UNIVERSITY PARK, Pa. — On Sept. 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 100 years prior. On that day 10 years ago, the twin detectors of the U.S. National Science Foundation Laser Interferometer Gravitational-Wave Observatory (NSF LIGO) made the first-ever direct detection of gravitational waves, whispers in the cosmos that had gone unheard until that moment.
Now, a decade later, researchers on this collaboration have “heard” the clearest evidence for a theorem proposed by Stephen Hawking in 1971 that merging black holes must grow their collective surface areas, despite energy loss and other competing factors. The team, which includes scientists at Penn State, published the finding today (Sept. 10) in Physical Review Letters.
It’s only the most recent finding resulting directly from the initial discovery of gravitational waves.
“I have seen first-hand the transformation of knowledge brought on by gravitational wave discoveries,” said Chad Hanna, professor of physics and of astronomy and astrophysics in the Eberly College of Science at Penn State, a co-hire of the Penn State Institute for Computational and Data Sciences (ICDS) and a leader of the Penn State LIGO group. Hanna published an article on the anniversary and the collaboration's achievements today (Sept. 10) in The Conversation. “Reflecting over the last 10 years I am reminded of a job interview which occurred years before the initial gravitational wave discovery. After delivering a lecture on searching for binary black hole mergers with a null result, one faculty member in the audience was quick to point out the flaw in my research — binary black holes detectable by LIGO don’t exist! Of course, we now know that binary black hole mergers do exist, are quite common in the universe, and are regularly detected by the LIGO-Virgo-KAGRA (LVK) collaboration, but before the gravitational wave discovery 10 years ago many scientists were skeptical that LIGO would be successful.”
The first historic discovery meant that researchers could sense the universe through three different means. Light waves, such as X-rays, optical, radio and other wavelengths of light as well as high-energy particles called cosmic rays and neutrinos had been captured before, but this was the first time anyone had witnessed a cosmic event through its gravitational warping of space-time.
“LIGO finally captured the unmistakable signature of a black hole,” said B.S. Sathyaprakash, Bert Elsbach Professor of Physics and professor of astronomy and astrophysics in the Eberly College of Science at Penn State and a leader of the Penn State LIGO group. “Black holes are believed to be ubiquitous, but distinguishing them from other exotic objects that might mimic their behavior has been a longstanding challenge. The decisive evidence lies in the unique spectrum of gravitational waves that only black holes can produce. For the first time, we confirmed, with high statistical confidence, that the observed signals indeed bear this unmistakable signature. After decades of pursuit, it was deeply gratifying to see LIGO, with its unmatched technology, finally capture the definitive proof of black holes.”
For this achievement, first dreamed up more than 40 years prior, three of the team's founders won the 2017 Nobel Prize in Physics: Massachusetts Institute of Technology’s (MIT) Rainer Weiss, professor emeritus of physics who died last month at age 92; California Institute of Technology’s (Caltech) Barry Barish, the Ronald and Maxine Linde Professor Emeritus of Physics; and Caltech's Kip Thorne, the Richard P. Feynman Professor Emeritus of Theoretical 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 collaboration, 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.
“Penn State and the Eberly College of Science have been involved in the LIGO Scientific Collaboration from its very beginning — more than a decade and half before the initial detection,” said Tracy Langkilde, Verne M. Willaman Dean of the Eberly College of Science. “In 2001, we were among the first to receive support from the NSF Physics Frontier Center program to establish the Center for Gravitational Wave Physics, and in 2003, with support from the NSF we built a supercomputer cluster dedicated to analyzing LIGO data. Now, thanks to the foresight of the NSF to fund this high-risk, high-reward science, the LIGO observations — many of which were detected by the search pipeline developed by Penn Staters — have transformed our view of the universe.”
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, Hanna said. The space-time distortions induced by gravitational waves are incredibly miniscule and LIGO remains by far the most precise ruler for making measurements ever created by humans. 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.
Penn State LIGO
“From its inception in 1993, the Gravity Group at Penn State has played a seminal role in gravitational wave science, first in analytical and numerical theory and, since the arrival of Professor Sathyaprakash, also in data analysis and phenomenology,” said Abhay Ashtekar, Atherton University Professor and Evan Pugh Professor Emeritus in the Eberly College of Science at Penn State. He founded Penn State’s Institute for Gravitation and the Cosmos and served as its director until 2021. “Even at the turn of the century, the field was fragmented. We at Penn State were awarded a Physics Frontier Center by the NSF in the very first year of the program to bring together experts from diverse communities, ranging from nuclear physics to quantum optics to computer science and general relativity, who rarely talked to each other.”
Through several workshops and focus sessions, the center served as a crucible to crystallize gravitational wave science into a single field, Ashtekar said.
Part of the Gravity Group eventually evolved into the Penn State LIGO group, which is part of the Center for Multimessenger Astrophysics in the Institute for Gravitation and the Cosmos, and focuses on all aspects of gravitational wave astronomy: detection of gravitational-wave signals, estimation of parameters of the source and exploiting the detected events to understand the cosmos. Researchers in the group develop and operate an analysis pipeline that analyzes the data from the observatories in real time and alerts scientists when a gravitational wave is detected.
This work requires significant computing resources with custom hardware and software environments, all of which has been enabled by Penn State and the ICDS team, Hanna said.
“LIGO is a top priority project for ICDS and among the largest computational projects at Penn State,” said Guido Cervone, ICDS director. “Through the use of high-performance computing, we can study data and perform simulations for scientific phenomena that change our understanding of the universe. Chad Hanna, who is an ICDS co-hire, has been a pioneer for the design and use of advanced computational methods and infrastructure to hasten discoveries in gravitational waves”
This cluster has been actively used for real-time observation since approximately 2018 and has consistently been used for 10% to 15% of the LVK’s total computing for many years, only behind Caltech. The cluster has also detected over 200 gravitational wave events in real-time, generating “nearly 1,000 astronomical communications known as GCNs among the transient astronomy community since 2023,” Hanna said.
At Penn State, research on black holes has continued to grow in multiple directions, according to Ashtekar.
“In particular, we discovered explicit equations that dictate the quantitative growth in the horizon area, going beyond Hawking’s seminal but qualitative prediction that it cannot decrease,” Ashtekar said. "Our research also addressed fundamental issues such as the quantum origin of the astonishingly large black hole entropy, and quantum evaporation of black holes. Our faculty has played a central role in the LVK collaboration, serving as leaders in major discovery papers. Professor Hanna’s group has created the computational infrastructure that can alert astronomers of gravity wave signals in less than a minute so they can do follow ups of the extreme event at various electromagnetic wavelengths, enabling multi-messenger astrophysics. Young researchers trained at Penn State have gone on to occupy leadership positions world-wide at the highest levels; among them are two of the seven spokespersons of the LIGO Science Collaboration.”
In addition to Hanna and Sathyaprakash, the Penn State LIGO group includes: Vaishak Prasad, Shio Sakon, Shomik Adhicary, Viviana Cacéres, Wendy Dang, Rossella Gamba, Alice Heranval, Yun-Jing Huang, James Kennington, Sanika Khadkikar, Nathaniel Kirby, Olivia Laske, Sayan Neogi, Victoria Niu, Leo Ng, Alexander Pace, Cort Posnansky, Ron Tapia and Yu-Cun Xie.
The clearest signal yet
LIGO's improved sensitivity is exemplified in the recent discovery of a black hole merger referred to as GW250114. The numbers denote the date the gravitational-wave signal arrived at Earth: Jan. 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 the Earth’s Sun. But thanks to 10 years of technological advances reducing instrumental noise, the GW250114 signal is dramatically clearer.
"The exceptional strength of the GW250114 signal, recorded in the two U.S. detectors, has allowed us to measure the properties of the black holes with unprecedented precision," said Prasad, Eberly Postdoctoral Scholar in physics at Penn State and a member of the LSC. “This precision has enabled a definitive test of Stephen Hawking’s celebrated black hole area theorem — confirming that the surface area of black holes never decreases in classical processes, with a failure rate of only a few parts per thousand. With GW250114, we stand at the threshold of a new era in precision gravitational-wave astronomy. "
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 Hawking’s black hole area theorem. When black holes merge, their masses combine, increasing the surface area. But they also lose energy in the form of gravitational waves. Additionally, the merger can cause the combined black hole to increase its spin, which leads to it having a smaller area. The black hole area theorem, proposed in 1971, states that despite these competing factors, the total surface area must grow in size.
Later, Hawking and physicist Jacob Bekenstein concluded that a black hole's area is proportional to its entropy, or degree of disorder. The findings paved the way for later groundbreaking work in the field of quantum gravity, which attempts to unite two pillars of modern physics: general relativity and quantum physics.
In essence, the LIGO detection — Virgo and KAGRA were offline during this particular observation — allowed the team to "hear" two black holes growing as they merged into one, verifying Hawking's theorem. The initial black holes had a total surface area of 240,000 square kilometers — roughly the size of Oregon, while the final area was about 400,000 square kilometers, or roughly the size of California. It was a clear increase. 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 that data was not as clean, so the results had a confidence level of 95% as compared to 99.999% for the new data.
In the new study, the researchers were able to precisely measure the details of the ringdown phase. This 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 first predicted by C.V. Vishweshwara in 1970, demonstrating that the black hole's ringdown occurred exactly as predicted by models developed by Saul Teukolsky, Subramanyam Chandrasekhar and Steven Detweile later in the 1970s.
Another study from the LVK, available as a preprint ahead of journal review, places limits on a predicted third, higher-pitch tone in the GW250114 signal, and performs some of the most stringent tests yet of general relativity's accuracy in describing merging black holes.
“Detecting gravitational waves is challenging, since the data we analyze is dominated by noise,” said Sakon, a graduate student in physics at Penn State and a member of the LSC. “Yet over the past decade we have identified more than 350 candidates, allowing us to probe the population of black holes and neutron stars across the universe. GW250114 stands out as the loudest gravitational-wave signal so far and it was plainly visible in the data from both LIGO detectors — a thrilling moment for those of us who design and operate search algorithms. Within just 15 seconds of the signal’s arrival, our detection pipeline — led by Penn State — flagged the event with high confidence, becoming the first in the world to identify this remarkable discovery.”
Pushing the limits
In the coming years, the scientists and engineers of LVK said they hope to further fine tune their machines, expanding their reach deeper and deeper into space. They also plan to use the knowledge they have gained to build another gravitational-wave detector, LIGO India. Having a third LIGO observatory would greatly improve the precision with which the LVK network can localize gravitational-wave sources. Looking farther into the future, the team is working on a concept for an even larger detector, called Cosmic Explorer, which would have arms 40 kilometers long. For comparison, the twin LIGO observatories have 4-kilometer arms. A European project, called Einstein Telescope, also plans to build one or two huge underground interferometers with arms of more than 10-kilometers long. Observatories on this scale would allow scientists to hear the earliest black hole mergers in the universe.
“This is just the beginning,” Sathyaprakash said. “With future observatories such as the U.S. Cosmic Explorer — planned to be 10 times bigger than LIGO — a signal similar to GW250114 would appear 25 times brighter, allowing us to detect black holes from an epoch before the first stars formed.”
Editor’s note: The original version of this press release was published by the LIGO Laboratory at Caltech.