The search for those elusive gravitational waves

1. LIGO-G060192-00-R The search for those elusive gravitational waves Recent results from interferometric detectors Nergis MavalvalaIAS, Oct. 2006 (on behalf of the LIGO Scientific Collaboration)

2. Global network of detectors GEO VIRGO LIGO TAMA AIGO LIGO • Detection confidence • Source polarization • Sky location LISA

3. Gravitational waves • Transverse distortions of the space-time itself  ripples of space-time curvature • Propagate at the speed of light • Push on freely floating objects  stretch and squeeze the space transverse to direction of propagation • Energy and momentum conservation require that the waves are quadrupolar  aspherical mass distribution

4. Astrophysics with GWs vs. E&M • Very different information, mostly mutually exclusive • Difficult to predict GW sources based on EM observations

5. GWs neutrinos photons now Astrophysical sources of GWs • Periodic sources • Pulsars  Spinning neutron stars • Low mass Xray binaries • Coalescing compact binaries • Classes of objects: NS-NS, NS-BH, BH-BH • Physics regimes: Inspiral, merger, ringdown • Burst events • Supernovae with asymmetric collapse • Stochastic background • Primordial Big Bang (t = 10-43 sec) • Continuum of sources • The Unexpected

6. R M M r h ~10-21 Strength of GWs:e.g. Neutron Star Binary • Gravitational wave amplitude (strain) • For a binary neutron star pair

7. Effect of a GW on matter

8. Measurement and the real world • How to measure the gravitational-wave? • Measure the displacements of the mirrors of the interferometer by measuring the phase shifts of the light • What makes it hard? • GW amplitude is small • External forces also push the mirrors around • Laser light has fluctuations in its phase and amplitude

9. GW detector at a glance L ~ 4 km For h ~ 10–21 DL ~ 10-18 m Seismic motion -- ground motion due to natural and anthropogenic sources Thermal noise -- vibrations due to finite temperature Shot noise -- quantum fluctuations in the number of photons detected

10. 3 0 3 ( ± 0 1 k 0 m m s ) LIGO: Laser Interferometer Gravitational-wave Observatory MIT WA 4 km Caltech 2 km LA 4 km

12. Initial LIGO Sensitivity Goal • Strain sensitivity < 3x10-23 1/Hz1/2at 200 Hz • Displacement Noise • Seismic motion • Thermal Noise • Radiation Pressure • Sensing Noise • Photon Shot Noise • Residual Gas • Facilities limits much lower

13. Gravitational-wave searches

14. Science runs and sensitivity S1 1st Science Run Sept 02 (17 days) S2 2nd Science Run Feb – Apr 03 (59 days) S3 3rd Science Run Nov 03 – Jan 04 (70 days) Strain (sqrt[Hz]-1) LIGO Target Sensitivity S5 5th Science Run Nov 05 onward (1 year integrated) S4 4th Science Run Feb – Mar 05 (30 days) Frequency (Hz)

15. Science Run 5 (S5) underway • Schedule • Started in November, 2005 • Get 1 year of data at design sensitivity • Small enhancements over next 3 years • Typical sensitivity (in terms of inspiral distance) • H1 12 to 14 Mpc (33 to 39 million light years) • H2 6 Mpc (16 million light years) • L1 12 to 14 Mpc (26 to 33 million light years) • Duty factor • 60% (L1), 72% (H1), 79% (H2) individual • 45% triple coincidence

16. S5 Run Sensitivity S5 Goal: one year of coincident data, at design sensitivity

17. Gravitational-wave searches Pulsars

18. Continuous Wave Sources • Nearly-monochromatic continuous GW radiation, e.g. neutron stars with • Spin precession at • Excited modes of oscillation, e.g. r-modes at • Non-axisymmetric distortion of shape at • Signal frequency modulated by relative motion of detector and source • Amplitude modulated by the motion of the antenna pattern of the detector • Search for gravitational waves from a triaxial neutron star emitted at

19. Known pulsar searches • S1 Setting upper limits on the strength of periodic GW from PSR J1939 2134 using time domain and frequency domain techniques • Phys. Rev. D 69 (2004) 082004 • S2 Limits on GW emission from 28 known pulsars in the time domain • Phys. Rev. Lett. 94 (2005) 181103 S1 Crab pulsar

20. Analysis method • Heterodyne time domain data using the known phase evolution of the pulsar to remove Doppler/spin-down effects • Joint Bayesian parameter estimation of unknown pulsar parameters: GW amplitude h0, initial phase f0, polarisation angle y and inclination angle i, using data from all interferometers • Produce probability distribution functions for unknown parameters and marginalise over angles to set 95% upper limit on h0 • Set limits on the ellipticity of the pulsar and compare with limits from spin-down arguments  assuming all energy lost as the pulsar spins-down is dissipated via GWs R. Jones: University of Glasgow

21. 76 known radio pulsars 32 isolated 44 in binary systems 30 in globular clusters Method Accurate timing data coherently follow phases Coherently combine data from all detectors S5 h0 frequency (Hz) S3 and S4

22. GW amplitude (PSR J1603-7202)f = 134.8 Hz, r = 1.6kpc Ellipticity (PSR J2124-3358)f = 405.6 Hz, r = 0.25kpc Spindown 2.1 x above spindown limit for Crab pulsarf = 59.6 Hz, dist = 2.0 kpc S5 S5 95% upper limits

23. Astrophysically interesting? • Upper limits for most of these pulsars are generally well above those permitted by spin-down constraints and neutron star eqns of state • Crab pulsar is nearing the spin-down upper limit • Some others within < 100 x • Provide the limits independent of the cluster dynamics for 29 globular cluster pulsars • Apparent spin-ups due to accelerations within the cluster • Cannot set spin-down limits • Most stringent ellipticity limits (4.0x10-7) are starting to approach the range of neutron star structures for some NPE models (B. Owen, PRL, 2005

24. Light reading tonight... B. Abbott et al. (LIGO Scientific Collaboration): • S1: Setting upper limits on the strength of periodic gravitational waves from PSR J1939 2134 using the first science data from the GEO 600 and LIGO detectorsPhysical Review D 69, 082004, (2004) • S2: Limits on gravitational wave emission from selected pulsars using LIGO data (LSC+M. Kramer and A. G. Lyne)Phys. Rev. Lett. 94, 181103 (2005) • S2: First all-sky upper limits from LIGO on the strength of periodic gravitational waves using the Hough transformPhys. Rev. D 72, 102004 (2005) • S3, S4, S5: in progress (S3 searched with Einstein@home)

25. Gravitational-wave Searches Binary Inspirals

26. Search for Inspirals • Sources • Binary neutron stars (~1 – 3 Msun) • Binary black holes (< 30 Msun) • Primordial black holes (< 1 Msun) • Search method • Waveforms calculable (depend on mass and spin), so use templates and optimalfiltering • Templates generated by population synthesis • Horizon distance (inspiral range) • The distance to which an optimally oriented and located (1.4 + 1.4) Msun NS binary is detectable with SNR = 8

27. S5 Inspiral Range

28. S5 Analysis pipeline • Tune pipeline using background, playground (10% of data) and injections • Measure background by applying time slides before coincidence • Add simulated signals to detector data to evaluate analysis performance • Search for double or triple coincident “triggers” • Estimate false alarm probability of resulting candidates: detection? • Compare with expected efficiency of detection and surveyed galaxies: upper limit

29. Rate < 47 per year per Milky-Way-like galaxy; 0.04 yr data, 1.27 Milky-Ways cumulative number of events signal-to-noise ratio squared Binary Neutron Stars • S3 search • 0.09 yr of data • ~3 Milky-Way like galaxies • S4 search • 0.05 yr of data • ~24 Milky-Way like galaxies Phys. Rev. D. 72, 082001 (2005)

30. Log( cum. # of events ) Rate < 38 per year per Milky-Way-like galaxy signal-to-noise ratio squared Binary Black Holes • S3 search • 0.09 yr of data • 5 Milky-Way like galaxies for 5 + 5 Msun • S4 search • 0.05 yr of data • 150 Milky-Way like galaxies for 5 + 5 Msun Phys. Rev. D. 73, 062001 (2006)

31. Rate < 63 per year per Milky-Way-like halo Rate / MW halo / yr Total mass (Msun) Primordial Black Holes • S3 search • 0.09 yr of data • 1 Milky-Way like halos for 0.5 + 0.5 Msun • S4 search • 0.05 yr of data • 3 Milky-Way like halos for 0.5 + 0.5 Msun Phys. Rev. D. 72, 082002 (2005)

32. Light reading tonight... • B. Abbott et al. (LIGO Scientific Collaboration) • S1: Analysis of LIGO data for gravitational waves from binary neutron starsPhys. Rev. D 69, 122001 (2004) • S2: Search for gravitational waves from primordial black hole binary coalescences in the galactic haloPhys. Rev. D 72, 082002 (2005) • S2: Search for gravitational waves from galactic and extra-galactic binary neutron starsPhys. Rev. D 72, 082001 (2005) • S2: Search for gravitational waves from binary black hole inspirals in LIGO dataPhys. Rev. D 73, 062001 (2006) • S2: Joint Search for Gravitational Waves from Inspiralling Neutron Star Binaries in LIGO and TAMA300 data (LIGO, TAMA collaborations)Phys. Rev. D, in press • S3: finished searched for BNS, BBH, PBBH: no detection • S4, S5: searches in progress

33. Gravitational-wave searches Stochastic Background

34. Astrophysical backgrounds due to unresolved individual sources E.g.: BH mergers, inspirals, supernovae Stochastic GW Backgrounds • Cosmological background from Big Bang (analog of CMB) WMAP 3-year data GW spectrum due to cosmological BH ringdowns (Regimbau & Fotopoulos)

35. Cosmological GW Background 10-22 sec 10+12 sec Waves now in the LIGO band were produced 10-22 sec after the Big Bang WMAP 2003

36. Stochastic Background of GWs • Given an energy density spectrum Wgw(f ), there is a GW strain power spectrum • For standard inflation (rc depends on present day Hubble constant) • Search by cross-correlating output of two GW detectors: L1-H1, H1-H2, L1-ALLEGRO • The closer the detectors, the lower the frequencies that can be searched (due to overlap reduction function)

37. Isotropic search procedure • Cross correlate two data streams x1 and x2 • For isotropic search optimal statistic is “Overlap Reduction Function” (determined by network geometry) Detector noise spectra g(f) frequency (Hz)

38. Test other Wa(f) Measured (S3, S4) Bayesian 90% upper limits Expected from S5 a = 0  inflationary or CS models a = 2  pre-BB cosmology a = 1  rotating NS

39. LIGO S1: Ω0< 44 PRD 69 122004 (2004) LIGO S3: Ω0< 8.4x10-4 PRL 95 221101 (2005) LIGO S4: Ω0< 6.5x10-5 (new) Initial LIGO, 1 yr data Expected Sensitivity ~4x10-6 CMB Adv. LIGO, 1 yr data Expected Sensitivity ~ 1x10-9 Predictions and Limits 0 Pulsar Timing -2 CMB+galaxy+Ly-aadiabatichomogeneous BB Nucleo- synthesis -4 -6 (W0) -8 Cosmic strings Log -10 Pre-BB model -12 Inflation -14 Cyclic model Slow-roll EW or SUSY Phase transition -16 -14 -12 -10 -8 -6 -4 -2 0 2 4 6 8 -18 10 Log (f [Hz])

40. Light reading tonight... • B. Abbott et al. (LIGO Scientific Collaboration): • S1: Analysis of first LIGO science data for stochastic gravitational waves Phys. Rev. D 69, 122004 (2004) • S3: Upper Limits on a Stochastic Background of Gravitational WavesPhys. Rev. Lett. 95, 221101 (2005)

41. Gravitational-wave Searches Transient or “burst’ events

42. GWs from burst sources • Brief transients: unmodelled waveforms • Time-frequency search methods • Upper limit on rate, and rate as a function of amplitude for specific shapes • Triggered searches • Use external triggers (GRBs, supernovae) • Untriggered searches • compact binary system coalescences… (SN1987A Animation: NASA/CXC/D.Berry)

43. Gravitational-Wave Bursts • Expected from catastrophic events involving solar-mass (1-100 Mo) compact objects • core-collapse supernovae • accreting/merging black holes • gamma-ray burst engines • other … ??? • Sources typically not well understood, involving complicated (and interesting!) physics • Dynamical gravity with event horizons • Behavior of matter at supra-nuclear densities • Lack of signal models makes GWBs more difficult to detect SN 1987 A

44. Burst Search Techniques Two main types of burst searches: • Untriggered: Scan ~all data, looking for excess power indicative of a transient signal • Robust way to detect generic waveforms • Triggered: Scan small amount of data around time of astronomical event (e.g., GRB), by cross-correlating data from pairs of detectors • Exploits knowledge of time of and direction to astronomical event Always: • Use techniques that make minimal assumptions about the signal • Be open to the unexpected!

45. Excess-Power Detection • Look for transient jump in power in some time-frequency region: • frequency ~ [60,2000] Hz (determined by noise curves of instruments) • duration ~ [1,100] ms (time scale associated with solar-mass COs) • Anderson et al. PRD 63 042003 (2001) • Many different implementations in LIGO: • Fourier modes, wavelets, Gaussian-modulated sinusoids • Multiple time-frequency resolutions • Provide redundancy & robustness • Also time-domain & optimal filter searches Simulated binary inspiral signal in S5 data Chatterji: Q Pipeline search for GWBs with LIGO

46. detector 1 detector 2 detector 3 time time time frequency frequency frequency Schematic Typically require coincident detection in all 3 LIGO interferometers: time, frequency coincidence range of time-freq resolutions 8 - 256 Hz  candidate event 1/128 – 1/4 s

47. Upper Limits No GWBs detected through S4. So, set limit on GWB rate vs. signal strength: h = upper limit on event number T = observation time e(hrss) = efficiency vs strength Progress: Central Frequency Lower rate limits from longer observation times Rate Limit (events/day) Lower amplitude limits from lower detector noise

48. (from HETE) Integration time (logarithmic steps) Segment time Gamma Ray Burst: GRB030329 • Targeted search NO detection • A supernova at z ~ 0.17 ~ 800 Mpc • H1 and H2 operational during S2 hrss 6 x 10-21 Hz-1/2 (waveform-dependent) Phys. Rev. D 72 (2005) 042002

49. Cross correlate data between pairs of detectors around time of event 25 – 100 ms target signal duration [-2,+1] min around GRB Compute probability of largest measured CC using distribution of CCs from neighboring times (no GRB, with time shifts). Improbably large CC equals candidate GWB Procedure: GRBs sample GRB lightcurve (BATSE) trigger time detector 2 detector 1

50. Statistical Tests • No loud signals seen. • Look also for weak cumulative effect from population of GRBs. • Use binomial test to compare to uniform distribution. • No significant deviation from expected distribution. • Max-likelihood test also used. • Mohanty, CQG 22 S1349 (2005) 54 GRB tests most significant excess at 9th least probable event: P≥ 9 (p9 ) = 0.102 Leonor / Sannibale, Session W11