Help me understand colliding galaxies!
January 21, 2011 2:04 PM   Subscribe

Expanding Universe Filter: Why will our galaxy collide with Andromeda in +/- 4 billion years?

I'm trying to get my head around the expanding universe and have found a few related posts on metafiler: x and y and z. However, I haven't found much about galaxy movement.

It seems to me that if all galaxies are speeding away or expanding away from each other at incredible speeds, that there should be no way for them to ever come in contact with each other. Is Andromeda simply catching up with us? Is there a curve in space that is putting us on a collision course? Help me understand colliding galaxies please!
posted by jjonajason to Science & Nature (8 answers total) 3 users marked this as a favorite
 
Very short answer: Andromeda and the Milky Way formed close together, and never were moving apart at faster than their escape velocity, so now they're coming back together. Take a look at this.
posted by lukemeister at 2:12 PM on January 21, 2011 [2 favorites]


An expanding universe does not mean that every single object in the universe is moving away from every other object. The Earth continues to orbit the sun, the moon continues to orbit the Earth. These objects are not moving away from each other. Similarly, the galaxies in our local cluster orbit around their mutual center of gravity, and therefore are not moving away from each other. And yes, their orbits can eventually intersect under the influence of gravitational attraction.
posted by grizzled at 2:13 PM on January 21, 2011


. . .if all galaxies are speeding away or expanding away from each other. . .

But they're not. Essentially--and this is often glossed over for simplicity--galaxy clusters are moving away from each other, on average. But just as Jupiter is bound to the Sun by gravity, and the Magellanic Clouds are bound to the Milky way, the Milky Way is bound to Andromeda.
posted by General Tonic at 2:16 PM on January 21, 2011


Unlike gravity, which is a relatively weak force, concentrated in areas with a lot of energy/mass, that gets weaker with distance, dark energy permeates the universe equally everywhere, but with a very weak force. At close distances, such as between galaxies in the same cluster, it barely has any effect at all. You have to go very far out before the acceleration from dark energy becomes stronger than gravity.
posted by empath at 2:53 PM on January 21, 2011


It seems to me that if all galaxies are speeding away or expanding away from each other at incredible speeds

This is happening on a far more macro scale than the normal effect of gravity, which still holds "micro" level sway over bodies that are still relatively close, like this particular pair of galaxies among the billions and billions of galaxies everywhere.
posted by Cool Papa Bell at 3:00 PM on January 21, 2011


So the answer is... yes, but don't worry about it.
posted by entropone at 3:20 PM on January 21, 2011


Is there a curve in space that is putting us on a collision course?

Well, yes! According to the relativistic description of gravity, mass curves space, and those curves control how mass moves. So you can imagine space as a sheet of rubber that is slowly being stretched, and the galaxies as weights forming dimples in that sheet. If the dimples were farther apart, they'd be carried away from each other by the stretching sheet; but the Milky Way and Andromeda are close enough that the dimples are going to merge before that can happen.
posted by nicwolff at 3:27 PM on January 21, 2011 [1 favorite]


Phil Plait devotes a chapter to this in his book which might have some more information for you.
posted by curious nu at 4:03 PM on January 21, 2011


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