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The Question

(Submitted April 21, 2011)

I have seen various reports that in approx. 5 billion years time the Andromeda galaxy will collide with the Milky Way.

I would like to know how this is possible in an expanding universe? I thought all galaxies were getting steadily further apart?

The Answer

Thanks for your question. Yes, in about five billion years the Andromeda and Milky Way galaxies will collide, forming an elliptical galaxy, though the collision itself will take a few billion years. In our current time, the expansion of the universe is only detectable on very large scales. Nearby galaxies and galaxy clusters can have peculiar motions far greater than cosmic expansion, given that the distances to them are relatively short in comparison to the total size of the observable universe. In fact, there are about 7000 galaxies with recorded blue-shifts, meaning these galaxies have a peculiar motion toward us. This is still less than 0.01% of catalogued galaxies, the vast majority of which are red-shifted due to the expansion of the universe.

However, the expansion of the universe is accelerating, and if this continues there will be a point in the future when the observable universe will actually begin to shrink. Light emitted from galaxies a certain distance from us will never be able to reach us due to the accelerating expansion of the space in between. So fewer and fewer galaxies will appear to be approaching us, or even observable at all. It's estimated that in about 100 billion years time this "cosmic horizon" will only include our local galaxy group, which by that point will have merged into one giant elliptical galaxy. The local gravitational attraction between galaxies will keep galaxy clusters together in the face of universal expansion, just as gravity keeps galaxies themselves together

Jack Hewitt
for Ask an Astrophysicist

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