Detection of Gravitational Waves with Interferometers

1. Detection of Gravitational Waves with Interferometers Giant detectorsPrecision measurement The search for the elusive waves Nergis Mavalvala(LIGO Scientific Collaboration)@ Northwestern UniversityOctober 2004

2. Global network of detectors GEO VIRGO LIGO TAMA AIGO LIGO • Detection confidence • Source polarization • Sky location LISA

3. Gravitational waves • Transverse distortions of the space-time itself  ripples of space-time curvature • Propagate at the speed of light • Push on freely floating objects  stretch and squeeze the space transverse to direction of propagation • Energy and momentum conservation require that the waves are quadrupolar  aspherical mass distribution

4. Astrophysics with GWs vs. E&M • Very different information, mostly mutually exclusive • Difficult to predict GW sources based on EM observations

5. Emission of gravitational radiation from PSR1913+16 due to loss of orbital energy period sped up 14 sec from 1975-94 measured to ~50 msec accuracy deviation grows quadratically with time Nobel prize in 1997  Taylor and Hulse Gravitational waves measured?

6. R M M r h ~10-21 Strength of GWs:e.g. Neutron Star Binary • Gravitational wave amplitude (strain) • For a binary neutron star pair

7. GWs neutrinos photons now Astrophysical sources of GWs • Coalescing compact binaries • Classes of objects: NS-NS, NS-BH, BH-BH • Physics regimes: Inspiral, merger, ringdown • Periodic sources • Pulsars • Low-mass Xray binaries • Burst events • Supernovae • Gamma ray bursts • Stochastic background • Primordial Big Bang (t = 10-43 sec) • Continuum of sources • The Unexpected

8. Effect of a GW on matter

9. Measurement and the real world • How to measure the gravitational-wave? • Measure the displacements of the mirrors of the interferometer by measuring the phase shifts of the light • What makes it hard? • GW amplitude is small • External forces also push the mirrors around • Laser light has fluctuations in its phase and amplitude

10. GW detector at a glance L ~ 4 km For h ~ 10–21 DL ~ 10-18 m Seismic motion -- ground motion due to natural and anthropogenic sources Thermal noise -- vibrations due to finite temperature Shot noise -- quantum fluctuations in the number of photons detected

11. 3 0 3 ( ± 0 1 k 0 m m s ) LIGO: Laser Interferometer Gravitational-wave Observatory WA 4 km 2 km LA 4 km

13. Initial LIGO Sensitivity Goal • Strain sensitivity < 3x10-23 1/Hz1/2at 200 Hz • Displacement Noise • Seismic motion • Thermal Noise • Radiation Pressure • Sensing Noise • Photon Shot Noise • Residual Gas • Facilities limits much lower

14. The LIGO Scientific Collaboration The Laser Interferometer Gravitational-wave Observatory LIGOLaboratory LIGOScientificCollaboration U.K.GermanyJapan Russia India Spain Australia U.S. National Science Foundation

15. Limiting Noise SourcesSeismic Noise • Motion of the earth is a few mm at low frequencies • Passive and active seismic isolation • Amplify mechanical resonances • Get isolation above a few Hz

16. FRICTION Limiting Noise SourcesThermal Noise • Suspended mirror in equilibrium with 293 K heat bath akBT of energy per mode • Fluctuation-dissipation theorem: • Dissipative system will experience thermally driven fluctuations of its mechanical modes: • Low mechanical loss (high Quality factor) • Suspension  no bends or ‘kinks’ in pendulum wire • Test mass  no material defects in fused silica Z(f) is mechanical impedance (loss)

17. Optics Suspension andControl • Suspension is the key to controlling thermal noise • Magnets and coils to control position and angle of mirrors

18. Core Optics Installation and Alignment • Cleanliness of paramount importance

19. Limiting Noise SourcesOptical Noise • Shot Noise • Uncertainty in number of photons detected • Higher circulating power Pbsa low optical losses • Frequency dependence a light (GW signal) storage time in the interferometer • Radiation Pressure Noise • Photons impart momentum to cavity mirrors • Fluctuations in the number of photons • Lower input power, Pbs • Frequency dependence a response of mass to forces  Optimal input power depends on frequency

20. Initial LIGO

21. Light bounces back and forth along arms ~100 times 20 kW DL = h L h ~ 10-21 Light is “recycled” ~50 times 300 W input test mass Optical Configuration end test mass Laser + optical field conditioning 6Wsingle mode at 1064 nm signal 4 km All cavities on resonance  interferometer is “locked”

22. Science Running Plan • Interferometer performance • Intersperse commissioning and data taking consistent with obtaining one year of integrated data at h = 10-21 by end of 2006 • Astrophysical searches • Science runs interleaved with commissioning • S1 Aug 2002 – Sep 2002 duration: 2 weeks • S2 Feb 2003 – Apr 2003 duration: 8 weeks • S3 Oct 2003 – Jan 2004 duration: 10 weeks • S4 planned for early 2005 (duration: few months) • Finish detector integration & design updates... • Engineering "shakedown" runs interspersed as needed • Advanced LIGO

23. S2 2nd Science Run Feb - Apr 03 (59 days) S1 1st Science Run Sept 02 (17 days) Strain (1/rtHz) LIGO Target Sensitivity S3 3rd Science Run Nov 03 – Jan 04 (70 days) Frequency (Hz) Science Runs and Sensitivity DL = strain x 4000 m 10-18 m rms

24. Gravitational-wave searches Pulsars

25. Continuous Wave Sources • Nearly-monochromatic continuous gravitational radiation, e.g. neutron stars with • Spin precession at • Excited modes of oscillation, e.g. r-modes at • Non-axisymmetric distortion of shape at • Signal frequency modulated by relative motion of detector and source • Amplitude modulated by the motion of the antenna pattern of the detector • Search for gravitational waves from a triaxial neutron star emitted at

26. Summary of pulsar search • Time-domain analysis of 28 known pulsars with 2 frot > 50 Hz completed for S2 data • Limit on strain amplitude • Results on h0 can be interpreted as upper limit on equatorial ellipticity • Distance to pulsar is known • Izz assumed to be typical 1045 g cm2 • Limit on ellipticity

27. Gravitational-wave searches Stochastic Background

28. Stochastic Background Waves now in the LIGO band were produced 10-22 sec after the Big Bang WMAP 2003

29. Stochastic Background • Strength specified by ratio of energy density in GWs to total energy density needed to close the universe • Search by cross-correlating output of two GW detectors

30. LIGO S2 data, preliminary Limits on Wgw from astrophysical observations , at design sensitivity H0 = 72 km/s/Mpc

31. Progression of SB sensitivity S1: W0h2100 < 23 (H2-L1) S2: W0h2100 < 0.017 (H1-L1) S3: W0h2100 ~ 5 x 10-4 (H1-L1) Expected sensitivity

32. Stochastic Background • Starting to establish direct upper limits on a stochastic background of gravitational waves with LIGO • Orders of magnitude below previous direct measurements • S1 W0h1002 ≤ 23 ± 4.6 • S2 W0h1002 ≤ 0.017(Preliminary) • S3 W0h1002 5 x 10-4 (Expected) • Cross-correlation of all 3 LIGO interferometer pairs • Complete analysis of systematic errors • Future projections • LIGO I  W0h1002 ~ 10-7 to10-6 • AdLIGO  W0h1002 ~ 10-10 to 10-9 + 0.008 - 0.006

33. Gravitational-wave Searches Binary neutron star inspirals

34. Search for Inspirals • Sources • Orbital-decaying neutron star binaries • Black hole binaries • MACHOs • Search method • Waveforms calculable • Use optimalmatched filtering correlate detector output with template waveform • Template inputs from population synthesis (NWU)

35. Results of the Inspiral Search • Upper limit on binary neutron star coalescence rate • Use maximum signal-to-noise ratio statistic to establish the rate limit • Express the rate as a rate per Milky-Way Equivalent Galaxies (MWEG) • Previous observational limits • TAMA (Japan)  R < 30,000/ yr / MWEG • Caltech 40m  R < 4,000/ yr / MWEG • S1 (2003) R < 170/ yr / MWEG • Theoretical prediction • R < 2 x 10-5 / yr / MWEG

36. Gravitational-wave Searches What’s next

37. S2, S3 and beyond • Summary of S2 searches • Inspirals • Limit on reach 8 Mpc • Limit on rate  50 / year / MWEG • Stochastic background • Limit on Wgw10-2 • Pulsars • Limit on h0 ~ few x 10-24 • Limit on ellipticity ~ few x10-6 • What about S4

38. S1 S2 S3 H1 noise history (science design)

39. H1 Noise Model

40. Advanced LIGO

41. Why a better detector? Astrophysics • Factor 10 to 15 better amplitude sensitivity • (Reach)3 = rate • Factor 4 lower frequency bound • NS Binaries • Initial LIGO: ~20 Mpc • Adv LIGO: ~350 Mpc • BH Binaries • Initial LIGO: 10 Mo, 100 Mpc • Adv LIGO : 50 Mo, z=2 • Stochastic background • Initial LIGO: ~3e-6 • Adv LIGO ~3e-9

42. Quantum LIGO I LIGO II Test mass thermal Suspension thermal Seismic A Quantum Limited Interferometer

43. How will we get there? • Seismic noise • Active isolation system • Mirrors suspended as fourth (!!) stage of quadruple pendulums • Thermal noise • Suspension  fused quartz; ribbons • Test mass  higher mechanical Q material, e.g. sapphire; more massive (40 kg) • Optical noise • Input laser power  increase to ~200 W • Optimize interferometer response signal recycling

45. Cavity forms compound output coupler with complex reflectivity. Peak response tuned by changing position of SRM ℓ Reflects GW photons back into interferometer to accrue more phase SignalRecycling Signal-recycled Interferometer 800 kW 125 W signal

46. Advance LIGO Sensitivity:Improved and Tunable broadband detunednarrowband thermal noise

47. LISA Laser Interferometer Space Antenna

48. Laser Interferometer Space Antenna (LISA) • Three spacecraft • triangular formation • separated by 5 million km • Formation trails Earth by 20° • Approx. constant arm-lengths • Constant solar illumination 1 AU = 1.5x108 km

49. LISA and LIGO

50. Science from gravitational wave detectors? • Tests of general relativity • Waves  direct evidence for time-dependent metric • Black hole signatures  test of strong field gravity • Polarization of the waves  spin of graviton • Propagation velocity  mass of graviton • Astrophysics • Predicted sources: compact binaries, SN, spinning NS • Inner dynamics of processes hidden from EM astronomy • Dynamics of neutron stars  large scale nuclear matter • The earliest moments of the Big Bang  Planck epoch • Precision measurements below the quantum noise limit