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Gravity Wave Detectors
Riccardo DeSalvo
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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
LIGO
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
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
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
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
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
GW detectors as seismic sensors
• They can detect soil strain
Detecting Earth’s Tidal Strain
Future generation of GW detectors
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Next generation
CLIO
LCGT
EGO
CEGO
Will be underground
How Small is
Meter?
-18
10
One meter ~ 40 inches
 10,000
100
Human hair ~ 100 microns
Wavelength of light ~ 1 micron
 10,000
Atomic diameter 10-10 m
 100,000
Nuclear diameter 10-15 m
 1,000
LIGO sensitivity 10-18 m
Can we measure
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-18
10
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
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)
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
Reducing Newtonian Noise
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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,
1.
2.
The movement of the rock surfaces or interfaces buffeted by the
seismic waves
The variations of rock density caused by the pressure waves
NN reduction underground
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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
seismic motion
leads to
NN reduction underground
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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
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(with the exception of the longitudinal dipole moment, which can be measured
and subtracted).
a tilting
leads to
fluctuating attraction force
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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
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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
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]
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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
calculated
correction
calculated
correction
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This subtraction is largely impeded on the surface by the fractal-like character of the
rubble composing surface soil
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]
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
LF Suspension and
Seismic Isolation
schematics
10-20 meter pendula
Between all stages
2-3 meter tall
Pre-isolator
In upper
LF Vertical filters
cave
marionetta
Composite
Mirror
Recoil mass
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)
Movie (click to start)
QuickTime™ and a
YUV420 codec decompressor
are needed to see this picture.
150 mHz
QuickTime™ and a
YUV420 codec decompressor
are needed to see this picture.
Spring tuning procedure, progressive radialcompression
frequency [mHz]
1200
1000
800
600
400
200
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compression [mm]
Attenuation performance of a GAS filter
Acoustic coupling >100 Hz
GAS spring demonstration
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Next two movies (click to start)
Watch the black flag on the wire
supporting the payload
1. Exciting the payload movement (gas
spring main resonance)
2. Applying an Earthquake
QuickTime™ and a
YUV420 codec decompressor
are needed to see this picture.
QuickTime™ and a
YUV420 codec decompressor
are needed to see this picture.
Tidal Strain
rejection
Reject common mode Tidal strain (<mHz)
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Clamp laser wavelength to common arm length
Reject differential mode Tidal strain (<mHz)
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Track strain with top of attenuation chain
All LF strain NOISE efficiently cancelled!
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
Items of common interest
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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
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
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