The Kerr black hole solutions gives an analytic continuation that is asymptotically flat. Some people have argued that this is another universe, but others state that the analytic continuation represent the same space-time region at different times.
The Kerr solution is an exact, analytic solution for an eternal black hole where formation is infinitely in the past.
Is the Kerr solution amenable to perturbative analysis to include the collapse itself in the solution for finite times? How does the analytic continuation looks when considering a non-eternal solution? If there exists one and it is understood well enough, How does it map to the external space-time?