Gravitational Wave Sources
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Gravitational waves are produced when there is a change in the curvature of spacetime. Since the shape of spacetime depends only on how mass is distributed, events that change the distribution of mass cause gravitational waves. It takes events with a lot of energy to make gravitational waves that we can detect because spacetime is not very elastic. Remember the bowling ball analogy? Space-time is like a stiff trampoline, one that only sinks when you put something very heavy on it.
Like ripples on a pond, gravitational waves lose strength as they move farther from where they started. This is why they are so difficult to detect on earth – we need heavy objects moving at speeds near the speed of light to create gravitational waves large enough to detect, but all of those objects are also far from the earth. The gravitational waves that scientists think we can detect with LIGO and GEO 600 are from things like binary neutron stars, supernovae, and colliding black holes.
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When two massive stars or black holes orbit each other, they form a binary system. The two objects gradually spiral inward and lose energy in the form of gravitational waves. As the objects get closer together, the gravitational waves they emit get stronger. When the two objects collide, they create an intense gravitational wave signal. Binary systems that emit gravitational waves can be made of stars, black holes, or a combination of the two. Left: The Butterfly Nebula, binary stars at center. Image courtesy of NASA. |
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A supernova is a violent explosion that happens to the most massive stars. When a heavy star has burned up all of its fuel, it collapses and the outer layers shoot off into space. If the collapse if not perfectly spherical, the supernova will give off an intense burst of gravitational waves.
Left: Three Great Eyes on Kepler's Supernova Remnant. Image courtesy of NASA. |
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A supernova can leave behind a dense, rapidly spinning core made almost completely of neutrons, called a neutron star. A neutron star that is not perfectly spherical and rotates rapidly will cause gravitational waves. Some neutron stars become pulsars, stars that send off pulses of radio waves. These can also cause gravitational waves. These waves, unlike those from other sources, are emitted steadily year after year. This lets the Einstein@Home project search for them even though they are weaker than those from binary stars and black holes. Left: Hubble sees a neutron star all alone in space. Image courtesy of NASA. |
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If a core continues collapsing beyond the neutron star stage, it may become a black hole. In this case, the gravitational attraction of the core is so strong that nothing can escape; the only information black holes emit is in the form of gravitational waves. Left: Dust disk around a black hole in galaxy NGC 4261. Courtesy NASA. |
Cosmic Gravitational Wave Background
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Scientists also hope to detect gravitational waves left over from the beginning of the universe. This requires multiple detectors because these waves are even weaker than the waves from binary stars and neutron stars. Also, these waves don't come from a single direction but are spread out all over the sky, much like the all-sky image of the cosmic microwave background at left. Left: Even distribution of the cosmic microwave background radiation left over from the beginning of the universe. Image curtesy of WMAP Science Team. |
