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Why are Andromeda and the Milky Way on a Collision Course Rather than Moving Away from Each Other?

-- | March 18, 2015

Question: If the universe is expanding and all galaxies are moving away from each other, how is it possible that the Andromeda and milky way galaxies are on a collision course? — Johnny

Answer: It is correct that on the largest scales that the universe is expanding such that all galaxies are moving away from each other. On smaller scales, though, there are so-called “peculiar motions” of galaxies, where one galaxy is found to be moving toward another galaxy due to local gravitational effects in the vicinity of the two galaxies.

Jeff Mangum

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