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.
“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.)
To read more, click here.