AJW, Caltech/LIGO, 6/20/02LIGO-G020007-00-RGravitational waves and LIGO§ Brief introduction to LIGO» What is a gravitational wave?» Astrophysical sources» Gravitational wave interferometers» LIGO and its sister projects§ Progress report on Engineering runs§ Data analysis – finding signals in the noiseAlan Weinstein, CaltechAJW, Caltech/LIGO, 6/20/02LIGO-G020007-00-RThe LIGO ProjectLIGO: Laser Interferometer Gravitational-Wave Observatory§ US project to build observatories for gravitational waves (GWs)» ...and laboratory to run them§ to enable an initial detection, then an astronomy of GWs§ collaboration by MIT, Caltech; other institutions participating » (LIGO Scientific Collaboration, LSC)» Funded by the US National Science Foundation (NSF)Observatory characteristics§ Two sites separated by 3000 km§ each site carries 4km vacuum system, infrastructure§ each site capable of multiple interferometers (IFOs)Evolution of interferometers in LIGO§ establishment of a network with other interferometers § A facility for a variety of GW searches§ lifetime of >20 years§ goal: best technology, to achieve fundamental noise limits for terrestrial IFOsAJW, Caltech/LIGO, 6/20/02LIGO-G020007-00-RGravitational WavesStatic gravitational fields are described in General Relativity as a curvature or warpage of space-time, changing the distance between space-time events.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 curvatureShortest straight-line path of a nearby test-mass is a ~Keplerian orbit.AJW, Caltech/LIGO, 6/20/02LIGO-G020007-00-REinstein’s Theory of Gravitationexperimental tests“Einstein Cross”The bending of light raysgravitational lensingQuasar image appears around the central glow formed by nearby galaxy. Such gravitational lensingimages are used to detect a ‘dark matter’ body as the central objectMercury’s orbitperihelion shifts forwardtwice Post-Newton theoryMercury's elliptical path around the Sunshifts slightly with each orbitsuch that its closest point to the Sun (or "perihelion") shifts forwardwith each pass.bending of lightAs it passes in the vicinity of massive objectsFirst observed during the solar eclipse of 1919 by Sir ArthurEddington, when the Sun was silhouetted against the Hyades star clusterAJW, Caltech/LIGO, 6/20/02LIGO-G020007-00-RStrong-field•Most tests of GR focus on small deviations from Newtonian dynamics (post-Newtonian weak-field approximation)•Space-time curvature is a tiny effect everywhere except:ØThe universe in the early moments of the big bangØNear/in the horizon of black holes•This is where GR gets non-linear and interesting!•We aren’t very close to any black holes (fortunately!), and can’t see them with lightBut we can search for (weak-field) gravitational waves as a signal of their presence and dynamicsAJW, Caltech/LIGO, 6/20/02LIGO-G020007-00-RSources of GWs§ Accelerating charge ⇒ electromagnetic radiation (dipole)§ Accelerating mass ⇒ gravitational radiation (quadrupole)§ Amplitude of the gravitational wave (dimensional analysis):§ = second derivative of mass quadrupole moment(non-spherical part of kinetic energy – tumbling dumb-bell)§ G is a small number!§ Need huge mass, relativistic velocities, nearby.§ For a binary neutron star pair,10m light-years away, solar masses moving at 15% of speed of light:rcfGMRhIrcGhorb4222442 πµνµν≈⇒=&&µνI&&Terrestrial sources TOO WEAK!kmAJW, Caltech/LIGO, 6/20/02LIGO-G020007-00-RNature of Gravitational RadiationGeneral Relativity predicts :• transverse space-time distortions, freely propagating at speed of light mass of graviton = 0• Stretches and squashes space between “test masses” – strain •Conservation laws:•cons of energy ⇒ no monopole radiation•cons of momentum ⇒ no dipole radiation•quadrupole wave (spin 2) ⇒ two polarizationsplus (⊕) and cross (⊗)Spin of graviton = 2Contrast with EM dipole radiation:xˆ(( ))))))yˆh = ∆L/LAJW, Caltech/LIGO, 6/20/02LIGO-G020007-00-RContrast EM and GW informationdetectors have large solid angle acceptancedetectors have small solid angle acceptancemeasure amplitudemeasure amplitude (radio) or intensity (light)103Hz and down106Hz and upvery small interaction; no shieldingabsorbed, scattered, dispersed by matterwavelength ~large compared to sources -poor spatial resolutionwavelength small compared to sources -imagescoherent motions of huge masses (or energy)incoherent superpositions of atoms, moleculesSpace-time itselfspace as medium for fieldGWE&M• Very different information, mostly mutually exclusive• Difficult to predict GW sources based on E&M observations• GW astronomy is a totally new and unique window on the universeAJW, Caltech/LIGO, 6/20/02LIGO-G020007-00-RObserving the Galaxy with Different Electromagnetic Wavelengthsλ = 1 x 10 -14 mhttp://cossc.gsfc.nasa.gov/cossc/egret/λ = 6 x 10 -13 mhttp://antwrp.gsfc.nasa.gov/apod/image/comptel_allsky_1to3_big.gifλ = 5 x 10 -10mhttp://antwrp.gsfc.nasa.gov/apod/image/xallsky_rosat_big.gifλ = 5 x 10 -7mhttp://antwrp.gsfc.nasa.gov/apod/image/SagSumMW_dp_big.gifλ = 2 x 10 -6mhttp://www.gsfc.nasa.gov/astro/cobeλ = 9 x 10 -1m//rsd-www. nrl.navy.mil/7213/lazio/GC/AJW, Caltech/LIGO, 6/20/02LIGO-G020007-00-RWhat will we see?A NEW WINDOWON THE UNIVERSEWILL OPEN UP FOR EXPLORATION.BE THERE!AJW, Caltech/LIGO, 6/20/02LIGO-G020007-00-RAstrophysicalSources of Gravitational WavesCoalescing compact binaries (neutron stars, black holes)Non-axi-symmetric supernova collapseNon-axi-symmetric pulsar (rotating, beaming neutron star)AJW, Caltech/LIGO, 6/20/02LIGO-G020007-00-RGWs from coalescing compact binaries (NS/NS, BH/BH, NS/BH)Compact binary mergersAJW, Caltech/LIGO, 6/20/02LIGO-G020007-00-RHulse-Taylor binary pulsarNeutron Binary SystemPSR 1913 + 16 -- Timing of pulsars••17 / sec~ 8 hr• A rapidly spinning pulsar (neutron star beaming EM radiation at us 17 x / sec)• orbiting around an ordinary star with 8 hour period • Only 7 kpc away• discovered in 1975, orbital parameters measured• continuously measured over 25 years!AJW, Caltech/LIGO, 6/20/02LIGO-G020007-00-RGWs from Hulse-Taylor binary§ Only 7 kpc away§ period speeds up 14 sec from 1975-94§ measured to ~50 msec accuracy§ deviation grows quadratically with time§ Merger in about 300M years §