There is no sound in space, but black holes can still sing.
When two black holes collide, their song ripples through the very fabric of existence, creating a thundering chorus of oscillations in spacetime that echo across the universe like the fading gong of a bell. Each cosmic duet is unique, and scientists have been faithfully recording these songs since they first detected gravitational waves in 2015. Now researchers think they can hear a hidden melody within the music: a newly predicted type of gravitational wave signal known as a direct wave.
What makes direct waves so fascinating is their origin. All of the gravitational wave signals scientists have seen so far—known as quasinormal modes—are produced after two black holes merge into a single larger one, and the warped spacetime around it settles. Direct waves appear to originate much closer to the new black hole’s event horizon: the point of no return beyond which nothing, not even light, can escape.
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“It’s almost a tug-of-war,” says Katerina Chatziioannou, a physicist at the California Institute of Technology. “You want to get closer to the horizon, but the closer you get, the harder it is to get any information about it.”
Indeed, anything created so close to a black hole’s event horizon seems almost destined to fall victim to its immense gravity. But black hole mergers are also among the most violent events in the cosmos. Their immense gravitational fields churn the surrounding spacetime like a spoon stirring coffee, theoretically allowing some of these signals to escape the cosmic maelstrom.
A new study published in Nature presents the first evidence for such ripples, using data from the clearest gravitational wave signal ever observed: a colossal black hole merger known as GW250114. (The same merger that made waves—pun intended—last year when it offered physicists a rare opportunity to dissect a black hole merger in unprecedented detail. Conclusions from that study include, among other things, that black holes are not only great vocalists but also bald.)
For study co-author Sizheng Ma, who helped develop the theory behind direct waves, GW250114’s timing couldn’t have been better. “Sometimes when you make a prediction, maybe people have to wait many years so that it can be proven,” he says. “Because this event is so loud, it allows us to prove our prediction immediately.”
The reason GW250114’s signal is so “loud” actually has little to do with the collision itself. Similar-strength gravitational waves have been observed before. What’s changed is the instrumentation. “It’s like hearing the same noise when your microphone has lower static,” says Chatziioannou, who was not involved in the study.
Put simply, a decade of technological advances transformed this cosmic duet into a true showstopper.
The musical metaphor is particularly apt because gravitational waves oscillate much like sound waves, allowing researchers to analyze them with many of the same mathematical tools. The collision of two black holes is often likened to the striking of a bell, which is why the fading signal that follows is known as a ringdown. “You can think of gravitational waves as the acoustics of spacetime,” Ma says.
If ordinary ringdown signals are the fading resonance of a bell, direct waves could tell us how the bell was struck in the first place. They may offer physicists a new way to probe some of the most extreme environments in the universe. Yet knowing for sure that astronomers have seen a direct wave is tricky.
“If you could observe this, then you will have a direct measurement of properties of the horizon,” says Emanuele Berti, a professor at Johns Hopkins University who wasn’t involved in the study. “The question is, can we really see this?”
The signal identified in GW250114 matches predictions for a direct wave, an encouraging sign. But matching a prediction is not the same as proving it. Some physicists are skeptical that such waves could escape the intense gravitational environment near a black hole’s event horizon, or that current instruments can reliably separate a direct wave signal from the surrounding noise. “It’s very difficult to observe these things, if they can be observed at all,” Berti says.
Nevertheless, “any observational evidence for black holes is welcome and a breakthrough,” says Vitor Cardoso, director of the Center of Gravity at the Niels Bohr Institute and a distinguished professor at the Instituto Superior Técnico in Lisbon, Portugal.
Physicists are eager to further examine the signal and to look for signs of direct waves hiding underneath previously discovered quasinormal modes.
“I am sure that much follow-up work will take place worldwide, and the approach will spur progress,” says Szabolcs Márka, a professor at Columbia University who was uninvolved with the study. “The more we observe, more confident we will become.”
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