Cavalier Fabien LAL Orsay IIHE Bruxelles 30 Avril 2004 The Quest for Gravitational Waves

Virgo and the quest for Gravitational Waves I.The interferometric detection of gravitational waves 1.Gravitational Waves: nature and effects 2.Sources 3.Principle of interferometric detection and improvements II.The Virgo Challenge 1.The infrastructures 2.The seismic noise and the super-attenuator 3.The thermal noise 4.The control system 5.The Virgo sensitivity III.The first experimental results 1.CITF 2.Fabry-Perot cavities 3.Recombined Interferometer IV.Interferometric search of GW in the world

Gravitational Waves A little bit of General Relativity In Special Relativity, the space-time interval is given by: ds 2 = dx 2 + dy 2 + dz 2 – c 2 dt 2 =   dx  dx where   is the Minkowski metric tensor In General Relativity, we have: ds 2 = g  dx  dx  with  g  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 source c4c4

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 h+h+ t = 0t = T/4t = T/2t = 3T/4t = T hxhx L L +  L

An Hertz Experiment ? sourcedistancehP (W) Steel Bar, 500 T,  = 2 m L = 20 m, 5 turn/s 1 m2 x H Bomb 1 megaton Asymmetry 10% 10 km2 x Supernova 10 M  asymmetry 3%10 Mpc Coalescence of 2 black holes 1 M  10 Mpc Einstein Quadrupole Formula: G/5c 5 ~ W -1 Quadrupole Moment

GW Amplitude  source asymmetry c 5 R s 2 v 6 R s Schwarzschild radius of the source R source radius v source typical speed  Cataclysmic Astrophysical Phenomena needed for production of detectable GW G R 2 c 6 P ~  2 © J. Weber (1974) G/c 5 very small, c 5 /G will be better

An Indirect Proof: PSR (Hulse & Taylor, Nobel’93) Gravitational Waves exist PSR : 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 detectors 1972 Feasibility of ITF detector (Weiss) and first prototype (Forward) 1974 PSR (Hulse&Taylor) Late 70s Bars cooled at 4 K, ITF prototypes (Glasgow, Garching, Caltech) 1980 First activities in France 1986 Birth of VIRGO collaboration(France+Italie) 1989 proposal VIRGO, proposal LIGO (USA) 1992 VIRGO FCD French Approval. LIGO approved 1993 VIRGO approved in Italy 1996 Start Construction VIRGO et LIGO VIRGO CITF. LIGO : engineering runs 200X VIRGO at its sensitivity

Recycling Mirror M rc The Interferometric Detection Laser Photodiode End Mirror M 22 End Mirror M 12 Beam-Splitter Mirror M bs Input Mirror M 11 Input Mirror M 21 Fabry-Perot 2 Fabry-Perot 1 Table Top experiment: h Min  Hz -1/2 Virgo : h Min  Hz -1/2

Virgo and the CITF

The 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 < mbar (H 2 ) P < for hydrocarbons Tube: Diameter 1,2 m 6 km long V  7000 m 3 Diffused Light light traps deflectors

Vacuum Chamber Beam Splitter Entry North Entry West Power Recycling Laser Lab Detection Lab

Seismic Noise Measurement: h seismic ( )  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 motion of the mirrors about one micron speed about few microns per second The Super-Attenuator

The thermal Noise Each suspension wire and each mirror behave as an oscillator excited by thermal agitation Characterized by  0 and Q quality factor Q Measurements: silica : 10 6 steel wire : 10 4 – 10 5 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 > 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 (SiO 2 )  = 35 cm and h = 10 or 20 cm

Position Control Fabry-Perot resonant:  L < m Recycling Cavity resonant:  l R < m Dark Fringe (coupling with laser power noise):  l DF < m Alignment End Mirrors: rad Alignment Entry Mirrors: rad Alignment Recycling Mirror: rad Fully Digital System running at 10 kHz for Locking and 500 Hz for Alignment

The errors signals Pound-Drever technique for 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 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: Michelson Control

CITF Engineering runs : results RunDateConfiguration E009/2001Simple Michelson E112/2001+ control SA E204/2002+ recycling E305/2002+ automatic alignment E407/2002+ injection system RunLosses of LockDuty CycleMax Duration E0198%51h E1185%27h E2398%41h E3498%40h E4473%14h 5 « runs », 3 days long from September 2001 to July 2002

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 First Sensitivity Shot noise Electronic noise (ADC) Frequency noise Control noise MC length control noise

North Arm with Automatic Alignment Linear alignment OFF Linear alignment ON

Arms Sensitivity during C1 and C2 runs

Frequency Stabilization on North Arm laser 50 Hz 300 kHz Ref Cav 50 Hz A free running laser has a too noisy frequency  You need to stabilize the frequency on a more stable reference The Virgo arms are the best available reference at high frequency

Frequency Stabilization on North Arm laser 50 Hz 20 kHz. 300 kHz Ref Cav 1 Hz Loop needed to stabilize the frequency in DC

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 B5B5 B1B1B2B2 “3 steps” strategy

Recombined Interferometer North lockedWest locked DC Error signal Correction Michelson locked

Near Past and Near Future 4 days C3 run last week North Cavity with Automatic Alignment and Frequency stabilization West Cavity with Automatic Alignment only Recombined Interferometer Analysis under progress New Hierarchical control under test Improvement of robustness for Recombined Interferometer Transition to full Virgo beginning of this summer

GEO TAMA 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

LIGO Sensitivities

LIGO Detection Range

Conclusions Lot of debugging and experience acquired on the CITF Commissioning of full VIRGO started in September 2003 After 8 months: North cavity locked, aligned and frequency stabilization OK West cavity locked, alignment robustness to be improved Recombined Interferometer locked, alignment under progress, frequency stabilization to be done. Robustness has to be improved Full Virgo locked in simulation, preliminary measurements under progress. Experimental work to be done First run with full Virgo at the end of the summer ? First science run in 2005 R&D effort started for next generation

GW: a never ending story The future of gravitational astronomy looks bright That the quest ultimately will succeed seems almost assured. The only question is when, and with how much further effort [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