Non-stationary wormholes with the presence of scalar fields and modified gravity

Some novel regular spacetimes are considered that show a non-stationary wormhole structure. A Simpson–Visser-like procedure is applied to reconstruct these regular spacetimes, free of time-like and space-like singularities. Such a procedure is also applied to describe a regular cosmological expansion, where the universe reaches a minimum scale and then rebounds. This type of regular spacetime is achieved by considering some scalar fields as sources, with the appropriate kinetic term and scalar potential. We show that all these sources become ghosts due to the wrong sign of the kinetic term. Nevertheless, these ghosts can be eliminated by constraints. The same procedure is also explored in the framework of modified gravities, particularly within the so-called [Formula: see text] gravity, where a new wormhole spacetime is also obtained that does not require a ghost scalar field.