A NASA computer simulation shows two supermassive black holes spiraling toward a collision.

By BlueShift and AFP | 2025-09-26

Gravitational waves that ripple through the universe when two black holes collide have confirmed a theory proposed by British theoretical physicist and cosmologist Stephen Hawking over 50 years ago, scientists said in September.

In his 1916 theory of general relativity, German-born physicist Albert Einstein predicted the cataclysmic merger of two black holes would produce gravitational waves that would ripple across the universe and eventually arrive at Earth.

These gravitational waves were detected for the first time on September 14, 2015 by the Laser Interferometer Gravitational-Wave Observatory (LIGO).

Supported by the US National Science Foundation, LIGO is operated by the California Institute of Technology (Caltech) and the Massachusetts Institute of Technology (MIT).

The gravitational waves carried information about a pair of remote black holes that had spiraled together and merged.

"The signal had traveled about 1.3 billion years to reach us at the speed of light -- but it was not made of light," LIGO said.

"It was a different kind of signal: a quivering of space-time called gravitational waves first predicted by Albert Einstein 100 years prior."

On that day 10 years ago, LIGO's twin detectors made the first-ever direct detection of gravitational waves, "whispers in the cosmos that had gone unheard until that moment," it said.

The historic discovery meant researchers could now sense the universe through three different means: light waves, high-energy particles called cosmic rays, and at last "through the gravitational warping of space-time."

Three of the team's founders won the 2017 Nobel Prize in Physics for this achievement, which verified Einstein's prediction.

Signal from distant universe

LIGO's detectors, which operate in two parts of the United States, now routinely observe roughly one black hole merger every three days, operating in coordination with two international partners.

The LVK network, which consists of LIGO, the Virgo gravitational-wave detector in Italy and KAGRA in Japan, has captured a total of about 300 black hole mergers, some of which are confirmed while others await further analysis.

But on January 14 this year, LIGO detected a signal from the distant universe that LIGO member Geraint Pratten of the UK's University of Birmingham described as "the loudest gravitational wave event we have detected to date."

"It was like a whisper becoming a shout," he said, and bore striking similarities to the first one detected a decade ago.

Both involved collisions of black holes with masses of between 30 and 40 times that of our sun that occurred around 1.3 billion light years away.

Technological improvements over the years have enabled scientists to greatly reduce the background noise, giving them much clearer data and enabling them to confirm a theory put forward by Hawking in 1971.

Hawking's Black Hole Area theory predicted that a black hole's event horizon -- the area from which nothing including light can escape -- cannot shrink.

This means that when two black holes merge, the new monster they create must have the same or larger surface area than the pair started out with.

Verifying Hawking's theory

Scientists analyzing January's merger, called GW250114, were able to show that Hawking -- who died in March 2018 -- was right.

The black holes collectively started out at 240,000 sq km wide. But after the collision, the resulting mega-black hole took up 400,000 sq km.

Working out the final merged surface area was "the trickiest part of this type of analysis," Caltech said in a statement.

"The surface areas of pre-merger black holes can be more readily gleaned as the pair spiral together, roiling space-time and producing gravitational waves," it said. But the signal gets muddier once the black holes start joining together.

This period is called the "ringdown phase," because the merged black hole rings like a struck bell -- another phenomenon Einstein predicted.

Scientists were able to measure different frequencies emanating from this rung bell, allowing them to determine the size of the new post-merger black hole.

In 1963, New Zealand mathematician Roy Kerr predicted that "two black holes with the same mass and spin are mathematically identical," a feature unique to black holes, Maximiliano Isi of Columbia University said in a statement.

"Given the clarity of the signal produced by GW250114, for the first time we could pick out two 'tones' from the black hole voices and confirm that they behave according to Kerr's prediction," said Gregorio Carullo of the University of Birmingham.

Scientists continue to study black hole mergers, with several new gravitational wave detectors planned for the coming years -- including one in India.