1. Cavalier Fabien LAL Orsay NIKHEF July, 3 rd 2006 The Quest for Gravitational Waves

2. 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.Experimental Results 1.Control of optical cavities 2.Sensitivities 3.Data analysis IV.Interferometric search of GW in the world V.The future of Gravitational Waves

3. 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

4. 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 L L L+dL L-dL

5. An Hertz Experiment ? sourcedistancehP (W) Steel Bar, 500 T,  = 2 m L = 20 m, 5 turn/s 1 m2 x 10 -34 10 -29 H Bomb 1 megaton Asymmetry 10% 10 km2 x 10 -39 10 -11 Supernova 10 M  asymmetry 3%10 Mpc10 -21 10 44 Coalescence of 2 black holes 1 M  10 Mpc10 -20 10 50 Einstein Quadrupole Formula: G/5c 5 ~10 -53 W -1 Quadrupole Moment

6. 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

7. 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

8. 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)

9. 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

10. 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 PSR1913+16 (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+Italy) 1989 proposal VIRGO, proposal LIGO (USA) 1992 VIRGO FCD French Approval. LIGO approved 1993 VIRGO approved in Italy 1996 Start Construction VIRGO et LIGO 2001-2002 VIRGO CITF. LIGO : engineering runs Fin 2005 LIGO reaches its nominal sensitivity 200X VIRGO at its nominal sensitivity

11. 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  10 -17 Hz -1/2 Virgo : h Min  10 -23 Hz -1/2

12. 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

14. Vacuum Chamber Pressure Fluctuations: P < 10 -7 mbar (H 2 ) P < 10 -14 for hydrocarbons Tube: Diameter 1,2 m 6 km long V  7000 m 3 Diffused Light light traps deflectors

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

16. 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

17. Performances mirror motion with few microns amplitude mirror speed about few microns per second The Super-Attenuator

18. 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 : 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

19. 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 (SiO 2 )  = 35 cm and h = 10 or 20 cm

20. Position Control Fabry-Perot resonant:  L < 5 10 -10 m Recycling Cavity resonant:  l R < 2.5 10 -10 m Dark Fringe (coupling with laser power noise):  l DF < 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

21. 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

22. The Virgo Sensitivity If all technical noises are under control

23. Virgo and the CITF

24. 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

25. CITF Engineering runs : results Alignment Noise Frequency Noise

26. 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)

27. Locking at the first trial first lock ~ 1 hour frequency noise and alignment noise Transmitted power Frequency noise reduction North Arm

28. North Arm with Automatic Alignment Linear alignment OFF Linear alignment ON

29. Hierarchical control: 3 points

30. 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

31. Recombined Interferometer B7_demod B8_demod north arm west arm B5B5 B1B1B2B2 “3 steps” strategy

32. Recombined Interferometer North lockedWest locked DC Error signal Correction Michelson locked

33. 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 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 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 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

34. Recycled Interferometer Power in recycling cavitiesPower in the arms Power on Side-Bands Automatic procedure lasting few minutes

35. The Various Sensitivities

36. Noise Sources

37. 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

38.  New Recycling mirror : better mechanical properties increased reflectivity to gain on power recycling 

39. 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

40. Data Analysis: Coalescing Binaries Horizon with detection threshold at SNR=8 supposing an optimal orientation

41. Data Analysis: Bursts Main activity : Definition of vetoes on auxiliary channels  Feedback to commissioning

42. 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

43. 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) HanfordLivingstonVirgo HLHVLV HL  HV  LV HLV Efficiency41 %22 % 60 %19 %

44. LIGO Sensitivities

45. 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 RangeDuty Cycle

46. 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

47. Virgo + (~2008) Monolithic Suspensions (fused silica) More powerful laser (50 W) Thermal Compensation Upgrade of the control system

48. The Sensitivity for 2 nd generation

49. 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 2 nd generation under definition and foreseen after 2010 R&D 3 rd generation starting

50. 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