When Two Black Holes Collide, the Result "Chirps" Not Once, But Multiple Times

Gravitational wave scientists reported that in the event of a collision and subsequent merging of two black holes, the resulting black hole "chirps" not just once, but multiple times.

Researchers led by members from the ARC Center of Excellence for Gravitational Wave Discovery (OzGrav) noted colliding black holes emit simple signals known as "chirps," generated from gravitational waves. The increasing frequency and amplitude of these sounds clue in scientists as to the speed and the size of the orbit of these objects.

Chirps Offering Insights on the Nature of Black Holes

British mathematician Roger Penrose, the recent Nobel Laureate for Physics, noted that black holes remain the "greatest puzzle" among astrophysicists, with a lot of details about their nature and the physics that occur within the remaining a mystery. At its surface alone, a phenomenon known as the event horizon generates an extraordinarily strong gravitational field - not even light can escape.

Black holes are also usually undetectable until an object is close enough to interact with it. In the event two black holes collide and merge, they create one of the most catastrophic events, but only for a fraction of a second. In a very short period, it releases very large amounts of energy-releasing gravitational waves - before settling into a more massive black hole.

In the new report from OzGrav researchers, they reported that the gravitational waves contain information about the shape of colliding black holes before they settle into its resulting form. "We performed simulations of black-hole collisions using supercomputers and then compared the rapidly changing shape of the remnant black hole to the gravitational waves it emits," said Christopher Evans, a graduate student from Georgia Institute of Technology and a co-author in the study." He added that their works showed these "chirp" signals are more "rich and complex" than previously thought, noting that these signals allow researchers to learn more about the resulting black hole.

Clues on Resulting Black Hole, Post-Collision

"The pitch and amplitude of the signal increases as the two black holes approach faster and faster," explained Calderón Bustillo, OzGrav alumnus. "After the collision, the final remnant black hole emits a signal with a constant pitch and decaying amplitude-like the sound of a bell being struck."

Furthermore, the characteristics of these chirps offer clues to the shape of the resulting black hole after the collision and merging, likening it to a "gravitational wave lighthouse." Bustillo explains that when the two original black holes in a collision are of different sizes, the resulting object takes on a "chestnut" shape at first, with a cusp on one end and a wider backside. He adds that black holes emit more intense gravitational waves in its curved regions or those that surround the "cusp" part. Since the resulting black hole also spins, with the cusp and backside of the proverbial chestnut pointing toward all observers, this results in multiple chirps.

According to Pablo Laguna, former chairperson of the School of Physics at Georgia Tech and also a co-author of the study explains that although the relationship between gravitational waves and the resulting black hole has been conjectured before, their study was able to present "the first explicit example" of this relationship.

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