High energy Astrophysics Mat Page Mullard Space Science Lab, UCL 13. Gravitational waves

12. Gravitational Waves This lecture: What are they? How are they made? Detection: indirect methods Detection: direct methods Gravitational wave astronomy Slide 2

Waves in the gravitational field We are used to tides on the earth. –differential gravitational forces due to the difference in distance from the moon to the near and far parts of the Earth. Imagine if the sun was a binary star rotating rapidly and you have on the earth a test particle that samples the gravitational field –It would oscillate due to the changing gravity –We can see that an oscillating body will produce a gravity wave, somewhat analogous to the way an oscillating charge produces an electromagnetic wave What are gravitational waves? Slide 3

Mass behaves different to charge. Momentum is conserved –Proper treatment of gravitational radiation is hideous – whole chapter of Misner, Thorne and Wheeler. –Whenever mass is accelerated one way, other masses experience equal and opposite change in momentum in the other direction –Radiation does not depend on the dipole moment, so it doesn’t just move things up and down. But gravitational waves are a little different Slide 4

Why are gravitational waves important? Predicted by Einstein’s general relativity –Important, testable prediction of the theory. Has important astrophysical consequences: –Can be a significant energy loss mechanism –Can cause compact orbits to decay Completely different means of astronomical observation –Potentially even more revolutionary than radio astronomy or X-ray astronomy! Before we get any further: Slide 5

Displacement of test particles by a gravity wave Slide 6

Just like electromagnetic waves, gravitational radiation is able to cause acceleration of external systems and so it carries energy. If the separation between orbiting systems is L, power emitted can be written: Energy of a gravity wave P GW ~ c5Gc5G GM c 2 L () 5 Slide 7

Now there is a maximum: L cannot be smaller than half the Schwarzschild radius So no pair of orbiting bodies can emit more than 4x10 52 W regardless of their mass For the Earth around the Sun, about 200W are emitted – so our orbit should last another years before it decays by gravitational radiation When dimensions of orbit approach the Schwarzschild radius, gravitational radiation is pretty serious. Slide 8

Very compact binary systems lose energy rapidly by gravitational radiation. If short  -ray bursts are caused by the coalescence of neutron stars or black holes, there should be a strong gravitational wave signal as the orbit rapidly decays. So: some obvious predicted sources and consequences of gravitational waves: Slide 9

Indirect detection Gravitational waves remove energy from an orbiting system. If we find a system with well defined orbital parameters that imply it should be radiating gravitational radiation –We can predict how fast the orbit should be slowing due to gravitational radiation –Measure and compare Slide 10

Binary pulsar Binary system of two orbiting neutron stars, one of which a pulsar, discovered by Joseph Taylor and Russel Hulse in –Orbital period 7.8 hours. –Pulse period 0.059s Pulses combined with general relativity allow very accurate determination of the binary parameters. Orbital period change prediction in agreement with measurement of rate of period change = x Secured 1993 Nobel prize! Slide 11

Ultracompact binaries The pulsar in the binary pulsar enabled accurate determination of the orbital parameters, but this is not the most extreme binary system in the galaxy. There are now (at least) 3 ultracompact binaries known, in which both stars are white dwarfs in which the orbital period is less than 10 minutes! All 3 were discovered as X-ray sources within the last 15 years. Two of these were discovered by Gavin Ramsay of MSSL and colleagues. They will make excellent test cases for gravitational radiation. Slide 12

Direct detection The principal observable effect of gravitational waves is the slight fractional distortion in the distance between two points. Called ‘gravitational strain’. For reference, strain expected from a maximally emitting source 1kpc distant, the 1kHz strain is only More realistic supernova producing a neutron star a Mpc away may produce a strain more like –Absurdly small – atomic nucleus added to a 1 AU baseline Two types of detectors: –Resonant bars –Laser interferometers Slide 13

Resonant bars Pioneered by J. Weber. Gravitational strain will make bars resonate, - like making a bell ring (but very small amplitude) Weber published detection of bursts of gravitational radiation – about 3 per day(!) using aluminium cylinders about 2m long and resonant frequency 1.6 kHz Levine and Garwin (1973) failed to detect any gravity waves with similar apparatus – Weber was not detecting gravity waves. Gravity waves have not been detected with resonant bars… Slide 14

Laser interferometers New gravitational wave experiments based on laser interferometers. Laser light is transmitted along 2 perpendicular paths, reflected at each end and recombined in the centre to produce interference fringes. Movement of the fringes indicates relative movement of the two arms. Slide 15

Geo 600 (600m German/UK) in Hannover Slide 16

Laser interferometer gravitational- wave observatory (LIGO) Slide 17

LIGO Hanford interferometer Slide 18

LIGO Two interferometers, Livingstone and Hanford in US. More perhaps to be added. The arms are 4km long. The whole light path is in vacuum (all 4km x2 at each station!) Two stations so that terrestrial interference such as small scale seismic activity is not confused with gravitational waves Slide 19

Schematic of LIGO Slide 20

One arm of the Livingstone interferometer Slide 21

The interferometer station at Livingstone Slide 22

So do we expect them to detect gravitational waves? Yes! But maybe not as soon as the gravitational wave people always claim! LIGO will only be sensitive to high- frequency gravitational waves. For slower gravitational waves (e.g. from supermassive black holes merging) the only option is space! Slide 23

Laser interferometer space antenna (LISA) Recently selected to be an ESA mission launching By going into space can get much longer interferometer Slide 24

LISA formation Slide 25

We can really do gravitational wave astronomy with LISA! Slide 26

Some key points: Gravitational radiation is a phenomena predicted from general relativity. (sort of) the gravitational equivalent of electromagnetic radiation. Causes small oscillations in the fractional displacement between points in space. Important energy loss mechanism in very close binaries. Detected indirectly in the binary pulsar Laser interferometers should be capable of detecting astronomical sources by gravitational radiation Could be truly revolutionary new astronomy Slide 27