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Experimental search for Gravitational Waves
Geppo [email protected]
INFN - Firenze
University of Glasgow
Physik-Institut der Universität Zürich/ETH 28th June 2006
28th June 2006 Physik-Institut der Universität Zürich / ETH 2 of 53
The GR prediction
Newton’s Theory“instantaneous action
at a distance”
Einstein’s Theoryinformation carried
by gravitational radiation at the speed of light
G= 8
28th June 2006 Physik-Institut der Universität Zürich / ETH 3 of 53
Sources of Gravitational Waves
• Compact object binaries
• Pulsars
• Neutron Star internal dynamics
• Non symmetrical supernovae
• Cosmological gravitational waves
Small amplitudeapproximation
Mass quadruplemoment
28th June 2006 Physik-Institut der Universität Zürich / ETH 4 of 53
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.
28th June 2006 Physik-Institut der Universität Zürich / ETH 5 of 53
Detection Principles -1
• In the reference frame of the lab (Fermi’s coordinates) the effect of GW is pure mechanical. The potential is:
• 3 types of detectors
– Resonators
– Interferometers
– RF cavities
)yx(mh41
)y,x( 22
Inertia Dimensions
28th June 2006 Physik-Institut der Universität Zürich / ETH 6 of 53
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
LL
L/L < 10 -21
Expected from astronomical sources
28th June 2006 Physik-Institut der Universität Zürich / ETH 7 of 53
Detection Principles -3
Two detectors fully developed:Resonant Masses Interferometers
Figure: S. Reid
28th June 2006 Physik-Institut der Universität Zürich / ETH 8 of 53
Theory of GW Detectors - 1
Detectorh x
Read-out
V
f
Detectorinternal
noise
Readoutinternal
noise
28th June 2006 Physik-Institut der Universität Zürich / ETH 9 of 53
First attempt of buildinga resonant detector
Joseph Weber(~1960)
Resonant barsuspended in the middle
Piezoelectrictransducers
Sensitivitypattern
28th June 2006 Physik-Institut der Universität Zürich / ETH 10 of 53
The Band Width of a resonant detector
x/h
Detector noise
Read-out noise
DetectorBW
28th June 2006 Physik-Institut der Universität Zürich / ETH 11 of 53
Bars
Resonant detectors today
Spheres
GW burstsexcite the
resonances of the test masses
mechanicalsignal
enhancement
Capacitive + SQUIDor optical readout
28th June 2006 Physik-Institut der Universität Zürich / ETH 12 of 53
Bar
SQUIDAmplifier
MatchingTransformer
CapacitiveResonantTransducer
DecouplingCapacitor
Transducer Charging Line
L Ls Li
CryogenicSwitch
Mi
CT
CdM
A capacitive Read-out systemof a resonant detector
28th June 2006 Physik-Institut der Universität Zürich / ETH 14 of 53
Interferometric detectors:the 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
28th June 2006 Physik-Institut der Universität Zürich / ETH 15 of 53
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
28th June 2006 Physik-Institut der Universität Zürich / ETH 16 of 53
The Band Width of an interferometric detector
Detector noise
Read-out noise
DetectorBW
x = h · L / 2 for each end mirror
)yx(mh21
)y,x(f
28th June 2006 Physik-Institut der Universität Zürich / ETH 17 of 53
Interferometers today - 1
LaserPhotodiode
Pendulumsuspensions
Beamsplitter
Fabry – Perotcavities
• End mirrors positioned in theDark Fringe condition: laser beam is frequency modulated, the sidebands are detected
• Multiple bouncingphase accumulation:laser power increasesfrom 20W to 1kW
• Power recycling: number ofphotons in the interferometerincreases
• Signal recycling:just the side bands are reflectedback in the interferometerGEO600 is the onlydetector that uses thistechnique to enhance the detector response in a narrow band
28th June 2006 Physik-Institut der Universität Zürich / ETH 18 of 53
Interferometers today - 2
LaserPhotodiode
Pendulumsuspensions
Beamsplitter
Fabry – Perotcavities
28th June 2006 Physik-Institut der Universität Zürich / ETH 19 of 53
Interferometers today - 3
LaserPhotodiode
Pendulumsuspensions
Beamsplitter
Fabry – Perotcavities
The optics and suspensions arein vacuum to minimize fluctuation of index of refraction
28th June 2006 Physik-Institut der Universität Zürich / ETH 20 of 53
Interferometers today - 4
TAMA600 m
300 m4 & 2 km
4 km
AIGO
3 km
28th June 2006 Physik-Institut der Universität Zürich / ETH 22 of 53
Real data from GEO600
100 1000 [Hz]
10 -19
10 -18
10 -17
Dis
pla
cem
en
t [m
]
28th June 2006 Physik-Institut der Universität Zürich / ETH 24 of 53
Detectors of 1st Generation
1 10 100 1k 10kFrequency [Hz]
10 -22
10 -23
10 -24
10 -25
10 -19
10 -20
10 -21
h
[ H
z –1
/2 ]
GEO600
LIGOVIRGO
1ST GENERATION
FOR INTERFEROMETERS•STEEL SUSPENSIONS (APART GEO600)•ROOM TEMPERATURE
FOR RESONATORS• Al or AlCu•100mK < T < 4K
AURIGANAUTILUS MiniGRAIL
Pulsars
NS-NS 14 MpcBH-BH 67 Mpc
Supernovae
NSvibration
1ST GENERATION IS CLOSE TO REACH THEDETECTION RANGE FOR NS-NS COALESCENCE
AT THE DISTANCE OF THEVIRGO CLUSTER (17MPc)
BUT THEEVENT RATEIS TOO LOW !!
1 EVENT/3 YRSMOST OPTIMISTICCASE
28th June 2006 Physik-Institut der Universität Zürich / ETH 25 of 53
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
28th June 2006 Physik-Institut der Universität Zürich / ETH 26 of 53
DUAL – the concept
read-out the differential deformations of two nested resonators
useful GW band
5.0 kHz
π Phase difference
The inner resonator is driven below resonance
The outer resonator is driven above resonance
28th June 2006 Physik-Institut der Universität Zürich / ETH 27 of 53
Mo Dual 16.4 ton height 3.0m 0.94m
SiC Dual 62.2 ton height 3.0m 2.9mQ/T=2x108 K-1
M. Bonaldi et al. Phys. Rev. D 68 102004 (2003)
Antenna pattern: like 2 IFOs colocated and rotated by
45°
DUAL performance
28th June 2006 Physik-Institut der Universität Zürich / ETH 28 of 53
Real data from Virgo
READOUTTHERMALNOISE
EARTHRELATEDNOISE
CONTROL RELATED NOISE
28th June 2006 Physik-Institut der Universität Zürich / ETH 29 of 53
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 P=1 MW F=3 mN F=1.5
N · ≥ 1/2
fN√Hz
28th June 2006 Physik-Institut der Universität Zürich / ETH 30 of 53
Readout noiseThe Standard Quantum Limit
For a simple Michelson interferometer (GEO HF parameters)
1 100Frequency [ Hz ]
10-21
10-23
Str
ain
[
1/√
Hz
]
Quantum limit onphase measurement
Rad
iation p
ressure n
oise
Quantum noisewith increased laser
power (x100)
SQL
RomanSchnabelMPG-AEIHannover
28th June 2006 Physik-Institut der Universität Zürich / ETH 31 of 53
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-AEIHannover
28th June 2006 Physik-Institut der Universität Zürich / ETH 32 of 53
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-AEIHannover
28th June 2006 Physik-Institut der Universität Zürich / ETH 33 of 53
Beyond the SQL: Squeezed Light
RomanSchnabelMPG-AEIHannover
1 100Frequency [ Hz ]
10-21
10-22
Str
ain
[
1/√
Hz
]
Quantum limit onphase measurement
Radia
tion p
ressu
re n
oise SQL
Noise reduction by squeezed light - 6 dB in variance
28th June 2006 Physik-Institut der Universität Zürich / ETH 34 of 53
Squeezed light demonstrations
[Chelkowski et al., Phys. Rev. A 71, 013806 (2005)].
[Vahlbruch et al., Phys. Rev. Lett., submitted (2005)].
28th June 2006 Physik-Institut der Universität Zürich / ETH 35 of 53
Intermediate frequencies
1 10 kFrequency [ Hz ]
10-19
10-25
Str
ain
[
1/√
Hz
]
THERMALNOISE
From the realm of Quantum to the realm of Statistical Physics
28th June 2006 Physik-Institut der Universität Zürich / ETH 36 of 53
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
TB
2
V
2B2
PV
TkVCTk
T
28th June 2006 Physik-Institut der Universität Zürich / ETH 37 of 53
Thermal noise reduction strategy
• Linear systems & thermal equilibrium
• Each dynamic variable <E>= kT
• Fluctuation-Dissipation theorem
)(Tk4)(S Bff R.K.PatriaStatistical MechanicsPergamon Press
Log f
Noise
Log
[S
xx (
) ]
Lower dissipation Lower thermal noise
Thermal noise forDamped HarmonicOscillator
Lower T Lower thermal noise
28th June 2006 Physik-Institut der Universität Zürich / ETH 38 of 53
The most severe limit for IFOs:thermal 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 m of coating produces morethermal noise than 10 cm of substrate 1 10 kFrequency [ Hz ]
10-19
10-25
Str
ain
[
1/√
Hz
]
QUANTUM
COATINGS
SUBSTRATESEGO
28th June 2006 Physik-Institut der Universität Zürich / ETH 39 of 53
Suspensions at room temperature
• Best material:silica (SiO2)
• Silicate bonding
• Tested on GEO600
28th June 2006 Physik-Institut der Universität Zürich / ETH 40 of 53
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
2.5e-65000
28th June 2006 Physik-Institut der Universität Zürich / ETH 41 of 53
• 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
Earth related noise - 1
28th June 2006 Physik-Institut der Universität Zürich / ETH 42 of 53
Earth related noise - 2:Isolation short-circuit
Newtonian noise
00( ) . ( )
( )
Gh f const x f
H f
Figure: M.Lorenzini
SEISMIC NOISE
The Newtonian noisewill be dominant below 10 Hz for
cryogenic detectors
Surface waves dieexponentially with
depth:
GO UNDERGROUND!
28th June 2006 Physik-Institut der Universität Zürich / ETH 43 of 53
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
28th June 2006 Physik-Institut der Universität Zürich / ETH 44 of 53
Detector Generations
1 10 100 1k 10kFrequency [Hz]
10 -22
10 -23
10 -24
10 -25
10 -19
10 -20
10 -21
h
[ H
z –1
/2 ]
GEO600
LIGOVIRGO
AURIGANAUTILUS MiniGRAIL Ad
VIRGO
Mo DUAL
SiC DUAL
3rd GENERATIONINTERFEROMETER
Distance
Rate
NS-NS 14 Mpc1/30ce
1/3yr
NS-BH 29 Mpc1/25ce
1/2yr
BH-BH 67 Mpc1/6ce
3/yr
NS-NS240 Mpc
3/yr
4/day
NS-BH500 Mpc
1/yr
6/day
BH-BH Z~0.31/month
30/day
28th June 2006 Physik-Institut der Universität Zürich / ETH 45 of 53
NS-NS coalescence range
GRB050509B
3RD GENERATIONINTERFEROMETER
2ND
GENERATION
1ST GENERATION
BH-BH coalescence range
28th June 2006 Physik-Institut der Universität Zürich / ETH 46 of 53
Audio band1 Hz – 10 kHz
Beyond Earth based detectors:LISA
28th June 2006 Physik-Institut der Universität Zürich / ETH 47 of 53
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°
28th June 2006 Physik-Institut der Universität Zürich / ETH 48 of 53
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
28th June 2006 Physik-Institut der Universität Zürich / ETH 50 of 53
LISA technology demonstration
10-15
10-14
10-13
10-12
F
NS
Hz
Torsion pendulum
Flight test
LISA
28th June 2006 Physik-Institut der Universität Zürich / ETH 51 of 53
LISA Path Finder Mission
Testing:
Inertial sensorCharge managementThrustersDrag-free controlLow frequency laser metrology 2
14 msa 3 10 f 5 mHz
Hz
Only one S/C with two test masses is needed
28th June 2006 Physik-Institut der Universität Zürich / ETH 52 of 53
LISA sensitivity curve
10-4 10-3 10-2 10-1 100
Frequency (Hz)
10-23
10-22
10-21
10-20
10-19
10-18
Detection th
reshold
106Mo z=1104Mo z=1
RXJ1914.4+2456
4U1820-30
wav
e am
plit
ud
e h
10
5
0
-5
-10
-15
app
aren
t m
agn
itu
de
(GW
flu
x)
10 M o + 10
6 M o BH z=1
LISA will see all the compact white-dwarfand neutron-star binaries in the Galaxy. (Schutz)