Producing the gravitational waves observed by the LIGO experiment last fall should take a cosmic event of nigh unfathomable extremity: the cataclysmic merger of two black holes, each about 30 times that of our Sun, in the distant universe. Gravity is such a diminutive force, relative to the other fundamental forces, that to see it rippling across space-time requires at least this much energy. It's hard to imagine what else in space could do the job, anyhow.
The LIGO observations offered evidence for a black hole merger in the form of what's known as a ringdown signal, which corresponds to the huge burst of gravitational radiation that can be expected following a merger of inward spiraling black holes. A GW signal has three parts, reflecting the three stages of the merger: the inward spiraling dance of the two black holes, the actual coalescence, and, finally, the ringdown.
In this final ringdown stage, the newly formed black hole "relaxes" into an equilibrium state, and life more or less goes back to normal, albeit with one big black hole instead of two.
A paper published this week in the Physical Review Letters offers the possibility for an alternative explanation for the ringdown signal registered via LIGO, however. That is, what if the observations were caused by something even stranger? That thing could be the gravastar predicted by some fringe theories that nix black holes from the universe entirely.
To be clear, the paper isn't trying to disprove black holes. It just notes that the LIGO detection doesn't carry sufficient data/resolution to prove their existence—merging black holes are far and away still the best explanation.
This is part of the significance of the LIGO observations in the first place: providing the first up-close direct evidence for the existence of black holes. We know well enough that they're predicted by theory and we see loads of evidence for their existence in the structures and evolution of the universe as we see it, but as far as just getting up next to one of the things and snapping a picture of an event horizon, that's something else entirely. Gravitational waves are the next best thing.
The new paper argues that the LIGO data only proves the existence of a "light ring" and not an event horizon. Light rings are a recent idea and consist of ultrabright spheres of photons trapped in orbit around a black hole. Crucially, light rings, if they exist, can be explained by things other than black holes, even if those things are very unlikely, like gravastars. A gravastar is a lot like a black hole, but without quite the singularity; all of the incoming matter winds up squished into a wad of Bose-Einstein condensate that kind of acts like a single giant atom.
Gravastars are pretty weird, and unlikely. But there is a small army of cranks and even real scientists out there who want black holes to not exist, or who are at least interested in rooting out each and every ambiguity as to their existence, which is fine because science is all about questioning itself. That's what you do if you want to really nail something to the wall: ask how it could be wrong. So, while LIGO's gravitational wave observation is as sure as anything gets in astrophysics, it seems a bit of ambiguity as to their origin has to persist, at least for now.
Anyhow, the Italian team behind the paper is looking forward to future gravitational wave results from better and high-resolution detectors. Better signals should enable astrophysicists to differentiate between actual event horizons and light rings.
"While [black holes] remain the most convincing Occam's razor hypothesis," the astrophysicists conclude, "it is important to bear in mind the elusive nature of an event horizon and the challenges associated with its direct detection."
An open-access version of the paper is available from the arXiv pre-print server.