Gravitational-wave Detection with Interferometers

1. Gravitational-wave Detection with Interferometers LIGO, LISA, and the like Nergis MavalvalaIAP, 2003

2. Global network of gravitational wave interferometers GEO VIRGO LIGO TAMA AIGO LIGO 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 aspherical astrophysical objects • Radiated by “dark” mass distributions  black holes, dark matter

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

7. 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?

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

9. 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

10. DL = h L (h ~ 10-21) LIGOmeasurement Earthdiameter Size of nucleus Short person Width of hair Earth-Sundistance Size of atom USA E-W Pea m 1011 107 104 100 10-2 10-5 10-10 10-15 10-18 GWs meet Interferometers • Laser interferometer

11. Power-recycled Interferometer Optical resonance: requires test masses to be held in position to 10-10-10-13meter end test mass Light bounces back and forth along arms ~100 times 30 kW Light is “recycled” ~50 times  300 W input test mass Laser + optical field conditioning signal 6W single mode

12. 3 0 4 km 3 ( ± 0 1 k 0 m m 2 km s ) The Laser Interferometer Gravitational-wave Observatory WA 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. Limiting Noise Sources:Seismic Noise • Motion of the earth few mm rms at low frequencies • Passive seismic isolation ‘stacks’ • amplify at mechanical resonances • but get f-2 isolation per stage above 10 Hz

15. 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: Z(f) is impedance (loss) • Low mechanical loss (high Quality factor) • Suspension  no bends or ‘kinks’ in pendulum wire • Test mass  no material defects in fused silica

16. 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

18. Displacement Sensitivity(Science Run 1, Sept. 2002)

19. The next-generation detectorAdvanced LIGO (aka LIGO II) • Now being designed by the LIGO Scientific Collaboration • Goal: • Quantum-noise-limited interferometer • Factor of ten increase in sensitivity • Factor of 1000 in event rate. One day > entire2-year initial data run • Schedule: • Begin installation: 2007 • Begin data run: 2009

20. Quantum LIGO I LIGO II Test mass thermal Suspension thermal Seismic A Quantum Limited Interferometer • Facility limits • Gravity gradients • Residual gas • (scattered light) • Advanced LIGO • Seismic noise 4010 Hz • Thermal noise 1/15 • Optical noise 1/10 • Beyond Adv LIGO • Thermal noise: cooling of test masses • Quantum noise: quantum non-demolition

21. f i (l) r(l).e Power Recycling l Signal Recycling Optimizing optical response: Signal Tuning Cavity forms compound output coupler with complex reflectivity. Peak response tuned by changing position of SRM Reflects GW photons back into interferometer to accrue more phase

22. Thorne… Advance LIGO Sensitivity:Improved and Tunable

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

24. LISA and LIGO

25. 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

26. New Instruments, New Field,the Unexpected…

27. Major research activities at the MIT LIGO Laboratory • Initial LIGO (now!) • Instrument science  lasers and optics, interferometry, optical metrology, optical resonant systems, photonics, control systems, low-noise electronics, vibration isolation systems, thermally induced dissipation, thermally adaptive optics • Data analysis and astrophysical searches  signals of known and unknown signatures buried in noise, astrophysical source signatures, computing challenges • Advance LIGO (beyond 2006) • Instrument development  design and prototyping of mechanical and optical subsystems (R&D and implementation) • Other things we work on • LISA (advisory role at present) • Quantum measurement  precision measurements at or below the quantum limit)