When two black holes merge, the resulting object emits gravitational waves that ring like a bell and disappear in a unique pattern called a ringdown. According to general relativity, this pattern should depend only on the mass and spin of the merging black holes. But if a black hole carries extra structure (often called a “hair”) from surrounding dark matter, exotic fields, or modified gravity, the hair should leave its own fingerprint on the signal. Physicists at Nagoya University and Kindai University have now developed a general method to predict this fingerprint.
Palomino Ira others. We discovered a way to check for black hole hair by using changes in ring-down waves. Image provided by: Nagoya University
“The black hole’s hair may represent a deviation from the matter surrounding the black hole or from the simplest type of black hole predicted by general relativity,” said the Nagoya University professor. student Ariadna Uxue Palomino Ylla, lead author of the study.
“These can change the ringdown signal slightly, so detecting or filtering out these changes could provide new ways to test gravity in this extreme region.”
In the study, the authors modeled the black hole hair as a thin anisotropic fluid superimposed on a regular Schwarzschild black hole and a Kerr black hole.
They then used the established relationship between a black hole’s ringdown and the trajectory of light bending around it to calculate how the hair changes the wave’s vibrational frequency and its rate of attenuation.
They found that these two traits did not shift by the same amount.
The gap between them is set by the local pressure of hidden matter on the photon trajectory. This means that the pattern of the ringdown frequency and its fade-out speed can reveal not only that a hair is present, but also what that hair is made of.
“Ringdown waves not only indicate that something special is affecting the black hole, but the way the signal changes may also give us clues about what this hidden material is actually like,” Palomino Yura said.
The method was tested on three known theoretical black holes and extended to rotating black holes. Light that orbits with or against the rotation reacts differently to hidden matter.
“The rotation makes the calculations more complicated because light orbiting with the black hole behaves differently than light orbiting against the black hole,” the researchers said.
“Depending on the direction of the spin, the hidden matter also changes its ring-down frequency and fade-out rate differently. The exact pattern depends on the type of hidden matter involved.”
“Instead of studying each possible type of black hole hair from scratch, this new method provides a common way to predict how extra matter or new physics will change a black hole’s ringdown.”
“Although the results are early estimates, this method can help us know what to look for if we discover something strange in the signals of real black holes.”
“In the future, this approach could help us use these waves to learn about the black hole’s size, rotation, and the hairs of nearby black holes.”
team’s work will appear in Journal of Cosmology and Astroparticle Physics.
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Ariadna Uxue Palomino Ira others. 2026. Ring-down waves from a hairy black hole. JCAP 09:046;Doi: 10.1088/1475-7516/2026/09/046
Source: www.sci.news
