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

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

International network (LIGO, Virgo, GEO, TAMA) of suspended mass Michelson-type interferometers on earth’s surface detect distant astrophysical sources

TerrestrialInterferometers

suspended test masses

free 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

Next generation

• CLIO

• LCGT

• EGO

• CEGO

Will be underground

How Small is 10-18 Meter?

Wavelength of light ~ 1 micron100÷

One meter ~ 40 inches

Human hair ~ 100 microns000,10÷

LIGO sensitivity 10-18 m000,1÷

Nuclear diameter 10-15 m000,100÷

Atomic diameter 10-10 m000,10÷

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

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 = 1 and = 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• 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. The movement of the rock surfaces or interfaces buffeted by the

seismic waves

2. The variations of rock density caused by the pressure waves

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

seismic motion leads to

Ground surface

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).

atilting leads to fluctuating attraction force

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

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

Reductionfactor

Cave radius [m]

5 Hz10 Hz20 Hz40 Hz

Calculation made forCentered Spherical CaveIn rock salt beds

Width Length

• 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

calculatedcorrection

calculatedcorrection

Contributions to NN

Cave radius [m]

Horizontal accelerometer on cave surface will gain a factor of 2Three-directional 3D matrix of accelerometers or density meters needed for further subtraction

Fraction of NN due toSurface Effects(balance from density waves)

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 pendulaBetween all stages

2-3 meter tallPre-isolatorIn uppercave

LF Vertical filters 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)

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frequency [mHz]

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Spring tuning procedure, progressive radialcompression

Attenuation performance of a GAS filter

Acoustic coupling >100 Hz

GAS spring demonstration

• 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

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Tidal Strain rejection

Reject common mode Tidal strain (<mHz)• Clamp laser wavelength to common arm lengthReject differential mode Tidal strain (<mHz)• 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

• 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