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Observations of colliding black holes from the LIGO-Virgo-KAGRA collaboration are shedding new light on the universe’s darkest inhabitants as well as on the nature of the universe itself.
Associate Research Scientist, Center for Computational Astrophysics
Flatiron Research Fellow, Center for Computational Astrophysics
In a series of new observations, astronomers have nearly doubled the number of black hole collisions ever identified. Detected through gravitational waves — ripples in the fabric of space-time caused by the movements of massive objects — the black hole collisions are shedding new light on the universe’s darkest inhabitants as well as on the nature of the universe itself.
The new observations, published in an online catalog and several papers submitted to the Astrophysical Journal and Astrophysical Journal Letters, include notable observations such as two black hole crashes that are the clearest and most precise gravitational wave measurements to date. The large new catalog has also enabled researchers to refine measurements of the universe’s expansion rate, discover two unique types of black holes, determine multiple pathways by which black holes are created and provide further evidence for Einstein’s theory of general relativity.
“With this catalog, we now have enough data that we can start to tease apart the properties of these black holes and to figure out where they came from,” says astrophysicist Maximiliano Isi, one of several researchers at the Simons Foundation’s Flatiron Institute in New York City involved with the international LIGO collaboration. “We’re also able to chart the expansion of the universe over its history, which is a super important unanswered question in cosmology right now.”
The catalog’s observations were made by the LIGO-Virgo-KAGRA collaboration, which has been hunting for gravitational wave signals since 2015. The new catalog, the fifth of its kind, contains 161 newly observed black hole mergers from April 2024 to January 2025. As a result of a series of upgrades to LIGO and the 2024 relaunch of Virgo in Italy, which had been offline for four years, the combined detectors have increased sensitivity, enabling them to capture as many as three to four gravitational wave signals per week.
That increase in detections is enabling astronomers to study black holes in ways never possible before, including probing their origins. Scientists theorize that black holes are likely created upon the death of gargantuan stars at least 20 times more massive than the sun. Subsequent mergers between these black holes are thought to create larger, ‘second-generation’ black holes. Previous observations of black holes have been too limited to test these theories, but that’s changing with the newly expanded catalog.
“From that data, we’re seeing a population of small, slowly spinning black holes and a separate population of ones heavier than 35 solar masses that spin much faster and appear consistent with these ‘second-generation’ black holes,” Isi says. Isi co-wrote one of the scientific papers on the results and worked alongside Asad Hussain of the Flatiron Institute, who determined the division between fast and slow spinning black holes. “This is a clear sign that there are multiple pathways to creating black holes.”
The detector upgrades have also increased the collaboration’s ability to pinpoint the location of gravitational wave sources. Among the new events is GW240615, observed on June 15, 2024, which set the record for the most precise localization of a gravitational wave event. The event, caused by the merger of two black holes with masses of around 26 and 30 times that of the sun, was located in a region in the sky just six square degrees across — about 30 times the size of the moon in the sky. This improved precision is helping scientists narrow down how fast the universe is expanding.
“This is a clear sign that there are multiple pathways to creating black holes.”
Scientists have known for nearly 100 years that our universe is expanding. But attempts over the years to determine exactly how fast this is happening have yielded different results. This disagreement suggests that either something is missing from the measurements or something larger is missing from our understanding of the universe. To figure out which, scientists are testing new ways of measuring the expansion of the universe, including using gravitational waves.
“With the new catalog, we’ve been able to significantly increase the accuracy of our measurements,” says Flatiron Institute astrophysicist and LIGO collaborator Konstantin Leyde, who co-led a paper about these results. “The LIGO-Virgo-KAGRA collaboration has never measured today’s expansion history to this precision.”
While the new findings vastly improve upon previous results, they are not yet precise enough to resolve the difference between the other measurement methods. However, the results demonstrate that gravitational waves offer an important independent check on the expansion rate. The findings also provide an independent check of how Albert Einstein’s theory of general relativity works on cosmological scales. As previous results have found, the gravitational wave research indicates that general relativity still applies on large scales.
The new catalog provides additional, smaller-scale evidence for general relativity in observations of the previously reported GW250114 event. Gravitational waves hold a wealth of information about the objects that created them. Just as a large iron bell makes different sounds than a smaller aluminum bell, the ‘sound’ a black hole merger makes is specific to the properties of the black holes involved. By listening to the tones, or frequencies, of the gravitational waves captured by the upgraded, more sensitive LIGO detector, the researchers were finally able to tease out the ringing clearly from the noise to learn about the properties of the black hole, like mass and spin. Analysis of these properties, led by Isi and his collaborator Will Farr of the Flatiron Institute, provided the strongest evidence yet that astrophysical black holes are the black holes predicted from general relativity.
The new catalog is massively expanding scientists’ understanding of the universe. But its findings are just the start.
“I’m super excited, because there will be another catalog released within the next year, and it may revolutionize what we understand about how stars die and become black holes, as well as the environments where black holes meet and merge,” Isi says. “With that update, we’ll start seeing even finer details of these properties of black holes, and it will help clarify a lot of important questions in astrophysics.”
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