A gravitational wave signal with increasing frequency and amplitude, characteristic of inspiraling compact objects.A chirp is the characteristic signal produced when two compact objects — such as black holes or neutron stars — spiral toward each other and merge. As they orbit, they radiate gravitational waves, which carry away energy and angular momentum. Losing energy forces them closer together, which makes them orbit faster, which makes them radiate even more strongly. The result is a runaway: both the frequency and the amplitude of the emitted waves rise rapidly, sweeping upward like the sound of a bird's chirp. The signal ends abruptly at the merger, sometimes followed by a brief ringdown as the final object settles into a stable shape.
The chirp matters because it encodes precise information about the source. The rate at which the frequency climbs depends on the masses of the two objects — heavier pairs merge at lower frequencies and chirp more briefly. The overall amplitude tells us how far away the event occurred. By matching the observed waveform against theoretical templates, researchers can weigh the objects, locate them on the sky, and test whether gravity behaves as general relativity predicts.
The first direct detection of gravitational waves, in 2015, was exactly such a chirp. Since then, chirps have become the standard signature that observatories look for, and their shape has turned gravitational-wave astronomy into a quantitative science rather than a search for a single event.
The chirp waveform was calculated from general relativity well before it was observed; the 2015 detection by LIGO of two merging black holes matched the predicted form and was reported in 2016.
Two compact bodies in a tight orbit distort spacetime in a pattern that rotates with them. These ripples travel outward at the speed of light. Because the orbit shrinks as energy is carried away, the orbital period shortens, so the ripples arrive at a detector more and more frequently. A detector like LIGO measures the stretching and squeezing of space by monitoring how laser light travels along long perpendicular arms; as a chirp passes, the arm lengths oscillate at the rising frequency.
The waveform's shape — how fast the frequency and amplitude increase — is set by the masses and spins of the objects. Comparing the measured strain with predicted templates reveals those parameters. The signal is faint by the time it reaches Earth, so detectors must be extraordinarily sensitive and shielded from seismic and other noise.
Chirp signals are the primary target of ground-based gravitational-wave observatories such as LIGO and Virgo, used to measure black hole and neutron star masses and to test general relativity.
The first detected chirp, from two black holes about 1.3 billion light-years away, lasted only about two-tenths of a second in the detectors' sensitive band.
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