When the densest objects in the universe collide and merge, the violence sets off gravitational waves that reverberate across space and time over hundreds of millions and even billions of years. By the time they pass through Earth, such cosmic ripples are barely discernible.
Thanks to a global network of gravitational-wave observatories—the US-based National Science Foundation-funded Laser Interferometer Gravitational-wave Observatory (LIGO), the Virgo interferometer in Italy, and the Kamioka Gravitational Wave Detector (KAGRA) in Japan—scientists can "listen" for faint wobbles in the gravitational field that could have come from far-off astrophysical smashups.
Now, the LIGO–Virgo–KAGRA (LVK) Collaboration is publishing its fourth major update to a catalog of detections since gravitational waves were first observed by LIGO in 2015. The latest findings, published in Astrophysical Journal Letters, indicate that the universe is echoing all over with a kaleidoscope of cosmic collisions.
"Each new gravitational-wave detection allows us to unlock another piece of the universe's puzzle in ways we couldn't just a decade ago," says Lucy Thomas, who led part of the analysis of the catalog and is a postdoc in the Caltech LIGO lab.
The LVK's Gravitational-wave Transient Catalog-4.0 (GWTC-4) comprises detections of gravitational waves from a portion of the observatories' fourth and most recent observing run. The observatories detected 128 new gravitational-wave events, meaning signals that are likely from exotic, far-off astrophysical sources. This newest crop more than doubles the size of the gravitational-wave catalog, which previously contained 90 events compiled from all three previous observing runs.
"The GWTC-4 catalog is a real benchmark for gravitational-wave astronomy," says David Reitze, the executive director of LIGO and a research professor at Caltech. "The abundance of black holes that LIGO and its partners have detected is beginning to have a real impact on our understanding of stellar evolution and black hole formation."
The merger of a pair of black holes was the source of the very first gravitational-wave detection and colliding black holes are the source of most of the gravitational waves detected since then. In addition to the black hole binaries, the updated catalog includes the heaviest black hole binary, a binary with black holes having asymmetric masses, and a binary where both black holes have exceptionally high spins. The catalog also holds two examples of black hole–neutron star mergers.
"The message from this catalog is: We are expanding into new parts of what we call 'parameter space' and a whole new variety of black holes," says paper co-author Daniel Williams, a research fellow at the University of Glasgow and a member of the LVK Collaboration. "We are really pushing the edges and are seeing things that are more massive, spinning faster, and are more astrophysically interesting and unusual."
Among the more unusual signals that LIGO detected in the first phase of the fourth observing run was an event called GW231123_135430. As previously reported, it is the heaviest black hole binary detected to date. Scientists estimate that the signal arose from the collision of two heavier-than-normal black holes, each roughly 130 times as massive as the Sun.
Another standout is GW231028_153006, which is a black hole binary with the highest recorded inspiral spin: Both black holes appear to be spinning at about 40 percent the speed of light. "This dataset has increased our belief that black holes that collided earlier in the history of the universe could more easily have had larger spins than the ones that collided later," says LVK member Salvatore Vitale, associate professor of physics at MIT and member of the MIT LIGO Lab.
The new detections have also allowed scientists to test Albert Einstein's general theory of relativity, which describes gravity as a geometric property of space and time, using an event called GW230814_230901, which is one of the "loudest" gravitational-wave signals observed to date. The surprisingly clear signal pushed the limits of scientists' tests of general relativity, passing most with flying colors.
In addition, the updated catalog is helping scientists to nail down a key mystery in cosmology: How fast is the universe expanding today? "It's incredibly exciting to think about what astrophysical mysteries and surprises we can uncover with future observing runs," Thomas says.
"Black holes are one of the most iconic and mind-bending predictions of general relativity," says co-author and LVK member Aaron Zimmerman (PhD '13), associate professor of physics at the University of Texas at Austin, adding that when black holes collide, they "shake up space and time more intensely than almost any other process we can imagine observing. When testing our physical theories, it's good to look at the most extreme situations we can, since this is where our theories are most likely to break down, and where we have the best chance of discovery."
Read the full version of this story from the LVK Collaboration.
The paper is titled "GWTC-4.0: An Introduction to Version 4.0 of the Gravitational-Wave Transient Catalog." LIGO is funded by the NSF and is operated by Caltech and MIT, which conceived and built the project. Financial support for the Advanced LIGO project was led by NSF with Germany (Max Planck Society), the UK (Science and Technology Facilities Council), and Australia (Australian Research Council) making significant commitments and contributions to the project. More than 1,600 scientists from around the world participate in the effort through the LIGO Scientific Collaboration, which includes the GEO Collaboration.
Additional studies describing the GWTC-4.0 methods and results are online.