Collisions between galaxies, geometry of expansion.

In summary, the galaxies in the universe are moving away from each other due to expansion, but this effect is only effective on very large scales. On smaller scales, gravity rules and can cause galactic collisions. The overall expansion rate is determined by the average density of material in the universe, but on smaller scales, the density can vary and cause local systems to move towards each other. This is due to the lumpy structure of the universe, which evolves over time through the process of gravitational growth.
  • #1
AdkinsJr
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Hubble discovered that galaxies are moving away at each other at a rate, today physicists believe that this rate is increasing. I've read about galactic collisions, and that the Andromeda galaxy will eventually collide with the milkyway. If the galaxies are moving away from each other, then how is it that galaxies can collide? I can't visualize the geometry of this.
 
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  • #2
Expansion [at present] is only effective on very large scales. Gravity rules local systems. It is otherwise difficult to explain why galaxies and solar system have held together.
 
  • #3
Chronos said:
Expansion [at present] is only effective on very large scales. Gravity rules local systems. It is otherwise difficult to explain why galaxies and solar system have held together.

It is fallacious to juxtapose 'expansion' with 'gravity'. One is a force the other is due to momentum. Gravity operates at all scales, and the overall expansion of the Universe is dictated by Gravity.

Instead, the key point is about the scale on which the material in the universe is distributed smoothly. To get a feel for this have a look at some of the great pics and movies on http://www.mpa-garching.mpg.de/galform/millennium/" [Broken] They are from simulations of the Universe (rather than data) but the results match our Universe very well, and its much easier to make pretty pics from computer simulations than real observations (just ask Hollywood :) ).

What you can see, is that on large scales, the material in the Universe is smoothly distributed, so that the rate of expansion is pretty much the same everywhere. The expansion rate is determined by the average density of material in the Universe and its properties as dictated by Einsteins theory of General Relativity, but the point is if the density is uniform so is the expansion.

As you go to smaller scales you can see that the Universe becomes more and more lumpy. This means that talking about the average expansion rate starts to make less sense, because the local density might be quite different from the average. In the case of the Milky Way and Andromeda, the density in our local area is more than the average, sufficiently so such that we are falling towards each other.

Note that the structure in the Universe evolves over time. Early on things were very smooth everywhere, but gravity acts to amplify any small difference, such that dense areas get denser and underdense areas get less dense as times goes by. This process of gravitational growth of the lumpiness takes place even though on average the whole Universe is expanding.
 
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1. How do galaxies collide?

Galaxies can collide due to the gravitational pull between them. As they get closer, their gravitational forces will cause them to merge and form a larger galaxy. This process can take millions of years.

2. What happens during a galactic collision?

During a collision between galaxies, the stars and gas clouds within the galaxies will interact and can cause disruptions in their orbits. This can lead to the formation of new stars and can also trigger the release of gas and dust into the surrounding space.

3. Are all galaxies destined to collide?

No, not all galaxies are destined to collide. The likelihood of a collision depends on the distance between galaxies and their relative speeds. Some galaxies are moving away from each other due to the expansion of the universe and will never collide.

4. How does the geometry of the universe affect galactic collisions?

The geometry of the universe can affect the likelihood and frequency of galactic collisions. In a flat universe, galaxies are more likely to collide due to their constant expansion. In a curved universe, the expansion may cause galaxies to move away from each other, reducing the chances of collisions.

5. Do galactic collisions have any impact on the expansion of the universe?

Galactic collisions do not have a significant impact on the overall expansion of the universe. However, they can affect the distribution of matter and energy in the universe, which may have small effects on the expansion rate in certain regions.

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