## principles behind galaxy formation

When things are kept at the simplest, I've heard that galaxies were formed because gravity pulls stuff together, but there's some things that are not fully making sense. If two objects are far away, and they get attracted by gravity, what happens is that they merely pass each others with some great velocity and fly far away again, according to the conservation of energy. So what is it really that makes clusters start forming out of homogeneous matter then?
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 Quote by jostpuur When things are kept at the simplest, I've heard that galaxies were formed because gravity pulls stuff together, but there's some things that are not fully making sense. If two objects are far away, and they get attracted by gravity, what happens is that they merely pass each others with some great velocity and fly far away again, according to the conservation of energy. So what is it really that makes clusters start forming out of homogeneous matter then?
Supposedly, quantum fluctuations in the early universe provided the seeds for (clusters of) galaxy formation. This an Interesting story that I don't understand fully.

http://www.physicsforums.com/showthr...18#post1818318

 Quote by jostpuur When things are kept at the simplest, I've heard that galaxies were formed because gravity pulls stuff together, but there's some things that are not fully making sense. If two objects are far away, and they get attracted by gravity, what happens is that they merely pass each others with some great velocity and fly far away again, according to the conservation of energy. So what is it really that makes clusters start forming out of homogeneous matter then?
You are neglecting friction in this scenario. As gas and dust collapse under the influence of gravity the particles collide more and more frequently as the density increases. Thus a lot of the kinetic energy is radiated away as heat leaving the particles moving more slowly than otherwise. Provided there is enough inhomogenaity to start with, friction does the rest.

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## principles behind galaxy formation

 Quote by George Jones Supposedly, quantum fluctuations in the early universe provided the seeds for (clusters of) galaxy formation.
Part of the story after this is given in

http://www.physicsforums.com/showthr...48#post1846548.
 The story about formation of original irregularities after big bang wasn't precisely what I was after, although that seems to be another interesting story too. I was wandering, that even if there is some irregularities to start with, how can gravity start causing clusters to emerge, when matter should be flying far away after first coming close to some other matter. I suppose the answer is the friction mentioned by paw then. Am I correct to guess that galaxies could not start forming out of randomly distributed pieces of rocks in the space, as easily as they start forming out of gas and dust? Because pieces of rocks have less chance of colliding, they just fly in hyperbola trajectories by each others? Gas and dust instead gets slowed down by friction more easily?

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