This week we have put a new paper out on the arXiv: Exotic Compact Objects and How to Quench their Ergoregion Instability, which is essentially the outcome of my student Elisa Maggio's master thesis, so I thought that summarizing our results was a good excuse to restart this blog.

*Exotic compact objects (ECOs)*is just a catchy name for several models that have been proposed as alternative to black holes. Black holes are the natural outcome of the gravitational collapse in classical general relativity, but a growing number of theoretical physicists is uneasy with some potential drawbacks associated with them, in particular with the consequences of forming an event horizon as the final state of the stellar collapse.

According to these arguments, the classical picture of black holes would be at clash with quantum mechanics, even for astrophysical objects for which, in principle, quantum corrections are expected to be small. Some of the potential problems include the famous information loss paradox, the recent firewall proposal, the huge entropy of a black hole, and the singularity that lurks in its interior, where general relativity

*has*to break down. There is no general consensus on whether or not these are serious problems that should be addressed in the context of classical gravity, but a growing community of theoreticians is looking for alternatives.

In brief, models of ECOs aim at reproducing all properties of compact dark objects that we observe (namely, a radius very close to the Schwarzschild radius and an arbitrarily large mass) and the theoretical properties (stability, formation as the end state of physical astrophysical processes,...) that characterize the black hole dynamics but

*without having an event horizon nor a singularity*. So far, none of the ECO models on the market succeeded in reproducing all these properties, which is already a strong result in support of the black hole picture. But we know that theoreticians are stubborn and they keep trying.

At the theoretical level, a possible problem with ECOs is the so-called

*ergoregion instability*, an instability that develops in compact objects which are spinning fast and that, unlike a Kerr black hole, do not have a horizon. In this paper, we showed two things:

a) The instability is rather generic and quite strong when the ECO has a surface that does not absorb any radiation (like a perfectly reflecting mirror). Although apparently unrealistic, some quantum-gravity models predict precisely this situation, and might be ruled out by this effect.

b) If the ECO can absorb a fraction of the radiation in its interior (for example by converting it in heat and thermalizing the perturbation), then it is possible to quench the instability. Whether this makes the model viable or not depends on two crucial issues, that we don't address in the paper: a) Are there ECO models that absorb enough radiation to quench the instability? b) What's the final state of the instability?

It is likely that the answer to the latter question is that an unstable ECO would simply slow down as a result of the instability, until it's not unstable anymore. In such case, one should compare theoretical models with observations of highly spinning black hole candidates, to see whether the instability is incompatible with observations.

A lot of work remains to be done but this is surely an exciting time for this kind of studies. As I write this, I'm at Frankfurt airport on my way back after having attended this workshop. I have found great interest among the participants related to the possibility of ruling out or detecting ECOs using GW observations (see also this other recent paper), which was something unimaginable just a few years ago, before the gravitational-wave revolution.

Anyway, truth is, i wrote this to have the opportunity also to share some funny pics from Elisa's graduation last December :-)

Elisa going through the wormhole, she entered as an undergrad and emerged from the throat as a grad student ready for her PhD |

From the left: Leonardo, Elisa, and Paolo celebrating Elisa's defense |