Unveiling Gravity Waves: Detectors, Sensitivity, and Noise Rejection

1. Gravity Wave DetectorsRiccardo DeSalvo - LIGO • Gravity waves • GW detectors • Strain measurement, sensitivity • Newtonian Noise • How do we throw away your signal • Rejection of Newtonian Noise • Rejection of Seismic Noise • Tidal rejection • Items of cross pollination

2. Gravity waves • Recipe to generate GW • You throw a couple of solar masses in Tahoe lake • You throw another couple of solar masses in Truckee city • You let them orbit until the fall on each other • The star-quake that ensues strains space-time and GW radiate out

3. Gravity waves • GW are quadrupolar, to conserve energy and impulse • They deform ellipsoidally a set of masses arranged on a circle • The amount of cyclic deformation on Earth for an inspiral in our galaxy is expected to be h~10-21

5. How to detect GW • You suspend 4 heavy mirrors, each two separated by ~4 Km and arrange them in a L configuration • You add a beam splitter at the corner and build a Michelson interferometer (with Fabry Perot light accumulators in the arms) • The differential signal is the GW signal • You make several for coincidence and triangulation

6. Terrestrial Interferometers free masses International network (LIGO, Virgo, GEO, TAMA) of suspended mass Michelson-type interferometers on earth’s surface detect distant astrophysical sources suspended test masses

7. The GW detectors • They are couple of large yard sticks laying on the ground • Virgo, 3+3 Km, in the fields of Pisa It EU • LIGO NW in the Hanford WA desert • LIGO SE in the Livingston LA forest

9. GW detectors as seismic sensors • They can detect soil strain

10. Detecting Earth’s Tidal Strain

11. Future generation of GW detectors Next generation • CLIO • LCGT • EGO • CEGO Will be underground

12. One meter ~ 40 inches Human hair ~ 100 microns Wavelength of light ~ 1 micron Atomic diameter 10-10 m Nuclear diameter 10-15 m LIGO sensitivity 10-18 m How Small is 10-18 Meter?

13. Can we measure 10-18 m • We built the instruments • It takes years to tune them up • The LIGO commissioning is quite advanced • Within a factor of 2 from design at 100 Hz • Virgo 2 years behind, but coming soon

15. What happens if you tune the detectors at lower frequency • Comparing Advanced LIGO with an interferometer tuned at the lower frequency band • When an interferometer is tuned below 30 Hz it starts being sensitive to Newtonian Noise (bifurcation on the left for b = 1 and b = 0.1)

17. The Newtonian Noise problem • The Newtonian Noise (also known as Gravity Gradient) is • a limit energized by seismic waves that generates a wall with ~ f4 slope • Changes of amplitude according to the local and dayly seismic activity

19. Reducing Newtonian Noise • NN derives from the varying rock density induced by seismic waves around the test mass • It generates fluctuating gravitational forces indistinguishable from Gravity Waves • It is composed of two parts, • The movement of the rock surfaces or interfaces buffeted by the seismic waves • The variations of rock density caused by the pressure waves

20. NN reduction underground • How to shape the environment’s surface to minimize NN? • The dominant term of NN is the rock-to-air interface movement • On the surface this edge is the flat surface of ground Ground surface

21. NN reduction underground • If the cavern housing the suspended test mass is shaped symmetrically along the beam line and around the test mass tilting and surface deformations, the dominant terms of NN, cancel out • (with the exception of the longitudinal dipole moment, which can be measured and subtracted).

22. NN reduction underground • Pressure seismic waves induce fluctuating rock density around the test mass • The result is also fluctuating gravitational forces on the test mass

23. NN reduction underground • Larger caves induce smaller test mass perturbations • The noise reduction is proportional to 1/r3 • The longitudinal direction is more important =>elliptic cave

24. NN reduction from size Cave radius [m] Reduction factor Calculation made for Centered Spherical Cave In rock salt beds 5 Hz 10 Hz 20 Hz 40 Hz Width Length

25. Additionally deep rocks, if uniform, elastic, transmitting and non dissipative, can be measured with a small number of seismometers (or better density meters) to predict its seismic induced density fluctuations and subtract them from the test mass movements • This subtraction is largely impeded on the surface by the fractal-like character of the rubble composing surface soil

26. Contributions to NN Fraction of NN due to Surface Effects (balance from density waves) Horizontal accelerometer on cave surface will gain a factor of 2 Three-directional 3D matrix of accelerometers or density meters needed for further subtraction Cave radius [m]

27. How do we get rid of your signal • We build seismic attenuation chains • The initial part is variable • All seismic attenuation systems end with alternating pendula and vertical springs

28. LF Suspension and Seismic Isolation schematics 10-20 meter pendula Between all stages 2-3 meter tall Pre-isolator In upper cave LF Vertical filters marionetta Composite Mirror Recoil mass

29. Examples of LF vertical springs • A payload (1/3 t) is suspended from a pair (or a crown for larger payloads) of cantilever leaf springs. • The vertical resonant frequency is reduced by radially compressing the leaf springs in antagonistic mode (Geometric Anti Springs)

31. Movie (click to start)

32. 150 mHz

33. Spring tuning procedure, progressive radialcompression

34. Attenuation performance of a GAS filter Acoustic coupling >100 Hz

35. GAS spring demonstration • Next two movies (click to start) • Watch the black flag on the wire supporting the payload • Exciting the payload movement (gas spring main resonance) • Applying an Earthquake

38. Tidal Strain rejection • Reject common mode Tidal strain (<mHz) • Clamp laser wavelength to common arm length • Reject differential mode Tidal strain (<mHz) • Track strain with top of attenuation chain • All LF strain NOISE efficiently cancelled!

39. How to recover strain signal • Below 50 mHz • Compare wavelength with the reference cavity or with a suitably stabilized laser • Keep track of the signal from the osition sensor at the top of the chain • Above 50 mHz • Install auxiliary interferometers between test masses and monuments • Intrusive if precision below 10-8 m needed

40. Items of common interest • Extracting signal from noise • Template strategies, model based • Extensive effort ongoing • Signal and correlation extraction techniques • Push Development of control techniques and digitalization techniques • Oversampling techniques • Cross timing techniques

41. Items of common interest • Can use seismic attenuation techniques developed for GW detection to characterize instruments, • Dedicated pilot station in Firenze • Geophysics interferometer in Napoli

42. GW seismic attenuators for Geophysics • TAMA-SAS, developed for the TAMA seismic attenuation upgrade, will equip its main mirrors implementing hierarchical controls and LF seismic attenuation like Virgo and Adv-LIGO • One of these towers being modified for University of Firenze as a seismometer testing facility • Three towers are being built for the Seismic Institute of the University of Napoli for a ground sensing interferometer