The universe is humming with ripples in spacetime: Scientists just doubled our catalog of black hole and neutron star collisions

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Our catalog of spacetime ripples "heard" by gravitational wave detectors here on Earth has doubled, scientists say, with newly discovered sources ranging from wobbly black hole mergers to the heaviest black hole collision detected to date.

Back in 1915, Albert Einstein predicted that when the most dense and extreme objects in the universe collide, these events would set the very fabric of space and time (united as a 4-dimensional entity called spacetime) ringing. Then, 100 years later, on Sept. 14, 2015, the Laser Interferometer Gravitational-Wave Observatory (LIGO) made the first detection of these spacetime ripples — they originated from colliding black holes over 1.3 billion light-years away.

Since then, LIGO and its partner gravitational wave detectors Virgo in Italy and KAGRA (Kamioka Gravitational Wave Detector) in Japan have detected a multitude of gravitational waves from colliding black holes, merging neutron stars, and even the odd "mixed merger" between a black hole and a neutron star. The latest data collection from the LIGO-Virgo-KAGRA (LVK) Collaboration reveals the universe is practically humming with gravitational waves from 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," LVK member Lucy Thomas of the California Institute of Technology (Caltech), said in a statement. "It's incredibly exciting to think about what astrophysical mysteries and surprises we can uncover with future observing runs."

More variety

The data that comprises this catalog, dubbed the Gravitational-Wave Transient Catalog-4.0 (GWTC-4), includes 128 incredibly distant gravitational wave sources. It was collected during the fourth observational run of these gravitational wave detectors, which was conducted between May 2023 and Jan. 2024.

Prior to this, and during the first three observing runs of LIGO, Virgo and KAGRA, scientists had only "heard" 90 potential gravitational wave sources. Excitingly, GWTC-4 could technically have been even larger, as around 170 other gravitational wave detections made by LIGO, Virgo and KAGRA haven't yet made their way into the catalog.

"In the past decade, gravitational wave astronomy has progressed from the first detection to the observation of hundreds of black hole mergers," LIGO spokesperson Stephen Fairhurst, a professor at Cardiff University in the U.K., said in the statement. "These observations enable us to better understand how black holes form from the collapse of massive stars, probe the cosmological evolution of the universe and provide increasingly rigorous confirmations of the theory of general relativity."

One aspect of GWTC-4 that really stands out is the variety of events that created these signals. Within this catalog are gravitational waves from mergers between the heaviest black hole binaries yet, each about 130 times as massive as the sun, lopsided mergers between black holes with seriously mismatched masses, and black holes that are spinning at incredible speeds of around 40% the speed of light. In these cases, scientists think the extreme characteristics of the black holes involved in these mergers are the result of prior collisions, providing evidence of merger chains that explain how some black holes grow to masses billions of times that of the sun.

"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," LVK member and MIT scientist Salvatore Vitale said in the statement.

GWTC-4 also includes two new mixed mergers involving black holes and neutron stars.

"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," LVK member Daniel Williams, of the University of Glasgow in the U.K., said in the statement. "We are really pushing the edges, and are seeing things that are more massive, spinning faster, and are more astrophysically interesting and unusual."

The catalog also demonstrates just how sensitive the LVK detectors have become. Some of the neutron star mergers occurred up to 1 billion light-years away, while some of the black hole mergers occurred up to 10 billion light-years away. These detections have allowed scientists to test the theory that first predicted the existence of both black holes and gravitational waves, Einstein's magnum opus theory of gravity, general relativity.

"Black holes are one of the most iconic and mind-bending predictions of general relativity. They shake up space and time more intensely than almost any other process we can imagine observing," LVK member Aaron Zimmerman, of the University of Texas at Austin, said in the statement. "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.

"So far, the theory is passing all our tests. But we’re also learning that we have to make even more accurate predictions to keep up with all the data the universe is giving us."

The LVK results will soon appear in a special edition of the Astrophysical Journal Letters.