Galaxies host supermassive black holes, which weigh millions to billions times more than the sun. When galaxies collide, pairs of supermassive black holes at their centers also lie on the collision course. It may take millions of years before two black holes slam into each other. When the distance between them is small enough, the black hole binary starts to produce ripples in space-time, which are called gravitational waves.
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Source: Phys.org