Tangential question. With blackholes merging, can more and more merge some time in future, creating a net gravitational pull to slow down the expansion of the universe and then may be eventually cause the universe to collapse into that continually merging mega black hole.
The gravity pull of the new black hole is essentially equal to the sum of the gravity pull of the two isolated black holes, and it's essentially equal to the gravity pull of the two stars before they transformed into black holes. So merging black holes don't increase the pull.
Actually, in each transformation there is an explosion and part of the mass goes away (try to not be very close). So the total mass in the final black hole is smaller than the mass in the initial stars, the rest are debris forming a nebula or something around the black hole.
(And, as the other commenter said, gravity is too weak.)
