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

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

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

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)

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

Seismic Noise (<50Hz) General characteristics: Seismic amplitudes are  m/  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)

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

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

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

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

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

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)

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).