Gravitation: A Decennial Perspective Confronting Theory with Gravitational Wave Observations B F Schutz Max Planck Institute for Gravitational Physics () Golm/Potsdam Germany
The AEI congratulates The CGPG On its first 10 years! 2
Gravitational Wave Observations: the last 10 years CGPG was opening its doors just as Hulse and Taylor were winning the Nobel Prize for the discovery and scientific exploitation of PSR1913+16 This system assures us that GR is reliable for most source calculations, but almost 30 years after its discovery, it remains our best test of the radiative properties of gravity. 3
Theory in the last 10 years Post-Newtonian approximation very mature (Blanchet) r-mode instability discovered and studied (Owen) Restricted 2-body problem nearing computational solution (Mino) Core collapse simulations reach 3D (Dimmelmeier) Black hole merger simulations begin to explore physics (Brügmann, many others) Neutron star solidity much better understood (Jones) New methods of producing a stochastic background (Flanagan) Exotic sources suggested: cosmic string kinks, binary MACHOs Intensive work in astrophysics on sources: binary pulsars, binary black holes (supermassive and stellar), isolated pulsars, gamma-ray bursts 4
Confronting theory with observations: the next 10 15 years Stretch time-frame to 15 yrs to include LISA Theory will be tested in two ways: astrophysics fundamental physics Observations will return two kinds of information: details of waveforms, intrinsic amplitudes, luminosities source population statistics (events per unit time per unit volume, or continuous sources per unit volume), some direct distance measurements 5
Worldwide Interferometer Network 32 msec 27 msec 6
The inverse problem for bursts: what do we learn? A short burst of GWs has 4 unknowns: θ,φ (position on sky) h (amplitude) function of time χ (polarization angle) function of time Each detector returns (at any time) response H, time of arrival T N detectors: 2N-1 data values 3 detectors get 5 data However, the two LIGO detectors are very similar in orientation, so problem is degenerate. Good for confidence, bad for inverse problem. LIGO on its own can determine a source location annulus on sky, cannot separate h and χ LIGO + (GEO or VIRGO) can return 4 data, just barely determine location LIGO + GEO + VIRGO can return 6 data, although short GEO-VIRGO baseline (3 ms) means only 5 are useful. Overdetermined, can test GR. Japanese or Australian detector would be very useful in reducing errors. 7
Inverse problem for CW sources R orbit λ wave detector A wave longer than ~ 30 mins will require corrections to remove Doppler effect due to motion of Earth Only spinning NSs are expected to produce such signals observable from the ground. Demodulation over a year gives arcsecond positions, polarization, amplitude from a single detector. Data from other detectors can confirm. LIGO/GEO has λ/r~ 10-6 ; LISA has ~ 10-1. 8
Data analysis by computer Ground-based detectors require computer processing of data even to recognize signals. Good prior information: use matched filtering, which is phase matched to signal. Best sensitivity. Random signals: use cross-correlation of 2 detectors, the closer the better (LIGO). Poorly predicted signals: use variety of criteria, based on total power, color, time-frequency, etc. Short bursts require coincidences for confidence Continuous signals need confirmation from other detectors. Sensitivity of a detector can be limited by data analysis capability. 9
Astronomical coincidences Important to see GW observations as part of astronomy. Almost all astronomical observing today is multiwavelength, either coordinated or as follow-up observations. Should expect this for GW too. Astronomical coincidence can in many cases be used to increase confidence in GW observation, but only if there is a reasonable the source model. There are a very wide variety of astronomical observing programs that can potentially be useful as triggers or for follow-up studies. 10
Some Astronomical Observing Projects Radio, LOFAR X-ray HSTSNO VLBI satellite Auger GLAST ALMA (Chandra) KAIT (robotic) SWIFT 11
First-generation interferometers and bar detectors Observations may reveal bh binaries, pulsars, nearby supernova, unexpected sources (binaries of MACHOs, cosmic string kink bursts, primordial bhs ) Astronomical triggers very helpful. Astronomical follow-up very important (emphasise, run through wave bands, cosmic rays). Source rates will be main observational return if S/N low. Polarization could be tested if 4 detectors work. Could challenge BH merger simulations Pulsar radiation would tell us about NS physics, not yet clear what. Nearby SN event very rich information 12
Advanced LIGO, observing 2009+ Expect to see NS coalescence, BH coalescence, better chance at pulsars, LMXBs, stochastic, nearby SNe, other unknown bursts. Learn if gamma bursts produce GWs, look for optical counterparts. Check r-mode instability. Standard candles useful. Stronger tests of numerical BH simulations. Timing of NS-NS coalescence can test photon mass, as shown by Will Narrow-banding and other processing on output can enhance sensitivity (Weiss). Serious problem: only 2 detectors currently planned, VIRGO s upgrade on this timescale uncertain, Japanese timescale uncertain. Degeneracy of LIGO detectors makes a particular problem. Could have better sensitivity but get less information!! Directions, distances, polarizations lost. 13
Future Ground-Based Detectors: 3rd Generation Interferometers Push as far as possible on the ground, using cryogenic mirrors, better suspensions, quantum detection Very expensive, might not fit into 2015 time-frame Could extend range of NS coalescences to z=1, survey whole universe for BH coalescences, study nearby BH-BH coalescences in great detail 14
Future Ground-Based Detectors: High-frequency Broad-band Interferometer Between 1-6 khz there are many NS normal modes, detecting would pin down EOS neutron star asteroseismology Two detectors could do very well on stochastic background Very difficult: high laser power, diffractive optics GEO considering this as its future after current phase. 15
LISA LISA covers low-frequency band with high sensitivity Most spectacular source: coalescences of MBH binaries in mass range 10 4-10 7 solar masses. Huge S/N > 1000. Gravitational captures of small stellar BHs Observations of binary systems in the Galaxy Respectable sensitivity to a stochastic background. 16
3C75 Massive Black Hole Coalescences (S Phinney) Detecting mergers of MBHs is rich in information. Relativity: strongly test numerical models Astronomy: illuminate galaxy formation, evolution. With identifications and redshifts, measure acceleration history of universe out to high redshifts 17
Gravitational capture example 10M /10 6 M circular equatorial orbit, fast spin [Finn/Thorne] h eff 1 yr before plunge: r=6.8 r Horizon 185,000 cycles left, S/N ~ 100 1 mo before plunge: r=3.1 r Horizon 41,000 cycles left, S/N ~ 20 1 day before plunge: r=1.3 r Horizon 2,300 cycles left, S/N ~ 7 f (Hz) 18
Gravitational Captures Strong test of BH uniqueness theorems Distant events will create confusion background, LISA will be able to distinguish only the nearest events. This is an Olbers limit. Similar to X-ray background in early days of X-ray astronomy. Recognition and removal of nearest events requires extensive searches over large parameter spaces, plus reliable ways to compute orbits of inspiralling objects: equation of motion in restricted two-body problem in GR. 19
Galactic Binaries and LISA LISA has poor angular resolution below 1 mhz, where there is a large population of WD-WD binaries in the Galaxy. These will not be resolved and will blend into a stochastic background noise. As with captures, the nearest and highestfrequency binaries will be resolved. All NS-NS and BH binaries with periods shorter than Hulse- Taylor will be detected. LISA will determine inclinations, distances Working with GAIA, LISA can calibrate size of Galaxy. 20
LISA follow-on NASA wants to begin a feasibility study for a follow-on mission before LISA is launched. What should the goals be? Higher sensitivity Deeper search for a stochastic background Better angular resolution on MBH coalsecences Probably involve multiple LISA-like configurations 21
What work do we need to do in next 10 years? Produce more accurate BH-BH, NS-NS, and NS-BH merger simulations and inspiral orbit models Develop better methods for searching for unknown signals and doing surveys for GW pulsars Develop closer connections with astronomical projects for triggers, follow-up Get the ground-based detectors working at full sensitivity!! Solve the restricted two-body problem in GR Understand how to filter for gravitational capture signals and how to make detections of overlapping signals. Study the dynamics of the evolution of SMBH binaries in central star clusters. Extract timescales, rates, observational predictions Make SMART-2 work and build LISA!! 22
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Neutron Star & Black Hole Binaries inspiral merger Spinning NS s LMXBs known pulsars previously unknown NS Birth (SN, AIC) tumbling convection Stochastic background big bang early universe Overview of Sources 24
LISA sensitivity curve (1-year observation) Ω gw = 10-10 vibration noise 10 2 +10 4 M o armlength shot noise 25