Universal signatures of singularity-resolving physics in photon rings of black holes and horizonless objects

Within quantum-gravity approaches and beyond, different mechanisms for singu-larity resolution in black holes exist. Under a set of assumptions that we spell out in detail, these mechanisms leave their imprint in shadow images of spherically symmetric black holes. We find that even current EHT accuracy is sufficient to place nontrivial constraints on the scale of new physics within one modified spacetime, if the EHT measurement of M87* is combined with an independent measurement of the black-hole mass. In other spacetimes, increased accuracy is required that the next-generation EHT may deliver.We show how the combination of n = 1 and n = 2 photon rings is a powerful probe of the spacetime geometry of regular black holes, even when considering astrophysical uncertainties in accretion disks. Further, we generate images containing a localized emission region, in-spired by the idea of hotspots in accretion flows. Finally, we investigate the photon-ring struc-ture of a horizonless object, which is characterized by either two or no photon spheres. We show how photon rings annihilate each other, when there is no photon sphere in the spacetime.

Automated summary

By observing the shadow images of black holes, we can investigate the effects of quantum gravity and other mechanisms on singularity resolution beyond traditional quantum gravity theories. Even the current accuracy of the Event Horizon Telescope (EHT) is sufficient to place meaningful constraints on the scale of new physics, when combined with an independent measurement of the black hole mass for M87*. Higher accuracy from the next-generation EHT is required for other spacetimes. Additionally, we study the concept of localized emission regions inspired by hotspots in accretion flows, as well as the photon-ring structure of horizonless objects where photon rings annihilate each other in the absence of photon spheres.