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Name: Tom
Status: other
Grade: other
Location: NV
Country: N/A
Date: N/A


Question:
The Andromeda Galaxy, being a blue-shifted object, is understood to be on a collision course with our own. There are other examples of two galaxies that have already collided. How can this be possible if the universe is expanding in all directions? Hasn't everything been moving away from everything else since the big bang? If galaxies are colliding, then their path since the big bang must have changed direction at some time in the past! What could cause this, gravity from another galaxy? I do not see how that could be--everything is moving away from everything else.


Replies:
Dear Tom,

At its largest scales, you are right-- everything is indeed moving away from everything else. But within clusters of galaxies, galaxies can approach and collide with one another. The red shift is intended as a distance yardstick for distant objects

Good question.

David Levy


Tom,

Think of it as two forces opposing each other: gravity that tends to bring things together, and the expansion of the universe which tends to spread things apart. In the case of Andromeda and the Magellanics, these galaxies are close enough to the Milky Way that the resultant gravity and the relatively close proximity of the galaxies to each other results in a strong enough attraction to resist the general expansion. In the cases where the galaxies are farther apart, the gravitational pull is not strong enough so that the galaxies drift away from each other.

Greg (Roberto Gregorius)



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