If all matter began from one infinitesimally small point, and flew outward from there. How can we have galaxies colliding? Did they make left hand turns or something? Or it is possible multiple galaxies were created by secondary coalescing and re-exploding of galactic material, flinging them in new directions? Wouldn't that increase the time-frame of the Universe's age?
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3$\begingroup$ The big bag didnt happen at a point $\endgroup$– TriatticusCommented Oct 4, 2023 at 19:47
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$\begingroup$ @Triatticus It's nevertheless true that the material from which any two observable galaxies originally formed was arbitrarily close in the distant past. $\endgroup$– StenCommented Oct 4, 2023 at 20:18
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$\begingroup$ Where did the Big Bang happen? $\endgroup$– mmesser314Commented Oct 4, 2023 at 21:33
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$\begingroup$ @Sten sure but it also wasn't an explosion as is commonly depicted. I was just providing this reference to clear that common misconception. $\endgroup$– TriatticusCommented Oct 4, 2023 at 21:49
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2 Answers
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Consider an expanding cloud of dust in space. In general, its expansion speed should slow over time, because of the cloud's gravitational attraction. The expansion could eventually stop, and then the cloud would collapse again. Alternatively, if the cloud is expanding fast enough, it might slow down but never stop.
On average, our universe's expansion rate and density seem to be precisely matched so that it is right on the threshold between "will expand forever" and "will eventually stop". (This is what it means for the universe to be "flat".) However, the universe is not exactly the same everywhere. There are regions of above-average density and regions of below-average density. Regions of above-average density eventually collapse. If they were sufficiently overdense, then they collapsed already. This is how galaxies formed, and this is how galaxies collide.
I neglected dark energy in the above discussion. Dark energy changes the story slightly, but it's only marginally relevant to galaxy formation and collisions because it has only started to become relevant very recently. Dark energy gravitationally repels everything, so that expanding regions have their expansion accelerated and collapsing regions have their collapse delayed. Dark energy would suppress the rate of galaxy collisions in the future.
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Individual galaxies possess what is called peculiar velocities which are deviations from the uniform recession velocities associated with Hubble expansion. Those deviations are produced by local nonuniformities in the distribution of matter in the universe, and can make galaxies collide.
Note also that a pair of galaxies will exert gravitational attraction upon each other, causing them to move towards each other and even to fall into orbit around their common center of mass. That motion caused by gravity can be significant enough to overwhelm the Hubble effect if the two galaxies are not too far apart.
In this way, galaxies can collide and even merge into one larger galaxy, a process which is very "messy"- but which has been telescopically observed.
answered Oct 5, 2023 at 4:59