A recent observation of two black holes merger has confirmed a major theory first proposed by Stephen Hawking in 1971: black holes grow larger when they collide. The phenomenon has given scientists the most direct evidence yet that the surface area surrounding black holes does not shrink — a key concept in understanding how these mysterious objects behave.
The event was detected earlier this year by the Laser Interferometer Gravitational-Wave Observatory (LIGO), which recorded the ripple-like gravitational waves produced by the cosmic crash. The study’s findings, published on Sept. 10 in Physical Review Letters, mark a clear milestone in black hole research. The lead author, Adrian G. Abac, is a doctoral researcher at the Max Planck Institute for Gravitational Physics in Germany.
Black hole merger confirms Stephen Hawking’s theory
The recorded event, officially named GW250114, showed two black holes combining to form one larger black hole. Before the merger, the total area of both black holes’ outer boundaries — known as event horizons — measured about 243,000 square kilometers, roughly the size of the U.S. state of Oregon. After merging, the new black hole’s surface area expanded to around 400,000 square kilometers, approximately the size of California.
This result aligns precisely with Hawking’s prediction that a black hole’s surface area can never shrink — a rule similar to how the overall disorder, or entropy, in a system naturally increases over time. In physics, this is compared to the second law of thermodynamics.
Maximiliano Isi, a physicist at Columbia University and co-author of the study, said that while Hawking’s law may seem straightforward, it holds powerful insights into how the universe works. “It’s not just about black holes getting bigger — it’s a sign of deep cosmic order,” he explained.
Listening to the echoes of a cosmic collision
To understand what happened during the black hole merger, scientists studied the gravitational waves — tiny ripples in space-time caused by the massive collision. These waves carry information about the size, mass, and spin of the black holes involved. Similar to how the sound of a drum changes depending on its size, black holes also leave a kind of “sound” in space when they merge.
Researchers focused on a specific part of the wave pattern called the “ringdown,” which is the fading echo left after two black holes combine. According to Katerina Chatziioannou, a physicist at the California Institute of Technology, this phase provided the most direct evidence that the final black hole had a larger surface area than the sum of the original two.
The study also supports earlier ideas from physicist Roy Kerr, whose work in the 1960s predicted how spinning black holes behave based on Einstein’s theories.
LIGO, operated jointly by Caltech and MIT, has two observatories in Washington and Louisiana. Together with partners in Europe and Japan, they make up the LIGO-Virgo-KAGRA collaboration — a global network designed to detect gravitational waves. By 2030, a new facility in India is expected to join the network, enhancing its ability to study black hole mergers more precisely.
Other large-scale projects, including the Cosmic Explorer in the U.S. and the Einstein Telescope in Europe, are currently in development. These future observatories aim to detect black hole collisions that occurred in the earliest days of the universe.