In the article "Stationary black holes and light rings" by P. Cunha and C. Herdeiro, published in Physical Review Letters, as Editor's Suggestion, a theorem is proven establishing that, under very generic conditions, any black hole must have a light ring.
Black holes remain one of the most mysterious objects in the Cosmos. The last five years have provided us with the first data on the strong gravity environment around astrophysical black holes, in the form of the gravitational waves detected by the LIGO-Virgo collaboration, and the first image of a black hole shadow produced by the Event Horizon Telescope collaboration.
None of these observations, however, is expected to probe the effective boundary of the black hole, the event horizon. Rather, they should contain a signature of a neighbourhood region slightly outside the horizon, wherein light is bent so strongly it forms closed orbits known as light rings. As a consequence, the properties of these light rings encode much of the relevant black hole properties.
A key question is then: does any black hole model, in any theory of gravity, need to have a light ring?
In this letter, a generic and mathematically innovative argument establishes that an equilibrium black holes must indeed have, as a rule, at least one standard light ring in each rotational sense. The argument is of topological nature and does not use the equations of motion of any theory of gravity, but only regularity requirements, together with horizon and far-away properties.
Indeed, one light ring is a rule to them all.
The article was chosen as PRL Editor's suggestion. About one letter in six is chosen for this highlighting. Editor's look for papers that they judge to be particularly important, interesting and well written.
See also: the coverage at Aveiro U. here and the coverage at the MPI here.