What are continuous gravitational waves?

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An artist’s impression of two black holes about to collide and merge – which would produce gravitational waves. Credit: MARK GARLICK / SCIENCE PHOTO / Getty images

It’s been five years since the groundbreaking detection of gravitational waves, and researchers are probing ever-deeper into these ripples in the fabric space-time.

But instead of searching for waves caused by the clash of cosmic titans, like two black holes or neutron stars, recent studies are focused on finding a softer signal – the “hum” of gravitational waves produced by a solitary neutron star.

Why can you hear gravitational waves?

Neutron stars are stellar corpses left over from supernova explosions: after the stars explode they collapse in on themselves, crushing atoms down into a super-dense ball of neutrons – essentially compressing a star about the size of the Sun to the size of a city, around 20 kilometres across.

These odd objects are not only the densest objects in the universe, but they’re also spinning hundreds of times per second. If a neutron star isn’t spot-on spherical, it will wobble as it rotates and produce a tiny ripple in space-time – a faint “hum” of gravitational waves.

OzGrav postdoctoral researcher, Karl Wette from the Australian National University (ANU), compares this phenomenon to wildlife sounds in the Australian bush.

“The gravitational waves from black hole and neutron star collisions we’ve observed so far are like squawking cockatoos – loud and boisterous, they’re pretty easy to spot,” he explains.

“A continuous gravitational wave, however, is like the faint, constant buzz of a faraway bee, which is much more difficult to detect.”

Wette is part of an international collaboration searching for these buzzing bees, and they’ve just released a paper explaining how they looked at 15 young neutron stars in supernova remnants during the third Advanced LIGO and Virgo observing run. The data spanned a period from 1 April 2019 to 27 March 27 2020.

They didn’t find any evidence of continuous gravitational waves just yet, but they’re getting closer.

How does LIGO look for gravitational waves?

According to OzGrav Associate Investigator Lilli Sun, also from ANU, in their most recent search they tried to listen for the waves at different frequencies.

“Since the interior configuration and structure of the neutron stars remain a mystery, there are possibilities that with some configurations, the continuous waves from the stars are emitted at two different frequencies,” she explains.

“In our analyses in the third observing run, for the first time, we look for signals from these young neutron stars that contain two frequency components.

“In the future, if we detect such a signal, it will bring us invaluable information about the neutron star structure.”

Artist’s impression of continuous gravitational waves generated by a spinning asymmetric neutron star. Credit: Mark Myers/OzGrav/Swinburne University.

This is one of the reasons gravitational waves make astrophysicists froth at the mouth – because they offer an excellent way to probe the properties of wacky objects like neutron stars, black holes and more. If the collaboration manages to hear a continuous gravitational wave, it will mean they can peer into the heart of a neutron star.

Already, these results have placed some constraints on what we know about these objects, according to OzGrav postdoctoral researcher Carl Blair from the University of Western Australia.

“We don’t know that much about neutron stars because they’re so small and strange,” he explains. “Are they hard or soft?  And when they spin fast as they collapse, do they wobble away that energy in the form of gravitational waves?

“While there is no evidence yet for continuous gravitational waves from neutron stars, limits have been placed on how wobbly a neutron star is from the fact that we haven’t measured gravitational waves from them yet.”

This recent study targeted just 15 specific supernova remnants, but scientists reckon there are billions of neutron stars strewn across the Milky Way that could be emitting a hum of gravitational waves. Other searches have also been conducted over the whole sky using the LIGO and Virgo Observatories, including a 2021 study by Wette.

Four further papers were based on data from this first half of the third observing run, including several looking at pulsars – neutron stars that emit bright flashes of energy as they spin, like lighthouses in space. Again, nothing was found, but it seems like only a matter of time.

As our detectors become even more sensitive and advanced, we might soon hear that tell-tale buzz.

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