Our catalog of ripples in spacetime “heard” by gravitational wave detectors here on Earth has doubled, researchers 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 matter of space and time (united as a 4-dimensional entity called space time) calls. Then, 100 years later, on September 14, 2015, the Laser Interferometer Gravitational-Wave Observatory (LIGO) made the first discovery of these space-time ripples – they originate from collisions black holes over 1.3 billion light years away.
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 is incredibly exciting to think about what astrophysical mysteries and surprises we can uncover with future observing runs.”
More variety
The data comprising this catalog, called the Gravitational-Wave Transient Catalog-4.0 (GWTC-4), includes 128 incredibly distant gravitational-wave sources. It was collected during the fourth observing run of these gravitational wave detectors, which was carried out between May 2023 and January 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 be even bigger, as around 170 other gravitational wave detections made by LIGO, Virgo and KAGRA have yet to enter the catalogue.
“Over the past decade, gravitational wave astronomy has progressed from the first discovery to the observation of hundreds of black hole mergers,” LIGO spokesperson Stephen Fairhurst, a professor at Cardiff University in the UK, 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 to date, each about 130 times as massive as sunlopsided mergers between black holes with severely mismatched masses, and black holes spinning at incredible speeds of around 40% the speed of light. In these cases, scientists believe that the extreme properties of the black holes involved in these mergers are the result of past collisions, providing evidence of fusion 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 universe’s history could more easily have had higher spins than those that collided later,” LVK member and MIT researcher 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 UK, said in the statement. “We’re really pushing the edges, seeing things that are more massive, spin faster, and are more astrophysically interesting and unusual.”
The catalog also shows 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 enabled scientists to test the theory that first predicted the existence of both black holes and gravitational waves, Einstein’s magnum opus theory of gravitation, general relativity.
“Black holes are one of the most iconic and thought-provoking 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 we test our physical theories, it’s good to look at the most extreme situations we can, since that’s where our theories are most likely to break down, and where we have the best chance of discovery.
“So far, the theory has passed all our tests. But we’re also learning that we need 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 issue of the Astrophysical Journal Letters.
