By now most people have heard of the success of the LIGO experiment in detecting gravitational waves, and know that they believe the waves were created by a pair of black holes that were merging into one. It is quite an achievement, and will likely lead to many interesting discoveries about violent and energetic events in the Universe.

However in the last week, several science journalists have been reporting on a paper that appeared in the astrophysics research community this week, in which it is argued that the gravitational waves that were detected could have been generated by something far more exotic - a gravastar.

As a result of this recent press coverage of a relatively unknown phenomenon, I have been fielding a number of inquiries on exactly what a gravastar is and whether I believe these new claims. And while this is not my particular area of expertise, I will try to provide a basic review of the primary concepts involved.

Most people have heard of black holes. Black holes are objects that have become so dense, and their gravity so strong, that even light cannot escape from their surface. This idea has been around for centuries, but really became a serious field of research after the development of general relativity and the realization that (a) if light cannot escape then nothing else will either and (b) every star that is more massive than a certain minimum will eventually collapse into a black hole. In the last two decades additional research in astronomy has suggested that most galaxies have a supermassive black hole at their center. Black holes are expected to be very common in the Universe!

But as so often happens in scientific research, there is a critical problem.

The theory of general relativity ignores the theory of quantum mechanics. For most systems this is not important, since quantum mechanics only has a significant effect on very small systems or very high energy systems. Planets, galaxies, and stars are basically classical objects are not affected by quantum mechanics. Black holes are very violent, high energy objects that will almost certainly be described by quantum theories instead of classical relativity. And we just don't know what quantum gravity looks like right now.

And that is where the concept of a gravastar comes into it.

One of the predictions of quantum mechanics is that there is a minimal length that can be measured - known as the Planck length. And so where a classical black hole has an event horizon, in which light on one side can escape and light on the other side cannot, a gravastar instead has a fuzzy region in which some photons might escape and some cannot, but there is no clear separation. Inside this region, the spacetime vacuum itself behaves differently. Some high energy virtual photons that fill the rest of spacetime will be missing from the gravastar shell, possibly even creating regions that have a lower energy than the vacuum (ie negative energy). 

Another interesting property is that gravastars might not absorb matter the way that black holes do. While this property is no universally accepted, it is believed that infalling matter will form a Bose-Einstein condensate just outside of the event horizon region, which would appear to an outside observer as a very cold, distributed form of matter. In contrast, matter falling into a classical black hole disappears behind the event horizon and is never seen again.

Unfortunately observers located far from the object cannot tell the difference between a black hole and a gravastar. They emit the same type and amount of radiation, and the objects behave in very similar ways. 

So did LIGO detect a black hole system or gravastars? There is no way of knowing, because the signals are identical. Do gravastars exist? Again the theories of quantum gravity are just not well developed enough for us to be able to answer that question.

But my own personal opinion is that what we will eventually find once quantum gravity is understood, is an exotic object that resembles both a black hole and a gravastar, but is very different from either in quite unpredictable ways!