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Advanced interferometers for astronomical observations

This presentation by Lee Samuel Finn from the Center for Gravitational Wave Physics at Penn State outlines the design, sensitivity limits, and technological advancements in advanced interferometers for astronomical observations. It discusses the challenges faced by LIGO in detecting gravitational waves, including seismic, thermal, and shot noise, while also proposing innovative solutions such as enhanced seismic isolation and thermal noise mitigation strategies. The implications of increased sensitivity on astrophysical phenomena are explored, highlighting how advanced designs can improve our understanding of compact binary inspirals and stochastic signals.

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Advanced interferometers for astronomical observations

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

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

  4. Building a better interferometer:Advanced LIGO • Seismic isolation • Thermal noise mitigation; high power optics • High power lasers • Tuning ifo response 40kg 2nd 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 2nd 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 a: 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 2nd Gravitational Wave Phenomenology Workshop

  7. Signal recycling mirror 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) • 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 cavity input mirrors cavity end mirror laser Interferometer arm (4km long) photodetector 2nd 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? 2nd 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 Sh(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 2nd Gravitational Wave Phenomenology Workshop

  10. Compact Binary Inspiral • Signal spectrum known • Measure: [dr2/dlnf ]1/2 • Favor low-frequency sensitivity • But balance against bandwidth • CBI range • Silica: • 130 Mpc (M/3M8)5/6 • Sapphire: • 190 Mpc (M/3M8)5/6 • But… • Chirp fmax ~ M-1 • Silica IFO more sensitive than sapphire when M > 26M8 (2x30M8 binary) 2nd 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: WGWh-2 < 2.6x10-9 • Sapphire: WGWh-2 < 5.0x10-9 • Note! • CBI: prefer sapphire • Stoch: prefer silica 2nd 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 e for normal, ms pulsars 2nd 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 Wh-2<10-9 @ 100Hz • (3x4Km IFO) inspiral sensitivity • NS/NS to ~330 Mpc • 2x10 M8 BH/BH to z~0.3 • 2x30 M8 BH/BH to z~0.5 • Pulsars @ 100 pc in 1 yr obs: e95% < • 3-5x10-8 @ 100 Hz • 3-5x10-9 @ 200 Hz • 1-2x10-9 above 300 Hz 2nd Gravitational Wave Phenomenology Workshop

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