1. Advanced interferometers for astronomical observations Lee Samuel Finn Center for Gravitational Wave Physics, Penn State

2. 9 November 20032nd Gravitational Wave Phenomenology Workshop 2 Goals and Outline Design sensitivity limits and next-generation technologies to overcome them Astrophysics and analysis implications of increased sensitivity

3. 9 November 20032nd Gravitational Wave Phenomenology Workshop 3 What limits LIGO’s sensitivity? Initial LIGO detectors: –Different f, different limit –< ~50Hz : seismic noise –50 - 200Hz : thermal noise –> 200Hz : “shot” noise Facility limits –Gravity gradients –Stray light –Residual gas

4. 9 November 20032nd Gravitational Wave Phenomenology Workshop 4 Seismic isolation Thermal noise mitigation; high power optics High power lasers Tuning ifo response 40kg Building a better interferometer: Advanced LIGO

5. 9 November 20032nd Gravitational Wave Phenomenology Workshop 5 Thermal noise contributions Suspensions: –kT energy in taut suspension wire violin modes Test masses: –Normal modes: kT energy in mirror modes –Thermoelastic: Temperature fluctuations and thermal expansion coefficient Noise proportional to mechanical losses: reduce losses –Initial LIGO: mirrors rest on wires –Advanced: mirrors bonded to ribbons

6. 9 November 20032nd Gravitational Wave Phenomenology Workshop 6 Thermal noise mitigation: test masses Material properties problem –Normal modes: Increase Young modulus: less motion for same thermal energy –Thermoelastic: Decrease coefficient thermal expansion  : less motion for same thermal fluctuations Laser spot diameter, profile –Fluctuations averaged over effective spot area –Increase area, reduce effective fluctuation –Initial LIGO: 25cm –Advanced LIGO: 35cm

7. 9 November 20032nd Gravitational Wave Phenomenology Workshop 7 Tuning the detector response Undisturbed interferometer operates on dark fringe –Response to gravitational waves is light at output port Introduce partially reflecting mirror at output port cavity end mirror Interferometer arm (4km long) cavity end mirror Interferometer arm (4km long) photodetector cavity input mirrors laser Signal recycling mirror –Make resonant cavity with rest of interferometer –Resonance enhances power at output port for excitation at resonant frequency –Higher power: lower shot noise Mitigate shot noise in (relatively) narrow band

8. 9 November 20032nd Gravitational Wave Phenomenology Workshop 8 Fused Silica v. Sapphire: Two Alternatives Fused silica –Pros: Broader bandwidth Better low-frequency performance –Cons: Higher in-band noise Sapphire –Pros: Lower in-band noise Better high frequency performance –Cons: Narrower bandwidth What does this mean for astrophysics?

9. 9 November 20032nd Gravitational Wave Phenomenology Workshop 9 “Ignorance” Sensitivity Ignorance? –Specific sources? Specific amplitude in specific bands General considerations –Generic burst character low- Q damped SHO –Measure? [f S h (f)] 1/2 Fused silica –Pros: Broader bandwidth; better low-f performance –Cons: Higher in-band noise Sapphire –Pros: Lower in-band noise, better high-f performance –Cons: Narrower bandwidth

10. 9 November 20032nd Gravitational Wave Phenomenology Workshop 10 Compact Binary Inspiral Signal spectrum known –Measure: [d  2 /dlnf ] 1/2 Favor low-frequency sensitivity –But balance against bandwidth CBI range –Silica: 130 Mpc (M/3M  ) 5/6 –Sapphire: 190 Mpc (M/3M  ) 5/6 But…But… –Chirp f max ~ M -1 –Silica IFO more sensitive than sapphire when M > 26M  (2x30M  binary)

11. 9 November 20032nd Gravitational Wave Phenomenology Workshop 11 Stochastic signal Detection involves pair of detectors –Sensitive to wavelengths greater than separation between detectors (f < 100 Hz for LIGO) Stochastic signal: prefer Silica –Silica:  GW h -2 < 2.6x10 -9 –Sapphire:  GW h -2 < 5.0x10 -9 Note! –CBI: prefer sapphire –Stoch: prefer silica

12. 9 November 20032nd Gravitational Wave Phenomenology Workshop 12 Pulsar periods Normal, millisecond pulsars are different populations –“Normal”: P > 100 ms –“MS”: P < 100 ms More normal than ms pulsars –Expect 160K normal, 40K ms pulsars in galaxy Preference? Name your game: –Aim for closest NS? Favor low-f performance –Aim for detected pulsar? Favor high-f performance –Fold in prejudice regarding  for normal, ms pulsars

13. 9 November 20032nd Gravitational Wave Phenomenology Workshop 13 Conclusions, or What does this all mean? Advanced LIGO is not a blunt instrument! –Subtle difference in science goals begin to make difference Ground-based “ifos” on-track for –Stochastic background sensitivity  h -2 <10 -9 @ 100Hz –(3x4Km IFO) inspiral sensitivity NS/NS to ~330 Mpc 2x10 M  BH/BH to z~0.3 2x30 M  BH/BH to z~0.5 –Pulsars @ 100 pc in 1 yr obs:   < 3-5x10 -8 @ 100 Hz 3-5x10 -9 @ 200 Hz 1-2x10 -9 above 300 Hz