Nearly a decade after the groundbreaking detection of gravitational waves, an international team of scientists, including astrophysicists from Northwestern University, has achieved another monumental milestone. By analyzing the frequencies of gravitational waves from a merger between two black holes, the team has verified Stephen Hawking’s 1971 black-hole area theorem, which posits that the total surface area of black holes cannot decrease.
The signal, detected by the U.S. National Science Foundation’s Laser Interferometer Gravitational-Wave Observatory (NSF LIGO), Virgo, and KAGRA (LVK) collaboration, is the clearest to date. This finding not only confirms a key aspect of Hawking’s theoretical work but also enhances our understanding of black holes, some of the universe’s most enigmatic objects. The study was published on September 10 in the Physical Review Letters, with contributions from about a dozen Northwestern coauthors.
A Decade of Gravitational Wave Discoveries
The announcement comes as the scientific community reflects on nearly ten years since the first detection of gravitational waves. Before this initial discovery in 2015, astrophysicists primarily relied on light waves, such as X-rays and radio waves, to study distant cosmic objects. The detection on September 14, 2015, marked a new era, as it was the first time a cosmic event was observed through the gravitational warping of space-time.
“Almost everything we currently know about the universe has been discovered with light of some kind,” said Vicky Kalogera, a senior member of the LIGO Scientific Collaboration. “Gravitational waves carry completely new information about black holes and other cosmic objects, and they will unlock a new part of the universe.”
Since then, gravitational wave observations have led to approximately 300 measurements of compact-object masses and several landmark discoveries, including the detection of the lightest black holes known, challenging the notion of a “mass gap” between neutron stars and black holes.
Verifying Hawking’s Theorem
With improved sensitivity, LVK recently discovered a black hole merger dubbed GW250114. Similar to the first-detected black-hole merger in 2015, GW250114 involved two colliding black holes about 1.3 billion light-years away. However, thanks to technological advances, the new signal was dramatically clearer, providing the best evidence yet to verify Hawking’s black hole area theorem.
When black holes merge, multiple factors influence the event. The masses combine, increasing the total surface area, while energy is lost as gravitational waves. Despite these competing factors, Hawking mathematically proved that the total surface area must still grow.
“This is the first incontrovertible confirmation of the law,” said Sylvia Biscoveanu, a NASA Einstein Fellow at CIERA and now an assistant professor at Princeton University. “This tells us that the null energy condition, weak cosmic censorship, and General Relativity all hold, confirming the theoretical simplicity of astrophysical black holes.”
Looking Ahead: The Future of Gravitational-Wave Astronomy
As the LVK team continues to refine their technology, plans are underway to expand their reach deeper into space. A new gravitational-wave detector, LIGO India, is in development, which will enhance the precision of localizing gravitational-wave sources.
Further into the future, the U.S. gravitational-wave community is conceptualizing an even larger detector, the Cosmic Explorer, with arms 40 kilometers long. This project, along with Europe’s Einstein Telescope, aims to allow scientists to detect black hole mergers from the universe’s earliest times.
“Over the past 10 years, Northwestern and CIERA scientists have contributed to every major milestone in gravitational-wave astronomy,” Kalogera said. “It’s incredibly rewarding to see how our contributions, together with our global partners, continue to push the boundaries of science.”
The move represents a significant step forward in our understanding of the universe, offering new insights into the fundamental laws of physics and the mysterious nature of black holes.
