The Gravitational Wave Signal May Not Have Come From Merging Black Holes

It could have originated from another very bizarre object.

Back in February, scientists from the Laser Interferometer Gravitational-Wave Observatory (LIGO) announced to the world that they had finally detected gravitational waves — ripples in spacetime as predicted by Albert Einstein’s Theory of Relativity over 100 years ago.

The signal was thought to come from two gigantic black holes merging into one, but a new group of researchers suggest it could have come from something even more bizarre — a gravastar.

“We’re not trying to say LIGO was wrong,” said Paolo Pani of the Sapienza University of Rome, Italy, to New Scientist. However, Pani and his colleagues propose that the signal may not have come from a black hole merger afterall.

Why? It has to do with how the LIGO signal breaks down into three phases. First, the inspiral phase tells you two objects are getting closer as they orbit each other, which changes the frequency of their gravitational waves. Second, the merger phase is where the signal builds in intensity and frequency. Finally, during the ringdown phase, there is a rapid drop-off as the merged black hole settles down and the wave fades.

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It is last phase that indicates the formation of a new event horizon — where the escape velocity (or speed required to escape the gravitational pull of the black hole) is equal to the speed of light.

“The common view is that when you see this ringdown, that is a signature of the horizon, because only black holes will vibrate in precisely that way,” explained Pani. However, in a new paper published in Physical Review Letters, he and his team show that there are other possibilities.

One alternative explanation to a black hole merger is a gravastar, which is a dense ball of matter kept inflated by a core of dark energy. Dark energy is a kind of reverse gravity that repels rather than attracts, pushing everything apart, and it makes up roughly 70 percent of all energy in the universe. And although we have never seen a gravastar, all the evidence we have for black holes could also support their existence.

However, there is one crucial difference: a gravastar lacks an event horizon. Rather, photons — particles of light — get trapped in a circular orbit around the gravastar, called a light ring.

“If an object is almost as compact as a black hole, even if it doesn’t have an event horizon, it will vibrate almost the same way,” said Pani. “The only difference appears at a very late time when the signal is small, so there is a chance LIGO will miss it.”

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So was the signal from a black hole or a gravastar? Right now, scientists are not sure.

“Our signal is consistent with both the formation of a black hole and a horizonless object — we just can’t tell,” explained B.S. Sathyaprakash of Cardiff University, UK, who is part of the LIGO team, to New Scientist.

To settle the debate, scientists will have to detect either a larger black hole merger or one that is closer to Earth. “That’s when we can conclusively say if the late-time signal is consistent with the merged object being a black hole or some other exotic object,” Sathyaprakash continued.

More than likely, the black hole explanation will be proven as the source, but it is always worth double checking as a scientist.

But considering it took 100 years to detect a single gravitational wave, it may be awhile before physicists spot another one.

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