What happens at the edge of a black hole? Astronomers may be about to find out

There is no sound in space, but black holes can still sing.

When two black holes collide, their song reverberates through the very fabric of existence, creating a thunderous chorus of oscillations in space-time that resonates through the universe like the gong of a fading 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 in music: a new type of gravitational wave signal called direct wave.

What makes direct waves so fascinating is their origin. All the gravitational wave signals observed so far by scientists – known as quasi-normal modes – are produced after two black holes merge into one larger one, and the distorted space-time around it stabilizes. The direct waves appear to originate much closer to the event horizon of the new black hole: 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 information about it.”

Indeed, anything created so close to a black hole’s event horizon almost seems 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 stir the surrounding space-time 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 of 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. The findings of this study include, among other things, that black holes are not only great singers, but also bald.)

For study co-author Sizheng Ma, who helped develop the theory behind direct waves, the timing of GW250114 couldn’t have been better. “Sometimes when you make a prediction, people may have to wait many years before it can be proven,” he says. “Because this event is so noisy, it allows us to immediately prove our prediction. »

The reason GW250114’s signal is so “strong” actually has little to do with the collision itself. Gravitational waves of similar strength have been observed before. What has changed is the instrumentation. “It’s like you’re hearing the same noise with a less static microphone,” says Chatziioannou, who was not involved in the study.

Simply put, a decade of technological advancements has transformed this cosmic duo into a spectacle.

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 ringing of a bell, which is why the fading signal that follows is called ringing. “You can think of gravitational waves as the acoustics of space-time,” says Ma.

If ordinary ringing signals are the attenuated resonance of a bell, direct waves could tell us how the bell was struck in the first place. They could offer physicists a new way to explore some of the most extreme environments in the universe. Still, it’s hard to know for sure that astronomers saw a direct wave.

“If you can observe this, you will get a direct measurement of the properties of the horizon,” says Emanuele Berti, a professor at Johns Hopkins University who was not involved in the study. “The question is: can we really see this?

The signal identified in GW250114 matches predictions of a direct wave, an encouraging sign. But matching a prediction is it’s not the same as proving it. Some physicists are skeptical that such waves can escape the intense gravitational environment near a black hole’s event horizon, or that current instruments can reliably separate a direct wave signal from surrounding noise. “It’s very difficult to observe these things, if they can be observed at all,” says Berti.

Nonetheless, “any evidence of black hole observations is welcome and a major step forward,” says Vitor Cardoso, director of the Center of Gravity at the Niels Bohr Institute and distinguished professor at the Instituto Superior Técnico in Lisbon, Portugal.

Physicists are eager to examine the signal in more detail and look for signs of direct waves hidden beneath previously discovered quasinormal modes.

“I am sure that a lot of follow-up work will take place around the world and that this approach will stimulate progress,” says Szabolcs Márka, a professor at Columbia University who was not involved in the study. “The more we observe, the more confident we will become. »