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Gravitational Wave Astronomy Dr. Giles Hammond Institute for Gravitational Research SUPA, University of Glasgow Universität Jena, August 2010

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Sensitivity Limits LASER test mass (mirror) photodiode beam splitter Seismic Noise Thermal Noise Wavelength & amplitude fluctuations Residual gas scattering Quantum Noise "Shot" noise Radiation pressure Beamtube bakeout (2000A => 160 o C) p water <10 -9 torr

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The design sensitivity predicted was reached in 2005 Interferometer Sensitivity seismic noise mirror thermal noise shot noise

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Noise Sources (<1Hz) Newtonian Gravity gradient noise (gravitational interaction between moving “masses” and free test masses). Cannot be shielded and effects low frequency (<10Hz) Sources: Density perturbations due to vehicles, clouds or seismic surface waves (S-waves)

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Charging Noise (<50Hz) Motion of surface charge on the silica test masses Some events at LIGO and GEO have already seen charging noise It is thought that charging could be a potential problem for 2 nd generation detectors Feb 2006 LLO charging event GEO electrostatic drive 1/f 3 GEO calibration error GEO ESD

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Seismic Noise (<50Hz) General characteristics: Seismic amplitudes are 10 -9 m/ Hz @10Hz Significant day-night and weather variations (wind/sea activity) 10Hz typically human (AC, fans, pumps, …..) Green: day (10am) Blue: Night Pink: Stormy weather Purple: Calm day Microseismic peak (rms contribution)

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Passive Seismic Isolation b Seismic isolation 1/ 2 for small low high resonance

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Passive Seismic Isolation LIGO I passive isolation Good isolation > 50Hz This sets lower frequency limit

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Thermal Noise ( 100Hz) What happens if we perform the following measurement? b Spectrum Analyser R

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Thermal Noise ( 100Hz) What happens if we perform the following measurement? Time SeriesFrequency Spectrum Spectrum Analyser R

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Thermal Noise ( 100Hz) The resistor has a mean-square voltage noise ( 130nV/ Hz for 1M ) called Johnson or Brownian noise. This is a white noise. The fluctuating voltage is due to the dissipation in the resistor and can be described by the fluctuation dissipation theorem: The response of a system in thermodynamic equilibrium to a small applied force is the same as its response to a spontaneous fluctuation. A mechanical system with dissipation has a mean-square fluctuating force noise b

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Thermal Noise ( 100Hz) Z is the mechanical impedance (the real part is equivalent to resistance in our electrical analogy) In terms of displacement the thermal noise is given by: It is convenient to introduce the loss ( ) into the spring constant and the displacement noise is (for f=1Hz)

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Thermal Noise ( 100Hz) The impedance then becomes or and the displacement noise is The shape of the thermal noise spectrum depends on the type of damping (external velocity) or friction (internal).

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