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Gravitational Waves

Gravitational Waves. ASTR 3010 Lecture 24. Propagation Speed of Information. EM waves : this should be propagated at c Object with mass : gravity  curvature in the space-time fabric change at A  how fast the change would be felt by B. B. A. V prop = ∞ Newtonian

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Gravitational Waves

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  1. Gravitational Waves ASTR 3010 Lecture 24

  2. Propagation Speed of Information • EM waves : this should be propagated at c • Object with mass : gravity  curvature in the space-time fabric • change at A  how fast the change would be felt by B B A Vprop= ∞ Newtonian Vprop= c General Relativity

  3. Gravitational Waves • Another confirmation of General Relativity • Hulse-Taylor Binary • PSR J1915+1606 • Binary pulsars • 1.4 Msun • 7.75 hours of orbit (1.1-4.8 Rsun) • 59 milli-sec of rotation • 1993 Nobel Physics Prize • Gravitational radiation takes away energy from the system • Orbital period decreases 76.5μsec / year (-3.5 m/year)

  4. Gravitational Waves versus EM Waves • Gravity is a weak force, but has only one sign of charge. Electromagnetism is much stronger, but comes in two opposing signs of charge. • Significant gravitational fields are generated by accumulating bulk concentrations of matter. Electromagnetic fields are generated by slight imbalances caused by small (often microscopic) separations of charge. • Gravitational waves, similarly, are generated by the bulk motion of large masses, and will have wavelengths much longer than the objects themselves. Electromagnetic waves, meanwhile, are typically generated by small movements of charge pairs within objects, and have wavelengths much smaller than the objects themselves. • Gravitational waves are weakly interacting, making them extraordinarily difficult to detect; at the same time, they can travel unhindered through intervening matter of any density or composition. Electromagnetic waves are strongly interacting with normal matter, making them easy to detect; but they are readily absorbed or scattered by intervening matter.

  5. Importance of Gravitational Waves • It can probe more distant (E~1/r) and bizarre places • Near to the black hole • Before the epoch of recombination of Universe : Universe was opaque to EM • t~300,000 years after Big Bang • z~1,100 WMAP cosmic microwave background map

  6. Gravitational Waves • Monopole : An object's gravitational monopole is just the total amount of its mass  conservation of mass (can’t radiate!) • Dipole : An object's gravitational dipole is a measure of how much that mass is distributed away from some center in some direction  conservation of momentum (can’t radiate!) • Quadrupole : The quadrupole represents how stretched-out along some axis the mass is. A sphere has zero quadrupole. • Power radiated by two orbiting bodies

  7. Gravitational Waves • Two objects orbiting each other in a Keplerianplanar orbit (basically, as a planet would orbit the Sun) will radiate. • A spinning non-axisymmetric planetoid — say with a large bump or dimple on the equator — will radiate. • A supernova will radiate except in the unlikely event that the explosion is perfectly symmetric. • An isolated non-spinning solid object moving at a constant speed will not radiate. This can be regarded as a consequence of the principle of conservation of linear momentum. • A spinning disk will not radiate. This can be regarded as a consequence of the principle of conservation of angular momentum. • A spherically pulsating spherical star (non-zero monopole moment or mass, but zero quadrupole moment) will not radiate

  8. Propagation of GW • Fluctuations in the X-Y plane, prop direction in +z • Amplitude of waves : h (“strain”) • For Earth-Sun, R~1Ly •  h~10-26

  9. Detecting Methods : resonant bar • Resonant Bars (“Weber Bar”) • Aluminum cylinder 2m x 1m • Resonant frequency of 1660Hz • Piezoelectric sensors to detect changes in length as small as 10-7 nano meter • Claimed to detect GW from SN1987A, but…

  10. Resonant Bar AURIGA gravity wave detector consists of a 3 m long one-metric-ton aluminum bar that is in thermal contact with a liquid helium reservoir.

  11. Detecting Methods : interferometers LIGO (Laser Interferometer Gravitational Wave Observatory) • Two 4km arms with Fabry-Perot cavities • Can detect GW as small as

  12. Mirror • LIGO (USA, Louisiana & Washington) • VIRGO (ITALY, Pisa) • TAMA (JAPAN) • GEO 600 (GERMANy, Postdam) • LISA (NASA-ESA, In space, 2016) Interferometers 4 km Vacuum Pipes Mirror Range of sensitivity on earth 10-1000 Hz In space 10^-4-1 Hz Laser 10 Watts Semi-transparent Mirror Photodetector

  13. Detecting Methods : Pulsar timing array Due to the passing GW, timings of pulsars change. • If changes are coherent across a large sky ares GW!

  14. The network of gravitational wave detectors LIGO/VIRGO/GEO/TAMA LISA ground based laser interferometers space-based laser interferometer (hopefully with get funded for a 20?? Launch) LIGO Hanford LIGO Livingston ALLEGRO/NAUTILUS/AURIGA/… Pulsar timing network, CMB anisotropy resonant bar detectors Segment of the CMB from WMAP AURIGA The Crab nebula … a supernovae remnant harboring a pulsar ALLEGRO

  15. Detection Sensitivity of LIGO

  16. Various noise sources

  17. Astronomical Sources of GW

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