Question: is there such thing as a white hole?

Uday Bangavadi answered on 20 Nov 2015:

In general relativity, a white hole is a hypothetical region of space-time which cannot be entered from the outside, although matter and light can escape from it. In this sense, it is the reverse of a black hole, which can only be entered from the outside, from which nothing, including light, can escape. White holes appear in the theory of eternal black holes.