## [0808.3419] Probing Dark Energy with Black Hole Binaries

Date: 25 Aug 2008

Abstract: The equation of state (EoS) of dark energy $w$ remains elusive despite enormous experimental efforts to pin down its value and its time variation. Yet it is the single most important handle we have in our understanding of one of the most mysterious puzzle in nature, dark energy. This letter proposes a new method for measuring the EoS of dark energy by using the gravitational waves (GW) of black hole binaries. The method described here offers an alternative to the standard way of large scale surveys.
It is well known that the mass of a black hole changes due to the accretion of dark energy but at an extremely slow rate. However, a binary of supermassive black holes (SBH) radiates gravitational waves with a power proportional to the masses of these accreting stars and thereby carries information on dark energy. These waves can propagate through the vastness of structure in the universe unimpeded. The orbital changes of the binary, induced by the energy loss from gravitational radiation, receive a large contribution from dark energy accretion. This contribution is directly proportional to $(1+w)$ and is dominant for SBH binaries with separation $R \ge 1000$ parsec, thereby accelerating the merging process for $w > -1$ or ripping the stars apart for phantom dark energy with $w < -1$. Such orbital changes, therefore $w$, can be detected with LIGO and LISA near merging time, or with X-ray and radio measurements of Chandra and VLBA experiments.

#### Aug 26, 2008

0808.3419 (/preprints)
2008-08-26, 17:36 

## [0808.2696] Time-delay interferometry and the relativistic treatment of LISA optical links

Authors: S. V. Dhurandhar

Date: 20 Aug 2008

Abstract: LISA is a joint space mission of the ESA and NASA for detecting low frequency gravitational radiation in the band $10ˆ{-5} - 1$ Hz. In order to attain the requisite sensitivity for LISA, the laser frequency noise must be suppressed below the other secondary noises such as the optical path noise, acceleration noise etc. This is achieved because of the redundancy in the data, more specifically, by combining six appropriately time-delayed data streams containing fractional Doppler shifts - time delay interferometry (TDI). The orbits of the spacecraft are computed in the gravitational field of the Sun and Earth in the Newtonian framework, while the optical links are treated fully general relativistically and thus, effects such as the Sagnac, Shapiro delay, etc. are automatically incorporated. We show that in the model of LISA that we consider here, there are symmetries inherent in the physics, which may be used effectively to suppress the residual laser frequency noise and simplify the algebraic approach to TDI.

#### Aug 21, 2008

0808.2696 (/preprints)
2008-08-21, 09:14 

## [0808.2624] Is there a Supermassive Black Hole at the Center of the Milky Way?

Authors: Mark J. Reid (1) ((1)Harvard--Smithsonian Center for Astrophysics)

Date: 19 Aug 2008

Abstract: This review outlines the observations that now provide an overwhelming scientific case that the center of our Milky Way Galaxy harbors a supermassive black hole. Observations at infrared wavelength trace stars that orbit about a common focal position and require a central mass (M) of 4 million solar masses within a radius of 100 Astronomical Units. Orbital speeds have been observed to exceed 5,000 km/s. At the focal position there is an extremely compact radio source (Sgr A*), whose apparent size is near the Schwarzschild radius (2GM/cˆ2). This radio source is motionless at the ~1 km/s level at the dynamical center of the Galaxy. The mass density required by these observations is now approaching the ultimate limit of a supermassive black hole within the last stable orbit for matter near the event horizon.

#### Aug 19, 2008

0808.2624 (/preprints)
2008-08-19, 23:42 

## [0808.2597] IMEX evolution of scalar fields on curved backgrounds

Authors: Stephen R. Lau (1), Harald P. Pfeiffer (2), Jan S. Hesthaven (1) ((1) Brown, (2) Caltech)

Date: 19 Aug 2008

Abstract: Inspiral of binary black holes occurs over a time-scale of many orbits, far longer than the dynamical time-scale of the individual black holes. Explicit evolutions of a binary system therefore require excessively many time steps to capture interesting dynamics. We present a strategy to overcome the Courant-Friedrichs-Lewy condition in such evolutions, one relying on modern implicit-explicit ODE solvers and multidomain spectral methods for elliptic equations. Our analysis considers the model problem of a forced scalar field propagating on a generic curved background. Nevertheless, we encounter and address a number of issues pertinent to the binary black hole problem in full general relativity. Specializing to the Schwarzschild geometry in Kerr-Schild coordinates, we document the results of several numerical experiments testing our strategy.

#### Aug 19, 2008

0808.2597 (/preprints)
2008-08-19, 22:39 

## [0808.2573] A method for characterization of coherent backgrounds in real time and its application in gravitational wave data analysis

Authors: E. J. Daw, M. R. Hewitson

Date: 19 Aug 2008

Abstract: Many experiments, and in particular gravitational wave detectors, produce continuous streams of data whose frequency representations contain discrete, relatively narrowband coherent features at high amplitude. We discuss the application of digital Fourier transforms (DFTs) to characterization of these features, hereafter frequently referred to as lines. Application of DFTs to continuously produced time domain data are achieved through an algorithm hereafter referred to as EFC for efficient time-domain determination of the Fourier coefficients of a data set. We first define EFC and discuss parameters relating to the algorithm that determine its properties and action on the data. In gravitational wave interferometers, these lines are commonly due to parasitic sources of coherent background interference coupling into the instrument. Using GEO 600 data, we next demonstrate that time domain subtraction of lines can proceed without detrimental effects either on features at frequencies separated from that of the subtracted line, or on features at the frequency of the line but having different stationarity properties.

#### Aug 19, 2008

0808.2573 (/preprints)
2008-08-19, 22:38 

## [0808.2510] Post-Newtonian Approximation in Maxwell-Like Form

Authors: Jeff D. Kaplan, David A. Nichols, Kip S. Thorne

Date: 19 Aug 2008

Abstract: The equations of the linearized first post-Newtonian approximation to general relativity are often written in "gravitoelectromagnetic" Maxwell-like form, since that facilitates physical intuition. This paper expresses the first post-Newtonian approximation, including all nonlinearities, in Maxwell-like form, focusing especially on the gravitational momentum and its conservation as expressed in terms of the Landau-Lifshitz pseudotensor. The authors and their colleagues are finding this formalism useful in developing physical intuition into numerical-relativity simulations of binary black holes with spin.

#### Aug 19, 2008

0808.2510 (/preprints)
2008-08-19, 22:38 

## [0808.2225] A Bayesian method to set upper limits on the strength of a periodic gravitational wave signal from the remnant of SN1987A: possible applications in LIGO searches

Authors: Richard Umstaetter, Renate Meyer, Nelson Christensen

Date: 16 Aug 2008

Abstract: We present a method that assesses the theoretical detection limit of a Bayesian Markov chain Monte Carlo search for a periodic gravitational wave signal emitted by a neutron star. Inverse probability yields an upper limit estimate for the strength when a signal could not be detected in an observed data set. The proposed method is based on Bayesian model comparison that automatically quantifies Occam s Razor. It limits the complexity of a model by favoring the most parsimonious model that explains the data. By comparing the model with a signal from a pulsar to the null model that assumes solely noise, we derive the detection probability and an estimate for the upper limit that a search, for example, for a narrow-band emission for SN1987a, might yield on data at the sensitivity of LIGO data for an observation time of one year.

#### Aug 19, 2008

0808.2225 (/preprints)
2008-08-19, 15:53 

## [0807.4697] Gravitational Radiation Driven Supermassive Black Hole Binary Inspirals as Periodically Variable Electromagnetic Sources

Authors: Zolt&#xe1;n Haiman (Columbia University), Bence Kocsis (Harvard-Smithsonian Center for Astrophysics and Etv&#xf6;s Lor&#xe1;nd University), Kristen Menou (Columbia University)

Date: 29 Jul 2008

Abstract: Supermassive black hole binaries (SMBHBs) produced in galaxy mergers are thought to complete their coalescence, below separations of r_GW=10ˆ{-3} (M_BH/10ˆ8 M_sun)ˆ{¾} pc, as their orbit decays due to the emission of gravitational waves (GWs). It may be possible to identify such GW-driven inspirals statistically in an electromagnetic (EM) survey for variable sources. A GW-driven binary spends a characteristic time T_GW at each orbital separation r_orb < r_GW that scales with the corresponding orbital time t_orb as T_GW = (const) t_orbˆ{8/3}. If the coalescing binary produces variations in the EM emission on this timescale, then it could be identified as a variable source with a characteristic period t_var = t_orb. The incidence rate of sources with similar inferred BH masses, showing near-periodic variability on the time-scale t_var, would then be proportional to t_varˆ{8/3}. Luminosity variations corresponding to a fraction f_Edd<0.01 of the Eddington luminosity would have been missed in current surveys. However, if the binary inspirals are associated with quasars, we show that a dedicated survey could detect the population of SMBHBs with a range of periods around tens of weeks. The discovery of a population of periodic sources whose abundance obeys N_var = (const) t_varˆ{8/3} would confirm (i) that the orbital decay is indeed driven by GWs, and (ii) that circumbinary gas is present at small orbital radii and is being perturbed by the BHs. Deviations from the t_varˆ{8/3} power-law could constrain the structure of the circumbinary gas disk and viscosity-driven orbital decay. We discuss constraints from existing data, and quantify the sensitivity and sky coverage that could yield a detection in future surveys.

#### Aug 08, 2008

0807.4697 (/preprints)
2008-08-08, 08:31 

## [0807.2985] The final mass and spin of black hole mergers

Authors: Wolfgang Tichy, Pedro Marronetti

Date: 18 Jul 2008

Abstract: We consider black holes resulting from binary black hole mergers. By fitting to numerical results we construct analytic formulas that predict the mass and spin of the final black hole. Our formulas are valid for arbitrary initial spins and mass ratios and agree well with available numerical simulations. We use our spin formula in the context of two common merger scenarios for supermassive galactic black holes. We consider the case of isotropically distributed initial spin orientations (when no surrounding matter is present) and also the case when matter closely aligns the spins with the orbital angular momentum. The spin magnitude of black holes resulting from successive generations of mergers (with symmetric mass ratio $\eta$) has a mean of $1.73\eta + 0.28$ in the isotropic case and 0.94 for the closely aligned case.

#### Aug 06, 2008

0807.2985 (/preprints)
2008-08-06, 13:00 

## [0807.5065] Detection of periodic gravitational wave sources by Hough transform in the frequency and spin down plane

Authors: F. Antonucci, P. Astone, S. D&#x27; Antonio, S. Frasca, C. Palomba

Date: 31 Jul 2008

Abstract: In the hierarchical search for periodic sources of gravitational waves, the candidate selection, in the incoherent step, can be performed with Hough transform procedures. In this paper we analyze the problem of sensitivity loss due to discretization of the parameters space vs computing cost, comparing the properties of the sky Hough procedure with those of a new frequency Hough, which is based on a transformation from the time - observed frequency plane to the source frequency - spin down plane. Results on simulated peak maps suggest various advantages in favor of the use of the frequency Hough. The ones which show up to really make the difference are 1) the possibility to enhance the frequency resolution without relevantly affecting the computing cost. This reduces the digitization effects; 2) the excess of candidates due to local disturbances in some places of the sky map. They do not affect the new analysis because each map is constructed for only one position in the sky. Pacs. numbers: 04.80Nn,07.05Kf,97.60Jd 1.

#### Aug 06, 2008

0807.5065 (/preprints)
2008-08-06, 13:00 

## [0808.0160] Numerical simulations of general gravitational singularities

Authors: David Garfinkle

Date: 1 Aug 2008

Abstract: This paper covers some of the current techniques and issues involved in performing numerical simulations of the formation of singularities.

#### Aug 06, 2008

0808.0160 (/preprints)
2008-08-06, 12:59