Antihorizon

White holes must therefore be surrounded by the opposite of an event horizon, something known as an antihorizon, which would allow one-way traffic out and never in. Unfortunately, antihorizons are very unstable and get converted to normal horizons in a matter of seconds after forming.

The Kruskal diagram of the Schwarzschild geometry showed it to possess not only a normal horizon but also an antihorizon, beyond which there was, supposedly, another Universe.

To distant observers the light appears, due to gravitational time dilation, to slow down as it approaches the antihorizon.