LIGO and the Quest for Gravitational Waves

Transcription

1 LIGO and the Quest for Gravitational Waves "Colliding Black Holes" Credit: National Center for Supercomputing Applications (NCSA) LIGO-G M Barry C. Barish Caltech UT Austin 24-Sept-03 1

2 A Conceptual Problem is solved! Newton s Theory instantaneous action at a distance G µν = 8πΤ µν Einstein s Theory information carried by gravitational radiation at the speed of light 2

3 Einstein s Theory of Gravitation a necessary consequence of Special Relativity with its finite speed for information transfer gravitational waves come from the acceleration of masses and propagate away from their sources as a space-time warpage at the speed of light gravitational radiation binary inspiral of compact objects 3

4 Einstein s Theory of Gravitation gravitational waves Using Minkowski metric, the information about space-time curvature is contained in the metric as an added term, h µν. In the weak field limit, the equation can be described with linear equations. If the choice of gauge is the transverse traceless gauge the formulation becomes a familiar wave equation 1 c t 2 2 ( ) h 2 2 µν = 0 The strain h µν takes the form of a plane wave propagating at the speed of light (c). Since gravity is spin 2, the waves have two components, but rotated by 45 0 instead of 90 0 from each other. h h ( t z / c) + hx ( t z / c) µν = + 4

5 The evidence for gravitational waves Hulse & Taylor 17 / sec Neutron binary system separation = 10 6 miles m 1 = 1.4m m 2 = 1.36m e = period ~ 8 hr Prediction from general relativity PSR Timing of pulsars spiral in by 3 mm/orbit rate of change orbital period 5

6 Indirect detection of gravitational waves PSR

7 Detection of Gravitational Waves Gravitational Wave Astrophysical Source Detectors in space LISA Terrestrial detectors Virgo, LIGO, TAMA, GEO AIGO 7

8 Frequency range for EM astronomy Electromagnetic waves over ~16 orders of magnitude Ultra Low Frequency radio waves to high energy gamma rays 8

9 Frequency range for GW Astronomy Audio band Gravitational waves over ~8 orders of magnitude Terrestrial and space detectors Space Terrestrial 9

10 International Network on Earth simultaneously detect signal LIGO GEO Virgo TAMA decompose detection locate the confidence polarization sources of gravitational waves AIGO 10

11 The effect Leonardo da Vinci s Vitruvian man Stretch and squash in perpendicular directions at the frequency of the gravitational waves 11

12 Detecting a passing wave. Free masses 12

13 Detecting a passing wave. Interferometer 13

14 The challenge. I have greatly exaggerated the effect!! If the Vitruvian man was 4.5 light years high, he would grow by only a hairs width Interferometer Concept 14

15 Interferometer Concept Laser used to measure relative lengths of two orthogonal arms causing the interference pattern to change at the photodiode Arms in LIGO are 4km Measure difference in length to one part in or meters As a wave passes, the arm Masses lengths change in different ways. Suspended 15

16 How Small is Meter? One meter ~ 40 inches 10,000 Human hair ~ 100 microns , ,000 1,000 Wavelength of light ~ 1 micron Atomic diameter m Nuclear diameter m LIGO sensitivity m 16

17 Simultaneous Detection LIGO Hanford Observatory MIT Caltech Livingston Observatory 17

18 LIGO Livingston Observatory 18

19 LIGO Hanford Observatory 19

20 LIGO Facilities beam tube enclosure minimal enclosure reinforced concrete no services 20

21 LIGO beam tube 1.2 m diameter - 3mm stainless 50 km of weld LIGO beam tube under construction in January ft spiral welded sections girth welded in portable clean room in the field 21

22 Vacuum Chambers vibration isolation systems» Reduce in-band seismic motion by 4-6 orders of magnitude» Compensate for microseism at 0.15 Hz by a factor of ten» Compensate (partially) for Earth tides 22

23 Seismic Isolation springs and masses Constrained Layer damped spring 23

24 LIGO vacuum equipment 24

25 Seismic Isolation suspension system suspension assembly for a core optic support structure is welded tubular stainless steel suspension wire is 0.31 mm diameter steel music wire fundamental violin mode frequency of 340 Hz 25

26 LIGO Optics fused silica Surface uniformity < 1 nm rms Scatter < 50 ppm Absorption < 2 ppm ROC matched < 3% Internal mode Q s > 2 x 10 6 Caltech data CSIRO data 26

27 Core Optics installation and alignment 27

28 Locking the Interferometers 28

29 Lock Acquisition 29

30 Making LIGO Work 30

31 Detecting Earthquakes From electronic logbook 2-Jan-02 An earthquake occurred, starting at UTC 17:38. 31

32 Detecting the Earth Tides Sun and Moon Eric Morgenson Caltech Sophomore 32

33 Tidal evaluation 21-hour locked section of S1 data Tidal Compensation Data Predicted tides Feedforward Feedback Residual signal on voice coils Residual signal on laser 33

34 Controlling angular degrees of freedom 34

35 What Limits LIGO Sensitivity? Seismic noise limits low frequencies Thermal Noise limits middle frequencies Quantum nature of light (Shot Noise) limits high frequencies Technical issues - alignment, electronics, acoustics, etc limit us before we reach these design goals 35

36 LIGO Sensitivity Livingston 4km Interferometer First Science Run 17 days - Sept 02 May 01 Jan 03 Second Science Run 59 days - April 03 36

37 Astrophysical Sources signatures Compact binary inspiral: chirps» NS-NS waveforms are well described» BH-BH need better waveforms» search technique: matched templates Supernovae / GRBs: bursts» burst signals in coincidence with signals in electromagnetic radiation» prompt alarm (~ one hour) with neutrino detectors Pulsars in our galaxy: periodic» search for observed neutron stars (frequency, doppler shift)» all sky search (computing challenge)» r-modes Cosmological Signal stochastic background 37

38 Compact binary collisions» Neutron Star Neutron Star waveforms are well described» Black Hole Black Hole need better waveforms» Search: matched templates chirps 38

39 Template Bank Covers desired region of mass param space Calculated based on L1 noise curve Templates placed for max mismatch of δ = templates Second-order post-newtonian 39

40 Optimal Filtering Transform data to frequency domain : Generate template in frequency domain : ~ h ( f ) ~ s ( f ) Correlate, weighting by power spectral density of noise: ~ s ( f ) h S ( f ~ * h z( t) = 4 0 z( t) ( f ) ) Then inverse Fourier transform gives you the filter output at all times: frequency domain ~ ~ s ( f ) h S ( f h ( f ) ) Find maxima of over arrival time and phase Characterize these by signal-to-noise ratio (SNR) and effective distance * e 2πi f t df 40

41 Matched Filtering 41

42 Loudest Surviving Candidate Not NS/NS inspiral event 1 Sep 2002, 00:38:33 UTC S/N = 15.9, χ 2 /dof = 2.2 (m1,m2) = (1.3, 1.1) Msun What caused this? Appears to be due to saturation of a photodiode 42

43 Sensitivity neutron binary inspirals Star Population in our Galaxy Population includes Milky Way, LMC and SMC Neutron star masses in range 1-3 Msun LMC and SMC contribute ~12% of Milky Way Reach for S1 Data Inspiral sensitivity Livingston: <D> = 176 kpc Hanford: <D> = 36 kpc Sensitive to inspirals in Milky Way, LMC & SMC 43

44 Results of Inspiral Search Upper limit binary neutron star coalescence rate LIGO S1 Data R < 160 / yr / MWEG Previous observational limits» Japanese TAMA R < 30,000 / yr / MWEG» Caltech 40m R < 4,000 / yr / MWEG Theoretical prediction R < 2 x 10-5 / yr / MWEG Detectable Range of S2 data will reach Andromeda! 44

45 Astrophysical Sources signatures Compact binary inspiral: chirps» NS-NS waveforms are well described» BH-BH need better waveforms» search technique: matched templates Supernovae / GRBs: bursts» burst signals in coincidence with signals in electromagnetic radiation» prompt alarm (~ one hour) with neutrino detectors Pulsars in our galaxy: periodic» search for observed neutron stars (frequency, doppler shift)» all sky search (computing challenge)» r-modes Cosmological Signal stochastic background 45

46 Detection of Burst Sources Known sources -- Supernovae & GRBs» Coincidence with observed electromagnetic observations.» No close supernovae occurred during the first science run» Second science run We are analyzing the recent very bright and close GRB NO RESULT YET Unknown phenomena» Emission of short transients of gravitational radiation of unknown waveform (e.g. black hole mergers). 46

47 Unmodeled Bursts GOAL search for waveforms from sources for which we cannot currently make an accurate prediction of the waveform shape. METHODS Raw Data Time-domain high pass filter Time-Frequency Plane Search TFCLUSTERS Pure Time-Domain Search SLOPE frequency 8Hz 0.125s time 47

48 Determination of Efficiency Efficiency measured for tfclusters algorithm To measure our efficiency, we must pick a waveform. h amplitude time (ms) 1ms Gaussian burst 48

49 Burst Upper Limit from S1 1ms gaussian bursts Result is derived using TFCLUSTERS algorithm 90% confidence Upper limit in strain compared to earlier (cryogenic bar) results: IGEC 2001 combined bar upper limit: < 2 events per day having h=1x10-20 per Hz of burst bandwidth. For a 1kHz bandwidth, limit is < 2 events/day at h=1x10-17 Astone et al. (2002), report a 2.2 σ excess of one event per day at strain level of h ~ 2x

50 Astrophysical Sources signatures Compact binary inspiral: chirps» NS-NS waveforms are well described» BH-BH need better waveforms» search technique: matched templates Supernovae / GRBs: bursts» burst signals in coincidence with signals in electromagnetic radiation» prompt alarm (~ one hour) with neutrino detectors Pulsars in our galaxy: periodic» search for observed neutron stars (frequency, doppler shift)» all sky search (computing challenge)» r-modes Cosmological Signal stochastic background 50

51 Detection of Periodic Sources Pulsars in our galaxy: periodic» search for observed neutron stars» all sky search (computing challenge)» r-modes Frequency modulation of signal due to Earth s motion relative to the Solar System Barycenter, intrinsic frequency changes. Amplitude modulation due to the detector s antenna pattern. 51

52 Directed searches NO DETECTION EXPECTED Crab Pulsar h =11.4 at present sensitivities ( f ) GW OBS 0 Sh /T Limits of detectability for rotating NS with equatorial ellipticity ε = δi/i zz : 10-3, 10-4, 8.5 kpc. PSR J Hz 52

53 Two Search Methods Frequency domain Time domain Best suited for large parameter space searches Maximum likelihood detection method + Frequentist approach Best suited to target known objects, even if phase evolution is complicated Bayesian approach First science run --- use both pipelines for the same search for cross-checking and validation 53

54 The Data time behavior < S h > < S h > days days < h S > S > < h days days 54

55 The Data frequency behavior S h S h Hz Hz S h S h Hz Hz 55

56 PSR J Frequency domain Fourier Transforms of time series Detection statistic: F, maximum likelihood ratio wrt unknown parameters use signal injections to measure F s pdf use frequentist s approach to derive upper limit Injected signal in LLO: h = 2.83 x Measured F statistic 56

57 PSR J Time domain time series is heterodyned noise is estimated Bayesian approach in parameter estimation: express result in terms of posterior pdf for parameters of interest Data 95% Injected signals in GEO: h=1.5, 2.0, 2.5, 3.0 x h = 2.1 x

58 Results: Periodic Sources No evidence of continuous wave emission from PSR J Summary of 95% upper limits on h: IFO Frequentist FDS Bayesian TDS GEO (1.94±0.12)x10-21 (2.1 ±0.1)x10-21 LLO (2.83±0.31)x10-22 (1.4 ±0.1)x10-22 LHO-2K (4.71±0.50)x10-22 (2.2 ±0.2)x10-22 LHO-4K (6.42±0.72)x10-22 (2.7 ±0.3)x10-22 Best previous results for PSR J : h o < (Glasgow, Hough et al., 1983) 58

59 Upper limit on pulsar ellipticity J h 0 = moment of inertia tensor 2 8π G 4 c I zz f R 2 0 ε gravitational ellipticity of pulsar h 0 < ε < R (M=1.4M sun, r=10km, R=3.6kpc) Assumes emission is due to deviation from axisymmetry:.. 59

60 Astrophysical Sources signatures Compact binary inspiral: chirps» NS-NS waveforms are well described» BH-BH need better waveforms» search technique: matched templates Supernovae / GRBs: bursts» burst signals in coincidence with signals in electromagnetic radiation» prompt alarm (~ one hour) with neutrino detectors Pulsars in our galaxy: periodic» search for observed neutron stars (frequency, doppler shift)» all sky search (computing challenge)» r-modes Cosmological Signal stochastic background 60

61 Signals from the Early Universe stochastic background Cosmic Microwave background WMAP

62 Signals from the Early Universe Strength specified by ratio of energy density in GWs to total energy density needed to close the universe: Ω (f) GW = dρgw d(lnf) Detect by cross-correlating output of two GW detectors: ρ First LIGO Science Data 1 critical Hanford - Livingston 62

63 Limits: Stochastic Search Interferometer Pair 90% CL Upper Limit T obs LHO 4km-LLO 4km LHO 2km-LLO 4km Ω GW (40Hz Hz) < 72.4 Ω GW (40Hz Hz) < hrs 61.0 hrs Non-negligible LHO 4km-2km (H1-H2) instrumental crosscorrelation; currently being investigated. Previous best upper limits:» Garching-Glasgow interferometers : Ω 5 GW (f) < 3 10» EXPLORER-NAUTILUS (cryogenic bars): ΩGW (907Hz) < 60 63

64 Gravitational Waves from the Early Universe E7 results projected S1 S2 LIGO Adv LIGO 64

65 Active Seismic Advanced LIGO improved subsystems Multiple Suspensions Sapphire Optics Higher Power Laser 65

66 Advanced LIGO Cubic Law for Window on the Universe Improve amplitude sensitivity by a factor of 10x number of sources goes up 1000x! Virgo cluster Initial LIGO Advanced LIGO 66

67 Advanced LIGO Enhanced Systems laser suspension seismic isolation test mass Rate Improvement ~ narrow band optical configuration 67

68 LIGO Construction is complete & commissioning is well underway New upper limits for neutron binary inspirals, a fast pulsar and stochastic backgrounds have been achieved from the first short science run Sensitivity improvements are rapid -- second data run was 10x more sensitive and 4x duration and results will be reported soon. Enhanced detectors will be installed in ~ 5 years, further increasing sensitivity Direct detection should be achieved and gravitational-wave astronomy begun within the next decade! 68

69 Gravitational Wave Astronomy LIGO will provide a new way to view the dynamics of the Universe 69