Black-bounce to traversable wormhole
So-called “regular black holes” are a topic currently of considerable interest in the general relativity and astrophysics communities. Herein we investigate a particularly interesting regular black hole spacetime described by the line element This spacetime neatly interpolates between the standard Schwarzschild black hole and the Morris-Thorne traversable wormhole; at intermediate stages passing through a black-bounce (into a future incarnation of the universe), an extremal null-bounce (into a future incarnation of the universe), and a traversable wormhole. As long as the parameter a is non-zero the geometry is everywhere regular, so one has a somewhat unusual form of “regular black hole”, where the “origin” r=0 can be either spacelike, null, or timelike. Thus this spacetime generalizes and broadens the class of “regular black holes” beyond those usually considered.