Hidden in the gravitational waves generated by the largest black hole collision ever detected, clues retrieved by Australian National University (ANU) researchers have helped them observe how black holes drag spacetime itself along with their spin.

As two black holes collide, they merge in a massive cosmic impact, forming a larger black hole and creating ripples in spacetime called gravitational waves. In the largest black hole merger observed, GW250114, the ANU team discovered a hidden direct wave for the first time, a feature expected by general relativity and revealed in a recent paper published in Nature.

Black Holes and Gravitational Waves

Some of the most intriguing objects in the universe, black holes have enormous power and effect on their surroundings, which scientists are still working to better understand. The gravitational waves produced by their collisions compress space as they travel, and highly sophisticated—and very precise—detectors are required to observe these waves, such as the United States’ Laser Interferometer Gravitational Wave Observatory (LIGO).

Since not even light can escape a black hole’s event horizon, gravitational waves’ ability to carry information about what lies just beyond the rim’s edge provides an important means of gaining perspective into one of a black hole’s most mysterious regions.

The most extreme black hole merger ever recorded, GW250114, came from a black hole perfectly positioned for Earth observers, allowing LIGO to capture faint signatures in the data. The team decoded the event’s direct wave, previously hidden in the data, revealing the black hole’s drag on spacetime.

Black Hole Frame Dragging

In his theory of relativity, Albert Einstein described black holes as producing a “frame dragging” effect in which they pull nearby spacetime with them as they rotate, like water circling a drain. Also, according to general relativity, the region where this frame-dragging occurs, just past the event horizon’s edge, should produce gravitational radiation that creates a direct wave, imprinting information about the cosmic whirlpool surrounding the black hole.

Event horizons have been essential to theoretical physics models for decades, but very little observational data on them has been collected due to the difficulty of observing light that close to a black hole. Although not a physical object and instead merely a boundary in spacetime, general relativity posits that the event horizon’s properties, such as gravitational strength and rotational speed, should be measurable. Therefore, having access to a data source so close to the black hole’s edge, as with a direct wave, is extremely valuable to understanding our universe.

Due to the extreme nature of the event and our prime viewing location on Earth, the extremely powerful gravitational wave produced by GW250114 was the perfect opportunity to hunt for a direct wave. Despite longstanding predictions of direct waves based on general relativity, this is the first time that one has ever been detected, providing new insight into how fast the black hole is spinning and the gravitational power of the event horizon.

Direct Wave Observation

The researchers employed a new technique to isolate the subtle direct wave in the data from the rest of the event. This first success proves the case for direct waves and how to detect them, opening an entirely new avenue for investigating black holes. By finally detecting these waves, the team provides future researchers with an essential technique for designing new tests of General Relativity, as direct waves, horizon rotations, and surface gravity should all be linked.

Scientists currently grapple with the apparent incompatibility between general relativity, which describes our universe at the macro scale, and quantum physics, which operates at the smallest scales of matter and energy.

Some of our most essential technologies, such as GPS, lasers, quantum computers, and semiconductors, are designed according to these theories and operate perfectly despite the seeming contradiction of how these two perspectives of our universe describe physical reality.

The difficulty of reconciling these ideas is one reason scientists are interested in distant black holes. The extreme conditions of spacetime at the edge of an event horizon may offer the best clues for finally reconciling these existing theories, or for upending them altogether in favor of something that more accurately describes the physical universe at all scales.

The paper, “GW250114 Reveals Signatures of Post-Merger Black-Hole Horizon,” appeared in Nature on June 24, 2026.

Ryan Whalen covers science and technology for The Debrief. He holds an MA in History and a Master of Library and Information Science with a certificate in Data Science. He can be contacted at ryan@thedebrief.org, and follow him on Twitter @mdntwvlf.

A new gravitational-wave catalogue reports 161 additional black hole mergers, bringing the total to 390 detections. Scientists at the University of Glasgow’s Institute for Gravitational Research are celebrating the release of a major new collection of gravitational wave discoveries, a milestone that highlights the rapid growth of gravitational astronomy. The latest Gravitational Wave Transient Catalogue [...]

Scientists believe an unusual LIGO detection may be evidence of a primordial black hole, potentially linking these long-theorized objects to the mystery of dark matter. Scientists at the University of Miami believe they may be closing in on evidence for one of the most elusive objects in the universe: primordial black holes. While definitive proof [...]

Editor’s Note: For the remainder of 2025, we’ll be looking at a few selections that we haven’t yet discussed on AAS Nova from among the most-downloaded articles published in AAS journals this year. The usual posting schedule will resume January 2nd.

Search for Gravitational Waves Emitted from SN 2023ixf

Published May 2025

Main takeaway:

The LIGO, Virgo, and KAGRA collaborations searched for gravitational waves from the core-collapse supernova SN 2023ixf. Though no significant gravitational wave events were detected during times when two or more gravitational wave detectors were online, the non-detection of gravitational waves from this nearby supernova places constraints on the amount of energy emitted by the explosion in the form of gravitational waves, the shape of the proto-neutron star produced in the collapse, and more.

Why it’s interesting:

When a massive star expires in a core-collapse supernova, the collapse of the stellar core into a neutron star or black hole produces gravitational waves. These gravitational waves, along with a stream of neutrinos, should arrive at Earth before the light from the explosion does; gravitational waves and neutrinos can easily escape the dense, roiling ejecta from the explosion, but photons take some time to claw their way through the debris. Gravitational waves from a supernova have never been detected, but the nearby supernova SN 2023ixf, which occurred in a galaxy about 20 million light-years away, offered the best recent opportunity to detect these waves.

What we learned from this non-detection:

Using the non-detection of SN 2023ixf, researchers placed stringent constraints on the gravitational wave energy and luminosity of a supernova explosion, but an even closer supernova is still needed to begin to rule out model predictions for these quantities. To estimate the distance out to which we can expect to detect gravitational waves from collapsing stars, the collaboration members injected synthetic supernova signals into their data. The majority of non-rotating explosions can be detected out to about 22,000 light-years — meaning events on the far side of our galaxy remain inaccessible to current detectors — while rapidly rotating explosions should be detectable out to nearly 100,000 light-years, pushing the detection horizon beyond the borders of the Milky Way.

Citation

A. G. Abac et al 2025 ApJ 985 183. doi:10.3847/1538-4357/adc681

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Black holes can have messy family histories, and a recent massive merger challenged astronomers to sort out just how many generations of black holes were involved in the event. Recent research, however, may have untangled the lineages of the two progenitors.

A Massive Collision

Earlier this year, the Laser Interferometer Gravitational-Wave Observatory (LIGO) detected the ripples in spacetime caused by the merger of two black holes somewhere in the distant universe. Though these detections have become increasingly common, this one in particular stood out among the more than 200 reported over the last decade. Named GW231123, this event must have caused more of a splash than a ripple since the two black holes that slammed together were unusually massive; both likely weighed more than 100 times the mass of the Sun. Not only that, each was also spinning unusually rapidly prior to the collision.

According to theory, black holes this large are not supposed to form on their own. When massive stars die, they can collapse into a black hole; but, when really massive stars die, the collapse is so violent that no material is left over to form the black hole. So, when researchers first measured the masses of the black holes involved in GW231123, they knew that the black holes involved couldn’t have formed directly from collapsed stars. Instead, they were likely second-generation black holes, meaning that they themselves formed from previous black hole–black hole mergers.

Merger Mystery

This result was celebrated as evidence of so-called “hierarchical formation,” in which black holes formed in dense environments swarm throughout their star clusters, merging with other black holes they encounter and growing ever larger.

However, there were a couple of complications with this picture. For one, theory predicts that it is challenging to form such rapidly spinning black holes from randomly oriented progenitors, since usually the spins wash out and leave the remnant rotating slowly. For another, the merging process tends to give the final black hole a “kick” that sends it flying rapidly away from the site of the impact, occasionally fast enough to leave the star cluster altogether, meaning second-generation black holes might not be able to merge into subsequent generations.

Born as a Pair

New research led by Jakob Stegmann, Max Planck Institute for Astrophysics, suggests a way around each of these problems. In the team’s model, the two black holes involved in the GW231123 merger didn’t form from isolated black holes in a cluster, but rather from binary stars embedded within that cluster. In this picture, two massive stars were born bound together, orbiting one another within a larger cluster of stars. Crucially for this scenario, the massive primordial stars have aligned spins — an expected outcome of massive binary star evolution.

After both stars in the binary collapsed into black holes, these black holes merged to form a rapidly spinning second-generation black hole. This second-generation black hole eventually encountered another black hole within the cluster and merged to produce the ripples in spacetime we observed as GW231123.

Since black holes born within binary systems are expected to have smaller kicks and faster spins, this scenario neatly explains the properties observed in GW231123. As LIGO keeps listening for more black hole mergers, hopefully similar events will provide more chances to untangle their complex family trees.

Citation

“Resolving Black Hole Family Issues Among the Massive Ancestors of Very High-Spin Gravitational-Wave Events like GW231123,” Jakob Stegmann et al 2025 ApJL 992 L226. doi:10.3847/2041-8213/ae0e5f

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Meet GW241011 and GW241101, two events from the fourth observing run of the LIGO, Virgo, and KAGRA (LVK) gravitational wave detectors. With rapid spins and mismatched black hole masses, both events provide strong evidence for black hole growth through hierarchical mergers.

A Decade of Discovery

Humanity is 10 years into its study of the gravitational wave universe, and detectors across the globe are about to wrap up their fourth observing run. The recently released fourth catalog of gravitational wave transients, GWTC-4.0, more than doubles the number of recorded events.

Today, we’re taking a look at two of the fourth observing run’s gravitational wave events, both of which point to black holes born in dense environments.

Two Detections in the Spotlight

GW241011 and GW241101 are gravitational wave signals that reached Earth in late 2024. Using Bayesian parameter estimation, members of the LVK collaboration determined that each of these two signals arose from the merger of a pair of black holes.

Though the LVK detectors have by now spotted dozens of coalescing black hole pairs, these two detections stand out. GW241011 is the third-loudest gravitational wave signal ever detected, and its primary (i.e., more massive) black hole has one of the largest and most precisely measured spins among black holes detected via gravitational waves.

GW241101 flips the script: its primary black hole also spins rapidly, but it appears to do so in a direction opposite from the direction in which the black holes orbit. Though its spin isn’t measured as precisely as that of the primary black hole in GW241011, this event still provides the best evidence so far that the spins of some merging black holes are misaligned with their orbital motion.

In addition to their standout spin parameters, these events are also remarkable for the masses involved. In both cases, the larger black hole has a mass of 15–20 solar masses, and the smaller black hole is significantly smaller — about 6 solar masses for GW241011 and 8 solar masses for GW241101.

A Black Hole Genealogy Project

The rapid spins and comparatively large masses of the primary black holes of GW241011 and GW241101 don’t line up with what’s expected for black holes arising in isolated binary systems, unsubjected to outside influences. But they do match expectations for hierarchical mergers, in which one or more of the black holes involved is itself the product of a merger.

If these signals arose from hierarchical mergers, that tells us something about the environment in which the black holes lived. When two black holes merge, the product is expected to spin rapidly and receive a “kick.” For a black hole pair in a loose grouping of stars, this kick would likely boot the merger product out of the group, preventing it from encountering other black holes and merging again.

But in dense environments like compact star clusters, even a good strong kick isn’t enough to overcome the cluster’s escape velocity and launch the resulting black hole on a lonely journey through space. Instead, these black holes are retained in the dense cluster environments of their birth, where they may dance and merge with another black hole partner. While it’s not possible to entirely rule out other origins, such as an isolated pair of massive binary stars, the data strongly support the hierarchical merger scenario.

This discovery yet again demonstrates the ability of our gravitational wave detectors to reveal the lives and histories of black holes throughout our universe. Stay tuned for even more black hole news from the current observing run!

Citation

GW241011 and GW241110: Exploring Binary Formation and Fundamental Physics with Asymmetric, High-Spin Black Hole Coalescences,” A. G. Abac et al 2025 ApJL 993 L21. doi:10.3847/2041-8213/ae0d54

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