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 earths 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 Earths Tidal Strain Future generation of GW detectors
Next generation CLIO LCGT EGO CEGO Will be underground How Small is 10-18 Meter? 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 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. 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
How to shape the environments 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 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
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] 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
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 1200 1000 800 frequency [mHz] 600
400 200 0 0 2 4
6 8 10 12 14
compression [mm] 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 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 (
Clamp laser wavelength to common arm length Reject differential mode Tidal strain (
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
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