Status Report: Gravitational Waves

Nine years after the very first detection of the feeble ripples in spacetime predicted by Albert Einstein, gravitational-wave astronomy has become a rich and promising field of astronomy. At the 32nd General Assembly of the International Astronomical Union in Cape Town, South Africa, researchers presented their latest results and expectations for the future.

Currently in the middle of their fourth observing run (which will last until June, 2025), the collaboration behind the gravitational-wave detectors in the U.S., Europe, and Japan (LIGO, Virgo, and KAGRA, respectively) has already observed more than 200 sources to date – something previously not expected to happen before early 2025. For comparison: At the start of the fourth observing run, in May 2023, the counter stood at just 90 events. “The detection rate is now a few per week on average,” says Sylvia Biscoveanu (Northwestern).

Most of these events are produced by merging black holes (or, in some cases, by mergers involving neutron stars) in galaxies a few hundreds of millions of light-years away. The measurements reveal the masses of the merging compact objects and hint at their formation histories. Some turn out to weigh in at more than 100 solar masses — too heavy to have been produced by regular supernova explosions. Most likely, these heavyweights are the result of earlier mergers, says Biscoveanu.

Then again, the very first detection in the current observing run, GW230529 (seen on May 29, 2023), originated from the collision and merger of a neutron star and an unexpectedly low-mass black hole, between 2.5 and 4.5 solar masses. Given the fact that such mergers are much harder to detect, as they produce less powerful gravitational waves, even this single case indicates that they must occur more often than theorists had thought — some didn’t even think these low-mass black hole could exist at all.

Kilonovae

Interestingly, simulations suggest that neutron stars can be tidally ripped apart when gobbled up by such low-mass black holes, temporarily leaving a cloud of superhot gas that would emit high-energy radiation. Based on small-number statistics, Biscoveanu now thinks that some 18% of mergers involving a neutron star might also produce an electromagnetic counterpart — three times more frequent than earlier estimates. (No counterpart was found for the May event, though.)

So far, only one gravitational-wave producing merger of two neutron stars (GW170817) has been observed by NASA’s Fermi Space Telescope as a burst of gamma-rays and by telescopes on the ground as a so-called kilonova. Most astronomers feel a second such event is much overdue, but Paul Groot (Radboud University, The Netherlands), who organized the gravitational-wave symposium at the International Astronomical Union (IAU) meeting, calls the delay “a blessing in disguise: We have now concentrated all our efforts on understanding this singular event.”

The Gravitational-Wave Background

Another way to observe gravitational waves is indirectly, by measuring the pulses from spinning neutron stars, known as pulsars. Distributed as they are over the sky, the arrival times of pulsars’ regular blips can change ever so slightly as they’re influenced by extremely low-frequency gravitational waves passing through interstellar space.

The tentative detection of a cosmic background of these nanohertz waves, announced last year, was largely put down to orbiting and merging supermassive black hole binaries in distant galaxies. There’s still a lot of wiggle room in the detection, though.

Results become more definitive as more rapidly spinning millisecond pulsars are added to the “array,” and as they are monitored for a longer period of time. Ryan Shannon (Swinburne University of Technology, Australia) told the IAU meeting that the relatively new MeerKAT radio observatory in South Africa is doing just that. Consisting of 64 dishes each 13.5 meters in diameter, MeerKAT is currently observing 83 millisecond pulsars every two weeks. “MeerKAT is rapidly catching up with the nanohertz results of other groups,” he says.

Shannon expects that the pulsar timing observations will reveal individual sources of low-frequency gravitational waves within four years or so. “The story is not finished yet,” he says. “We need more time.”

Meanwhile, the American pulsar timing array consortium, known as NANOGrav, experienced a huge setback with the collapse of the 305-meter Arecibo radio telescope in late 2020. But, says Scott Ransom (National Radio Astronomy Observatory), the future DSA-2000 observatory will devote a quarter of its time to observing pulsars. “This will be one of the earliest science [results] coming from DSA-2000,” Ransom says. Its completion is expected in late 2026.

NANOGrav may soon be as sensitive to nanohertz gravitational waves as the MeerKAT pulsar timing array. “It replaces the lost Arecibo sensitivity,” Ransom says. That’s partly because DSA-2000 is also expected to discover a few hundred new millisecond pulsars. If the source of the gravitational waves is supermassive black hole pairs, Ransom estimates that within the next 10 to 15 years, NANOGrav will detect dozens of them as individual sources.

“The future is loud” for gravitational-wave astronomy, says Biscoveanu, referring to the often-used metaphor that measuring gravitational waves is like “listening” to the universe. Symposium organizer Groot couldn’t agree more. “This is the first IAU General Assembly that hosted a gravitational-wave symposium,” he says, “but it will definitely not be the last.”

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This story was made possible through a travel grant of the Dutch VWN Trip-fund.