Introduction to Gravitational Waves Bernard Schutz Albert Einstein Institute – Max Planck Institute for Gravitational Physics, Golm, Germany and Cardiff University, Cardiff, UK http://www.aei.mpg.de schutz@aei.mpg.de

Gravitational Wave Astronomy Gravitational waves are the most important prediction of Einstein that has not yet been verified by direct detection. The Hulse-Taylor pulsar system PSR1913+16 gives very strong indirect confirmation of the theory. Gravitational waves carry huge energies, but they interact very weakly with matter. These properties make them ideal probes of some of the most interesting parts of the Universe, now that we have learned how to make sufficiently sensitive detectors. Unlike in most of electromagnetic astronomy, gravitational waves will be observed coherently, following the phase of the wave. This is possible because of their relatively low frequencies (most interest is below 10 kHz). This makes detection strategies very different: instead of bolometric (energy) detection in hardware, gravitational wave detection will be by data analysis, in software. Bernard F Schutz Albert Einstein Institute 19 May 2003 Frascati: Introduction to Gravitational Waves

Tidal gravitational forces By the equivalence principle, the gravitational effect of a distant source can only be felt through its tidal forces – inhomogeneous part of gravity. Gravitational waves are traveling, time-dependent tidal forces. Tidal forces scale with size, typically produce elliptical deformations. Bernard F Schutz Albert Einstein Institute 19 May 2003 Frascati: Introduction to Gravitational Waves

Frascati: Introduction to Gravitational Waves Polarisation Gravitational waves have 2 independent polarisations, illustrated here by the motions of free “test” particles. Interferometers are linearly polarised detectors. Distortions follow the motions of the source projected on the sky. A measurement of the degree of circular polarisation determines the inclination of a simple binary orbit. If the orbit is more complex, as for strong spin-spin coupling, then the changes in polarisation tell what is happening to the orbit. Bernard F Schutz Albert Einstein Institute 19 May 2003 Frascati: Introduction to Gravitational Waves

Frascati: Introduction to Gravitational Waves Nautilus Bar detectors Allegro The first detector was the Weber bar, operated at room temperature. Currently there are five main cryogenic bars, including the ultra-cyrogenic Nautilus and Auriga. They operate the ICEG collaboration for searching for coincident bursts. Narrow-bandwidths at relatively high frequencies. Niobe Auriga Bernard F Schutz Albert Einstein Institute 19 May 2003 Frascati: Introduction to Gravitational Waves

Frascati: Introduction to Gravitational Waves Bar sensitivity Bars have better sensitivity at resonance but bandwidth determined by sensor/amplifier. Aim of future development is to widen bandwidth. Bernard F Schutz Albert Einstein Institute 19 May 2003 Frascati: Introduction to Gravitational Waves

Frascati: Introduction to Gravitational Waves Strange Events? Coincidences were seen between Explorer and Nautilus. See P Astone, et al, Class. Quantum Grav. 19 5449 No claim has been made that they are gravitational waves, because they are marginally significant and difficult to understand on any expected model. More data coming soon from interferometers and the two bars! Bernard F Schutz Albert Einstein Institute 19 May 2003 Frascati: Introduction to Gravitational Waves

Worldwide Interferometer Network Bernard F Schutz Albert Einstein Institute 19 May 2003 Frascati: Introduction to Gravitational Waves

Large Interferometers: the 1st Generation Bernard F Schutz Albert Einstein Institute 19 May 2003 Frascati: Introduction to Gravitational Waves

Progress in commissioning of LIGO Bernard F Schutz Albert Einstein Institute 19 May 2003 Frascati: Introduction to Gravitational Waves

The Technology of Laser Interferometers GEO600 must measure mean motions of mirrors over distances of 10-21 of 600 m, or 6 x 10-19 m, on timescales of milliseconds. Detection is all about excluding other sources of mirror motion on these timescales. Noise source External vibrations Mirror thermal vibrations Pendulum thermal vibrations Photon counting statistics How it is mini-mized Multi-stage pendulum suspension for mirrors, mechanical filter, f >1 Hz Sets lower frequency limit on observing. Make mirror substrate of high-Q material so kT energy is concentrated near mode frequencies, above 2 kHz. Need Q~108 in fused silica. Make suspension with high-Q so kT is concentrated near 1 Hz pendulum frequency. Need Q~106. Use drawn silica fibres, hydroxide bonding to mirrors Need 100kW of laser power in arms, use power recycling so that laser input (5W) only replaces mirror losses (10-6 per reflection). Limited by thermal lensing. Bernard F Schutz Albert Einstein Institute 19 May 2003 Frascati: Introduction to Gravitational Waves

Data: Massive Volume, Massive Analysis LIGO and GEO have jointly developed data analysis software and are doing joint analysis of current data for upper limits. New software have come from this: Triana quick-look system (GEO) Hough-transform hierarchical methods for all-sky surveys (GEO-VIRGO) Grid efforts increasing: GriPhyN, DataGrid, Triana/GridOneD GEO600 will record 15 TB per year, LIGO maybe 200 TB. Most of this is “housekeeping”. Signal data around 500 GB/y. Real-time matched filtering requires ~100 Gflops. All-sky surveys for pulsars need far more: > 1020 filters 4 months long. Bernard F Schutz Albert Einstein Institute 19 May 2003 Frascati: Introduction to Gravitational Waves

Detectors Today and Tomorrow Chances Bar detectors operate at cyrogenic temperatures with sensitivity better than 10-19 today. Their relatively narrow bandwidth excludes some sources. ?? Future resonant-mass detectors could take the form of large omni-directional spheres, or concentric cylidrical shells. Could be competitive with interferometers at higher frequencies. 1st-generation interferometric detectors (LIGO, GEO, VIRGO) are nearing design sensitivity of 10-21 at frequencies above 40 Hz. ? The 2nd-generation Advanced LIGO upgrade (partnership with GEO) is seeking funding ( 10 gain in sensitivity by 2009, frequency range extended to 10 Hz). 3rd-generation interferometer technology in research.  LISA is aiming for a launch in 2011 as a joint ESA-NASA mission. It will open the low-frequency window (below 1 Hz). NASA envisions a succession of space detectors. They will be the workhorses of gravitational wave astronomy. The frequency range (down to 0.1 mHz) is a good match to the timescale (hours) of many astronomical systems.  Bernard F Schutz Albert Einstein Institute 19 May 2003 Frascati: Introduction to Gravitational Waves

Gravitational Waves in a Post-Newtonian Nutshell Coupling: h is the amplitude. Generation: internal potential Newtonian potential Energy Flux: all classical field theories dimensional factor Bernard F Schutz Albert Einstein Institute 19 May 2003 Frascati: Introduction to Gravitational Waves

Gravitational Dynamics / Frequency Luminosity very strong dependence on compactness / Timescale Chirp time  is a measure of light- crossing time Bernard F Schutz Albert Einstein Institute 19 May 2003 Frascati: Introduction to Gravitational Waves

Frascati: Introduction to Gravitational Waves Examples High frequency: neutron star with r = 20 Mpc, R = 10 km F = 0.6 W m-2s-1 > FMoon! But if L = 4 km, then h = 10-21 is the 1st detector goal. Low f: 2 BHs, each 106 M at z = 1 (r = 4 Gpc) Merger takes 4 minutes, but in-spiral takes months to move through observation band from 0.1 to 14 mHz. Bernard F Schutz Albert Einstein Institute 19 May 2003 Frascati: Introduction to Gravitational Waves

Detectors Measure Distances: Chirping Binaries are Standard Candles If a detector measures not only f and h but also  for a binary, then it can determine its distance r. For a circular binary, upper bounds are attained, so: Combining this with f itself gives us M and R, and then the value of h gives us r, the distance (luminosity distance ). If a chirping massive black-hole binary is identified so that a redshift can be obtained, then one can do cosmology: H0, q0. LISA can measure f, , and h to 0.1% accuracy. Bernard F Schutz Albert Einstein Institute 19 May 2003 Frascati: Introduction to Gravitational Waves

GW physics across the spectrum 2 x 100 M BHs coalesce in 1 yr from ~ 0.1 Hz A chirping system is a GW standard candle: if position is known, distance can be inferred. Bernard F Schutz Albert Einstein Institute 19 May 2003 Frascati: Introduction to Gravitational Waves

The High-Frequency Sky in 2003 Coalescing neutron-star binaries may cause gamma-ray bursts. They should be seen by advanced detectors. But the binary black-hole coalescence rate may be higher (made efficiently in globular clusters), so first interferometers may see them. Supernovae uncertain. Neutron-star r-modes are unstable by the CFS mechanism. May explain why LMXB spin periods are all near 300 Hz. Likely source for advanced detectors. Standard inflation sets a difficult target for observing a cosmological background. But superstring-inspired cosmologies (Veneziano et al) or brane scenarios (Hogan) may generate more radiation detectable by LISA or Advanced LIGO. Pulsars and unseen (young?) NSs may be cw-emitters; could be seen by first interferometers or bars, likely by advanced interferometers. NS normal modes would be probes of NS interior. Need broadband high-frequency detector. Bernard F Schutz Albert Einstein Institute 19 May 2003 Frascati: Introduction to Gravitational Waves

Looking for signals with matched filtering Matched filtering concentrates signal power while spreading out noise. Must know the signal waveform. Classic example: Fourier transform. F.t. of data set {xj} of length N This picks out sine-wave because we multiply exactly by sine-wave General matched filter for signal {sj} in data set {xj} If {sj} is a member of a family, must do filter separately for each member. May overwhelm computer! Bernard F Schutz Albert Einstein Institute 19 May 2003 Frascati: Introduction to Gravitational Waves

Conventions on Source/Sensitivity Plots Assume the best search algorithm now known Set Threshold so false alarm probability = 1% Broadband Waves Signal/Threshold in Df = f in 4 months integration Narrowband Waves Stochastic in Df=f & 4 months Bernard F Schutz Albert Einstein Institute 19 May 2003 Frascati: Introduction to Gravitational Waves

Frascati: Introduction to Gravitational Waves Overview of Sources NS & BH Binaries inspiral merger Spinning NS’s LMXBs known pulsars unknown NS Birth (SN, AIC) tumbling convection Stochastic big bang early universe Bars Bernard F Schutz Albert Einstein Institute 19 May 2003 Frascati: Introduction to Gravitational Waves

Frascati: Introduction to Gravitational Waves Neutron Star / Neutron Star Inspiral (our most reliably understood source) ~10 min ~3 sec ~10,000 cycles 20 Mpc Initial IFOs, Range: 20 Mpc 1 / 3000 yrs to 1 / 3yrs 1.4 Msun / 1.4 Msun NS/NS Event rates V. Kalogera, R. Narayan, D. Spergel, J.H. Taylor astro-ph/0012038 300 Mpc Advanced IFOs, Range: 300Mpc 1 / yr to 2 / day Bernard F Schutz Albert Einstein Institute 19 May 2003 Frascati: Introduction to Gravitational Waves

Science From Observed Inspirals Relativistic effects are very strong -- e.g. Frame dragging by spins  precession  modulation Tails of waves modify the inspiral rate Information carried: Masses (a few %), Spins (?few%?), Distance [not redshift!] (~10%), Location on sky (~1 degree) Mchirp = l3/5 M2/5 to ~10-3 Search for EM counterpart, e.g. c-burst. If found: Learn the nature of the trigger for that c-burst deduce relative speed of light and gw’s to ~ 1 sec / 3x109 yrs ~ 10-17 Bernard F Schutz Albert Einstein Institute 19 May 2003 Frascati: Introduction to Gravitational Waves

Neutron Star / Black Hole Inspiral and NS Tidal Disruption 43 Mpc inspiral NS disrupt Initial IFOs Range: 43 Mpc 1 / 2500 yrs to 1 / 2yrs 1.4Msun / 10 Msun NS/BH Event rates Population Synthesis [Kalogera’s summary] 140 Mpc NS Radius to 15% -Nuclear Physics- NEED: Reshaped Noise, Numerical Simulations 650 Mpc Advanced IFOs Range: 650 Mpc 1 / yr to 4 / day Bernard F Schutz Albert Einstein Institute 19 May 2003 Frascati: Introduction to Gravitational Waves < ~

Black Hole / Black Hole Inspiral and Merger 10Msun / 10 Msun BH/BH Event rates Based on population synthesis [Kalogera’s summary of literature] 100 Mpc inspiral merger Initial IFOs Range: 100 Mpc 1 / 300yrs to ~1 / yr z=0.4 inspiral merger Advanced IFOs - Range: z=0.4 2 / month to ~10 / day Bernard F Schutz Albert Einstein Institute 19 May 2003 < Frascati: Introduction to Gravitational Waves ~

BH/BH Mergers: Exploring the Dynamics of Spacetime Warpage Numerical Relativity Simulations Are Badly Needed! Bernard F Schutz Albert Einstein Institute 19 May 2003 Frascati: Introduction to Gravitational Waves

Massive BH/BH Mergers with Fast Spins Advanced Interferometers Lower Frequency Bernard F Schutz Albert Einstein Institute 19 May 2003 Frascati: Introduction to Gravitational Waves

Frascati: Introduction to Gravitational Waves BH Merger Simulations Improving all the time: More stable forms of the field equations Gauge conditions improved Run times lengthening Initial data must be improved: subtle Boundary conditions not yet satisfactory EU- funded network “Sources of Gravitational Waves” pushing all of these issues. Still hungry for computer time. The Discovery Channel funded AEI’s longest simulation to date, and its visualization. (Seidel, Benger, et al, AEI) Bernard F Schutz Albert Einstein Institute 19 May 2003 Frascati: Introduction to Gravitational Waves

Spinning NS’s: Pulsars Crab Spindown Upper Limit Known Pulsars: First Interferometers:  3x10-6 (1000Hz/f) x (distance/10kpc) Narrowband Advanced  2x10-8 (1000Hz/f)2 x (distance/10kpc) e = 10-7, 10kpc e = 10-6, 10kpc e = 10-5, 10kpc NS Ellipticity: Crust strength => e < ~10-6; possibly 10-5 Unknown NS’s - All sky search: Sensitivity ~5 to 15 worse Bernard F Schutz Albert Einstein Institute 19 May 2003 Frascati: Introduction to Gravitational Waves

Spinning Neutron Stars: Low-Mass X-Ray Binaries Rotation rates ~250 to 700 revolutions / sec Why not faster? Bildsten: Spin-up torque balanced by GW emission torque If so, and steady state: X-ray luminosity  GW strength Combined GW & EM obs’s => information about: crust strength & structure, temperature dependence of viscosity, ... Signal strengths for 20 days of integration Sco X-1 Bernard F Schutz Albert Einstein Institute 19 May 2003 Frascati: Introduction to Gravitational Waves

NS Birth: Tumbling Bar; Convection Born in: Supernovae Accretion-Induced Collapse of White Dwarf If very fast spin: Centrifugal hangup Tumbling bar - episodic? (for a few sec or min) If modeling gives enough waveform information, detectable to: Initial IFOs: ~5Mpc (M81 group, ~1 supernova/3yr) Advanced IFOs: ~100Mpc (~500 supernovae/yr) If slow spin: Convection in first ~1 sec. Advanced IFOs: Detectable only in our Galaxy (~1/30yrs) GW / neutrino correlations! Bernard F Schutz Albert Einstein Institute 19 May 2003 Frascati: Introduction to Gravitational Waves

Stochastic Background from Very Early Universe GW’s are the ideal tool for probing the very early universe Present limit on GWs From effect on primordial nucleosynthesis W = (GW energy density)/(closure density) 10-5 Bernard F Schutz Albert Einstein Institute 19 May 2003 Frascati: Introduction to Gravitational Waves

Stochastic Background from Very Early Universe Detect by cross correlating output of Hanford & Livingston 4km IFOs W = 10-7 W = 10-9 W = 10-11 Good sensitivity requires (GW wavelength) 2x(detector separation) f 40 Hz Initial IFOs detect if W 10-5 Advanced IFOs: W 5x10-9 Bernard F Schutz Albert Einstein Institute 19 May 2003 Frascati: Introduction to Gravitational Waves

Gravitational Waves from Very Early Universe. Waves from standard inflation: W~10-15: much too weak BUT: Crude superstring models of big bang suggest waves might be strong enough for detection by Advanced LIGO Energetic processes at (universe age) ~ 10-25 sec and (universe temperature) ~ 109 Gev => GWs in LIGO band phase transition at 109 Gev excitations of our universe as a 3-dimensional “brane” (membrane) in higher dimensions: Brane forms wrinkled When wrinkles “come inside the cosmological horizon”, they start to oscillate; oscillation energy goes into gravitational waves LIGO probes waves from wrinkles of length ~ 10-10 to 10-13 mm If wave energy equilibrates: possibly detectable by initial IFOs Example of hitherto UNKNOWN SOURCE Bernard F Schutz Albert Einstein Institute 19 May 2003 Frascati: Introduction to Gravitational Waves

LISA – Shared Mission of ESA & NASA ESA & NASA have exchanged letters of agreement. ESA/ESTEC and NASA/GSFC jointly manage mission. Launch 2011, observing 2012+. Mission duration up to 10 yrs. SMART-2 technology demonstrator (ESA: 2006) Project scientists: Karsten Danzmann (AEI) and Tom Prince (NASA: JPL/Caltech). Joint 20-strong LIST: LISA International Science Team Bernard F Schutz Albert Einstein Institute 19 May 2003 Frascati: Introduction to Gravitational Waves

Gravitational wave spectrum Gravity gradient noise on the Earth Space detector far from Earth GAP! Bernard F Schutz Albert Einstein Institute 19 May 2003 Frascati: Introduction to Gravitational Waves

Frascati: Introduction to Gravitational Waves LISA in Orbit Bernard F Schutz Albert Einstein Institute 19 May 2003 Frascati: Introduction to Gravitational Waves

Frascati: Introduction to Gravitational Waves LISA interferometry - 1 Each S/C carries 2 lasers, 2 telescopes, 2 test masses Local lasers phase-locked Lasers on distant S/C phase-locked to incoming light laser transponder – effectively an “active mirror” reference laser beams main transponded laser beams Bernard F Schutz Albert Einstein Institute 19 May 2003 Frascati: Introduction to Gravitational Waves

Frascati: Introduction to Gravitational Waves LISA interferometry - 2 Laser beams reflected off free-flying test masses, insensitive to spacecraft motion. Effectively 2 Michelsons Long arms  displacements in picometer range, much easier than ground-based interferometry reference laser beams main transponded laser beams Bernard F Schutz Albert Einstein Institute 19 May 2003 Frascati: Introduction to Gravitational Waves

The Technology of LISA LISA must measure mean motions of mirrors over distances of 10-21 of 5 x 106 km, or 5 nm, on timescales of seconds. Detection is all about excluding other sources of mirror motion on these timescales. Noise source External disturbances (Solar radiation) Relative motion of S/C Photon counting statistics How it is mini-mized Drag-free sensing, where S/C protects free-flying proof mass from external forces. Require micro-thrusters, good accelerometer. Sets lower frequency limit. Causes rapid motion of fringes (MHz). Count fringe rate using on-board ultra-stable oscillator and compensate in on-board computers. Transmit data to Earth at only 10 bits/s average. Cannot use mirror reflection, too much diffraction loss. Use active mirrors (laser transponder) to re-transmit incoming beam back to source with correct phase. . LISA’s technology will be tested in a joint NASA-ESA mission called ST-3 in 2005. Bernard F Schutz Albert Einstein Institute 19 May 2003 Frascati: Introduction to Gravitational Waves

SMART-2: Testing free fall in space Only one S/C with 2 test masses is needed Testing: Inertial sensor Charge management Thrusters Drag-free control Low frequency laser metrology Launch 2006 with ESA and NASA test packages Bernard F Schutz Albert Einstein Institute 19 May 2003 Frascati: Introduction to Gravitational Waves

LISA science goals Compact objects orbiting massive black holes formation, binary orbit, and coalescence White dwarf, neutron star, and other compact binary systems Bernard F Schutz Albert Einstein Institute 19 May 2003 Frascati: Introduction to Gravitational Waves

LISA sensitivity curve (1-year observation) Wgw = 10-10 102 + 104 Mo vibration noise armlength shot noise Bernard F Schutz Albert Einstein Institute 19 May 2003 Frascati: Introduction to Gravitational Waves

Frascati: Introduction to Gravitational Waves Low-Frequency Sky Merging supermassive black holes (SMBH) in galactic centers Formation, growth, relation to galaxy formation and mergers, indicators from other observations, cosmological information, numerical modelling, clean removal of signals so weaker events are detectable. Signals from gravitational capture of small BHs by SMBHs Event rates, evolution of clusters near SMBHs, modelling of very complex waveform (radiation-reaction), signal extraction from background of distant events, accuracy of tests of BH uniqueness theorems of general relativity Survey of all galactic binaries with sufficiently short periods Population statistics, confusion by large population at lower frequencies, confusion limit on signal extraction, information extraction from observations Backgrounds, astrophysically generated and from the Big Bang Strength and spectrum of astrophysical backgrounds, production of early-universe radiation, relation to fundamental physics (string theory, branes, …) Bursts, unexpected sources Formation of BHs of intermediate to large mass, possible sources in dark matter Bernard F Schutz Albert Einstein Institute 19 May 2003 Frascati: Introduction to Gravitational Waves

Frascati: Introduction to Gravitational Waves Galactic binaries All compact-object binaries (WD, NS, BH) in galaxy with large enough frequency will be observed. GAIA observations can help identify individual binaries. LISA will provided masses, distances (if needed), orbit inclination. Population statistics will make key contributions to understanding binary and stellar evolution. For f < 0.001 Hz, only nearest binaries will be resolved; most form an anisotropic noise. Even at higher frequencies, binary signals must be removed accurately to see other weak sources. Bernard F Schutz Albert Einstein Institute 19 May 2003 Frascati: Introduction to Gravitational Waves

Massive Black Holes in Galaxies Most galaxies near enough to be studied contain central black holes, 106 to > 109 solar masses. The Milky Way is one of the most convincing cases: it contains 2.6  106 M in a region not much bigger than our solar system. (Movie by Eckart & Genzel.) All observations show only a mass concentration. GWs are the only radiation actually emitted by black holes. LISA will literally listen to these black holes as they merge. MPE Garching Bernard F Schutz Albert Einstein Institute 19 May 2003 Frascati: Introduction to Gravitational Waves

Massive Black Holes Merge Detected masses from 106 to 109 M. Smaller masses possible. Galaxy mergers should produce BH mergers. Rate un certain: 1/yr for 106 M at z=1? Protogalaxy mergers may be richer. Phinney: possibly 103/yr for 105 M at z = 7. Stellar BHs fall into massive BHs more often, but weaker radiation. (S Phinney) Bernard F Schutz Albert Einstein Institute 19 May 2003 Frascati: Introduction to Gravitational Waves

Frascati: Introduction to Gravitational Waves Coalescences of Massive Black Holes: How Signal/Noise Grows Week by Week The high S/N at early times enables LISA to predict the time and position of the coalescence event, allowing the event to be observed simultaneously by other telescopes. Each point shows the gain in signal-to-noise ratio(SNR) after a further week of data on each signal. The vertical axis is power SNR, which is what is typically used in other branches of astronomy. The curves are labelled with the masses of the binary black holes; in each case two equal masses are assumed. The legend shows how the sensitivity depends on source location (with respect to the ecliptic) and how it is affected by the galactic binary noise. Note the dramatic increase in SNR during the last week of observing. Bernard F Schutz Albert Einstein Institute 19 May 2003 Frascati: Introduction to Gravitational Waves

Issue: Cosmology with SMBH Mergers Position uncertainties of SMBH mergers are significant, error boxes of order several degrees likely. This dominates uncertainty in range too, makes it impossible on position alone to find galaxy in which merger took place. Can other observations identify galaxy or cluster where merger is about to occur? NGST, LOFAR, X-ray activity? Cosmology with SMBHs. If the merger can be associated with a galaxy or cluster, then the uncertainty in position and distance error are drastically reduced, only dominated by random velocities of galaxies and gravitational lensing. This would allow tracking of the acceleration history of the Universe as far back as SMBH mergers occur. Bernard F Schutz Albert Einstein Institute 19 May 2003 Frascati: Introduction to Gravitational Waves

Frascati: Introduction to Gravitational Waves Gravitational capture example 10M/106M circular equatorial orbit, fast spin [Finn/Thorne] 1 yr before plunge: r=6.8 rHorizon 185,000 cycles left, S/N ~ 100 1 mo before plunge: r=3.1 rHorizon 41,000 cycles left, S/N ~ 20 heff 1 day before plunge: r=1.3 rHorizon 2,300 cycles left, S/N ~ 7 f (Hz) Bernard F Schutz Albert Einstein Institute 19 May 2003 Frascati: Introduction to Gravitational Waves

Issue: How well can we study gravitational captures? Potential for very fundamental results, mapping spacetime near a Kerr black hole Nightmare for matched filtering: Huge parameter space for orbits, perhaps 1030 or more distinguishable sets of parameters Radiation-reaction problem in strong-field Kerr not yet solved Approximate, hierarchical scheme will be needed Filtering must be good, because: Signals from galactic WD binaries and SMBH mergers need to be removed to avoid contamination Distant capture events provide background: “Olbers limit” Bernard F Schutz Albert Einstein Institute 19 May 2003 Frascati: Introduction to Gravitational Waves

Issue: cosmological background One of the most fundamental goals of GW detection is the cosmological background from the Big Bang. Best observational evidence we are likely to get about fundamental physics. LISA limit around Wgw ~ 10-10. Standard inflation predicts very weak radiation (Wgw < 10-14), but alternative scenarios can produce more in the LISA band (branes, pre-Big-Bang cosmology, …). Some alternatives produce no radiation at all, e.g. ekpyrotic universe. Some scenarios of symmetry breaking can produce observable radiation. Backgrounds from astrophysical sources restrict observing range if Wgw < 10-10. Possible window 0.1-10 Hz (the Gap) would be target of a follow-on mission. Bernard F Schutz Albert Einstein Institute 19 May 2003 Frascati: Introduction to Gravitational Waves

Frascati: Introduction to Gravitational Waves Expect the Unexpected! Within this decade, gravitational wave detectors will begin to make observations routinely. Although we can predict some sources, the most interesting may be the unexpected, unimagined. The launch of X-ray, gamma-ray, infrared, and ultraviolet observatories has time and again revealed new unanticipated objects. Our understanding of the Universe is very different from the one that depended only on optical telescopes. 90% of the Universe is dark, emitting no electromagnetic radiation. But it interacts gravitationally, so does at least some of it emit gravitational waves? Bernard F Schutz Albert Einstein Institute 19 May 2003 Frascati: Introduction to Gravitational Waves

Frascati: Introduction to Gravitational Waves Further Information You can find information about the projects on these web sites: LIGO: http://www.ligo.caltech.edu VIRGO: http://www.virgo.infn.it/ GEO: http://www.geo600.uni-hannover.de/ TAMA: http://tamago.mtk.nao.ac.jp/ LISA: http://www.estec.esa.nl/spdwww/future/html/lisa.htm and http://lisa.jpl.nasa.gov/ Bars: linked from IGEC site, http://igec.lnl.infn.it/ Further information about software and collaborations: Cactus: http://www.cactuscode.org/ Gridlab: http://www.gridlab.org/ EU GW Astrophys: http://www.eu-network.org/ Triana: http://www.triana.co.uk/ Bernard F Schutz Albert Einstein Institute 19 May 2003 Frascati: Introduction to Gravitational Waves