What happens when black holes collide? Does one black hole “consume” the other? Do they become a larger black hole? Does it get more dense or just larger?
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As I understand it, black holes are defined by three quantities: mass, spin, and charge.
I'm assuming that these quantities will be additive post-merger.
Edit: "The black holes appear to be spinning very rapidly—near the limit allowed by Einstein's theory of general relativity."
Perhaps the additive spin becomes asymptotic. Alternately, the gravitational waves might have departed with the energy of the excess spin.