Sources and Types of Gravitational Waves
Every massive object that accelerates produces gravitational waves. This includes humans, cars, airplanes etc., but the masses and accelerations of objects on Earth are far too small to make gravitational waves big enough to detect with our instruments. To find big enough gravitational waves, we have to look far outside of our own solar system.
It turns out that the Universe is filled with incredibly massive objects that undergo rapid accelerations that by their nature, generate gravitational waves that we can actually detect. Examples of such things are orbiting pairs of black holes and neutron stars, or massive stars blowing up at the ends of their lives. LIGO scientists have defined four categories of gravitational waves based on what generates them: Continuous, Compact Binary Inspiral, Stochastic, and Burst. Each category of objects generates a unique or characteristic set of signal that LIGO's interferometers can sense, and that researchers can look for in LIGO’s data. Read on to learn more about the different objects and events that LIGO is looking for.
Continuous Gravitational Waves
Continuous gravitational waves are thought to be produced by a single spinning massive object like a neutron star. Any bumps on or imperfections in the spherical shape of this star will generate gravitational waves as it spins. If the spin-rate of the star stays constant, so too are the gravitational waves it emits. That is, the gravitational wave is continuously the same frequency and amplitude (like a singer holding a single note). That's why these are called “Continuous Gravitational Waves”. Researchers have created simulations of what an arriving continuous gravitational wave would sound like if the signal LIGO detected was converted into a sound. Click on "Continuous Gravitational Wave Signal" below to hear what the gravitational waves from a spinning neutron star would "sound" like to LIGO.
(Credit: SXS Collaboration, http://www.black-holes.org)
Compact Binary Inspiral Gravitational Waves
The next class of gravitational waves LIGO is hunting for is Compact Binary Inspiral gravitational waves. So far, all of the objects LIGO has detected fall into this category. Compact binary inspiral gravitational waves are produced by orbiting pairs of massive and dense ("compact") objects like white dwarf stars, black holes, and neutron stars. There are three subclasses of "compact binary" systems in this category of gravitational-wave generators:
- Binary Neutron Star (BNS)
- Binary Black Hole (BBH)
- Neutron Star-Black Hole Binary (NSBH)
Each binary pair creates a unique pattern of gravitational waves, but the mechanism of wave-generation is the same across all three. It is called "inspiral".
Inspiral occurs over millions of years as pairs of dense compact objects revolve around each other. As they orbit, they emit gravitational waves that carry away some of the system's orbital energy. As a result, over eons, the objects orbit closer and closer together. Unfortunately, moving closer causes them to orbit each other faster, which causes them to emit stronger gravitational waves, which causes them to lose more orbital energy, inch ever closer, orbit faster, lose more energy, move closer, orbit faster... etc. The objects are doomed, inescapably locked in a runaway accelerating spiraling embrace.
This accelerating spin process is analogous to a spinning figure skater. Imagine that the skater's outstretched fists are neutron stars or black holes, and the skater's body is the force of gravity binding them together. As the spinning skater pulls their fists in toward their body (i.e., as the objects orbit closer and closer), they spin faster and faster. Unlike the skater, however, the pairs of neutron stars or black holes cannot halt their rotation. The process of emitting gravitational waves and orbiting closer and closer sets off an unstoppable sequence of events that can only end with the two objects colliding.
LIGO's instruments are designed to detect a specified range of frequencies of gravitational waves, just as human ears are sensitive to a certain range of sound frequencies. This means that LIGO cannot detect objects orbiting at rates that fall outside of this range of frequencies (either too low or too high). However, as the orbiting objects move closer together, they orbit faster and faster, which means eventually, the objects will begin orbiting each other fast enough that the gravitational waves they emit fall within our sensitive range. But the time they spend orbiting in that range of frequencies is typically very brief.
The masses of the objects involved dictate how long they emit detectable gravitational waves. Heavy objects, like black holes, move through their final inspiral phase much more rapidly than 'lighter' objects, like neutron stars. This means that black-hole merger signals are much shorter in LIGO than neutron star merger signals, and the differences are quite striking. For example, the first pair of merging black holes that LIGO detected produced a signal just two-tenths of a second long. In contrast, the first neutron star merger LIGO detected in August 2017 generated a signal over 100 seconds long in our instruments.
LIGO can convert its space-time distortion signals into an audible sound called a "chirp" so we can all, in a sense, 'hear' the final moments of the lives of two black holes and two neutron stars. These objects had been orbiting each other for billions of years; LIGO captures the last fraction of a second or few seconds of that lifetime together. The first video below shows the evolution of the signal in the instrument along with the chirp of our first black hole merger detection (the signal is played several times, repeating the chirp first in its natural frequency--the low 'thump'--and then increased to make it easier to hear). The second video is the chirp of the 2017 neutron star merger (only the last 32 seconds of the signal are included in the video).
These two examples of actual gravitational waves illustrate how different systems of merging objects display unique signatures in the interferometers. Shorter signals mean more massive objects, like black holes, were involved; longer signals suggest lower-mass objects, like neutron stars.
To date, LIGO has published the detection of gravitational waves generated by 10 pairs of merging black holes and two pairs of colliding neutron stars. Since April 1, 2019, dozens more detections have been made, keeping LIGO scientists busy analyzing the data to understand the true natures of those detections.
LIGO's enormous success belies the fact that its instruments must strain to sense anything over the constant din generated by everything on Earth from internal fluctuations of the laser beam itself, to traffic on nearby roads, to weather, and to earthquakes happening all over the world. For a more realistic sense of what LIGO contends with in its search for gravitational waves, click on the link below to hear how a simulated neutron star merger would sound like buried in all that noise. You'll have to listen very carefully to hear the chirp:
(Credit: SXS Collaboration, http://www.black-holes.org)
Stochastic Gravitational Waves
Astronomers predict that there are so few significant sources of continuous or binary inspiral gravitational waves in the Universe that LIGO doesn't worry about the possibility of more than one passing by Earth at the same time (producing confusing signals in the detectors). However, we do presume that many small gravitational waves are passing by from all over the Universe all the time, and that they are mixed together at random. These small waves from every direction make up what is called a “Stochastic Signal”, so called because the word, 'stochastic' means having a random pattern that may be analyzed statistically but not predicted precisely. These will be the smallest and most difficult gravitational waves to detect, but it is possible that at least part of this stochastic signal may originate from the Big Bang. Detecting relic gravitational waves from the Big Bang will allow us to see farther back into the history of the Universe than ever before. Click on the image at left to hear a simulated "Stochastic Signal" (Audio simulation credit: SXS Collaboration, http://www.black-holes.org).
Burst Gravitational Waves
The search for 'burst gravitational waves' is truly a search for the unexpected—both because LIGO has yet to detect them, and because there are still so many unknowns that we really don’t know what to expect! For example, sometimes we don’t know enough about the physics of a system to predict how gravitational waves from that source will appear.
We also expect to detect gravitational waves from systems we never knew about before. To search for these kinds of gravitational waves, we cannot assume that they will have well-defined properties like those of continuous and compact binary inspiral waves. This means we cannot restrict our analyses to searching only for the signatures of gravitational waves that scientists have predicted.
Searching for burst gravitational waves requires being utterly open-minded. For these kinds of gravitational waves, scientists must recognize a pattern of signals even when such a pattern has not been modeled (what we think a signal may look like) before. If you don’t know what you’re looking for, it’s really hard to find it. While this makes searching for burst gravitational waves difficult, detecting them has the greatest potential to reveal revolutionary information about the Universe.