Gravitational-wave observatories have released their latest catalogue of cosmic collisions, bringing their total number of detections to 90. The new crop of 35 events includes one featuring the lightest neutron star ever seen, as well as two clashes involving surprisingly large black holes.

The detections come from the two Laser Interferometer Gravitational-Wave Observatory (LIGO) sites, in Louisiana and Washington State, and their sister detector, Virgo, in Italy. They were recorded during 21 weeks of operations, beginning on 1 November 2019, that racked up an average detection rate of one event every 4.2 days. Since then, the collaboration has expanded to include the KAGRA detector in Japan, which started making observations in February 2020. The LIGO–Virgo–KAGRA collaboration describes its results in a paper posted on the arXiv preprint repository.

Gravitational waves are ripples in the fabric of space-time that are produced when large masses accelerate. Like the detections previously reported by LIGO–Virgo, the latest ones are all attributed to pairs of dense stellar remnants spiralling into each other and merging. The vast majority, including LIGO’s first historic detection in 2015, have involved pairs of black holes, but in a few cases one or both of the objects were neutron stars.

The collaboration initially released data on only high-confidence detections, but the latest catalogue—as well as the previous one, released in October 2020 — includes any detections that have better-than-even chances of being genuine gravitational waves. The team estimates that around 10–15% of the latest candidates in the catalogue are false alarms, “caused by instrumental noise fluctuations”.

Monster mash

From the shape and frequency of the waves produced by mergers, researchers can calculate the details of a host of features for the objects involved, including their masses and their distance from Earth. The latest 35 events varied in distance from roughly 245 million to more than 2.2 billion parsecs (800 million to more than 7 billion light years) away.

And they include some real monsters: two events involved black holes with masses more than 60 times that of the Sun. For astrophysicists, the mere existence of these black holes is problematic. Typically, black holes are thought to form from the collapse of a very massive star at the end of its life. But prevailing theories predict that some dying stars should explode rather than collapse, which should leave a dearth of black holes in the range of roughly 65–120 solar masses.

A merger that LIGO and Virgo picked up on 21 May 2019, revealed in last year’s catalogue, had already challenged that assumption because it involved a black hole of 85 solar masses. Now that the team has identified two more outlier events, it seems less likely that the earlier one was a fluke.

Alessandra Buonanno, a LIGO astrophysicist at the Max Planck Institute for Gravitational Physics in Potsdam, Germany, says one possible explanation is that these massive black holes might have come about as a result of an earlier merger, rather than the collapse of a single star. “What we are seeing could be a second-generation binary,” she says.

The observatories spotted another intriguing event on 19 December 2019, involving a black hole 30 times the mass of the Sun swallowing a minuscule neutron star. At just 1.17 solar masses, it is the one of the lightest known neutron stars and the lowest-mass object ever detected by LIGO–Virgo. But Buonanno warns that this was one of the lower-confidence detections, so could represent background noise rather than a genuine event.

Pattern recognition

Another LIGO astrophysicist, Daniel Holz at the University of Chicago in Illinois, says that the bounty of black-hole mergers is getting large enough for researchers to be able to see patterns emerging. The most conspicuous of these is that black-hole mergers tend to happen more often in galaxies that are farther away from us in space and time.

The upshot is that black-hole mergers have been getting less common as the Universe matures. “This is consistent with expectations from theory,” Holz says. “There were more stars being created earlier in the Universe, and therefore it is natural to expect that there would be more black holes created, and therefore more black-hole mergers.”

LIGO and Virgo shut down on 27 March 2020—earlier than planned owing to the COVID-19 pandemic—and have been undergoing major upgrades. They are expected to reopen in late 2022 for another run of observations, which together with KAGRA could double the bounty yet again. Researchers expect that once they have hundreds of events to compare, they will be able to see trends that point to the origins and history of the binary systems involved in the mergers, as well as the history of the Universe itself. "When we get down to the end of that, it’s going to be amazing," says Holz.

Meanwhile, astronomers around the world are still hoping for a repeat of the August 2017 merger of two neutron stars, which is the only gravitational-wave event so far to have also been seen by conventional observatories.

This article is reproduced with permission and was first published on November 9 2021.