Stephen Hawking Was Right: Black Holes Always Grow in Area
Researchers celebrate 10th anniversary of gravitational wave discovery, announce verification of a Hawking theorem.
Two black holes spiral together and violently merge in this simulation of an event that was detected by the LIGO-Virgo-KAGRA collaboration on January 14, 2025. The simulation was developed by Deirdre Shoemaker and Pablo Laguna at The University of Texas at Austin. Credit: Deborah Ferguson, Derek Davis, Rob Coyne (URI) / LVK / MAYA Collaboration. Simulation performed with NSF’s TACC Frontera supercomputer.
As gravitational wave astronomy turns 10, an international consortium that includes physicists from The University of Texas at Austin is revealing the best observational evidence captured to date for what is known as the black hole area theorem. The idea, put forth by Stephen Hawking in 1971, says the total surface areas of black holes cannot decrease.
Last January, the consortium detected the merger of two black holes with a total surface area of 240,000 square kilometers. After the merger, their final area was about 400,000 square kilometers.
“This discovery shows how far we’ve come in 10 years,” said Aaron Zimmerman, associate professor of physics at UT Austin and a member of the LIGO Scientific Collaboration. “Our detectors are better than ever, meaning the signal is so clear we can carry out precision tests we never could have before. In this case we get a detailed look into the ringing of the black hole leftover when two collide to become one, and we can test a remarkable property of black holes theorized by Hawking in a way that was out of reach when we first started.”
Celebrating 10 Years
On September 14, 2015, a signal arrived on Earth, carrying 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. It was a different kind of signal: a quivering of space-time called gravitational waves first predicted by Albert Einstein a century earlier. On that day 10 years ago, the twin detectors of the US National Science Foundation Laser Interferometer Gravitational-Wave Observatory (NSF LIGO) made the first-ever direct detection of gravitational waves.
“To see what we had been predicting as the signature of two black holes colliding as reality gave me goosebumps,” said Deirdre Shoemaker, director of the Center for Gravitational Physics and professor at UT Austin and a member of the LIGO Scientific Collaboration, dating back to that first detection. “Since then, LIGO has gone beyond my expectations, and we have only just begun to probe our understanding of gravity through gravitational waves. I’m looking forward to the next decade.”
Several UT faculty members collaborated on aspects of the research. David Reitze, a UT physics alum and LIGO’s director got to announce the historic discovery, which allowed researchers now to sense the universe through three different means: light, high-energy particles and the gravitational warping of space-time. For this achievement, three of the LIGO team’s founders won the 2017 Nobel Prize in Physics.
Today, LIGO, which consists of detectors in both Hanford, Washington and Livingston, Louisiana, routinely observes roughly one black hole merger every three days. LIGO now operates in coordination with two international partners, the Virgo gravitational-wave detector in Italy and KAGRA in Japan. Together, the gravitational-wave-hunting network, known as the LVK (LIGO, Virgo, KAGRA), has captured a total of about 300 black hole mergers, some of which are confirmed while others await further analysis. During the network’s current science run, the fourth since the first run in 2015, the LVK has discovered about 220 candidate black hole mergers, more than double the number caught in the first three runs.
The dramatic rise in the number of LVK discoveries over the past decade is owed to several improvements to their detectors—some of which involve cutting-edge quantum precision engineering. The LVK detectors remain by far the most precise rulers for making measurements ever created by humans. The space-time distortions induced by gravitational waves are incredibly miniscule. For instance, LIGO detects changes in space-time smaller than 1/10,000 the width of a proton. That’s 700 trillion times smaller than the width of a human hair.
Credit: Lucy Reading-Ikkanda/Simons Foundation
The Clearest Signal Yet
LIGO’s improved sensitivity is exemplified in a recent discovery of a black hole merger referred to as GW250114 (the numbers denote the date the gravitational-wave signal arrived at Earth: January 14, 2025). The event was not that different from LIGO’s first-ever detection (called GW150914)—both involve colliding black holes about 1.3 billion light-years away with masses between 30 to 40 times that of our sun. But thanks to 10 years of technological advances reducing instrumental noise, the GW250114 signal is dramatically clearer.
“We can hear it loud and clear, and that lets us test the fundamental laws of physics,” said LIGO team member Katerina Chatziioannou, Caltech assistant professor of physics and William H. Hurt Scholar, and one of the authors of a new study on GW250114 published in the Physical Review Letters.
By analyzing the frequencies of gravitational waves emitted by the merger, the LVK team was able to provide the best observational evidence captured to date for the black hole area theorem.
In essence, the LIGO detection allowed the team to “hear” two black holes growing as they merged into one, verifying Hawking’s theorem. (Virgo and KAGRA were offline during this particular observation.) This is the second test of the black hole area theorem; an initial test was performed in 2021 using data from the first GW150914 signal, but because that data was not as clean, the results had a confidence level of 95 percent as compared to 99.999 percent for the new data.
Nobel Laureate Kip Thorne recalls Hawking phoning him to ask whether LIGO might be able to test his theorem immediately after he learned of the 2015 gravitational-wave detection. Hawking died in 2018 and sadly did not live to see his theory observationally verified.
“If Hawking were alive, he would have reveled in seeing the area of the merged black holes increase,” Thorne said.
The trickiest part of this type of analysis had to do with determining the final surface area of the merged black hole. 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. But after the black holes coalesce, the signal is not as clearcut. During this so-called ringdown phase, the final black hole vibrates like a struck bell.
In the new study, the researchers were able to precisely measure the details of the ringdown phase, which allowed them to calculate the mass and spin of the black hole, and subsequently determine its surface area. More precisely, they were able, for the first time, to confidently pick out two distinct gravitational-wave modes in the ringdown phase. The modes are like characteristic sounds a bell would make when struck; they have somewhat similar frequencies but die out at different rates, which makes them hard to identify. The improved data for GW250114 meant that the team could extract the modes, demonstrating that the black hole’s ringdown occurred exactly as predicted by mathematical models.
