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An international collaboration has for the first time simulated in detail what these whispers – called late-time gravitational wave tails – might "sound" like.

"So far, we've only seen tails in simplified models, not in full simulations of numerical relativity," said Leo Stein, University of ²ÝÝ®ÊÓƵ associate professor of physics and astronomy and co-author of the study. "These are the first fully numerical simulations where we saw tails clearly."

Stein and Marina De Amicis, a postdoctoral researcher and fellow at the , were among a team of 20 researchers that published their findings in .

As gravitational waves move, they slightly stretch space-time. The tail that researchers are looking for is the slowest "sound" of space-time settling down after a wave passes.

"The tail is interesting because it's not just a set of discrete frequencies," De Amicis said. "It's more like a low hum. It is a collection of continuous frequencies that are very small as space-time is trying to relax after the wave stretches it.

"While the initial oscillations are dissipated really quickly, this decay is really, really slow. It's the final whimper of space-time returning to equilibrium."

Gravitational waves carry in them data that researchers use to better understand black holes and their collisions. Late-time tails, however, carry information not only about the black holes but also about all the surrounding space the waves have traveled through.

"What we are trying to do is understand general relativity and understand better the universe we live in," said De Amicis, the project's lead researcher. "With our initial frequencies that we have right after the black hole merger, we can look at the small scale of everything that is happening really close to the black hole.

"What's amazing is that the tails give completely complementary information. Tails really depend on the large structure of space-time, so they can give us a bigger picture of what we see happening in the universe."

But detecting and verifying the tails has proved difficult. Because these frequencies are low and easily disrupted, they can often interfere with one another.

To capture tails in simulations, the researchers had to make them as "loud" as possible in black hole collisions. This meant taking the black holes' usual merger – a relatively circular orbit – and stretching it into what is essentially a head-on collision.

The resulting crash was "loud" enough to show researchers what tails would look like, but the chances of a head-on collision of black holes is unlikely, Stein said.

"Is nature going to give us any super high-eccentricity events? Probably not," the Ole Miss physicist said. "If we do get very highly eccentric events, then it would make it easier to spot tails and observe the data. But we can't ask nature to do that. We have to use what we have."

By having these initial simulations, however, researchers can begin to move closer to understanding what actual late-time tails in space may look like.

"Now we have the machinery to fine-tune these simulations," De Amicis said. "The next step is to add some orbital motion and see how tails behave in more realistic settings."

Even the existence of late-time tails is another reinforcement of .

"Going from special relativity to general relativity was us finally being able to describe gravity in a way that's relativistic," Stein said. "And in general relativity, space-time can be curved. You don't get tails in flat space-time."

Gravitational waves were first predicted in Albert Einstein's 1915 theory of general relativity and first observed in 2015. Using detectors in the or collaborations, researchers rely on simulations of gravitational waves to know what to look for in their data.

The collaboration's most recent simulation means that – for the first time – researchers know what to look for when searching for evidence of late-time tails.

Top: Black holes exist beyond the ability for scientists to see with even the most advanced telescopes. Instead, researchers must 'listen' for the evidence of their activity with large detectors such as LIGO and LISA. A new study by physicists at UM and the Perimeter Institute for Theoretical Physics is helping researchers better understand how black holes interact with the space around them. Photo by Robert Jordan/Ole Miss Digital Imaging Services