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Expanding Universe and Galactic Collisions
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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Update: June 2012
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