Nearly ten years after the first detection of gravitational waves, an international team of scientists, including astrophysicists from Northwestern University, has provided the clearest evidence yet supporting Stephen Hawking’s black-hole area theorem. The latest findings come from a recent observation made by the U.S. National Science Foundation Laser Interferometer Gravitational-Wave Observatory (NSF LIGO), Virgo, and KAGRA (LVK) collaboration.
The research team analyzed gravitational waves produced by a merger between two black holes and confirmed Hawking’s 1971 prediction that the total surface area of black holes cannot decrease. The study was published in Physical Review Letters on September 10 and includes contributions from about a dozen Northwestern coauthors.
“It’s remarkable to celebrate nearly a decade since our first detection with a discovery that confirms one of Stephen Hawking’s famous predictions,” said Vicky Kalogera, senior member of the LIGO Scientific Collaboration (LSC) at Northwestern. “This is exactly the kind of breakthrough that shows how gravitational-wave astronomy is reshaping our understanding of black holes, the universe and our place within it.”
Kalogera is recognized for her expertise in black hole formation and evolution as well as gravitational-wave data analysis. She serves as Daniel I. Linzer Distinguished University Professor at Northwestern’s Weinberg College of Arts and Sciences, director of CIERA, and director of the NSF-Simons National AI Institute for the Sky (SkAI Institute).
Gravitational waves were first detected in 2015 when LIGO observed signals from merging black holes located about 1.3 billion light-years away. This marked a shift in how astrophysicists could observe cosmic events—using distortions in space-time rather than traditional light-based methods.
“Almost everything we currently know about the universe has been discovered with light of some kind,” Kalogera said in 2015. “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, observations have led to approximately 300 measurements involving compact-object masses and revealed several key phenomena: collisions between neutron stars; mergers involving neutron stars and black holes; asymmetrical mergers; discoveries challenging existing ideas about mass gaps between neutron stars and black holes; and records for both smallest and largest known merging black holes.
Northwestern researchers have played roles in many milestones—such as analyzing gap sources that question whether there truly is a mass gap between neutron-star and black-hole masses—and have identified possible observational biases related to X-ray source detection.
Recent technological improvements to LVK detectors allowed for greater sensitivity when observing GW250114—a merger similar to that seen in 2015 but with much clearer data due to reduced noise over ten years of upgrades. The clarity enabled direct empirical verification of Hawking’s theorem: while energy loss during merger might suggest shrinking areas due to increased spin or emission as gravitational waves, calculations showed overall surface area still grew—from roughly 240,000 square kilometers before merging to around 400,000 square kilometers afterward.
“This is the first incontrovertible confirmation of the law,” said Sylvia Biscoveanu, formerly at CIERA during this work and now an assistant professor at Princeton University. “This tells us that the null energy condition, weak cosmic censorship and General Relativity all hold, which means that astrophysical black holes are indeed the simple objects posited theoretically many decades before they were observationally confirmed.”
Looking ahead, LVK aims to further improve its technology for deeper space exploration. Plans include constructing another detector—LIGO India—which would enhance localization precision for future observations. Additionally, projects such as Cosmic Explorer in the U.S., featuring arms up to ten times longer than current LIGO facilities, are under development alongside Europe’s Einstein Telescope initiative.
“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 partners around the globe, continue to push the boundaries of science.”