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

Understand

Within quantum-gravity approaches and beyond, different mechanisms for singularity 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.