When two black holes merge, the resulting object rings like a struck bell, radiating gravitational waves that fade in a distinctive pattern called ringdown. According to general relativity, this pattern should depend only on the merged black hole’s mass and spin. But if a black hole carries extra structure — often called ‘hair’ — from surrounding dark matter, exotic fields, or modified gravity, the hair should leave its own fingerprint on the signal. Physicists from Nagoya University and Kindai University have now worked out a general method for predicting this fingerprint.
Palomino Ylla et al. found a way to check for black hole hair using changes in ringdown waves. Image credit: Nagoya University.
“Black hole hair may represent matter surrounding the black hole, or deviations from the simplest kind of black hole predicted by general relativity,” said Nagoya University Ph.D. student Ariadna Uxue Palomino Ylla, lead author of the study.
“Because these may slightly change the ringdown signal, detecting or ruling out these changes could give us a new way to test gravity in this extreme region.”
In the study, the authors modeled black-hole hair as a thin anisotropic fluid layered onto ordinary Schwarzschild and Kerr black holes.
They then used the well-established link between a black hole’s ringdown and the orbits of light bent around it to calculate how the hair would shift the wave’s oscillation frequency and its damping rate.
They found that these two properties don’t shift by the same amount.
The gap between them is set by the local pressure of the hidden matter at the photon orbit, meaning the pattern of a ringdown’s frequency versus its fade-out speed could reveal not just that hair is present, but something about what it’s made of.
“The ringdown waves may not only show that something extra is affecting the black hole; the way the signal changes could also give us clues about what this hidden matter is actually like,” Palomino Ylla said.
The method was tested on three known theoretical black holes and extended to spinning black holes, where light orbiting with or against the spin responds differently to the hidden matter.
“Rotation makes calculations more complex because light circling with the black hole’s spin would behave differently from light circling against it,” the researchers said.
“Hidden matter would also change the ringdown’s frequency and fade-out speed differently, depending on the spin direction. The exact pattern depends on the type of hidden matter involved.”
“Instead of studying each possible type of black hole hair from scratch, the new method gives us a common way to predict how extra matter or new physics could change a black hole’s ringdown.”
“While the results are early estimates, this method helps us know what to look for if we ever spot something strange in a real black hole’s signal.”
“In the future, this approach may help us use these waves to learn about a black hole’s size, spin, and any black hole hair nearby.”
The team’s work appears in the Journal of Cosmology and Astroparticle Physics.
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Ariadna Uxue Palomino Ylla et al. 2026. Ringdown waves from hairy black holes. JCAP 09: 046; doi: 10.1088/1475-7516/2026/09/046
