Gravitational wave ‘radar’ could help map the invisible universe

It sounds like the setup for a joke: if radio waves give you radar and sound your sonar, what do you get from gravitational waves?

The answer could be “GRADAR” – gravitational wave “radar” – a potential future technology that could use gravitational wave reflections to map the invisible universesay researchers in a paper accepted to Physical Assessment Letters† By looking for these signals, scientists can find dark matter or faint, exotic stars and learn more about their deep insides.

Astronomers routinely use gravitational waves – first traveling ripples in the fabric of space and time itself detected in 2015 — to view catastrophic events that are difficult to study with light alone, such as the merging of two black holes (SN: 2/11/2016

But physicists also know of a seemingly useless property of gravitational waves: They can change course. Einstein’s theory of gravity says that spacetime is distorted by matter, and any wave passing through these distortions will change course. As a result, when something sends out gravitational waves, some of the signal comes straight to Earth, but some of it can arrive later — like an echo — after taking longer paths that curve around a star or something more massive.

Scientists have always thought that these later signals, called “gravity glitter”, would be too faint to detect. But physicists Craig Copi and Glenn Starkman of Case Western Reserve University in Cleveland, Ohio, took a leap: Using Einstein’s theory, they calculated how strong the signal would be when waves scatter through the gravitational field in a star itself.

“The shocking thing is that you seem to get a much bigger result than you expected,” says Copi. “It’s something we’re still trying to understand, where that’s coming from — whether it’s believable, even, because it just seems too good to be true.”

If the glints of gravity could be that strong, astronomers could potentially use them to trace the insides of stars, the team says. Researchers could even look for massive bodies in space that would otherwise be impossible to detect, such as blobs of dark matter or lone neutron stars on the other side of the observable universe.

“That would be a very exciting probe,” said Maya Fishbach, an astrophysicist at Northwestern University in Evanston, Illinois, who was not involved in the study.

However, there are still reasons to be cautious. If this phenomenon passes through more detailed research, Fishbach says, scientists would need to understand it better before using it — and it will likely be difficult.

“It’s a very difficult calculation,” says Copi.

But similar challenges have been overcome before. “The whole story of gravitational wave detection has been like this,” Fishbach says. It was a struggle to do all the math needed to understand their measurements, she says, but now the field startsSN: 1/21/21)† “Now is the time to get really creative with gravitational waves.”

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