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

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 Reduction factor Calculation made for
Centered Spherical Cave
In rock salt beds
5 Hz
10 Hz
20 Hz
40 Hz
Width Length
Cave radius [m]

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