Once again:
The point that I've made with the supermassive black hole coalescence is that, according to the typical definition that you're using, these cannot be considered "Different" black holes in the case that they will merge in the future, and I already told you, more than once, that...
About the case of a collapsing FLRW universe:
A physically realistic collapsing universe is a huge mess, and that's exactly what R. Penrose stresses in many of his lectures about cosmology:
Due to unavoidable inhomogeneities (one needs extreme fine tuning for an idealised homogeneous collapse)...
Your comment about the possible future merging of the supermassive black holes is not at all relevant to the point I've made! It's totally irrelevant if these holes will actually merge or not.
It is possible that such a thing will happen, and that's enough!
The point is that when everybody today...
The collapse of a closed FLRW universe is very similar, in fact, with a merging of lots of individual black holes in a realistic case!
As for a reference: Roger Penrose's lectures about cosmology, gravitational entropy etc.(there are lots of them).
About the coalescence of S A* with Andromeda's...
Concerning your fourth objection: No, I was not thinking about Kerr. In the unperturbed, eternal Kerr spacetime only timelike singularities could exist (mathematically). And its well known, as you already pointed out, that due to blueshift/ mass inflation instabilities, instead of cauchy...
Thank you for the answer!
I don't really disagree with most of the replies you gave in this thread, but i think that you're using a very restricted definition for a black hole spacetime.
a)In a closed, recollapsing universe ( like the k=1FLRW model), there is no future null infinity/ asymptotic...
By the way, one can also have black holes "inside" other bhs in the cases of Bag of gold or Baby universe spacetimes.
These are not related, of course, with Grasshopper's question, but i think they're worth mentioning.
One could imagine a whole galaxy with millions of black holes collapsing inside its Schwarzschild radius.
Although, technically speaking, there is, classically, one global event horizon and a future spacelike singularity (as already mentioned), and r= constant hypersurfaces inside the...