Photon surfaces as pure tension shells: Uniqueness of thin shell wormholes

Understand

Thin shell wormholes are constructed by joining two asymptotically flat spacetimes along their inner boundaries.

• The junction conditions imposed on the spacetimes specify the equation of state of the matter called thin shell distributed along the joined boundaries.
• Barcelo and Visser (2000) reported that spherically symmetric thin shell wormholes have their shells, namely the wormhole throats, on the photon spheres if the wormholes are $Z_2$-symmetric across the throats and the shells are of pure tension.
• In this paper, first, we consider general joined spacetimes (JSTs) and show that any $Z_2$-symmetric pure-tensional JST (Z2PTJST) of $\Lambda$-vacuum has its shell on a photon surface, a generalized object of photon spheres, without assuming any other symmetries.