Detecting a Stochastic Background of Gravitational Radiation: Signal Processing Strategies and Sensitivities

PHYSICAL REVIEW D, VOLUME 59, 102001 Detecting a stochastic background of gravitational radiation: Signal processing strategies and sensitivities Bruce Allen* and Joseph D. Romano† Department of Physics, University of Wisconsin-Milwaukee, Milwaukee, Wisconsin 53201 ~Received 28 October 1997; published 31 March 1999! We analyze the signal processing required for the optimal detection of a stochastic background of gravitational radiation using laser interferometric detectors. Starting with basic assumptions about the statistical properties of a stochastic gravity-wave background, we derive expressions for the optimal filter function and signal-to-noise ratio for the cross-correlation of the outputs of two gravity-wave detectors. Sensitivity levels required for detection are then calculated. Issues related to ~i! calculating the signal-to-noise ratio for arbitrarily large stochastic backgrounds, ~ii! performing the data analysis in the presence of nonstationary detector noise, ~iii! combining data from multiple detector pairs to increase the sensitivity of a stochastic background search, ~iv! correlating the outputs of 4 or more detectors, and ~v! allowing for the possibility of correlated noise in the outputs of two detectors are discussed. We briefly describe a computer simulation that was used to ‘‘experi- mentally’’ verify the theoretical calculations derived in the paper, and which mimics the generation and detection of a simulated stochastic gravity-wave signal in the presence of simulated detector noise. Numerous graphs and tables of numerical data for the five major interferometers ~LIGO-WA, LIGO-LA, VIRGO, GEO- 600, and TAMA-300! are also given. This information consists of graphs of the noise power spectra, overlap reduction functions, and optimal filter functions; also included are tables of the signal-to-noise ratios and sensitivity levels for cross-correlation measurements between different detector pairs. The treatment given in this paper should be accessible to both theorists involved in data analysis and experimentalists involved in detector design and data acquisition. @S0556-2821~99!02708-3# PACS number~s!: 04.80.Nn, 04.30.Db, 07.05.Kf, 95.55.Ym I. INTRODUCTION gravity-wave detectors. One might also be able to detect a faint stochastic background of gravitational radiation, pro- The design and construction of a number of new and more duced very shortly after the big bang. These detections may sensitive detectors of gravitational radiation is currently un- happen soon after the detectors go ‘‘on-line’’ or they may derway. These include the two Laser Interferometric Gravi- require a decade of further work to increase the sensitivity of tational Wave Observatory ~LIGO! detectors being built in the instruments. But it is fairly safe to say that eventually, Hanford, WA and Livingston, LA by a joint Caltech-MIT when their sensitivity passes some threshold value, the collaboration @1#, the VIRGO detector being built near Pisa, gravity-wave detectors will find sources. Even more exciting Italy by an Italian-French collaboration @2#, the GEO-600 is the prospect that the detectors will discover new sources of detector being built in Hanover, Germany by an Anglo- gravitational radiation—sources which are different from those mentioned above, and which we had not expected to German collaboration @3#, and the TAMA-300 detector being find. It promises to be an exciting time. built near Tokyo, Japan @4#. There are also several resonant The subject of this paper is a stochastic ~i.e., random! bar detectors currently in operation, and several more refined background of gravitational radiation, first studied in detail bar and interferometric detectors presently in the planning by Michelson @5#, Christensen @6#, and Flanagan @7#. and proposal stages. Roughly speaking, it is the type of gravitational radiation The operation of these detectors will have a major impact produced by an extremely large number of weak, indepen- on the field of gravitational physics. For the first time, there dent, and unresolved gravity-wave sources. The radiation is will be a significant amount of experimental data to be ana- stochastic in the sense that it can be characterized only sta- lyzed, and the ‘‘ivory tower’’ relativists will be forced to tistically. As mentioned above, a stochastic background of interact with a broad range of experimenters and data ana- gravitational radiation might be the result of processes that lysts to extract the interesting physics from the data stream. took place very shortly after the big bang. But since we ‘‘Known’’ sources such as coalescing neutron star ~or black know very little about the state of the universe at that time, it hole! binaries, pulsars, supernovae, and other periodic and is impossible to say with any certainty. A stochastic back- transient ~or burst! sources should all be observable with ground of gravitational radiation might also arise from more recent processes ~e.g., radiation from many unresolved bi- nary star systems!, and this more recent contribution might *Email address: [email protected] overwhelm the parts of the background that contain informa- †Permanent address: Department of Physical Sciences, University tion about the state of the early universe. In any case, the of Texas at Brownsville, Brownsville, Texas 78521 ~Email address: properties of the radiation will be very dependent upon the [email protected]! source. For example, one would expect a stochastic back- 0556-2821/99/59~10!/102001~41!/$15.0059 102001-1 ©1999 The American Physical Society BRUCE ALLEN AND JOSEPH D. ROMANO PHYSICAL REVIEW D 59 102001 ground of cosmological origin to be highly isotropic, tiple detector pairs when searching for a stochastic back- whereas that produced by white dwarf binaries in our own ground of gravitational radiation. Section VI A also includes galaxy would be highly anisotropic. We will just have to a graph of the ‘‘enhanced’’ LIGO detector noise curves, wait and see what the detectors reveal before we can decide which track the projected performance of the LIGO detector between these two possibilities. design over the next decade. This paper will focus on issues related to the detection of In Sec. VII, we describe a computer simulation that mim- a stochastic background of gravitational radiation. ~We will ics the generation and detection of a simulated stochastic not talk much about possible sources.! We give a complete gravity-wave signal in the presence of simulated detector and comprehensive treatment of the problem of detecting a noise. The simulation was used to verify some of the theo- stochastic background, which should be accessible to both retical calculations derived in the previous sections. theorists involved in the data analysis and experimentalists Section VIII concludes the paper with a brief summary involved in detector design and data acquisition. and lists some topics for future work. The outline of the paper is as follows: Note that throughout the paper, we use c to denote the In Sec. II, we begin by describing the properties of a speed of light and G to denote Newton’s gravitational 10 28 stochastic background of gravitational radiation—its spec- constant (c52.998310 cm/sec and G56.673310 3 2 trum, statistical assumptions, and current observational con- cm /g sec ). straints. In Sec. III, we describe how one can correlate the outputs II. STOCHASTIC BACKGROUND: PROPERTIES of two gravity-wave detectors to detect ~or put an upper limit A. Spectrum on! a stochastic gravity-wave signal. Section III B includes a detailed derivation of the overlap reduction function that A stochastic background of gravitational radiation is a covers the case where the two arms of a detector are not random gravity-wave signal produced by a large number of perpendicular ~e.g., GEO-600! and corrects a typographical weak, independent, and unresolved gravity-wave sources. In error that appears in the literature. Section III C includes a many ways it is analogous to the cosmic microwave back- rigorous derivation of the optimal signal processing strategy. ground radiation ~CMBR!@9#, which is a stochastic back- Most of the material in Secs. II and III has already appeared ground of electromagnetic radiation. As with the CMBR, it in the literature. Interested readers should see Ref. @8# for is useful to characterize the spectral properties of the gravi- more details, if desired. tational background by specifying how the energy is distrib- In Sec. IV, we ask the following questions: ~i! How do we uted in frequency. Explicitly, one introduces the dimension- decide, from the experimental data, if we’ve detected a sto- less quantity chastic gravity-wave signal? ~ii! Assuming that a stochastic gravity-wave signal is present, how do we estimate its 1 drgw Vgw~ f !ª , ~2.1! strength? ~iii! Assuming that a stochastic gravity-wave signal rcritical d ln f is present, what is the minimum value of V0 required to detect it 95% of the time? This leads to a discussion of signal where drgw is the energy density of the gravitational radia- detection, parameter estimation, and sensitivity levels for tion contained in the frequency range f to f 1df, and rcritical stochastic background searches, adopting a frequentist point is the critical energy density required ~today! to close the 95%,5% universe: of view. The calculation of V0 in Sec. IV D corrects an error that has appeared in the literature. 3c2H2 ergs In Sec. V, we return to the problem of detection by ad- 0 28 2 rcritical 5 '1.6310 h100 . ~2.2! dressing a series of subtle issues initially ignored in Sec. III. 8pG cm3 These include ~i! calculating the signal-to-noise ratio for arbitrarily large stochastic backgrounds, ~ii! performing the H0 is the Hubble expansion rate ~today!, data analysis in the presence of nonstationary detector noise, km 1 ~iii! combining data from multiple detector pairs to increase 218 H05h1003100 5 3.2310 h100 the sensitivity of a stochastic background search, ~iv! corre- sec Mpc sec lating the outputs of 4 or more detectors, and ~v! allowing for 1 the possibility of correlated noise in the outputs of two de- 228 51.1310 ch100 , ~2.3! tectors.

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