and deduce the secular precession of the perihelion of planets induced by this effect. We find that the precession effect is rather large for outer gaseous planets, and in the case of Saturn is comparable to, and in some cases marginally excluded by published residuals of precession permitted by the best planetary ephemerides.

#### Jun 01, 2011

1105.5815 (/preprints)
2011-06-01, 11:53 

## [1105.6269] Testing gravity law in the solar system

Authors: B. Lamine, J.-M. Courty, S. Reynaud, M.-T. Jaekel

Date: 31 May 2011

Abstract: The predictions of General relativity (GR) are in good agreement with observations in the solar system. Nevertheless, unexpected anomalies appeared during the last decades, along with the increasing precision of measurements. Those anomalies are present in spacecraft tracking data (Pioneer and flyby anomalies) as well as ephemerides. In addition, the whole theory is challenged at galactic and cosmic scales with the dark matter and dark energy issues. Finally, the unification in the framework of quantum field theories remains an open question, whose solution will certainly lead to modifications of the theory, even at large distances. As long as those "dark sides" of the universe have no universally accepted interpretation nor are they observed through other means than the gravitational anomalies they have been designed to cure, these anomalies may as well be interpreted as deviations from GR. In this context, there is a strong motivation for improved and more systematic tests of GR inside the solar system, with the aim to bridge the gap between gravity experiments in the solar system and observations at much larger scales. We review a family of metric extensions of GR which preserve the equivalence principle but modify the coupling between energy and curvature and provide a phenomenological framework which generalizes the PPN framework and "fifth force" extensions of GR. We briefly discuss some possible observational consequences in connection with highly accurate ephemerides.

#### Jun 01, 2011

1105.6269 (/preprints)
2011-06-01, 11:52