The Virgo interferometer for Gravitational Wave detection Francesco Fidecaro EPFL, November 8, 2010
Outline • Gravitational waves: sources and detection • The Virgo interferometer • The global network • Some LSC-Virgo results (for the LSC and Virgo collaborations) • Advanced Virgo • Perspective
Gravitational waves • Tiny perturbations of spacetime geometry • Predicted by Einstein as consequence of General Relativity • Propagate at the speed of light • Non relativistic approximation: generated by accelerated masses (quadrupole formula) • Amplitude h decreases as 1/R (field, as opposed to 1/R2 for energy or particle counting) • Order of magnitude: RS/R • Detectable by measuring invariant separation between free falling masses
B3 B2 t B1 A3 A2 A1 x t B3 B2 A3 B1 A2 A1 x Gravitational wave detection • Measure variations in curvature of space time • Use clocks on geodetics as markers • Be careful of pitfalls of Relativity! Measure only well defined, invariant quantities • Need one precise clock in one place: laser • Need precise clocks in different places: • pulsar and atomic clock
Compact binary systems chirp
Horizon and event rate > 1 ev/yr Predictions for the rates of compact binary coalescences observable … CQG, 10.1088/0264-9381/27/17/173001
Stellar core collapse (Supernova) Impulsive events, final evolution of big mass stars Core collapses to NS or BH, GW emitted only in non-spherical collapse Big uncertainties, waveform “unpredictable” Coincidence detection necessary Amplitude: optimistic h~10-21 at 10 Mpc non-axisymmetric collapse Rate: several/year in the VIRGO cluster (how many detectable?) GW emitted
Pulsars 1000 galactic pulsars known Possible sources of GW
Pulsars: rotating neutron stars Non-axisymmetric rotating NS emit periodic GW at f=2fspinbut…weak SNR increases with observation time T as T1/2, T can be months But… Df ~ 10-6 Doppler correction of Earth motion: Df/f ~ 10-4 function of source position: Blind search limited by computing power 109 NS in the galaxy, ~1000 known Ellipticity determination: EOS nuclear matter. Strange stars?
CMBR Relic neutrinos Relic gravitons Relic stochastic background Imprinting of the early expansion of the universe Need two correlated ITFs Standard inflation produces a background too low String models ?
The Gravitational Wave Spectrum Dick Manchester, CSIRO LIGO/VIRGO
Early efforts Brillet (optics) Giazotto (suspensions) Collaboration started in 1992 LAPP Annecy EGO Cascina Firenze-Urbino Genova Napoli OCA Nice NIKHEF Amsterdam LAL Orsay LMA Lyon APC Paris – ESPCI Paris Perugia Pisa Roma La Sapienza Roma Tor Vergata Trento-Padova IM PAN Warsaw RMKI Budapest LKB Paris 18 groups About 200 authors The Virgo Collaboration
Seismic isolation • Super-attenuators: multi-stage passive seismic isolation system MODEL
marionetta mirror Superattenuator performance • Excitation at top • Use Virgo sensitivity and stability • Integrate for several hours • Upper limit for TF at 32 Hz:1,7 10-12 • In some configurations a signal was found, but also along a direction perpendicular to excitation: compatible with magnetic cross talk
GW interferometers • Isolated/suspended mirrors: • sz at 10 Hz ~ 10-18 m • sz at 100 Hz ~ 10-21 m • Differential measurement to cancel phase noise • Effective L ~ 102 km • l = 1 mm • Effective power ~ 1 kW ~ 1022g • Measurement noise ~ 10-11 rad • for a 1 s measurement • Record a signal, if high SNR there is a large information content L Light source
200 mm fused silica suspension fibre pioneered by Glasgow/GEO600 Mirror coating Beam size High power laser Mirror thermal lensing compensation for high power Signal recycling Use of non standard light Seismic attenuation Local gravity fluctuations Issues in sensitivity (Virgo example) • h ~ 3 x 10-21 Hz-1/2 @ 10 Hz • h ~ 7 x 10-23 Hz-1/2 @ 100 Hz
The European Gravitational Observatory PURPOSE • The Consortium shall have as its purpose the promotion of research in the field of gravitation in Europe. • In this connection and in particular, the Consortium pursues the following objectives: • ensures the end of the construction of the antenna VIRGO, its operation, maintenance and the upgrade of the antenna as well as its exploitation; • ensures the maintenance of the related infrastructures, including a computer centre and promotes an open co-operation in R&D; • ensures the maintenance of the site; • carries out any other research in the field of gravitation of common interest of the Members; • promotes the co-operation in the field of the experimental and theoretical gravitational waves research in Europe; • promotes contacts among scientists and engineers, the dissemination of information and the provision of advanced training for young researchers.
EGO • 5 year renewal approved this year • Current members: CNRS, INFN participating equally to budget (ca 10 M€ / year) • Management: • EGO Council and its President • EGO Director • Board of auditors • Currently 48 staff, EGO Scientific Director, Adminstrative Head • Scientific and Technical Advisory Committee • Experts of the field or of related questions • VESF:Virgo-EGO Scientific Forum • Implementation of one of the EGO purposes • Gathers people interested in gravitational waves and their detection
Noise understanding • Noise sources and coupling are well understood • Low frequency shows more structures • Noise reduction in advanced detectors achieved with proper design • Virgo+ in 2010: fused silica suspensions and higher Finesse • risk reduction for Advanced detectors
Virgo sensitivity progress VSR1: May 18-Sep 30 2007 4 month continuous data taking simultaneously with LIGO Analysis in progress
Stability • Robust interferometer • 95% Science Mode duty cycle • Good sensitivity • Stable horizon: 8-8.5 Mpc (1.4-1.4 Ns-Ns) - averaged 42-44 Mpc (10-10 BH-BH) - averaged • fluctuating with input mirror etalon effect • Low glitch rate: factor 10 lower than VSR1 • Preparing for installation of monolithic suspensions
Environmental noises studies Investigations to understand the sources and the path to dark fringe Coupling (paths) to dark fringe - diffused light from in air optical benches - diffused light related to Brewster window - beam jitter on injection bench Sources of environmental noise: - air conditioning - electronic racks • Need to work both on: • reduction of coupling • reduction of environmental noise End benches Elec racks Injection bench Laser Beam jitter Brewster window DAQ room Detection suspended bench External bench
Motivation for a Global GW Detector Network • Time-of-flight to reconstruct source position t3 t5 t1 t4 t2 GEO VIRGO LIGO TAMA t6 AIGO
source location Motivation for a Global GW Detector Network • Source location: • Ability to triangulate (or ‘N-angulate’) and more accurately pinpoint source locations in the sky • More detectors provides better source localization Multi-messenger astronomy • Network Sky Coverage: • GW interferometers have a limited antenna pattern; a globally distributed network allows for maximal sky coverage • Detection confidence: • Redundancy – signals in multiple detectors • Maximum Time Coverage - ‘Always listening’: • Ability to be ‘on the air’ with one or more detectors • Source parameter estimation: • More accurate estimates of amplitude and phase • Polarization - array of oriented detectors is sensitive to two polarizations • Coherent analysis: • Combining data streams coherently leads to better sensitivity ‘digging deeper into the noise’ • Also, optimal waveform and coordinate reconstruction
LIGO Abbott, et al., “The laser interferometer gravitational-wave observatory”http://stacks.iop.org/0034-4885/72/076901
Large Cryogenic Gravitational wave Telescope LCGT is almost entirely financed to be built underground at Kamioka, where the prototype CLIO detector is placed.
World wide GW network: LV agreement • “Among the scientific benefits we hope to achieve from the collaborative search are: • better confidence in detection of signals, better duty cycle and sky coverage for searches, and better source position localization and waveform reconstruction. In addition, we believe that the intensified sharing of ideas will also offer additional benefits.” • Collaborations keep their identities and independent governance
LV Agreement (I) • “All data analysis activities will be open to all members of the LSC and Virgo Collaborations, in a spirit of cooperation, open access, full disclosure and full transparency with the goal of best exploiting the full scientific potential of the data.” • Joint committees set up to coordinate data analysis, review results, run planning, and computing. The makeup of these committees decided by mutual agreement between the projects. • Joint publication of observational data whether data from Virgo, or LIGO (GEO) or both
Some results from LV • MoU for data sharing: now common data analysis groups (Bursts, Coalescing Binaries, Periodic Sources, Stochastic Background), weekly (and more) telecons • An Upper Limit on the Amplitude of Stochastic Gravitational-Wave Background of Cosmological Origin • Joint searches for GRBs (LV) • GRB 070201 (LSC) • Crab spindown limit (LSC) and Vela (Virgo)
Stochastic Background (SB) • A stochastic background can be • a GW field which evolves from an initially random configuration: cosmological background • the result of a superposition of many uncorrelated and unresolved sources : astrophysical background) • Typical assumptions • Gaussian, because sum of many contributions • Stationary, because physical time scales much larger than observational ones • Isotropic (at least for cosmological backgrounds) If these are true, SB is completely described by its power spectrum
Detection method • It is stochastic and presumably overwhelmed by noise • Need (at least) two detectors to check for statistical correlations • Optimal filtering Uncorrelated (?) noises Signals
Detection performance • Sensitivity improves as T1/2 • Better performances when coherence is high ( ) • detectors near each other compared to l • detectors aligned
Isotropic search: results • Data collected during S5 run (one year integrated data of LIGO interferometers) • Point estimate of Y: no evidence of detection integrating over 40-170 Hz (99% of sensitivity)
Isotropic search: results • Now we are beyond indirect BBN and CMB bounds • We are beginning to probe models
Joint LIGO/Virgo Search for GRBs • Gamma Ray Bursts (GRBs) - brightest EM emitters in the sky • Long duration (> 2 s) bursts, high Z progenitors are likely core-collapse supernovae • Short duration (< 2 s) bursts, distribution about Z ~ 0.5 progenitors are likely NS/NS, BH/NS, binary merger • Both progenitors are good candidates for correlated GW emissions! • 212 GRBs detected during S5/VSR1 • 137 in double coincidence (any two of LIGO Hanford, LIGO Livingston, Virgo) • No detections, we place lower limits on distance assuming EGW= 0.01 Mc2
GRB 070201 Refs: GCN: http://gcn.gsfc.nasa.gov/gcn3/6103.gcn3 X-ray emission curves (IPN) M31The Andromeda Galaxy by Matthew T. Russell Date Taken:10/22/2005 - 11/2/2005Location:Black Forest, COEquipment:RCOS 16" Ritchey-ChretienBisque Paramoune MEAstroDon Series I FiltersSBIG STL-11000M http://gallery.rcopticalsystems.com/gallery/m31.jpg
Inspiral Exclusion Zone 25% 50% 75% 90% 99% GRB070201: Not a Binary Merger in M31! • Burst search: • Cannot exclude an SGR in M31 • SGR in M31 is the current best explanation for this emission • Upper limit: 8x1050 ergs (4x10-4 Mc2) (emitted within 100 ms for isotropic emission of energy in GW at M31 distance) Abbott, et al. “Implications for the Origin of GRB 070201 from LIGO Observations”, Ap. J., 681:1419–1430 (2008). • Inspiral (matched filter search: • Binary merger in M31 (770 kpc) scenario excluded at >99% level • Exclusion of merger at larger distances GRB 2007
The Crab Pulsar: Beating the Spin Down Limit! • Remnant from supernova in year 1054 • Spin frequency nEM = 29.8 Hz • ngw = 2 nEM = 59.6 Hz • observed luminosity of the Crab nebula • accounts for < 1/2 spin down power • spin down due to: • electromagnetic braking • particle acceleration • GW emission? • early S5 result: h < 3.9 x 10-25 ~ 4X below • the spin down limit (assuming restricted priors) • ellipticity upper limit: e < 2.1 x 10-4 • GW energy upper limit < 6% of radiated energy is in GWs Abbott, et al., “Beating the spin-down limit on gravitational wave emission from the Crab pulsar,” Ap. J. Lett. 683, L45-L49, (2008).