The rise of gravitational waves leaves scientists with more questions than answers

The rise of gravitational waves leaves scientists with more questions than answers

New data release more than doubles the number of candidate gravitational wave events and reveals unexpected complexities related to black hole mergers

By KR Callaway edited by Lee Billings

A soaring cosmic symphony surrounds us; its notes emerge from massive celestial objects crashing hundreds of millions or even billions of light years away. But scientists only listened to this music of the spheres about a decadethanks to sophisticated observatories custom-built to capture these reverberations – gravitational waves – which otherwise propagate unnoticed through the fabric of space-time. And with each new note, the symphony becomes more complex – and, for now, perhaps more confusing.

Since astronomers announced first detection of gravitational waves in 2016, they carefully tweaked their detectors to detect more mergers. Today, four facilities combine to form a global network of observatories, namely the two Laser Interferometer Gravitational-wave Observatory (LIGO) stations in the United States and the single Virgo and Kamioka Gravitational-wave Detector (KAGRA) stations in Italy and Japan, respectively. The LIGO-Virgo-KAGRA (LVK) collaboration has proven particularly successful in recent years; the network’s fourth observation period yielded more gravitational wave detections than the previous three combined. The total number of candidate events observed amounts to 218, according to a published catalog earlier this month.

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

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Researchers are excited about gravitational waves because these space-time ripples represent a whole new way to study the universe, independent of the electromagnetic radiation (light) that most other astronomical observations rely on. Emerging from the inaccessible cores of collapsing stars and the tumultuous space-time churning of merging black holes and neutron stars, gravitational waves provide deep and fundamental ideas on these distant astrophysical systems which are otherwise unavailable. But analyzing the gravitational waves from these events still leaves researchers with more questions than answers.

The waves produced by merging pairs of black holes, in particular, are a treat for data-hungry theorists. By guessing the spins, orbits, and masses of progenitor black holes from their emitted gravitational waves, researchers can better understand how black holes formed in the first place and how they and the universe around them subsequently evolved. Most of the merging black holes seen by LVK are thought to have originated via 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 massive stars. We can go back in time and use that to learn something about how stars lived.”

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

Some of the more recent editions of the catalog include GW231123caused by the collision of two abnormally heavy black holes whose final mass is approximately 225 times that of our sun; GW231028a merger of two black holes in which each rotates at about 40% the speed of light; And GW241011 and GW241110each of which appears to have arisen from mergers where the progenitor black holes were wildly mismatched in terms of mass and the alignment of their respective orbits and spins. These events all suggest complex formation processes in which the black holes themselves formed as a result of multiple previous mergers.

Yet despite all this data, researchers say the field of gravitational-wave astronomy is at a point where the flood of discoveries is providing more new possibilities rather than excluding old ones.

“There are clues, but they are by far not definitive proof,” says Salvatore Vitale, a member of the LVK collaboration and a physicist at MIT. “Astrophysics is really complicated, and so it turns out there are multiple ways to create these features.”

Researchers have not yet identified all the celestial bodies whose merger can produce gravitational waves detectable by LVK. They also haven’t reached a consensus on what causes some of the unique features of atypical black holes, or what a given set of waves can reveal about its 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 pace of discoveries, which is accelerating but remains hampered by the limited sensitivity of the LVK network and the fact that the network has long offline periods planned in advance for maintenance and upgrades.

LIGO, Virgo, and KAGRA are all large L-shaped observatories, with each arm of the “L” formed by a several-kilometre-long vacuum tube insulated against sources of ambient noise such as earthquakes, as well as against breaking waves on beaches and passing trucks on highways in close geographic proximity. The laser beams passing through each arm and bouncing between the mirrors at the ends are combined to reveal extremely slight differences in their travel times, which can occur when space-time 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 exceed the capabilities of even a fully optimized LVK network. Collect new melodies in this heavenly symphony, like gravitational waves from merging supermassive black holesor the cosmic background of primordial gravitational waves produced shortly after the Big Bang – probably requires building bigger and better “ears”.

“If you want to see smaller signals, you’ll need, first of all, a much more sophisticated experiment with very low noise,” says Arushi Bodas, who theorizes about primordial gravitational waves as a physics doctoral student at the University of Maryland. “Some people are looking at larger versions of LIGO, basically…, or there’s an idea to put [an observatory] actually in space.

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

“It’s really like a detective’s work, where you look for every possible clue and try to see if they point one way versus another,” Vitale says. “There will be progress. It will probably be slower than we imagined ten years ago, but that’s good. It means there is work to be done.”