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Cavalier FabienLAL Orsay
NIKHEFJuly, 3rd 2006
The Quest for Gravitational Waves
Virgo and the quest for Gravitational WavesI. The interferometric detection of gravitational waves
1. Gravitational Waves: nature and effects2. Sources3. Principle of interferometric detection and improvements
II. The Virgo Challenge1. The infrastructures2. The seismic noise and the super-attenuator3. The thermal noise 4. The control system5. The Virgo sensitivity
III. Experimental Results1. Control of optical cavities2. Sensitivities3. Data analysis
IV. Interferometric search of GW in the world
V. The future of Gravitational Waves
Gravitational Waves
• A little bit of General Relativity
• In Special Relativity, the space-time interval is given by: ds2 = dx2 + dy2 + dz2 – c2 dt2 = dx dx where is the Minkowski metric tensor
• In General Relativity, we have: ds2 = g dx dxwithg metric tensor which follows Einstein’s equation
• Weak Field Approximation
g = + h with || h || << 1 and
h can follow a propagation equation
�2 h = - 16 G T where T is related to the sourcec4
Gravitational Waves
• Properties
• Helicity 2• Celerity c• Dimensionless amplitude h• Quadrupolar emission Can be generated only by motions without axial symmetry
• Effect of free particles
• h ~ L/L• Differential effect
LL
L+dL
L-dL
An Hertz Experiment ?
source distance h P (W)
Steel Bar, 500 T, = 2 m
L = 20 m, 5 turn/s
1 m 2x10-34 10-29
H Bomb 1 megaton
Asymmetry 10%
10 km 2x10-39 10-11
Supernova 10 M asymmetry 3% 10 Mpc 10-21 1044
Coalescence of 2 black holes 1 M 10 Mpc 10-20 1050
Einstein Quadrupole Formula: QQcGP 5
5
G/5c5 ~10-53 W-1 Quadrupole Moment
• GW Amplitude source asymmetry
c5 Rs2 v6 Rs Schwarzschild radius of the
source R source radius v source typical speed
Cataclysmic Astrophysical Phenomena needed for production of detectable GW
G R2 c6P ~ 2
© J. Weber (1974)G/c5 very small, c5/G will be better
An Indirect Proof: PSR 1913+16(Hulse & Taylor, Nobel’93)
Gravitational Waves exist
PSR 1913+16 : binary pulsar (couple of 2 neutron stars) tests of gravitation in strong field and dynamic regime
Loss of energy due to GW emission: orbital period decreases
• Coalescence of binary systems• Neutron Star-Neutron Star• Neutron Star-Black Hole• Black Hole-Black Hole
The Sources
• Precise theoretical prediction of the waveform before merging phase• Huge incertitude on annual rate• Duration from few seconds to few minutes (for Virgo)
• Supernovae • Signal poorly predicted• Rate: 1/30 year per galaxy• Duration : few milliseconds
• Black Hole formation • formation poorly predicted• Good predictions for Ringdown phase • Rate: ?• Duration : few milliseconds
•Pulsars : • Periodic signal• If they have a quadrupolar moment
• Stochastic Background• Incoherent sum of individual sources
• Cosmological Background
(like 2.7 K CMB for photons)
The Sources
Historical View
1960 First detector (Weber)1963 Idea of ITF detector (Gersenshtein&Pustovoit, Weber)1969 First false alarm (Weber)197X Golden Age for Weber-like detectors1972 Feasibility of ITF detector (Weiss) and first prototype (Forward)1974 PSR1913+16 (Hulse&Taylor)Late 70s Bars cooled at 4 K, ITF prototypes (Glasgow, Garching, Caltech)1980 First activities in France1986 Birth of VIRGO collaboration (France+Italy)1989 proposal VIRGO, proposal LIGO (USA)1992 VIRGO FCD French Approval. LIGO approved1993 VIRGO approved in Italy1996 Start Construction VIRGO et LIGO2001-2002 VIRGO CITF. LIGO : engineering runsFin 2005 LIGO reaches its nominal sensitivity200X VIRGO at its nominal sensitivity
Recycling Mirror Mrc
The Interferometric Detection
Laser
Photodiode
End Mirror M22
End Mirror M12
Beam-SplitterMirror Mbs Input Mirror M11
Input Mirror M21
Fabry-Perot 2
Fabry-Perot 1
Table Top experiment: hMin 10-17 Hz-1/2
PLhMin
11Virgo : hMin 10-23 Hz-1/2
The “Historical” Laboratories• LAL Orsay:
• Vacuum• Laser Control• Global Control• Simulation
• LAPP Annecy:• Detection• Standard Electronic Components• Tower • Data Acquisition• Simulation
• Nice Observatory:• Laser• Input Optics
• IPN Lyon : Mirror Coating• ESPCI Paris : Mirror Metrology
• INFN Pisa: • Super-attenuator• Vacuum• Infrastructure
• INFN Florence : Super-attenuator
• INFN Naples :• Acquisition• Environmental Monitoring
• INFN Perugia : Suspension wires• INFN Frascati : Alignment• Univ. Rome :
• Local Controls• Marionette
Vacuum Chamber
• Pressure Fluctuations:• P < 10-7 mbar (H2)• P < 10-14 for hydrocarbons
• Tube:• Diameter 1,2 m• 6 km long• V 7000 m3
• Diffused Light • light traps• deflectors
Vacuum Chamber
Beam Splitter Entry North
Entry WestPower Recycling
Laser Lab
Detection Lab
Seismic Noise
• Measurement:h seismic ( ) 10-10 -2 Hz-1/2
• Isolation Principle:
• chain of pendulums with internal dissipation• each pendulum behaves as a low pass filter:
H( ) = ( 0 / )2 for > 0
Performances
• mirror motion with few microns amplitude• mirror speed about few microns per second
The Super-Attenuator
The thermal Noise
• Each suspension wire and each mirror behaves as an oscillator excited by thermal agitation
• Characterized by 0 and Q quality factor
• Q Measurements:• silica : 106
• steel wire : 104 – 105
• Limiting factor between 3 and 500 Hz
• Mirror weight: 30 kg (noise when M )
• Test of new materials (sapphire, silicon)
• Monolithic suspensions
The mirrors
• Reflectivity defined better than 0,01 %• Reflectivity of end mirrors > 0.9998• Losses (absorption, diffusion) about few ppm• High Radius of curvature (3400 m) and defined with 3 % precision• Surface defined with /40 precision over 30 cm of diameter
• Coating realized by SMA at IPN Lyon• Metrology made at ESPCI
Solution : silica mirrors (SiO2) = 35 cm and h = 10 or 20 cm
Position Control
• Fabry-Perot resonant: L < 5 10-10 m• Recycling Cavity resonant: lR < 2.5 10-10 m• Dark Fringe (coupling with laser power noise): lDF < 10-10 m
• Alignment End Mirrors: 3 10-9 rad• Alignment Entry Mirrors: 2 10-8 rad• Alignment Recycling Mirror: 10-7 rad
• Fully Digital System running at 10 kHz for Locking and 500 Hz for Alignment
The errors signals
Pound-Drever techniquefor Fabry-Perot cavity
• phase modulation of laser frequency• side-bands anti-resonant• use reflected beam
Generalization for Virgo
Use all signals coming out of the ITF
The Virgo Sensitivity
If all technical noises are under control
Virgo and the CITF
The CITF(Central area InTerFerometer)
Central Part (no kilometric arm) used from June 2001 to July 2002.
Tests and validation : super attenuators electronic and software data acquisition output mode cleaner injection optics
Main Output: Learn how to control a suspended interferometer with digital systems
CITF Engineering runs : results
Alignment Noise
Frequency Noise
The Virgo Commissioning
• Started in September 2003 after the upgrade to full Virgo
• Strategy:• North arm
• Lock acquisition
• Frequency stabilization
• Auto Alignment
• Hierarchical control (top stage, marionette, reference mass)
• West arm (same activities)
• Recombined ITF (no recycling mirror) (same activities)
• full ITF (same activities)
• Locking at the first trial
• first lock ~ 1 hour
• frequency noise and alignment noise
Transmitted power
Frequency noise
reduction
North Arm
North Arm with Automatic Alignment
Linear alignment OFF
Linear alignment ON
Hierarchical control: 3 points
Fast corrections (f > 70 mHz)
Slow corrections (f < 70 mHz)
3.5 mN
Force applied to mirror
No feedback to top stage
with feedback to top stage
Hierarchical control: top stage
Done during the CITF commissioning
Recombined Interferometer
B7_demod
B8_demod
north arm
west arm
B5
B1B2
“3 steps” strategy
Recombined InterferometerNorth locked West locked
DC
Error signal
Correction
Michelson locked
B2_3f_ACp B1p_DC
B8_ACp
10 μrad
B2_3f_ACp PR (PRCL)B1p_DC BS (MICH)B7_ACp FP Nord B8_ACp FP Ouest
LASER
B7_ACp
0 0.2 0.4 0.6 0.8 1 1.2 1.4 1.6 1.8 20
0.2
0.4
0.6
0.8
1
0 0.2 0.4 0.6 0.8 1 1.2 1.4 1.6 1.8 2-0.5
0
0.5
1
Offset sur B1p_DC
B2_3f_ACp B1p_DC
B8_ACp
10 μrad
B2_3f_ACp PR (PRCL)B1p_DC BS (MICH)B5_ACp SSFS (CARM)B8_ACp NE-WE (DARM)
LASER
B5_ACp
0 0.2 0.4 0.6 0.8 1 1.2 1.4 1.6 1.8 20
0.2
0.4
0.6
0.8
1
0 0.2 0.4 0.6 0.8 1 1.2 1.4 1.6 1.8 2-0.5
0
0.5
1
Offset sur B1p_DC
B2_3f_ACp B1p_DC
B8_ACp
B2_3f_ACp PR (PRCL)B1p_DC BS (MICH)B5_ACp SSFS (CARM)B8_ACp NE-WE (DARM)
LASER
B5_ACp
0 0.2 0.4 0.6 0.8 1 1.2 1.4 1.6 1.8 20
0.2
0.4
0.6
0.8
1
0 0.2 0.4 0.6 0.8 1 1.2 1.4 1.6 1.8 2-0.5
0
0.5
1
Offset sur B1p_DC
Recycled Interferometer
• Misalignment of Recycling Mirror • Lock ITF half distance from Dark Fringe• Start Second Stage of Frequency Stabilization• Alignment of Recycling Mirror• Decrease Dark Fringe offset• Switch to error signal
Recycled Interferometer
Power in recycling cavities Power in the armsPower on Side-Bands
Automatic procedure lasting few minutes
The Various Sensitivities
Noise Sources
C7 Limitations
• Several problems forced us to run with reduced input laser power• Mechanical problems with Recycling mirror
new injection bench for the laser source:• Faraday isolator to avoid problems with the light reflected by the ITF towards the laser• Better mechanical properties• Better optics
New Recycling mirror :• better mechanical properties• increased reflectivity to gain on power recycling
• Restart in December 2005• First stable lock since June (10 hours)• Impinging power on Beam-Splitter increased by a factor 10 (280W)• 10 alignment loops closed (7 with low bandwidths)• Noise hunting restarted• Science Run at the end of the year
After the modifications
Data Analysis: Coalescing Binaries
Horizon with detection threshold at SNR=8 supposing an optimal orientation
Data Analysis: Bursts
Main activity :
Definition of vetoes on auxiliary channels
Feedback to commissioning
GEOTAMA
AIGO
VIRGO
The other ITF detectors for GW
3 kilometric antennas :• VIRGO (3 km)• LIGO (2 antennas, 4 km) Coincidences and position reconstruction
GW Astronomy needs at least 3 detectors
LIGO
Why a network of detectors ?
• Mandatory for Stochastic
• Reduce false alarm rate (bursts and coalescences)
• Increase detection probability (bursts and coalescences)
• Reconstruct the source position (precision about one degree)
• Reconstruct the gravitational waveform
Antenna Pattern(Sensitivity as a function of the source position)
Hanford Livingston Virgo
HL HV LV HL HVLV HLV
Efficiency 41 % 22 % 22 % 60 % 19 %
LIGO Sensitivities
LIGO Detection Range and Duty Cycle (February 2006)
L1 ~12 Mpc
H1 ~14.5 Mpc
H2 ~7 Mpc
L1 55.1%
H1 63.9%
H2 72.5%
Any two 66.7%
Triple 38.4%
NS-NS Detection Range Duty Cycle
The Future of Gravitational Waves
Increase the sensitivity by a factor 10 Gain a factor 10 on detection distance Gain a factor 1000 on the volume of possible sources Start GW astronomy
The « second generation » detectors are mandatory (~2010):
• studies started in Virgo … but not yet a complete design for « advanced Virgo »
• « white paper » in preparation
• Advanced LIGO on the track
Virgo + (~2008)
1 10 100 1000 1000010-23
10-22
10-21
10-20
10-19
h(f
) [1
/sq
rt(H
z)]
Frequency [Hz]
(a) Virgo + (b) Virgo + (old mirror th. noise model) (c) Nominal Virgo (d) Pendulum Thermal Noise (e) Mirror Thermal Noise (f) Optical Readout Noise
(a)
(b)
(c)
(d)
(e)
(f)
• Monolithic Suspensions (fused silica)• More powerful laser (50 W)• Thermal Compensation• Upgrade of the control system
The Sensitivity for 2nd generation
10 100 1000 1000010-24
10-23
10-22
10-21
10-20
h
(f)
[1
/sq
rt(H
z)]
Frequency [Hz]
AdvVirgo Virgo Virgo semi-advanced Virgo semi-advanced "new mirror model" Advanced LIGO Advanced Virgo without thermo-refractive
Conclusions
• Significant improvements in 2005• After several difficult months in 2006, we reach C7 level with full power• A factor 20 to gain at high frequency to reach nominal sensitivity • Data Analysis really started, mainly focused on detector behavior• Science Run at the end of the year
• LIGO at its nominal sensitivity and will run to get one integrated year of data
• Joint analysis in preparation : • working group LIGO-Virgo• MoU soon signed for data exchange
• Virgo + foreseen for 2008
• 2nd generation under definition and foreseen after 2010
• R&D 3rd generation starting
GW: a never ending story
The future of gravitational astronomy looks bright. 1972
That the quest ultimately will succeed seems almost assured.The only question is when, and with how much further effort. 1983
[I]nterferometers should detect the first waves in 2001 or several years thereafter (…) 1995
Kip S. Thorne
Km-scale laser interferometers are now coming on-line, and it seems very likely that they will detect mergers of compact
binaries within the next 7 years, and possibly much sooner. 2002
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