How I wonder what you are Julian Stratenschulte/DPA/Corbis

IT’S one surprise after another. The detection of gravitational waves announced earlier this year sent ripples through the world of physics. The signal was thought to come from two gigantic black holes merging into one, but now a group says it could have come from something even more exotic – a gravastar.

“An object almost as compact as a black hole, but with no event horizon, will vibrate in almost the same way“

No one is disputing the first detection of gravitational waves. The Laser Interferometer Gravitational-Wave Observatory (LIGO) team announced in February that it had seen these ripples in space-time predicted by Albert Einstein’s theory of relativity (see “How we found them: Inside a giant gravitational wave detector“).


“We’re not trying to say LIGO was wrong,” says Paolo Pani of the Sapienza University of Rome, Italy. But Pani and his colleagues say the signal might not have come from a black hole merger.

That’s because the LIGO signal breaks down into three phases. First there is the inspiral, which tells you two objects are getting closer as they orbit each other, changing the frequency of their gravitational waves. Next, there is the merger itself, in which the signal ramps up in intensity and frequency. Finally there’s the ringdown, a rapid drop-off as the merged black hole settles down and the wave fades.

In particular, this last phase would indicate the formation of a new event horizon, the region of space from which not even light can escape a black hole’s clutches.

“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,” says Pani. But his team shows there are other possibilities (Physical Review Letters, doi.org/bfrm).

One is a proposed alternative to black holes called a gravastar, a dense ball of matter kept inflated by a core of dark energy. We have never seen one, but all the evidence we have for black holes could also support their existence. A crucial difference is that gravastars lack an event horizon. Instead, photons can 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,” says Pani. “The only difference appears at a very late time when the signal is small, so there is a chance LIGO will miss it.”

“Our signal is consistent with both the formation of a black hole and a horizonless object – we just can’t tell,” says B. S. Sathyaprakash of Cardiff University, UK, who is part of the LIGO team. But if we can detect larger black holes merging, or a pair that is closer to us, it should settle the matter, he says. “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.”

Ultimately, the black hole explanation is likely to win out, but it is worth double-checking, says Pani. “As scientists, we try to play the devil’s advocate and not believe in paradigms without observational evidence.”

This article appeared in print under the headline “Have we glimpsed a gravastar?”