Gravitational wave detection. K.A. Strain

Transcription

1 Gravitational wave detection K.A. Strain

2 Contents gravitational waves: introduction sources of waves, amplitudes and rates basics of GW detection current projects future plans and hopes

3 Gravitational Waves: the basic physics General relativity predicts gravitational waves that travel at the speed of light they travel (in the far field) as weak transverse modulations of space-time since mass has one sign (or equivalently a spin 2 graviton) there is no dipole radiation - quadrupole is the lowest order there are two independent polarisations X and + at 45 degrees to each other

4 Gravitational waves Produced in the form of bursts (compact binaries, stellar collapse) continuous waves (pulsars) stochastic background (early Universe)

5 How the waves are detected Gravitational waves, very weak at Earth due to the extreme stiffness of space-time, can be detected by their oscillating effect on the local space-time curvature they produce strains in space, so longer detectors generate larger signals up to the wavelength of the waves (~100 km for audio frequency waves) the waves are expected to produce strains of < (except for rare larger events) detectors use frequency selectivity to help improve the signal to noise ratio for such very weak effects

6 Effect of a gravitational wave Modulation of the proper distance between free test particles A gravitational wave of amplitude h, will produce a strain apart L h = L 2 between masses a distance L

7 Sources of waves, amplitudes and rates Some sources are guaranteed: radiation from short period galactic binaries must be seen by the proposed space interferometer (LISA) Other sources have predicted event rates some of these have known (or approximately known) signal waveforms and amplitudes as a function of distance others have known (or approximately known) event rates but unknown amplitudes

8 The gravitational wave spectrum Ground-based detectors are noise-limited to operation above ~10 Hz ; space-based detectors are required for lower frequency observations Gravity gradient wall

9 The detectors Initial ground based interferometers reasonable chance of detection test the basic technology (performance, reliability) Advanced interferometers ~15x better sensitivity upgrade of the initial interferometers LISA planned ESA/NASA space based interferometer promised detections if it operates correctly

10 Sources (selection) Neutron Star & Black Hole Binaries inspiral, merger Spinning NS s LMXBs known pulsars radio-quiet neutron stars Stochastic background big bang, early universe

11 Standards of detection assume the best search algorithm now known for an array of detectors (usually 3) for continuous wave sources assume 4 months integration for stochastic background assume 4 months integration set thresholds so that the false alarm probability=1% in a 10 year observation

12 Neutron Star Binary Inspiral Latest: new galactic, binary pulsar system observed (now 4 systems observed) Surprisingly close to coalescence (85 Myr) Changes expected population of NS-NS systems (Kalogera et al) Initial interferometers range: 20 Mpc 1/60 yrs to 1/3 yrs Advanced interferometers range: 300Mpc 80/yr to 5/day

13 Science From Inspirals: NS/NS, NS/BH, BH/BH Information carried: masses (a few %), Spins (?few%?), Distance [not redshift!] (~10%), Location on sky (~1 degree) Mchirp = 3/5 M2/5 to ~10-3 Search for EM counterpart, e.g. γ-burst. If found: learn the nature of the trigger for that -burst deduce relative speed of light and GW s to ~ 1 sec / 3x109 yrs ~ encouraged by at least one Swift event that looks like a NS:NS merger

14 Black Hole-Black Hole Inspiral and Merger Event rates Population extrapolated from NS-NS rate, relatively uncertain Initial interferometers range: 100 Mpc latest estimate 1/250yrs to 3/yr Advanced interferometers range: z=0.4 estimate 2/month to 30/day Note BH-BH rate is larger than NSNS, although less certain

15 BH/BH Mergers: Exploring the Dynamics of Spacetime Warping figure credit Kip Thorne

16 Spinning Neutron Stars Continuous-wave (CW) radiation; expect low amplitudes, require long integration times Many objects with known frequency and position (pulsars), some more with known positions (X-ray sources) Great interest in detecting radiation: physics of such stars is poorly understood. after 4 decades we still don t know what, exactly, makes pulsars pulse. interior properties not understood: equation of state, superfluidity, superconductivity, solid core, source of magnetic field.

17 Spinning Neutron Stars: Pulsars Crustal asymmetries NS ellipticity based on current understanding of crust strength and equation of state: ε < Known Pulsars: first interferometers limits ε ~ (f khz )-2 r10kpc advanced interferometers limits ε ~ (f khz )-2 r10kpc

18 Spinning Neutron Stars: LMXBs Rotation rates ~250 to 700 rev/sec Spin-up torque balanced by GW emission torque Combined GW & EM Sco X-1 observations Signal strengths for 20 days of integration

19 Primordial Gravitational Waves Production: Fundamental physics in the early universe - Inflation Phase transitions Topological defects String-inspired cosmology Brane-world scenarios Spectrum: slope, peaks give masses of key particles & energies of transitions.

20 Primordial Gravitational Waves Strength: Expressed as fraction of closure energy Ωgw density, it is poorly constrained: < Ωgw<10 5 2nd generation detectors will reach ~ for f > 10 Hz. LISA will reach at mhz frequencies

21 Detection: how to achieve good sensitivity measure with frequency selectivity filter out large low Fourier frequency signals engineer the system to provide minimum noise in the desired band ~10 Hz to ~2 khz for groundbased detectors ~0.1 mhz to 1 Hz for LISA after that each noise contribution must be studied and reduced - fundamental limits allow useful sensitivity

22 Interferometry: the problems sensing imagine perfectly quiet test masses (mirrors) require extremely sensitive sensing of the path lengths test masses must be isolated from ground vibration use conventional vibration isolation techniques (active around 1 Hz, passive above 1 Hz in the GW band) thermal noise (Brownian motion) must be minimised use massive rigid mirrors on very soft, low dissipation suspensions (fine engineering)

23 How interferometers operate: the basic Michelson GW produce differential mirror length changes can be thought of as phase modulation of the light carrier held at dark fringe differential modulation signals reach detector where they are measured using homodyne or heterodyne techniques laser and injection optics beamsplitter detector main limit is shot noise in the detected photocurrent

24 Enhancements to the basic interferometer: Power recycling Most of the light mirror (>99.99%) is reflected back to the laser place a power recycling mirror to catch this and add it in phase with the ingoing light enhances circulating light field ~1000 times laser and injection optics beamsplitter detector limit now due to thermal distortion of the optics (at >>10 kw CW power)

25 Signal recycling signal recycling catches the mirror phase modulation sidebands leaving the interferometer they are reinjected and resonate within the optical cavity formed GW produces phase modulation of light in arms laser and injection optics beamsplitter the transmittance of the SR mirror determines the bandwidth its position determines the tuning detector The SR mirror + interferometer form a resonant cavity

26 Resonant arm cavities Optical cavities can mirror be added in the arms the light makes multiple passes the phase change is enhanced the interferometer is harder to control laser and injection optics beamsplitter detector

27 Interferometrically sensed gravitational wave detectors (ground based) 4 detector systems worldwide: LIGO (USA) 2 detectors of 4km arm length + 1 detector of 2km arm length operating near design sensitivity!!! VIRGO (Italy/France) 1 detector of 3km arm length (now being commissioned) GEO 600 (UK/Germany) 1 detector of 600m arm length (now at advanced stage of commissioning) TAMA 300 (Japan) 1 detector of 300m arm length (now operating)

28 GW Detectors: Under Construction and Planned GEO LIGO VIRGO TAMA ACIGA

29 GEO 600: the German - British detector

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31 GEO 600 Initial GEO 600 strategy: to build a low cost detector of comparable sensitivity to the initial LIGO and VIRGO detectors to take part in gravitational wave searches in coincidence with these systems Unique GEO 600 design technology to make this possible: advanced suspension technology for low thermal noise advanced optics configuration signal recycling

32 Monolithic silica suspensions GEO600 is the first interferometer to use such suspensions to reduce thermal noise the technology offers ~10 x lower noise than the alternative (steel wire) designs that are used in the other initial interferometers

33 Silicate bonding GEO600 GEO600 coated mirror Coatings protected by vacuum end caps Bonding surfaces cleaned Silica ears bonded to masses Fibres welded to ears and monolithic stage installed in interferometers using catchers

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35 Timescales first detectors GEO and LIGO 1st coincident run took place over New Year nd run took place in August/September rd Science run over the winter 03/04 4th Science run spring 2005 results being analysed now 5th run starts in a few months, lasting ~1 year probably marks start of serious observing

36 GEO 600 Duty Cycle (S3II period) better than we ever dared to hope

37 Recent progress at LIGO Sep 02 Apr 03 Jan 04 Aug 04 goal

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39 LISA the Mission LISA is a mission to detect gravitational waves in space joint mission: NASA and ESA launch planned ~ spacecraft in solar orbits, one launch

40 Orbits Orbital inclination provides stability of the formation

41 The Spacecraft

42 LISA Interferometry (1) reference beams laser transponder operation: each s/c sends and receives two beams each incoming beam is heterodyned separately with a local laser main beams

43 Drag free LISA needs a purely gravitational orbit test masses have to be shielded from solar wind float free inside spacecraft capacitive sensing of the test masses relative to spacecraft feedback loop to propulsion - FEEP thrusters with micro-newton thrust

44 10-18 wave amplitude h Mo z= Mo z= RXJ apparent magnitude (GW flux) LISA Sensitivity (examples) 6Mo M o U z BH =1 tio c te e D 10-2 Frequency (Hz) n 10-1 h s e th r old

45 From Initial to Advanced LIGO signal recycling is added to upgrade the interferometer hrms = h(f) f ~10 h(f) configuration Initial interferometers silica suspension technology and multiple stage Open up wider band 15 in h ~3000 in rate pendulums replace the current wire-loop single stage suspensions Advanced interferometers higher laser power (200W laser 20J stored in system) and 40kg mirrors Kip S. Thorne California Institute of Technology used with permission Reshape noise

46 UK Involvement in Advanced LIGO Glasgow, Birmingham, RAL and Cardiff Project to build key mirror-suspensions and optical systems is funded by PPARC This will give the UK a full share in the first GW observatory (for a fraction of the total cost). Project now well under way in UK US funding for R&D in place, construction funding now has National Science Board approval to go to President s Budget 2008

47 Conclusion There is a chance (no promise) that the first GW detections will occur within the next 3 years on the first generation systems It is highly probable that within a decade Advanced LIGO and LISA will be making routine observations of gravitational waves