Status of the search for Gravitational Waves � Gravitational waves � Detection of GW’s � The LIGO project and its sister projects � Astrophysical sources � Recent results “Merging Neutron Stars“ (Price & Rosswog) � Conclusions No discovery to report here! Alan Weinstein, Caltech for the LIGO Scientific Collaboration 1 LIGO-G0900681
Gravitational Waves Static gravitational fields are described in General Relativity as a curvature or warpage of space-time, changing the distance between space-time events. G μν = 8 πΤ μν Shortest straight-line path of a nearby test-mass is a ~Keplerian orbit. If the source is moving (at speeds close to c), eg, because it’s orbiting a companion, the “news” of the changing gravitational field propagates outward as gravitational radiation – a wave of spacetime curvature 2
Nature of Gravitational Radiation General Relativity predicts that rapidly h = Δ L / L changing gravitational fields produce ripples of curvature in fabric of spacetime • Stretches and squeezes space between “test masses” – strain h = Δ L / L • propagating at speed of light • mass of graviton = 0 • space-time distortions are transverse to direction of propagation • GW are tensor fields (EM: vector fields) Contrast with EM dipole radiation: two polarizations: plus ( ⊕ ) and cross ( ⊗ ) )) (EM: two polarizations, x and y ) (( )) Spin of graviton = 2 )) 3
Sources of GWs Accelerating charge ⇒ electromagnetic radiation (dipole) � Accelerating mass ⇒ gravitational radiation (quadrupole) � Energy-momentum conservation: energy cons ⇒ no monopole radiation � Amplitude of the gravitational wave (dimensional analysis): momentum cons ⇒ no dipole radiation ⇒ lowest multipole is quadrupole wave � = second derivative of mass quadrupole moment (non-spherical part of kinetic energy – tumbling dumb-bell) � G is a small number! M ~ 10 30 kg (space-time is stiff ). � Waves can carry huge energy R ~ 20 km km with minimal amplitude f ~ 400 Hz � Need huge mass, r ~ 10 23 m relativistic velocities, nearby. � For a binary neutron star pair, 10m light-years away, solar masses Terrestrial sources TOO WEAK ! moving at 15% of speed of light: 4
Indirect Evidence for GWs from Hulse-Taylor binary emission of gravitational waves by compact binary system 17 / sec • • Binary pulsar PSR 1913 + 16 • Discovered in 1974 • orbital parameters measured continuously measured over 30 years! • � Only 7 kpc away ~ 8 hr � 8 hr period speeds up 35 sec from 1975-2005 � measured to ~50 msec accuracy � deviation grows quadratically with time � shortening of period ⇐ orbital energy loss � Compact: negligible loss from friction, material flow (Weisberg & Taylor, 2005) � beautiful agreement with GR prediction � Apparently, loss is due to GWs! � Nobel Prize, 1993 � Merger in about 300M years (<< age of universe!) � GW emission will be strongest near the end – 5 Coalescence of black holes!
A NEW WINDOW ON THE UNIVERSE The history of Astronomy: new bands of the EM spectrum opened → major discoveries! GWs aren’t just a new band, they’re a new spectrum, with very different and complementary properties to EM waves. • Vibrations of space-time, not in space-time • Emitted by coherent motion of huge masses moving at near light-speed; not vibrations of electrons in atoms • Can’t be absorbed, scattered, or shielded. GW astronomy is a totally new, unique window on the universe 6
Interferometric detection of GWs mirrors GW acts on freely falling masses: laser Beam splitter For fixed ability to measure Δ L , make L Dark port photodiode as big as possible! Antenna pattern: (not very directional!) 7
Interferometric GW detectors � Quadrupolar radiation pattern � Michelson interferometer “natural” GW detector � Suspended mirrors in “free-fall” Fabry-Perot cavity � Broad-band response 4km ~50 Hz to few kHz � Waveform detector e.g., chirp reconstruction h = Δ L / L � Goal: get h ≤ 10 -22 ; can build L = 4 km; GW output must measure power recycling port Δ L = h L ≤ 4×10 -19 m mirror 8
Limits to Initial LIGO Sensitivity Seismic Noise test mass (mirror) Thermal Residual gas (Brownian) scattering Noise Beam splitter LASER Wavelength Radiation & amplitude photodiode pressure fluctuations "Shot" noise Quantum Noise 9
Global network of interferometers GEO VIRGO LIGO 600m 3 km TAMA 4 km & 2 km 300m AIGO- R&D facility • Simultaneous detection • Detection confidence • Source polarization LIGO • Sky location 4 km • Duty cycle • Verify light speed propagation • Waveform extraction 10
Event Localization With An Array of GW Interferometers SOURCE SOURCE GEO TAMA VIRGO LIGO Hanford LIGO Livingston SOURCE SOURCE cos θ = δ t / (c D 12 ) Δθ ~ 0.5 deg Δ L = δ t/c θ D 1 2 11
Frequency range of GW Astronomy Electromagnetic waves Audio band � over ~16 orders of magnitude � Ultra Low Frequency radio waves to high energy gamma rays Gravitational waves � over ~8 orders of magnitude � Terrestrial + space detectors Space Terrestrial 12
The Laser Interferometer Space Antenna LISA Three spacecraft in orbit The center of the triangle formation about the sun, will be in the ecliptic plane with 5 million km baseline 1 AU from the Sun and 20 degrees behind the Earth. LISA (NASA/JPL, ESA) may fly in the next 10 years! 13
Cryogenic Resonant detectors Explorer (at CERN) Nautilus (at Frascati) Univ. of ROME ROG group Univ. of ROME ROG group sensitivity: h rms ~ 10 -19 ; excellent duty cycle ALLEGRO, LSU (Baton Rouge) AURIGA, LNL (Padova) 14
LIGO: Laser Interferometer Gravitational-wave Observatory LHO MIT Hanford, WA 4 km (H1) Caltech + 2 km (H2) LLO 4 km L1 Livingston, LA 15
16 LIGO Scientific Collaboration
17
Despite a few difficulties, science runs started in 2002. 18
Science Runs Binary neutron star A Measure of Inspiral Range Progress: Milky Way 4/02: E8 ~ 5 kpc Andromeda 10/02: S1 ~ 100 kpc 4/03: S2 ~ 0.9Mpc 11:03: S3 ~ 3 Mpc Virgo Cluster Design~ 18 Mpc 19
Best Performance to Date …. The design sensitivity predicted in the 1995 LIGO Science Requirements Document was reached (from 60 Hz up) in 2005 h ~ 2 × 10 -23 / √ Hz Δ x ~ 8 × 10 -20 m/ √ Hz 20
LIGO → eLIGO → AdvLIGO S5 S5 Begin S6 End S6 Start End Enhanced LIGO decomm. LIGO Begin AdvLIGO 2006 2007 2008 2009 2010 Observations! 2008 2009 2010 2011 2012…2014 AdvLIGO AdvLIGO Const. begins Installation NOW Improve amplitude sensitivity by a factor of 10x, and… ⇒ Number of sources goes up 1000x! 21
What will we see? GWs from the most energetic processes in the universe! • Compact Binary Coalescences: black holes orbiting each other and then merging together • GW bursts of unknown waveform: Supernovas, SGRs, GRB engines • Continuous waves from pulsars, Analog from cosmic microwave background -- rapidly spinning neutron stars WMAP 2003 • Stochastic GW background from vibrations from the Big Bang 22
Frequency-Time Characteristics of GW Sources time Ringdowns Broadband Background frequency Bursts CW (quasi-periodic) Chirps Earth’s orbit Earth’s rotation frequency δ f frequency − 4 f ≈ 2.6 × 10 δ f − 6 f ≈ 4 × 10 time time
GWs from coalescing compact binaries (NS/NS, BH/BH, NS/BH) Compact binary mergers K. Thorne 24
Understanding Inspiral-Merger-Ringdown � The key to optimal detection is a well-modeled waveform, especially the phase evolution � Low-mass systems (BNS) merge above ~1500 Hz, where LIGO noise is high - we see the inspiral � Higher-mass systems (BBH) merge or ring down in-band. These systems are unique: highly relativistic, � dynamical, strong-field gravity – exactly where Einstein’s equations are most non-linear, intractable, interesting, and poorly-tested. � Numerical relativity is devoted to deriving waveforms for such systems, to aid in detection and to test our understanding of strong-field gravity. � HUGE progress in the last few years! 25
Mass space for template-based search • The more massive the Black hole system, the lower the ringdown GW frequency templates • Binary neutron star (BNS) waveforms are in LIGO band during inspiral. • Higher-mass Binary black Inspiral-merger-ringdown hole (BBH) waveforms “EOBNR” templates merge in-band “SPA” PN templates •Above ~100 M sun , all LIGO can see is the merger and ringdown 26
Horizon distance is a strong function of mass Horizon distance (Mpc) Horizon distance (Mpc) versus mass (M sun ) versus mass (M sun ) for ringdowns Inspiral-Merger-Ringdown iLIGO ⇒ eLIGO ⇒ aLIGO Initial LIGO 27
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