Could the Big Bang have emerged from a black hole forming in another external universe?

Who knows? I cannot even begin to imagine how we could experimentally verify such a proposition, and without such a possibility, we are not speaking about scientific investigation.

If we want to engage in pure speculation, then I suppose there is nothing to prevent such a scenario, but I am not a relativist.

The big problem is that black hole singularities are not of the same class as the one which produced our universe ( space-like versus time-like) so I would not expect a black hole in another spacetime to produce a universe.

Still, people like Stephen Hawkings have speculated that black holes in our universe create universes. Physicist Lee Smolin even speculates that in some larger arena, our universe is similar to others because black holes only happen in universe like ours, and so these universes spawn others like them with very high efficiency.

Copyright 1997 Dr. Sten Odenwald