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Gravitational wave ‘radar’ can help map the invisible universe

It sounds like setting up a joke: If radio waves give you radar and sound gives you sonar, what do gravitational waves get?

The answer may be “GRADAR” – gravitational waves “radar” – a potential future technology that could use gravitational wave reflections to map the unseen universe, say researchers in a paper they accepted Physical Review Letters. By looking for these signals, scientists may be able to find dark matter or strange dark stars and learn about their deep interior.

Astronomers routinely use gravitational waves — traveling ripples in the very fabric of space and time, first discovered in 2015 — to watch catastrophic events that are difficult to study with light alone, such as the merger of two black holes (SN: 2/11/2016).

But physicists also know about a seemingly useless property of gravitational waves: they can change course. Einstein’s gravitational theory says that matter distorts spacetime, and any wave that passes through these distortions will change its path. The upshot is that when something emits gravitational waves, part of the signal comes directly to Earth, but some may arrive later — like echoes — after taking longer paths that curve around a star or other heavy object.

Scientists have long believed that these later signals, called “gravitational glints,” must be too weak to be detected. But physicists Craig Copi and Glenn Starkman of Case Western Reserve University in Cleveland, Ohio, jumped: Working on Einstein’s theory, they calculated how strong the signal is when waves propagate through the gravitational field within the star itself.

“The shocking thing is that you seem to get a much greater result than you expected,” Kobe says. “It’s something we’re still trying to understand, where that comes from – whether it’s believable, even because it sounds too good to be true.”

The team says that if gravitational flashes are too strong, astronomers can use them to track what’s inside the stars. Researchers can even search for massive objects in space that would have been impossible to detect, such as balls of dark matter or lone neutron stars on the other side of the visible universe.

“This is going to be a very exciting probe,” says Maya Fischbach, an astrophysicist at Northwestern University in Evanston, Illinois, who was not involved in the study.

However, there are still reasons to be careful. If this phenomenon faces more detailed scrutiny, Fischbach says, scientists will have to understand it better before they can use it — and that may be difficult.

“It’s a very difficult math,” Kobe says.

But similar challenges have been overcome before. “The whole story of the discovery of gravitational waves was like this,” Fischbach says. She was having a hard time doing all the math needed to understand their measurements, she says, but the field is taking off now (SN: 1/21/21). “This is the time to get really creative with gravitational waves.”

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