Experimental search for Gravitational Waves

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Experimental search for Gravitational Waves

• Geppo Cagnoli
• cagnoli_at_fi.infn.it
• INFN - Firenze
• University of Glasgow
• Physik-Institut der Universität Zürich/ETH 28th
  June 2006

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The GR prediction
Newtons Theory instantaneous action at a
distance
Gmn 8pTmn
Einsteins Theory information carried by
gravitational radiation at the speed of light
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Sources of Gravitational Waves

• Compact object binaries
• Pulsars
• Neutron Star internal dynamics
• Non symmetrical supernovae
• Cosmological gravitational waves

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New potential sources
January 05 A swarm of 10,000 or more black holes
may be orbiting the Milky Way's supermassive
black hole, according to new results from NASA's
Chandra X-ray Observatory. This would represent
the highest concentration of black holes anywhere
in the Galaxy.
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Detection Principles -1

• In the reference frame of the lab (Fermis
  coordinates) the effect of GW is pure mechanical.
  The potential is
• 3 types of detectors
• Resonators
• Interferometers
• RF cavities

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Detection Principles -2
Effect of a sinusoidal gravitational wave going
through the slideon the space-time frame and on
a circular distribution of free masses
Figure M.Lorenzini
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Detection Principles -3
Two detectors fully developed Resonant Masses
Interferometers
Figure S. Reid
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Theory of GW Detectors - 1
Detector
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First attempt of buildinga resonant detector
Joseph Weber(1960)
Resonant barsuspended in the middle
Piezoelectrictransducers
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The Band Width of a resonant detector
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Resonant detectors today
GW bursts excite the resonances of the test masses
Capacitive SQUIDor optical readout
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A capacitive Read-out systemof a resonant
detector
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(No Transcript)
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Interferometric detectorsthe concept

• Monitoring the distances between free-flying
  masses with laser interferometer

• The background noise comes from the readout
  and from the internal motion of the masses

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A bit of history

• Gertsenshtein M E and Pustovoit V I 1962 Sov.
  Phys.JETP 16 433
• Moss G E, Miller L R and Forward R L 1971 Appl.
  Opt. 10 2495b
• Weiss R 1972 Q. Prog. Rep. Res. Lab. Electron.
  105 54

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The Band Width of an interferometric detector
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Interferometers today - 1

• End mirrors positioned in theDark Fringe
  condition laser beam is frequency modulated,
  the sidebands are detected
• Multiple bouncingphase accumulationlaser power
  increasesfrom 20W to 1kW
• Power recycling number ofphotons in the
  interferometerincreases
• Signal recyclingjust the side bands are
  reflectedback in the interferometerGEO600 is
  the onlydetector that uses thistechnique to
  enhance the detector response in a narrow band

Pendulumsuspensions
Beamsplitter
Photodiode
Laser
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Interferometers today - 2
Pendulumsuspensions
Beamsplitter
Photodiode
Laser
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Interferometers today - 3
Pendulumsuspensions
Beamsplitter
Photodiode
The optics and suspensions are in vacuum to
minimize fluctuation of index of refraction
Laser
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Interferometers today - 4
600 m
TAMA
4 2 km
300 m
AIGO
4 km
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Real data from LIGO
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Real data from GEO600
10 -17
Displacement m
10 -18
10 -19
100
Hz
1000
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Real data from Virgo
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Detectors of 1st Generation
h
Pulsars
Hz 1/2
Supernovae
NSvibration
BUT THEEVENT RATEIS TOO LOW !! 1 EVENT/3
YRS MOST OPTIMISTICCASE
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Future Detectors of Gravitational Waves

• DUAL
• Nested hollow cylinder resonant detector
• AURIGA collaboration
• Construction planned starting on 2009
• Ad. LIGO, Ad. Virgo and GEO HF
• 2nd generation interferometers
• Virgo GEO600 collaboration
• Commissioning starts on 2009
• 3rd Generation Interferometer
• Cryogenic and underground interferometer
• Construction envisaged by 2014

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DUAL the concept
read-out the differential deformations of two
nested resonators
The inner resonator is driven below resonance
The outer resonator is driven above resonance
p Phase difference
5.0 kHz
useful GW band
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DUAL performance
M. Bonaldi et al. Phys. Rev. D 68 102004 (2003)
Mo Dual 16.4 ton height 3.0m 0.94m SiC
Dual 62.2 ton height 3.0m 2.9m
Antenna pattern like 2 IFOs colocated and
rotated by 45
Q/T2x108 K-1
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Real data from Virgo
CONTROL RELATED NOISE
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Readout noise shot noise

• A fundamental limit to phase measurement is due
  to the quantum nature of light
• Phase measurements to a level of 10 -13 rad
  require about 1 MW of laser power in the optical
  cavities
• But more power more fluctuating radiation
  pressure P1 MW ?? F3 mN ?? dF1.5

DN Dj 1/2
fN
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Readout noiseThe Standard Quantum Limit

• For a simple Michelson interferometer (GEO HF
  parameters)

RomanSchnabelMPG-AEI Hannover
10-21
Quantum noise with increased laser power (x100)
10-23
1
100
Frequency Hz
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Beyond the SQL Squeezed Light

• In one representation of the EM field the two
  orthogonal states are the Amplitude Quadrature
  X1 and the Phase Quadrature X2

RomanSchnabelMPG-AEI Hannover
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Beyond the SQL Squeezed Light

• In one representation of the EM field the two
  orthogonal states are the Amplitude Quadrature
  X1 and the Phase Quadrature X2

RomanSchnabelMPG-AEI Hannover
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Beyond the SQL Squeezed Light
10-21
RomanSchnabelMPG-AEI Hannover
Quantum limit onphase measurement
Radiation pressure noise
SQL
10-22
1
100
Frequency Hz
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Squeezed light demonstrations
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Intermediate frequencies
From the realm of Quantum to the realm of
Statistical Physics
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Thermal noise

• Non isolated system shows uncorrelated
  fluctuations of volume and temperature
• The equipartition principle states that each
  observable has a mean energy equal to kBT/2
• The observable
• Optical readout part of the mirror sensed by the
  laser
• Capacitive readout the average position of the
  capacitor plates

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Thermal noise reduction strategy

• Linear systems thermal equilibrium
• Each dynamic variable ltEgt kT
• Fluctuation-Dissipation theorem

Lower T ?? Lower thermal noise
Thermal noise for Damped HarmonicOscillator
Lower dissipation ?? Lower thermal noise
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The most severe limit for IFOsthermal noise
from the coatings

• Alternate layers of transparent materials with
  different index of refraction
• Impedance mismatch andinterference produce
  highcoefficient of reflectivity
• Its structure is not compact as the
  substrateDeposition with DIBS
• 10 mm of coating produces morethermal noise than
  10 cm of substrate

QUANTUM
COATINGS
EGO
SUBSTRATES
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Suspensions at room temperature

• Best materialsilica (SiO2)
• Silicate bonding
• Tested on GEO600

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Silicon for mirrors and suspensions at low T

• Thermal expansion null at 124K and 18K ? main
  source of thermal noise is ruled out
• High thermal conductivity
• Monocrystal ingots up to 45cm diameter
• Possibility of monolithic suspensions
• Diffractive as well as transmissive
  interferometry allowed

k
5000
2.5e-6
a
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Earth related noise - 1

• Test masses have to behave like free flying
  objects, yet they have to be suspended against
  gravity
• Seismic motion always present has to be filtered

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Earth related noise - 2Isolation short-circuit
The Newtonian noisewill be dominant below 10 Hz
for cryogenic detectors Surface waves
die exponentially with depth GO UNDERGROUND!
Figure M.Lorenzini
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Further considerations

• Building the most perfect inertial reference
  system
• A system subjected to the quantum problem of
  measurement
• All the fundamental parameters of the detector
  have to be CONTROLLED without introducing a
  significant noise

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Detector Generations
Distance Rate
NS-NS 14 Mpc 1/30ce 1/3yr
NS-BH 29 Mpc 1/25ce 1/2yr
BH-BH 67 Mpc 1/6ce 3/yr



h
Hz 1/2
NS-NS 240 Mpc 3/yr 4/day
NS-BH 500 Mpc 1/yr 6/day
BH-BH Z0.3 1/month 30/day
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NS-NS coalescence range
BH-BH coalescence range
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Beyond Earth based detectorsLISA
LISA
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A collaborative ESA NASA mission

• Cluster of 3 S/C in heliocentric orbit
• Trailing the earth by 20 (50 Mio km)
• Equilateral triangle with 5 Mio km arms
• Inclined against ecliptic by 60

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The spacecraft

• LISA needs a purely gravitational orbit
• Test masses have to be shielded from solar
  wind
• Capacitive sensing of the test masses

• Feedback loop to propulsion
• FEEP thrusters with micro-Newton thrust

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The Payload
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LISA technology demonstration
10-12
10-13
10-14
10-15
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LISA Path Finder Mission
Only one S/C with two test masses is needed
Testing Inertial sensor Charge
management Thrusters Drag-free control Low
frequency laser metrology  
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LISA sensitivity curve
LISA will see all the compact white-dwarf and
neutron-star binaries in the Galaxy. (Schutz)
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Conclusions
A new way to observe the Universe