**Authors**: Matt Visser (Victoria University of Wellington)

**Date**: 29 Mar 2011

**Abstract**: Horava gravity is a relatively recent (Jan 2009) idea in theoretical physics for trying to develop a quantum field theory of gravity. It is not a string theory, nor loop quantum gravity, but is instead a traditional quantum field theory that breaks Lorentz invariance at ultra-high (presumably trans-Planckian) energies, while retaining approximate Lorentz invariance at low and medium (sub-Planckian) energies. The challenge is to keep the Lorentz symmetry breaking controlled and small - small enough to be compatible with experiment. I will give a very general overview of what is going on in this field, paying particular attention to the disturbing role of the scalar graviton.

1103.5587
(/preprints)

2011-03-30, 08:04
**[edit]**

**Authors**: David Merritt

**Date**: 28 Mar 2011

**Abstract**: Chandrasekhar's most important contribution to stellar dynamics was the concept of dynamical friction. I briefly review that work, then discuss some implications of Chandrasekhar's theory of gravitational encounters for motion in galactic nuclei.

1103.5446
(/preprints)

2011-03-28, 22:47
**[edit]**

**Authors**: Cosimo Bambi

**Date**: 26 Mar 2011

**Abstract**: There is robust observational evidence supporting the existence of $5 - 20$ $M_\odot$ compact bodies in X-ray binary systems and of $10ˆ5 - 10ˆ9$ $M_\odot$ bodies at the center of many galaxies. All these objects are commonly interpreted as black holes, even is there is no direct evidence that they have an event horizon. A fundamental limit for a black hole in 4-dimensional general relativity is the Kerr bound $|a_*| \le 1$, where $a_*$ is the spin parameter. This is just the condition for the existence of the event horizon. The accretion process can spin a black hole up to $a_* \approx 0.998$ and some super-massive objects in galactic nuclei could be rapidly rotating black holes with spin parameter close to this limit. However, if these super-massive objects are not black holes, the Kerr bound does not hold and the accretion process can spin them up to $a_* > 1$. In this paper, I consider compact bodies with non-Kerr quadrupole moment. I study the evolution of the spin parameter due to accretion and I find its equilibrium value. Future experiments like the gravitational wave detector LISA will be able to test if the super-massive objects at the center of galaxies are the black holes predicted by general relativity. If they are not black holes, some of them may be super-spinning objects with $a_* > 1$.

1103.5135
(/preprints)

2011-03-28, 22:47
**[edit]**

**Authors**: Michael Boyle

**Date**: 25 Mar 2011

**Abstract**: A general method is presented for estimating the uncertainty in hybrid models of gravitational waveforms from binary black-hole systems with arbitrary physical parameters, and thence the highest allowable initial orbital frequency for a numerical-relativity simulation such that the combined analytical and numerical waveform meets some minimum desired accuracy. The key strength of this estimate is that no prior numerical simulation in the relevant region of parameter space is needed. The method is demonstrated for a selection of extreme physical parameters. It is shown that optimal initial orbital frequencies depend roughly linearly on the mass of the binary, and therefore useful accuracy criteria must depend on the mass. The results indicate that accurate estimation of the parameters of stellar-mass black-hole binaries in Advanced LIGO data or calibration of waveforms for detection will require much longer numerical simulations than are currently available or more accurate post-Newtonian approximations -- or both -- especially for comparable-mass systems with high spin.

1103.5088
(/preprints)

2011-03-28, 22:46
**[edit]**

**Authors**: Nicolás Yunes, Bence Kocsis, Abraham Loeb, Zoltán Haiman

**Date**: 23 Mar 2011

**Abstract**: We study the effects of a thin gaseous accretion disk on the inspiral of a stellar--mass black hole into a supermassive black hole. We construct a phenomenological angular momentum transport equation that reproduces known disk effects. Disk torques modify the gravitational wave phase evolution to detectable levels with LISA for reasonable disk parameters. The Fourier transform of disk-modified waveforms acquires a correction with a different frequency trend than post-Newtonian vacuum terms. Such inspirals could be used to detect accretion disks with LISA and to probe their physical parameters.

1103.4609
(/preprints)

2011-03-24, 22:06
**[edit]**

**Authors**: Francesco Pannarale, Luciano Rezzolla, Frank Ohme, Jocelyn S. Read

**Date**: 17 Mar 2011

**Abstract**: The strong tidal forces that arise during the last stages of the life of a black hole-neutron star binary may severely distort, and possibly disrupt, the star. Both phenomena will imprint signatures about the stellar structure in the emitted gravitational radiation. The information from the disruption, however, is confined to very high frequencies, where detectors are not very sensitive. We thus assess whether the lack of tidal distortion corrections in data-analysis pipelines will affect the detection of the inspiral part of the signal and whether these may yield information on the equation of state of matter at nuclear densities. Using recent post-Newtonian expressions and realistic equations of state to model these scenarios, we find that point-particle templates are sufficient for the detection of black hole-neutron star inspiralling binaries, with a loss of signals below 1% for both second and third-generation detectors. Such detections may be able to constrain particularly stiff equations of state, but will be unable to reveal the presence of a neutron star with a soft equation of state.

1103.3526
(/preprints)

2011-03-24, 16:33
**[edit]**

**Authors**: D. F. Mota, V. Salzano, S. Capozziello

**Date**: 22 Mar 2011

**Abstract**: We investigate whether there are any cosmological evidences for a scalar field with a mass and cou- pling to matter which change accordingly to the properties of the astrophysical system it "lives in", without directly focusing on the underlying mechanism that drives the scalar field scale-dependent- properties. We assume a Yukawa type of coupling between the field and matter and also that the scalar field mass grows with density, in order to overcome all gravity constraints within the solar system. We analyse three different gravitational systems assumed as "cosmological indicators": su- pernovae type Ia, low surface brightness spiral galaxies and clusters of galaxies. Results show that: a. a quite good fit to the rotation curves of low surface brightness galaxies only using visible stellar and gas mass components is obtained; b. a scalar field can fairly well reproduce the matter profile in clusters of galaxies, estimated by X-ray observations and without the need of any additional dark matter; c. there is an intrinsic difficulty in extracting information about the possibility of a scale-dependent massive scalar field (or more generally about a varying gravitational constant) from supernovae type Ia.

1103.4215
(/preprints)

2011-03-24, 16:33
**[edit]**

**Authors**: Luca Baiotti, Thibault Damour, Bruno Giacomazzo, Alessandro Nagar, Luciano Rezzolla

**Date**: 20 Mar 2011

**Abstract**: Binary neutron-star systems represent one of the most promising sources of gravitational waves. In order to be able to extract important information, notably about the equation of state of matter at nuclear density, it is necessary to have in hands an accurate analytical model of the expected waveforms. Following our recent work, we here analyze more in detail two general-relativistic simulations spanning about 20 gravitational-wave cycles of the inspiral of equal-mass binary neutron stars with different compactnesses, and compare them with a tidal extension of the effective-one-body (EOB) analytical model. The latter tidally extended EOB model is analytically complete up to the 1.5 post-Newtonian level, and contains an analytically undetermined parameter representing a higher-order amplification of tidal effects. We find that, by calibrating this single parameter, the EOB model can reproduce, within the numerical error, the two numerical waveforms essentially up to the merger. By contrast, analytical models (either EOB, or Taylor-T4) that do not incorporate such a higher-order amplification of tidal effects, build a dephasing with respect to the numerical waveforms of several radians.

1103.3874
(/preprints)

2011-03-22, 14:06
**[edit]**

**Authors**: Christian Röver, Chris Messenger, Reinhard Prix

**Date**: 15 Mar 2011

**Abstract**: While gravitational waves have not yet been measured directly, data analysis from detection experiments commonly includes an upper limit statement. Such upper limits may be derived via a frequentist or Bayesian approach; the theoretical implications are very different, and on the technical side, one notable difference is that one case requires maximization of the likelihood function over parameter space, while the other requires integration. Using a simple example (detection of a sinusoidal signal in white Gaussian noise), we investigate the differences in performance and interpretation, and the effect of the "trials factor", or "look-elsewhere effect".

1103.2987
(/preprints)

2011-03-17, 21:20
**[edit]**

**Authors**: Z. L. Wen, F. A. Jenet, D. Yardley, G. B. Hobbs, R. N. Manchester

**Date**: 14 Mar 2011

**Abstract**: Pulsar timing observations are used to place constraints on the rate of coalescence of supermassive black-hole (SMBH) binaries as a function of mass and redshift. In contrast to the indirect constraints obtained from other techniques, pulsar timing observations provide a direct constraint on the black-hole merger rate. This is possible since pulsar timing is sensitive to the gravitational waves (GWs) emitted by these sources in the final stages of their evolution. We find that upper bounds calculated from the recently published Parkes Pulsar Timing Array data are just above theoretical predictions for redshifts below 10. In the future, with improved timing precision and longer data spans, we show that a non-detection of GWs will rule out some of the available parameter space in a particular class of SMBH binary merger models. We also show that if we can time a set of pulsars to 10ns timing accuracy, for example, using the proposed Square Kilometre Array, it should be possible to detect one or more individual SMBH binary systems.

1103.2808
(/preprints)

2011-03-16, 09:49
**[edit]**

**Authors**: S. J. Waldman (the LIGO Scientific Collaboration)

**Date**: 14 Mar 2011

**Abstract**: The Advanced LIGO gravitational wave detectors will be installed starting in 2011, with completion scheduled for 2015. The new detectors will improve the strain sensitivity of current instruments by a factor of ten, with a thousandfold increase in the observable volume of space. Here we describe the design and limiting noise sources of these second generation, long-baseline, laser interferometers.

1103.2728
(/preprints)

2011-03-16, 09:47
**[edit]**

**Authors**: Martin D. Weinberg

**Date**: 9 Nov 2009

**Abstract**: Computation of the marginal likelihood from a simulated posterior distribution is central to Bayesian model selection but is computationally difficult. I argue that the marginal likelihood can be reliably computed from a posterior sample by careful attention to the numerics of the probability integral. Posing the expression for the marginal likelihood as a Lebesgue integral, we may convert the harmonic mean approximation from a sample statistic to a quadrature rule. As a quadrature, the harmonic mean approximation suffers from enormous truncation error as consequence . In addition, I demonstrate that the integral expression for the harmonic-mean approximation converges slowly at best for high-dimensional problems with uninformative prior distributions. These observations lead to two computationally-modest families of quadrature algorithms that use the full generality sample posterior but without the instability. The first algorithm automatically eliminates the part of the sample that contributes large truncation error. The second algorithm uses the posterior sample to assign probability to a partition of the sample space and performs the marginal likelihood integral directly. This eliminates convergence issues. The first algorithm is analogous to standard quadrature but can only be applied for convergent problems. The second is a hybrid of cubature: it uses the posterior to discover and tessellate the subset of that sample space was explored and uses quantiles to compute a representive field value. Neither algorithm makes strong assumptions about the shape of the posterior distribution and neither is sensitive outliers. [abridged]

0911.1777
(/preprints)

2011-03-14, 15:04
**[edit]**

**Authors**: Priscilla Canizares, Carlos F. Sopuerta

**Date**: 10 Mar 2011

**Abstract**: When a stellar-mass compact object is captured by a supermassive black hole located in a galactic centre, the system losses energy and angular momentum by the emission of gravitational waves. Subsequently, the stellar compact object evolves inspiraling until plunging onto the massive black hole. These EMRI systems are expected to be one of the main sources of gravitational waves for the future space-based Laser Interferometer Space Antenna (LISA). However, the detection of EMRI signals will require of very accurate theoretical templates taking into account the gravitational self-force, which is the responsible of the stellar-compact object inspiral. Due to its potential applicability on EMRIs, the obtention of an efficient method to compute the scalar self-force acting on a point-like particle orbiting around a massive black hole is being object of increasing interest. We present here a review of our time-domain numerical technique to compute the self-force acting on a point-like particle and we show its suitability to deal with both circular and eccentric orbits.

1103.2149
(/preprints)

2011-03-13, 18:23
**[edit]**

**Authors**: Robert H. Brandenberger (McGill University)

**Date**: 11 Mar 2011

**Abstract**: Observational cosmology is in its "golden age" with a vast amount of recent data on the distribution of matter and light in the universe. This data can be used to probe theories of the very early universe. It is small amplitude cosmological fluctuations which encode the information about the very early universe and relate it to current data. Hence, a central topic in these lectures is the "theory of cosmological perturbations", the theory which describes the generation of inhomogeneities in the very early universe and their evolution until the current time. I will apply this theory to three classes of models of the very early universe. The first is "Inflationary Cosmology", the current paradigm for understanding the early evolution of the universe. I will review the successes of inflationary cosmology, but will also focus on some conceptual challenges which inflationary cosmology is facing, challenges which motivate the search for possible alternatives. I will introduce two alternative scenarios, the "Matter Bounce" model and "String Gas Cosmology", and I will discuss how cosmological fluctuations which can explain the current data are generated in those models.

1103.2271
(/preprints)

2011-03-13, 18:22
**[edit]**

**Authors**: R. van Haasteren, Y. Levin, G.H. Janssen, K. Lazaridis, M. Kramer B.W. Stappers, G. Desvignes, M.B. Purver, A.G. Lyne, R.D. Ferdman, A. Jessner, I. Cognard, G. Theureau, N. D'Amico, A. Possenti, M. Burgay, A. Corongiu, J.W.T. Hessels, R. Smits, J.P.W. Verbiest

**Date**: 2 Mar 2011

**Abstract**: Direct detection of low-frequency gravitational waves ($10ˆ{-9} - 10ˆ{-8}$ Hz) is the main goal of pulsar timing array (PTA) projects. One of the main targets for the PTAs is to measure the stochastic background of gravitational waves (GWB) whose characteristic strain is expected to approximately follow a power-law of the form $h_c(f)=A (f/\hbox{yr}ˆ{-1})ˆ{\alpha}$, where $f$ is the gravitational-wave frequency. In this paper we use the current data from the European PTA to determine an upper limit on the GWB amplitude $A$ as a function of the unknown spectral slope $\alpha$ with a Bayesian algorithm, by modelling the GWB as a random Gaussian process. For the case $\alpha=-2/3$, which is expected if the GWB is produced by supermassive black-hole binaries, we obtain a 95% confidence upper limit on $A$ of $6\times 10ˆ{-15}$, which is 1.8 times lower than the 95% confidence GWB limit obtained by the Parkes PTA in 2006. Our approach to the data analysis incorporates the multi-telescope nature of the European PTA and thus can serve as a useful template for future intercontinental PTA collaborations.

1103.0576
(/preprints)

2011-03-09, 23:08
**[edit]**

**Authors**: I. M. Anderson, C. G. Torre

**Date**: 8 Mar 2011

**Abstract**: DifferentialGeometry is a Maple software package which symbolically performs fundamental operations of calculus on manifolds, differential geometry, tensor calculus, Lie algebras, Lie groups, transformation groups, jet spaces, and the variational calculus. These capabilities, combined with dramatic recent improvements in symbolic approaches to solving algebraic and differential equations, have allowed for development of powerful new tools for solving research problems in gravitation and field theory. The purpose of this paper is to describe some of these new tools and present some advanced applications involving: Killing vector fields and isometry groups, Killing tensors and other tensorial invariants, algebraic classification of curvature, and symmetry reduction of field equations.

1103.1608
(/preprints)

2011-03-09, 23:08
**[edit]**

**Authors**: E. Goetz, K. Riles

**Date**: 7 Mar 2011

**Abstract**: Rapidly spinning neutron stars with non-axisymmetric mass distributions are expected to generate quasi-monochromatic continuous gravitational waves. While search methods for unknown isolated spinning neutron stars are approaching maturity, there have been no previous searches for unknown sources in binary systems. Since current search methods for unknown, isolated neutron stars are already computationally limited, expanding the parameter space searched to include binary systems is a formidable challenge. We present a new hierarchical binary search method called TwoSpect, which exploits the periodic orbital modulations of the continuous waves by searching for patterns in doubly Fourier-transformed data. We will describe the TwoSpect search pipeline, including its mitigation of detector noise variations and corrections for Doppler frequency modulation caused by changing detector velocity. Tests on Gaussian noise and on a set of simulated signals will be presented.

1103.1301
(/preprints)

2011-03-08, 09:42
**[edit]**

**Authors**: Umberto Cannella

**Date**: 5 Mar 2011

**Abstract**: In this PhD thesis I make use of the effective field theory approach to General Relativity to investigate theories of gravity and to take a different point of view on the physical information that can be extracted from experiments. In the first work I present, I study a scalar-tensor theory of gravity and I address the renormalization of the energy-momentum tensor for point-like and string-like sources. The second and third study I report are set in the context of testing gravity. So far experiments have tested dynamical regimes only up to order (v/c)ˆ5 in the post-Newtonian expansion, i.e. the very first term of the radiative sector in General Relativity. In contrast, by means of gravitational-wave astronomy, one aims at testing General Relativity up to (v/c)ˆ(12)! It is then relevant to envisage testing frameworks which are appropriate to this strong-field/radiative regime. In the last two chapters of this thesis a new such framework is presented. Using the effective field theory approach, General Relativity non-linearities are described by Feynman diagrams in which classical gravitons interact with matter sources and among themselves. Tagging the self-interaction vertices of gravitons with parameters it is possible, for example, to translate the measure of the period decay of Hulse-Taylor pulsar in a constraint on the three-graviton vertex at the 0.1% level; for comparison, LEP constraints on the triple-gauge-boson couplings of weak interactions are accurate at 3%. With future observations of gravitational waves, higher order graviton vertices can in principle be constrained through a Fisher matrix analysis.

1103.0983
(/preprints)

2011-03-08, 09:42
**[edit]**

**Authors**: Alejandro Bohe

**Date**: 3 Mar 2011

**Abstract**: We investigate the effect of a large number of kinks on the gravitational power radiated by cosmic string loops. We show that the total power radiated by a loop with N left-moving and right-moving kinks is proportional to N and increases with the typical kink angle. We then apply these results to loops containing junctions which give rise to a proliferation of the number of sharp kinks. We show that the time of gravitational decay of these loops is smaller than previously assumed. In light of this we revisit the gravitational wave burst predictions from a network containing such loops. We find there is no parameter regime in which the rate of individual kink bursts is enhanced with respect to standard networks. By contrast, there remains a region of parameter space for which the kink-kink bursts dominate the stochastic background. Finally, we discuss the order of magnitude of the typical number of sharp kinks resulting from kink proliferation on loops with junctions.

1103.0768
(/preprints)

2011-03-03, 20:45
**[edit]**

**Authors**: Niels Warburton, Leor Barack

**Date**: 1 Mar 2011

**Abstract**: We present a numerical code for calculating the self force on a scalar charge moving in a bound (eccentric) geodesic in the equatorial plane of a Kerr black hole. We work in the frequency domain and make use of the method of extended homogeneous solutions [Phys.\ Rev.\ D {\bf 78}, 084021 (2008)], in conjunction with mode-sum regularization. Our work is part of a program to develop a computational architecture for fast and efficient self-force calculations, alternative to time-domain methods. We find that our frequency-domain method outperforms existing time-domain schemes for small eccentricities, and, remarkably, remains competitive up to eccentricities as high as $\sim 0.7$. As an application of our code we (i) compute the conservative scalar-field self-force correction to the innermost stable circular equatorial orbit, as a function of the Kerr spin parameter; and (ii) calculate the variation in the rest mass of the scalar particle along the orbit, caused by the component of the self force tangent to the four-velocity.

1103.0287
(/preprints)

2011-03-02, 23:59
**[edit]**

**Authors**: Eric V. Linder

**Date**: 1 Mar 2011

**Abstract**: Gravitation governs the expansion and fate of the universe, and the growth of large scale structure within it, but has not been tested in detail on these cosmic scales. The observed acceleration of the expansion may provide signs of gravitational laws beyond general relativity. Since the form of any such extension is not clear, from either theory or data, we adopt a model independent approach to parametrising deviations to the Einstein framework. We explore the phase space dynamics of two key post-GR functions and derive a classification scheme and an absolute criterion on accuracy necessary for distinguishing classes of gravity models. Future surveys will be able to constrain the post-GR functions' amplitudes and forms to the required precision, and hence reveal new aspects of gravitation.

1103.0282
(/preprints)

2011-03-02, 23:59
**[edit]**

**Authors**: C. Messenger, A. Lommen, P. Demorest, S. Ransom

**Date**: 2 Mar 2011

**Abstract**: The increasing sensitivities of pulsar timing arrays to ultra-low frequency (nHz) gravitational waves promises to achieve direct gravitational wave detection within the next 5-10 years. While there are many parallel efforts being made in the improvement of telescope sensitivity, the detection of stable millisecond pulsars and the improvement of the timing software, there are reasons to believe that the methods used to accurately determine the time-of-arrival (TOA) of pulses from radio pulsars can be improved upon. More specifically, the determination of the uncertainties on these TOAs, which strongly affect the ability to detect GWs through pulsar timing, may be unreliable. We propose two Bayesian methods for the generation of pulsar TOAs starting from pulsar "search-mode" data and pre-folded data. These methods are applied to simulated toy-model examples and in this initial work we focus on the issue of uncertainties in the folding period. The final results of our analysis are expressed in the form of posterior probability distributions on the signal parameters (including the TOA) from a single observation.

1103.0518
(/preprints)

2011-03-02, 23:58
**[edit]**

**Authors**: K. J. Lee, N. Wex, M. Kramer, B. W. Stappers, C. G. Bassa, G. H. Janssen, R.Karuppusamy, R. Smits

**Date**: 1 Mar 2011

**Abstract**: Abbreviated:

We investigate the potential of detecting the gravitational wave from individual binary black hole systems using pulsar timing arrays (PTAs) and calculate the accuracy for determining the GW properties. This is done in a consistent analysis, which at the same time accounts for the measurement of the pulsar distances via the timing parallax.

We find that, at low redshift, a PTA is able to detect the nano-Hertz GW from super massive black hole binary systems with masses of $\sim10ˆ8 - 10ˆ{10}\,M_{\sun}$ less than $\sim10ˆ5$\,years before the final merger, and those with less than $\sim10ˆ3 - 10ˆ4$ years before merger may allow us to detect the evolution of binaries.

We derive an analytical expression to describe the accuracy of a pulsar distance measurement via timing parallax. We consider five years of bi-weekly observations at a precision of 15\,ns for close-by ($\sim 0.5 - 1$\,kpc) pulsars. Timing twenty pulsars would allow us to detect a GW source with an amplitude larger than $5\times 10ˆ{-17}$. We calculate the corresponding GW and binary orbital parameters and their measurement precision. The accuracy of measuring the binary orbital inclination angle, the sky position, and the GW frequency are calculated as functions of the GW amplitude. We note that the "pulsar term", which is commonly regarded as noise, is essential for obtaining an accurate measurement for the GW source location.

We also show that utilizing the information encoded in the GW signal passing the Earth also increases the accuracy of pulsar distance measurements. If the gravitational wave is strong enough, one can achieve sub-parsec distance measurements for nearby pulsars with distance less than $\sim 0.5 - 1$\,kpc.

1103.0115
(/preprints)

2011-03-01, 23:29
**[edit]**

© M. Vallisneri 2012 — last modified on 2010/01/29

*Tantum in modicis, quantum in maximis*