Colliding Galaxies... WT_?

[Garden]

I have some trouble seeing these two things. Lets say the big bang did happen, and the universe is expanding... ie all galaxies are moving away from each other. If all galaxies are moving away from each other, how can two galaxies collide?

If its because of gravity then wont eventually the colliding galaxies pull in more and more galaxies at an exponential rate?

What happens when the two Massive black holes meet(because at the center of each galaxy is a super massive black hole right?)?

 

[ebbixx]

I have some trouble seeing these two things. Lets say the big bang did happen, and the universe is expanding... ie all galaxies are moving away from each other. If all galaxies are moving away from each other, how can two galaxies collide?

Not the best example, but look at a river. All the water is moving away from the source at more or less the same rate. Yet local irregularities also create eddys, whirlpools and other local phenomena, some extreme enough to give the appearance of moving in retrograde.

Colliding galaxies are similarly local exceptions to a much larger scale trend. If the majority of the motion is expansive, collisions will happen, but will, for the most part, be unlikely to cascade to the degree needed to create a massive gravity well. Galaxies are not evenly or regularly distributed, they tend to "clump" on the larger scale, but those clumps are tiny features relative to the scale on which the universe is expanding and such clumps are being pulled apart.

 

[Garden]

Ok right, the cooling after the big bang happened unevenly and that's where clumps of galaxies formed and while the clumps are expanding the galaxies within the clumps are free to interact with one another?

Im really hoping the contracting universe theory is the right one because then the universe just restarts, an ever expanding cooling universe were everything eventually hits 0 kelvin is pretty boring.

 

[ebbixx]

Im really hoping the contracting universe theory is the right one because then the universe just restarts, an ever expanding cooling universe were everything eventually hits 0 kelvin is pretty boring.

Unfortunately, our votes don't have an impact on physics. While not ruling out a contracting universe, given the remaining unknowns in cosmology, what will be, will be. Except under M-string theory... maybe. Personally I'm a big fan of entropy. I don't expect humans to be around long enough to find out for certain, though.

 

[Armondikov]

The example I heard refers to the universe as the surface of a balloon and galaxies being dots that have been drawn on it. As you blow up the balloon, the dots move away (space expands) but imagine that the dots aren't static, but animated and moving based on some more localised distubances across the surface. On average, they'd all move away from each other (due to expansion), but one or two may actually move towards each other and a few may even collide.

I think that's slightly clearer than the river example.

 

[PuppetXeno]

Andromeda is going to collide with the Milky Way (3 billion years from now) and in fact we're already slurping up the Sagittarius Elliptical Dwarf Galaxy (see picture)

And yeh, what the others said about expansion/local turbulence...