Detecting Gravitational-waves with Interferometers

1. Detecting Gravitational-waves with Interferometers Nergis MavalvalaUniversité de Montréal, 2004

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

3. Universal gravitation Three laws of motionand law of gravitation (centripetal force) eccentric orbits of comets cause of tides and variations precession of the earth’s axis perturbation of motion of the moon by gravity of the sun Solved most problems of astronomy and terrestrial physics known then Unified the work of Galileo, Copernicus and Kepler Fg Newton’s gravity • Worried about instantaneous action at a distance (Aristotle) • How could objects influence other distant objects?

4. TheSpecial Theory of Relativity (1905) said outrageous things about space and time Relative to an observer traveling near the speed of light space and time are altered The General Theory of Relativity and theory of Gravity(1916) No absolute motion  only relative motion Space and time not separate  four dimensional space-time Gravity is not a force acting at a distance  warpage of space-time Gravitational radiation (waves) Einstein’s gravity Distances stretched and Clocks tick more slowly

5. Gravitational Waves • GR predicts transverse space-time distortions propagating at the speed of light • In TT gauge and weak field approximation, Einstein field equations  wave equation • Conservation laws • Conservation of energy  no monopole radiation • Conservation of momentum  no dipole radiation • Lowest moment of field  quadrupole (spin 2) • Radiated by non-spherical astrophysical objects with strong gravitational fields(including “dark” mass distributions) • Carry energy

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

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  Spinning neutron stars • Burst events • Supernovae  asymmetric collapse • Stochastic background • Primordial Big Bang (t = 10-43 sec) • Continuum of sources • The Unexpected

8. Neutron Stars colliding head on Courtesy of WUStL GR group

9. Neutron Stars spiraling toward each other Courtesy of WUStL GR group

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

11. 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?

12. DL = h L LIGOmeasurement Earthdiameter Size of nucleus Short person Width of hair Earth-Sundistance f DL Size of atom USA E-W Pea m 1011 107 104 100 10-2 10-5 10-10 10-15 10-18 Principle of Interferometric Measurement • Laser interferometer • Suspend mirrors on pendulums  “free” mass

13. 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

14. 3 0 4 km 3 ( ± 0 1 k 0 m m 2 km s ) The Laser Interferometer Gravitational-wave Observatory WA LA 4 km

15. 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

16. Limiting Noise Sources:Seismic Noise • Motion of the earth few mm rms at low frequencies • Passive (and active) seismic isolation • amplify at mechanical resonances • but get isolation above 10 Hz (0.10 Hz)

17. FRICTION Limiting Noise Sources:Thermal 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 impedance (loss)

18. Limiting Noise Sources:Quantum Noise • Shot Noise • Uncertainty in number of photons detected a • Higher input power Pbsa need low optical losses • (Tunable) interferometer response  Tifo depends on light storage time of GW signal in the interferometer • Radiation Pressure Noise • Photons impart momentum to cavity mirrorsFluctuations in the number of photons a • Lower input power, Pbs  Optimal input power for a chosen (fixed) Tifo

19. 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 Power-recycled Interferometer end test mass Laser + optical field conditioning signal 6Wsingle mode 4 km All cavities on resonance  interferometer is “locked”

21. Operations 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 • “Upper limit” runs (at unprecedented early sensitivity) are interleaved with commissioning • S1 Aug -- Sep 2002 duration: 2 weeks • S2 Feb -- Apr 2003 duration: 8 weeks • S3 Oct -- Dec 2004 duration: 9 weeks • Coincidence with GEO600, TAMA and Allegro Bar • First search run (S4) planned for mid-2004 (duration: 6 months) • Finish detector integration & design updates... • Engineering "shakedown" runs interspersed as needed • Advanced LIGO

22. Strain Sensitivity during S1 and S2 L1 S1 S2

23. Summary of upper limits for S1 • Stochastic backgrounds • Upper limit gwh1002 < 23(H1- L1 pair) • Neutron star binary inspiral • Range of detectability  200 kpc(1.4 – 1.4 M NS binary with SNR = 8) • Coalescence Rate for Milky Way equivalent galaxy < 170/yr 90% CL • Previous limits  30000/yr/MWEG (TAMA) and 4000/yr/MWEG (40m) • Expected rate  10-5/yr • Periodic sources PSR J1939+2134 at 1283 Hz • GW radiation h0 ~ 10-22 90% CL (expect h0 ~ 10-27 if pulsar spindown entirely due GW emission) • Burst sources • 1.6 events/day with 90% CL at hrss 10-18 1/rtHz Standard Inflation Prediction < 10-15 Limit from Big Bang Nucleosynthesis < 10-5

24. LIGO Science Has Started • LIGO has started taking data • First science run (S1) summer 2002 • Collaboration has carried out first analysislooking for • Bursts • Compact binary coalescences • Stochastic background • Periodic sources • Second science run (S2) last winter • Sensitivity is ~10x better than S1 • Duration is ~ 4x longer • Bursts4x lower rate limit & 10x lower strain limit • Inspirals  reach > 1 Mpc -- includes M31 (Andromeda) • Stochastic background  limits on Wgw < 10-2 • Periodic sources  limits on hmax ~ few x 10-23(e ~ few x 10-6 @ 3.6 kpc)

25. LLO S2 Sensitivity

26. The Task Ahead • Science runs • Upper limits • Scientific searches • Commissioning remaining subsystems • Factor of few (above 200 Hz) to 100 (at 40 Hz) improvement needed to reach design sensitivity • High power operation • Coupling to environment, e.g. • Acoustic mitigation • Trains twice a day at Livingston • Anthropogenic noise  seismic pre-isolation system • Complete alignment control • Electronics improvements • Advanced LIGO proposed and R&D well underway

27. The next-generation detectorAdvanced LIGO (aka LIGO II) • Now being designed by the LIGO Scientific Collaboration • Goal: • Quantum-noise-limited interferometer • Factor of 15 increase in sensitivity • Factor of 3000 in event rate One day > entire2-year initial data run • Schedule: • Begin installation: 2007 • Begin data run: 2009

28. Quantum LIGO I Ad LIGO Test mass thermal Suspension thermal Seismic A Quantum Limited Interferometer

29. Thorne… Advance LIGO Sensitivity:Improved and Tunable

30. 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 • Optical noise • Input laser power  increase to ~200 W • Optimize interferometer response signal recycling

32. 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

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

34. 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

35. LISA – space detector

36. Wgw = 10-10 LISA sensitivity curve(1-year observation) 102+104Mo

37. LISA and LIGO

38. Spacecraft • Two optical assemblies • Proof mass and sensors • 30 cm telescope • Interferometry: 20 pm/√Hz • 1 W, 1.06 µ Nd:YAG lasers • Drag-free control • Positioning to 10 nm/√Hz • Attitude to 3 nrad/√Hz

39. Payload

40. Science from gravitational wave detectors? • Test 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 • Different view of the Universe • 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 at and below the quantum limit set by Heisenberg on photons

41. New Instruments, New Field,the Unexpected…