Stable photon orbits in stationary axisymmetric spacetimes with an electromagnetic field and a cosmological constant

Abstract: Stable light rings, which are associated with spacetime instabilities, are known to exist in four-dimensional stationary axisymmetric spacetimes that solve the Einstein-Maxwell equations (so-called electrovacuum solutions, with Faraday tensor $F_{\mu \nu} \neq 0$); however, they are not permitted in pure vacuum ($F_{\mu \nu} = 0$). In this work, we extend this result to spacetimes with a non-zero cosmological constant $\Lambda$. In particular, we demonstrate that stable light rings are permitted in $\Lambda$-electrovacuum ($F_{\mu \nu} \neq 0$, $\Lambda \neq 0$), but ruled out in $\Lambda$-vacuum ($F_{\mu \nu} = 0$, $\Lambda \neq 0$).