When two black holes crash into each other, the event is one of the most violent and powerful collisions in the universe.
The two objects spiral together, merge into a larger black hole, and release enormous amounts of energy in the form of gravitational waves — tiny ripples that spread through spacetime itself.
But the story does not end with the collision. After the merger, the newly formed black hole continues to vibrate and “ring” for a short time as it settles into a stable shape.
Scientists compare this process to the sound of a bell after it has been struck or the vibration of a guitar string after it is plucked.
Now, researchers at University of Cambridge have developed a new technique that could help scientists better understand these mysterious vibrations and uncover hidden details about black holes.
The study, published in Physical Review Letters, focuses on the special frequencies produced during this “ringdown” phase. These vibrations are called quasinormal modes, and each black hole has its own unique set of frequencies depending on its mass and spin. In a way, these frequencies act like a fingerprint for the black hole.
Scientists are especially interested in these signals because they offer one of the best ways to test Albert Einstein’s theory of general relativity under the most extreme conditions in the universe.
Until now, researchers could usually detect only the strongest vibration from a black hole merger. The weaker signals, known as overtones, fade away very quickly and are much harder to identify in gravitational wave data.
The Cambridge team created a new method to sort through complicated computer simulations of black hole collisions and identify these faint vibrations more accurately. Their approach uses Bayesian analysis, a statistical technique that helps determine the most likely explanation for a set of data.
Lead researcher Richard Dyer said scientists have long debated which vibration modes are truly present in black hole mergers and when they appear. The new method provides a more systematic and reliable way to answer those questions.
The researchers also discovered unusual “nonlinear modes,” which happen when different vibration frequencies interact with each other. They compare these effects to the distorted and layered sounds produced by an electric guitar. These signals are extremely subtle and difficult to separate from background noise.
To test their method, the researchers examined a large collection of detailed simulations of black hole mergers involving different sizes and spinning speeds. They mapped out which vibration modes could be detected in different situations and when they became visible.
The findings could help scientists better understand data from major gravitational wave observatories such as LIGO Scientific Collaboration and Virgo Collaboration. Future detectors with greater sensitivity may be able to capture even more of these hidden vibrations.
By listening more carefully to the “ringing” of black holes, researchers hope to test whether Einstein’s equations truly describe gravity in the most extreme corners of the cosmos.
