What's a white hole?

Astronomer Matt Bothwell sucked us in with this answer...

Matt - Um, so the answer is quite simple in a number of ways. A white hole is basically a mirror image of a black hole. You think about what a black hole is, it's an object in space that can take in matter in and light and there's this boundary around it that we call the event horizon. And once you cross the event horizon then you can never come out. Um, a white hole is basically the flip side of that, right? So it's an object in space from which light and matter can escape. So you'd say white holes, you know, could easily just kind of spew matter and light out constantly.

Chris - I spoke to Michio Kaku, and when he came to Cambridge he was promoting a book and he gave a talk, and his contention was that perhaps a black hole in one universe sucks stuff in and ejects it through a white hole in a parallel universe. Does that sound plausible?

Matt - I may think even the concept of parallel universes is, uh, heavily up for debate, so I think I might pass on that one. But the point of the white hole is that they're surrounded by this boundary, like an opposite of event horizon, which you could never cross, no matter how powerful your real rocket engines.

Chris - Michio Kaku, his contention was that that could be a big bang in a new universe. So in the same way that we have a big bang that we think created our universe, and led to the conversion of all that energy into the materials that now make up the universe we inhabit, that perhaps a white hole is spawning a new universe somewhere else at the, to put it bluntly, the ass end of a black hole in this universe! Does that seem at all reasonable?

Matt - Um, yeah, I think it certainly seems reasonable. You can construct solutions to Einstein's field equations that do this kind of thing. Um, yeah, whether it is true in reality or not, I think remains to be seen, but it's definitely a nice idea.