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If the universe is expanding, why do galaxies collide with each other?

Bill Ziegler, West Chicago, Illinois

Frank Summers, an astrophysicist at the Space Telescope Science Institute in Baltimore, replies:

Astronomers have known for 75 years that galaxies are streaming away from one another. This rushing apart is caused by the overall expansion of the universe, a stretching of space-time that began with the Big Bang about 13.7 billion years ago. If galaxies did not interact with each other, the expansion of space would push them all farther and farther apart.

But galaxies do interact, because their huge masses create powerful gravitational forces that pull on other galaxies. When two or more galaxies are near each other, gravitational attraction can overcome the expansion of space and cause them to approach each other. (Put another way, cosmic expansion is canceled out where gravity is strong; the stretching of space occurs mostly in vast, nearly empty regions where gravity is comparatively weak.) Sometimes this leads to galaxies orbiting around one another, as in the case of the Magellanic Clouds, satellite galaxies orbiting our Milky Way. In other instances, two galaxies can come crashing together. Such collisions distort the galaxies’ shapes and lead to their merging into a single larger galaxy, as dramatically documented in many Hubble Space Telescope images. This type of cosmic smashup is the likely fate of the Milky Way and the Andromeda galaxies. It should be a spectacular sight, but it will not happen for several billion years.

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