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