|
|
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
|
