The fate of the light-ring instability
In the paper "The fate of the light-ring instability" Gr@v members P. Cunha, C. Herdeiro and E. Radu, together with former member, currently at the U. Valencia, N. Sanchis-Gual, have unveiled the mystery of the fate of a large class of horizonless ultracompact objects, that could be potential black hole foils.
A long standing intriguing possibility is if the astrophysical black hole candidates could be some other kind of mysterious objects but without event horizons, the defining property of black holes.