Transcript PowerPoint Presentation - ASTR498E High energy

More tests of GR &
Gravitational Radiation
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Finish discussion of solar-system tests
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Precession of orbit of Mercury
Rotation and dragging of inertial frames Gravity Probe B
Strong gravity & Gravitational waves
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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
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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
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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.
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Precession of Mercury’s orbit is sensitive to
two of the post-Newtonian parameters,  and 
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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
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III.10 : Geodetic and
Lens-Thirring precession
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Two effects predicted by GR…
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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)
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In terms of post-Newtonian parameters…
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This effect has not yet been directly detected
in a “clean” experiment…
NASA’s Gravity Probe B
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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
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III.11 : On to strong gravity…
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A situation involves “strong” gravitational
fields when the simple post-Newtonian
expansion is no longer appropriate
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Means that at least one of the following is
true…
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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
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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
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Two orbiting objects
Gravitational waves
carry energy and
angular momentum
away from orbit… causes
orbit to decay
Hulse-Taylor binary pulsar
PSR 1913+16
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59ms pulsar in eccentric
orbit around another
neutron star (7.75 hour
period)
Can determine orbits
accurately by timing
pulsar…
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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…
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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
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Laser Interferometer Gravitational-wave Observatory (LIGO)
4km, 2 armed interferometer
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Laser Interferometer Space Antenna (LISA)
NASA/ESA
5 million km ; triangular closed loop interfermeter
What kinds of objects/events
will we see?
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LIGO - detects high frequency GWs
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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
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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
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Black holes spiraling together because of gravitational
waves… [what’s wrong with this movie?]
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Testing strong-field GR with
gravitational waves
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The merger of two black holes is a violent
process that emits very strong gravitational
waves.
Three stages…
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Inspiral of the two black holes
Merging of their event horizons
“Ringdown” to a final relaxed black hole
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Predictions of gravitational wave forms
(i.e. amplitude vs time) for black hole
merger depend on full solution to
Einstein’s equations
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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)