What do hundreds of gravitational wave events reveal about the universe?

A boom in gravitational waves leaves scientists with more questions than answers

By KR Callaway edited by Lee Billings

A soaring cosmic symphony surrounds us; its notes come from massive celestial bodies crashing together hundreds of millions or even billions of light years away. But scientists have only been tuning into this music of the spheres for about a decade, thanks to sophisticated observatories purpose-built to pick up these reverberations—gravitational waves—that otherwise ripple unnoticed through the fabric of spacetime. And with each newfound note, the symphony becomes more complex – and, for now, perhaps more confusing.

Ever since astronomers announced the first gravitational wave detection in 2016, they have been carefully fine-tuning their detectors to catch more mergers. Today, four facilities are combined to form a global network of observatories – namely the two stations of the Laser Interferometer Gravitational-Wave Observatory (LIGO) in the USA and the individual stations of the Virgo and Kamioka Gravitational-Wave Detector (KAGRA) in Italy and Japan respectively. The LIGO-Virgo-KAGRA (LVK) collaboration has proved particularly successful in recent years; the network’s fourth observing period yielded more gravitational wave detections than the previous three combined. The total number of candidate events observed is up to 218, according to a catalog released earlier this month.

“We learn a lot of things that are qualitative and phenomenological from the catalog,” says Jack Heinzel, a member of the LVK collaboration and a graduate student in physics at the Massachusetts Institute of Technology. “Starting to see all these different structures emerge is quite fascinating.”

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Scientists are excited about gravitational waves because these space-time waves represent an entirely new way of studying the universe, independent of the electromagnetic radiation (light) that most other astronomical observations rely on. Gravitational waves, emanating from the inaccessible hearts of collapsing stars and from the tumultuous spacetime crunches of merging black holes and neutron stars, provide deep, fundamental insights into these distant astrophysical systems that are otherwise unavailable. But analyzing the gravitational waves from these events still leaves scientists with more questions than answers.

Waves produced by merging pairs of black holes are especially a feast for data-hungry theorists. By predicting the spins, trajectories and masses of the progenitor black holes from their emitted gravitational waves, scientists can better understand how the black holes formed in the first place – and how they and the universe around them have subsequently evolved. Most of the merging black holes glimpsed by the LVK are thought to have been born through the death of massive stars.

“Gravitational wave astrophysics is almost like paleontology,” says Ilya Mandel, a theoretical astrophysicist at Monash University in Australia. “Black holes are the fossils of the massive stars. We can rewind the clock and use it to learn something about how the stars lived.”

The catalog of observations now includes many “typical” gravitational wave events – high-energy collisions between two black holes of roughly the same mass – as well as waves caused by unusual mergers.

Some of the catalog’s latest editions include GW231123, caused by the collision of two abnormally heavy black holes with a terminal mass about 225 times that of our Sun; GW231028, a merger of two black holes, each spinning at about 40 percent the speed of light; and GW241011 and GW241110, each of which appears to have sprung from mergers in which the progenitor black holes have been wildly mismatched in mass and in the alignment of their respective orbits and spins. These events all suggest intricate formation processes in which the black holes themselves were formed through several previous mergers.

Still, despite all this data, researchers say the field of gravitational wave astronomy is at a point where the flood of discoveries is opening up more new possibilities rather than ruling out old ones.

“There are clues, but they are far from a ‘smoking gun,'” says Salvatore Vitale, a member of the LVK collaboration and a physicist at MIT. “Astrophysics is very messy, and so it turns out there are several ways you can make these features.”

Scientists still haven’t pinned down the full range of celestial bodies whose mergers can produce gravitational waves that can be detected by the LVK. Nor have they reached consensus on what causes some of the unique properties of atypical black holes, and how much a given set of waves can reveal about their immediate cosmic surroundings.

Vitale notes that understanding the complex formation of gravitational waves is “inherently a very difficult problem,” but that further observations should eventually provide the answers scientists need. The main obstacle is the detection rate, which is increasing, but is still hampered by the limited sensitivity of the LVK network and the fact that the network has extensive, pre-planned offline periods for maintenance and upgrades.

LIGO, Virgo, and KAGRA are all large, L-shaped observatories, with each arm of the “L” formed by a mile-long vacuum tube isolated from sources of environmental noise such as earthquakes—as well as pounding surf on beaches and passing trucks on highways in their geographic vicinity. Laser beams crossing each arm and bouncing between mirrors at the ends are combined to reveal extremely small differences in their travel times, which can be produced when spacetime stretches and contracts due to the passage of a gravitational wave.

Expanding the catalog by finding significantly weaker gravitational waves from more distant or less energetic sources may be beyond the capabilities of even a fully optimized LVK network. Picking up new tunes in this celestial symphony—like gravitational waves from merging supermassive black holes, or the cosmic background of primordial gravitational waves produced shortly after the big bang—probably requires building bigger, better “ears.”

“If you want to see smaller signals, you first need a much more sophisticated experiment that has very low noise,” says Arushi Bodas, who theorizes about primordial gravitational waves as a graduate student in physics at the University of Maryland. “Some people envision larger versions of LIGO, really…, or there’s an idea of ​​actually putting (an observatory) in space.”

Such larger observatories are probably still many years in the future, researchers say. In the meantime, they hope to piece together more of the gravitational wave puzzle with deeper analysis of existing data – and soon with data from the next observation period, which will start later this year.

“It’s really like a detective’s work, where you look for all the clues you can and try to see if they point one way over the other,” says Vitale. “There will be progress. It will probably be slower than people imagined 10 years ago, but that’s good. It means there’s work to be done.”