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LIGO's black hole detection survives the gravatstar test

Ever since the historic discovery of gravitational waves by the Laser Interferometry Gravitational wave Observatory (LIGO), scientists have been trying to amend and refine that data. The original idea was that the first confirmed detection of gravitational waves was the result of a binary black hole event: two black holes meeting in space, spiraling inward, and eventually merging. The dynamics of this largely hypothetical process were thought to be quite well understood — but there have been discrepancies between observation and expectation that imply the event may have been quite different than originally imagined.

Now, a new test has examined the idea that what LIGO actually saw was not the merger of two black holes, but the merger of two gravastars. The test did not confirm this hypothesis, but it’s an interesting enough concept to be worth going over, regardless.

A gravastar is a hypothetical star-ish astronomical body that’s basically composed of a ball of exotic matter, thought to be much like dark energy, with a shell of normal star matter surrounding it on all sides. The exotic core in this hybrid body would prevent the collapse of the normal matter into a traditional black hole, essentially keeping the whole thing inflated. From the outside it would look quite similar to a black hole, but gravastars do not have an event horizon, so photons can technically get stuck in a near-infinite orbit around the outside called a “light ring.”

The other way gravastars differ is in the dynamics of a hypothetical merger. All mergers of super-massive bodies seem to occur in three basic phsaes: the spiral-inward phase, the merger phase, and the “ringdown” phase where the signal quickly falls off after the event. It’s this ringdown that had the potential to distinguish between a black hole merger and a gravastar merger, as two gravastars should have produced a noticeably different ringdown patterns than expected. This international team, based out of Germany and Brazil, set out to test check the LIGO detection for predicted gravastar values.

blackholeintstelIn the team’s own opinion, their findings don’t support the idea that LIGO actually detected a gravastar merger without knowing it. In all likelihood, the gravitational waves were the product of a boring old collision between two black holes traveling at relativistic speeds. Remember, though, that the first detection by LIGO was not the only detection, so the concept of a gravastar collision likely won’t go anywhere soon. LIGO’s second detection was announced a few months ago, and it could present just as likely a target. The procedures refined here can be applied to later observations, down the road.

LIGO’s original assumptions haven’t always fared so well, however. Earlier this year, it became clear that an explanation was needed for the burst of gamma ray radiation that was observed alongside the gravitational waves. Two black holes interacting should release nothing as light, yet here was a huge release of high-energy radiation. One hypothesis could explain this quite handily: black holes do release lots of gamma rays when they’re born.

So it was both a black hole merger and the birth of a black hole? Yep.

The idea is that instead of two black holes spiraling inward to merge, two stars did so, long ago, and each was individually large enough to one-day form a black hole itself, upon collapse. Once merged, their orbital speed around an empty point would become rotational speed around the new star’s axis — and it should be very speedy indeed. If it was fast enough, the star could spin so its core, where most of the mass resides, elongated into a “barbell” shape and eventually split into two separate cores.

twin black holes 12These cores would rapidly orbit one another inside the mega-star until one-day the star cooled and began to collapse. Each core could become its own black hole while still orbiting the other inside the dying star, then spiral down and merge with the other in barely more than an instant. The result would be a black hole merger event of the sort originally predicted, but with the outer layers of a massive super-star all around it. The gamma ray burst thus came from the same source as all such bursts during black hole formation: soon after collapse, the black hole sucks up the star’s outer layers. As the matter falls over the black hole’s event horizon, a portion of its mass is converted to energy and released — thus, the observed bursts of energy.

Gravastars are intriguing to many physicists not only because they’re fantastical new-physics chimeras, but because if they are real then they get around the requirement that a black hole is a place where the laws of physics seem not to exist. By their very nature they would be difficult to distinguish from black holes — but their behavior is distinct enough that they will respond differently to the most exotic events. Ever since gravastars were first proposed, it’s been known that an event like this could distinguish between them and black holes, and now such an event has been detected.

Falsifying or amending LIGO’s observations will be an ongoing process. The more accurately we understand the event LIGO detected, the more we’ll be able to say about the accuracy of its measurements. Gravity is the premiere medium in which we might be able to learn about exotic objects like gravastars and dark energy stars and quantum puffballs, and it’s only been possible to stay for about a year. It’s no wonder that the few observations we do have are subject to so much scrutiny.

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