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Transcript
More tests of GR & Gravitational Radiation Finish discussion of solar-system tests Precession of orbit of Mercury Rotation and dragging of inertial frames Gravity Probe B Strong gravity & Gravitational waves What do we mean by strong gravity? Gravitational radiation The Hulse-Taylor binary pulsar Prospects for the direct detection of gravitational waves III.9 : Precession of Mercury’s orbit Mercury orbit about Sun is rather elliptical (e=0.206) Orbit precesses around Sun by 5600 arcsec/century… most of this is due to perturbations from other planets 43 arcsec/century unaccounted for by Newtonian effects of known planets This was a discovered in 1859 and was certainly known to Einstein Various Newtonian effects to explain this (planet Vulcan, ring of planetoids, breakdown of inversesquare law) all unsuccessful GR provided very natural explanation for precisely this difference. QuickTime™ and a TIFF (Uncompressed) decompressor are needed to see this picture. Precession of Mercury’s orbit is sensitive to two of the post-Newtonian parameters, and GR has =1 and =1 Solar oblateness effect very small. From radar measurements of Mercury’s precession (anomalous precession known to 0.1% accuracy), we have III.10 : Geodetic and Lens-Thirring precession Two effects predicted by GR… Curvature of spacetime causes a moving gyroscope to precess relative to “fixed stars” (Geodetic precession) The rotation of a massive object causes inertial frames to be “dragged around” with it (Lens-Thirring precession) In terms of post-Newtonian parameters… This effect has not yet been directly detected in a “clean” experiment… NASA’s Gravity Probe B Contains very accurate gyroscopes Attempt to detect precession of gyroscopes relative to the stars (observed with telescope) Launched 20th April 2004; currently flying and collecting data QuickTime™ and a Sorenson Video 3 decompressor are needed to see this picture. QuickTime™ and a Sorenson Video decompressor are needed to see this picture. III.11 : On to strong gravity… A situation involves “strong” gravitational fields when the simple post-Newtonian expansion is no longer appropriate Means that at least one of the following is true… The system contains objects bound together by strong gravitational potentials The system contains relativistic orbits (GM/R~v2~c2) Gravitational waves are important to the evolution of the system III.12 : Gravitational waves GR predicts existence of propagating waves of spacetime curvature travelling at speed of light Emitted by any (nonspherical) changing distribution of masses Most important example Two orbiting objects Gravitational waves carry energy and angular momentum away from orbit… causes orbit to decay Hulse-Taylor binary pulsar PSR 1913+16 59ms pulsar in eccentric orbit around another neutron star (7.75 hour period) Can determine orbits accurately by timing pulsar… Find that orbit is decaying; period is decreasing Fits prediction of GR perfectly… GWs are carrying away orbital energy Only unambiguous detection of the effects of gravitational radiation Direct detection of gravitational waves… Numerous experiments currently being built to detect gravitational waves All based on detecting characteristic changes in the separation of a several masses as a GW passes by Measure the strength of a gravitational wave via the “strain” h=x/x. Known sources produce GWs with small strains as measured at Earth… h<10-21 Most modern attempts based on laser interferometry QuickTime™ and a TIFF (Uncompressed) decompressor are needed to see this picture. Laser Interferometer Gravitational-wave Observatory (LIGO) 4km, 2 armed interferometer YUV420 codec decompressor are needed to see this picture. Laser Interferometer Space Antenna (LISA) NASA/ESA 5 million km ; triangular closed loop interfermeter What kinds of objects/events will we see? LIGO - detects high frequency GWs Final stages of merging neutron star binaries Final stages of stellar mass BHs merging with other stellar mass BHs or neutron stars (Possibly) core collapse supernovae LISA - detects lower frequency GWs Merging supermassive black holes Infall of a stellar-mass BH or neutron star into a supermassive black hole Galactic Binary star system (esp. binary white dwarfs) NASA QuickTime™ and a YUV420 codec decompressor are needed to see this picture. Black holes spiraling together because of gravitational waves… [what’s wrong with this movie?] YUV420 codec decompressor are needed to see this picture. Testing strong-field GR with gravitational waves The merger of two black holes is a violent process that emits very strong gravitational waves. Three stages… Inspiral of the two black holes Merging of their event horizons “Ringdown” to a final relaxed black hole Predictions of gravitational wave forms (i.e. amplitude vs time) for black hole merger depend on full solution to Einstein’s equations Very powerful probe of correctness of GR Very difficult calculations! First full-blown calculation of BH-merger was only completed last month (by NASA-Goddard group)! Now just need the data… ! Baker et al. (2006; gr-qc/0602026)