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1b) if there would have been a merger the images would not have shown an decrease only a steady increase.


Have you taken into account the distinct possibility that what was observed was not a "head on" collision, but that the objects orbited each other; in that case the last seconds would have given rise to fluctuating waves, rather than a steady increase.

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are we really supposed to believe that these two massive
black holes merged in only the 15 seconds depicted on the
images?


It has been estimated that in the final seconds, objects such as two massive black holes could be orbiting at about 100 times per Sec. and travelling close to the speed of light. In that sort of scenario, a lot can happen in fifteen Secs.


There never was nothing.