- · Northwestern scientists have made key contributions to every major gravitational-wave milestone
- · After a decade of upgrades, the observatory received its clearest gravitational-wave signal to date
- · Signal verifies Hawking’s black-hole area theorem, stating the total surface areas of black holes cannot decrease
EVANSTON, Ill. — Nearly a decade has passed since an international team of scientists, including Northwestern University astrophysicists, first detected gravitational waves — a historic discovery that confirmed Albert Einstein’s 100-year-old prediction of these subtle quivers in space-time and the mere existence of merging black holes.
Now, the team has received perhaps the best anniversary gift possible.
By analyzing the frequencies of gravitational waves from a merger between two black holes, the team verified Stephen Hawking’s 1971 black-hole area theorem, which states the total surface area of black holes cannot decrease. The signal is the clearest to date detected by the U.S. National Science Foundation Laser Interferometer Gravitational-Wave Observatory (NSF LIGO), Virgo and KAGRA (LVK) collaboration. The finding sheds further light on the mysterious nature of black holes, one of the most extreme objects in the universe.
The study was published on Wednesday (Sept. 10) in the Physical Review Letters. The new paper 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 Northwestern’s Vicky Kalogera, a senior member of the LIGO Scientific Collaboration (LSC). “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.”
An expert in the formation and evolution of black holes and other stellar remnants in binary systems and in gravitational-wave data analyses, Kalogera has been a member of LIGO for 25 years. She also is the Daniel I. Linzer Distinguished University Professor of Physics and Astronomy at Northwestern’s Weinberg College of Arts and Sciences, director of the Center for Interdisciplinary Exploration and Research in Astrophysics (CIERA) and the NSF-Simons National AI Institute for the Sky (SkAI Institute).
A decade of advancement
Before the initial detection of gravitational waves in 2015, astrophysicists had only detected distant objects with light waves, such as X-rays, optical light, infrared radiation and radio waves. Then, on Sept. 14, 2015, LIGO detected a signal 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 Earth at the speed of light — but the signal itself did not comprise light. It marked the first time anyone witnessed a cosmic event through its gravitational warping of space-time.
“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 that initial achievement, gravitational wave observations have provided about 300 measurements of compact-object masses. Other LVK scientific discoveries include the first detection of collisions between two neutron stars; mergers between one neutron star and one black hole; asymmetrical mergers, in which one black hole is significantly more massive than its partner black hole; the discovery of the lightest black holes known, challenging the idea that there is a “mass gap” between neutron stars and black holes; and the most massive black hole merger seen yet with a merged mass of 225 solar masses. For reference, the previous record-holder for the most massive merger had a combined mass of 140 solar masses.
Northwestern and CIERA played roles in many of these landmark discoveries. Kalogera and Wen-Fai Fong’s groups played key roles in the first detection of a collision between two neutron stars. Fred Rasio’s group pioneered the understanding of how dense star clusters can naturally form very massive black hole binaries, potentially producing some of the heaviest gravitational-wave mergers observed.
Kalogera and her team also made major contributions to the discovery, analysis and astrophysical interpretation of mystery compact objects right in the middle of the purported “mass gap” between the well-established regimes of neutron-star and black-hole masses. The discovery of such gap sources has called into question whether the gap is a true feature of nature. Kalogera’s group has not only uncovered further evidence that the low-mass gap does not appear in the gravitational-wave observations but also offered a verifiable explanation for why the gap shows up in X-ray sources — pointing to observational biases in how those systems are detected.
The accelerated pace of discoveries is owed to several improvements to LVK’s detectors, some of which involve cutting-edge quantum precision engineering. Gravitational waves distort space-time by a miniscule amount, sometimes slighter than one-ten-thousandth of the width of a proton. That’s 700 trillion times smaller than the width of a human hair.
Verifying Hawking’s theorem
With this improved sensitivity, LVK recently discovered a black hole merger dubbed GW250114. Similar to the first-detected black-hole merger in 2015, GW250114, too, involved two colliding black holes about 1.3 billion light-years away from Earth. But, thanks to 10 years of technological advances to reduce noise, the new signal was dramatically clearer. So distinct, in fact, that it provided the best observational evidence captured to date to verify Stephen Hawking’s black hole area theorem.
When black holes merge, multiple factors are at play. The holes’ masses combine to increase the overall surface area, but they also lose energy in the form of gravitational waves. The merger also can cause the combined black hole to increase its spin, shrinking the surface area. Despite these competing factors, Hawking proved mathematically that the total surface area still must grow in size.
The LIGO detection enabled scientists to “hear” two black holes growing as they merged into one, verifying empirically Hawking’s theorem. After the two objects coalesce, the final black hole rings like a struck bell. (Virgo and KAGRA were offline during this observation.) The total surface area of the initial black holes reached 240,000 square kilometers, roughly the size of Oregon. The final area, after the merger, increased to 400,000 square kilometers, about the size of California.
“This is the first incontrovertible confirmation of the law,” said Sylvia Biscoveanu, who was a NASA Einstein Fellow at CIERA at the time of the work and now is 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
In the coming years, LVK’s team aims to further fine tune their technology, expanding its 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 will greatly improve the precision with which the LVK network can localize gravitational-wave sources.
Looking further into the future, the U.S. gravitational-wave community is developing a concept for an even larger detector, called Cosmic Explorer, which would have arms 40 kilometers long (the twin LIGO observatories have 4-kilometer arms). Kalogera leads the expert advisory committee that reviewed and recommended to this project to the NSF. 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 black hole mergers all the way back to the earliest times in the universe’s origins.
“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.”