Expanding Universe & Colliding Galaxies

Here’s one for Cecil and the Cosmologists (hey, isn’t that a 50’s a capella group?):

I hear over and over that all galaxies are racing away from one another as a result of the expanding universe which is a result of the Big Bang. No matter where you are, all other galaxies show that Doppler red shift characteristic of something fleeing you.

So… if that’s the case, then how come Hubble’s got pictures of galaxies colliding? And if you point out local effects of gravity, then how come our local galaxies ain’t affecting us? Whatsamatter? Ain’t we got enough pull?



I’m not a cosmetician, or anything, – good question though – it looks like the Big Bang through a few curve balls with those galaxies. This isn’t the first time such a thing has happened either. You think there was enough space between them.

The expansion of the universe is an overall effect, but there are local variations due to gravity.

Not all galaxies are flying away from each other. This has never been thought to be the case. Any galaxy that is very, very distant from us will certainly be moving away.

We aren’t flying away from the Sun. Why not? Because the gravitational pull of the Sun has long since overwhelmed any momentum we might have had pulling us away from it.

Galaxies don’t fly apart. Why? Again, because the gravitational pull of the galaxy has overwhelmed the momentum imparted by the expansion of the universe, that might have pulled the apart.

Galaxies are not isolated entities. They often exist in large clusters. Just as a galaxy, or a star system is held together by gravity, the clusters are also held together. Just as a comet can strike a planet within our solar system, two galaxies within a cluster can collide.

Our local galaxies are affecting us. IIRC our local group contains about a dozen galaxies dominated by our Milky Way and the Andromeda galaxy. Galaxies in our local group are not moving away from each other but are instead orbiting their common center of gravity.

The Local Group is a small cluster compared to some, which may contain hundreds of galaxies. In those, the gravitational situation is complex enough that the orbits of some galaxies intersect. Hence, colliding galaxies.

So, galaxies are not moving away from each other, but clusters of galaxies are moving away from each other. Each cluster is a case where mutual gravitational attraction has overcome the general expansion of the universe.

(There are also “superclusters” which are clusters of clusters, but you get the idea.)

“You can’t run away forever; but there’s nothing wrong with getting a good head start.” — Jim Steinman

Dennis Matheson — Dennis@mountaindiver.com
Hike, Dive, Ski, Climb — www.mountaindiver.com