2.2 Sources of Gravitational Waves
According to theory, any non-spherical accelerating matter produces gravitational waves.
- Einstein realized that most ordinary (human-scale) mass produces spacetime disturbances that are immeasurably small.
- However, cataclysmic astrophysical events involving large accelerating masses can produce detectable gravitational waves.
The early universe itself is an important source of gravitational waves.
- In the early universe all matter was located in a relatively small volume, but it was not completely homogeneous. Quantum fluctuations meant that matter was moving and therefore creating gravitational waves in the young, expanding spacetime.
- These early gravitational waves are expected to produce a constant signal, like a rumble. They are a relic of the early universe, like the cosmic microwave background, though the gravitational wave background is much older.
- The most promising way to detect the gravitational wave background is to look for their “imprint” on the polarization of the cosmic microwave background, but the search has not yet been successful.
Some stars explode at the ends of their lives in a type of event called a supernova.
- Supernovas happen about once every 100 years in the Milky Way galaxy.
- There are different kinds of supernovas. Some are caused by core collapse, an event in which a star’s gravity wins out over the nuclear reactions inside it. This causes the star to contract and then bounce back, creating the characteristic explosion.
- If that explosion is not perfectly symmetrical it will produce gravitational waves. Those waves will be brief, difficult-to-detect blips. Supernovas are symmetrical to about 1 part in 1000.
- Core collapse supernovas can sometimes leave behind neutron stars or, if there is too much compression, black holes. Both are potential sources of gravitational waves themselves.
Neutron stars are the most compact stars known.
- Neutron stars are made of subatomic particles and are extremely dense. The density of a neutron star is comparable to compressing the mass of the Sun to the radius of Manhattan.
- Some neutron stars, called pulsars, emit constant electromagnetic beams. These beams are usually on a different axis from the star’s axis of rotation, causing the beams to sweep through space as the star rotates. This causes the radio beams to appear as regular radio blips or flashes to observers on Earth. The regularity of these radio blips can be as precise as atomic clocks.
- These stars are the smoothest known objects in the universe, but even they are not perfectly spherical. According to theory, this means they will produce gravitational waves.
- Gravitational waves produced by neutron stars would have constant amplitude and frequency, like a sine wave. This frequency is related to the rotational frequency of the star.
Neutron stars and black holes are often found in binary systems.
- About 5% of all stars are in binary systems. Our galaxy contains about 100 million neutron stars, so many millions of them should be found in binary systems.
- Binary systems are often composed of pairs of neutron stars, black holes, or a neutron star and a black hole.
- Remember from earlier that such systems will emit gravitational waves and lose energy as the stars orbit each other, causing them to get closer together.
- As the stars approach each other the frequency and amplitude of their gravitational waves will increase until the stars collide or merge. The resultant gravitational signal is like a bird’s chirp, rising in pitch as it rises in amplitude.
- Binary systems provide the best evidence that gravitational waves exist because they have been observed getting closer together over time, as expected if they are actually radiating energy as gravitational waves. Joseph Taylor and Russell Hulse won the Nobel Prize in Physics in 1993 for their observations of the decay of a binary system of neutron stars.