Event horizons and apparent horizons in spherically symmetric geometries

Spherical configurations that are very massive must be surrounded by apparent horizons. These, in turn, when placed outside a collapsing body, have a fixed area and must propagate outward with a velocity equal to the velocity of radially outgoing photons. That proves, within the framework of the 1+3 formalism and without resorting to the Birkhoff theorem, that apparent horizons coincide with event horizons in an electrovacuum. The existence of the maximal slicing of electrovacuum is proved and an explicit line element is found in the maximal foliation.