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Dark Energy, Universal Expansion, and Peculiar Galaxy Motions

-- | February 3, 2014

Question: I have been hearing lately in some space documentaries that black energy, which makes the universe expand and at an increasing speed, separate and repulse objects such as galaxies from one another, am I correct? But i also heard that the Andromeda galaxy is heading straight for us and should merge with the milky way, isn’t it contradictory? Thank you in advance for taking the time. — David

Answer: I suspect that what you mean by “black energy” is really Dark Energy, which is one of several explanations for the acceleration of the expansion of the Universe. The folks at NASA have a very nice page which describes what we think Dark Energy is, which you should certainly check out. You are correct that Dark Energy is theorized to act like a repulsive force which pushes galaxies apart. As for the Andromeda galaxy’s peculiar motion toward us, this is due to the mutual gravitational pull between our galaxy and Andromeda. This is a “small-scale” affect, while the overall expansion of the Universe happens on a larger scale.

Jeff Mangum

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