How can it be that all galaxies and stars are moving away from each other yet they still collide? From what I have researched it as an open universe (or many open universes) yet the idea of stars and galaxies colliding boggles me.
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$\begingroup$ scientificamerican.com/article/how-can-galaxies-collide $\endgroup$– ApproachingDarknessFishAug 15, 2016 at 22:26
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1$\begingroup$ That being said, in a nutshell, the universe is expanding, but its expansion compared to the proximity between most stars is miniscule. On a Galactic scale, most galaxies are moving away from the Milky Way because their distance from us is very significant, with one major exception being Andromeda, which is very close, relatively speaking. For Galaxies that are close to one another, collision is possible only because they are so close and their proximity outweighs the Expansion. $\endgroup$– MozerShmozerAug 15, 2016 at 22:27
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3 Answers
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The expansion is overridden on a small scale by gravity. Our galaxy is not expanding, and the stars are bound together. In fact, a whole group of galaxies don't notice the overall expansion.
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$\begingroup$ Clusters and supercluster are only compressed in comoving coordinates. In physical coordinates they expand. $\endgroup$– pelaAug 22, 2016 at 13:51
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$\begingroup$ What is “physical” coordinates? Which is used by somone in that cluster making a map? $\endgroup$– JDługoszAug 22, 2016 at 21:18
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$\begingroup$ Physical coordinates are the distances you would measure at some point in time, if you could freeze the expansion of the Universe and lay out measuring rods. That is, it is what people normally think of as a distance, but is imposissible to measure in practise, since the Universe keeps expanding while you try to measure. In contrast, comoving coordinates expand with the Universe, so that the comoving distance between two galaxies (that are not too close) is constant (modulus a small change over time from peculiar velocities). The "normalization" is such that today, [cont'd below] $\endgroup$– pelaAug 23, 2016 at 7:13
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$\begingroup$ …the two types of coordinates coincide, but in the past, at a time corresponding to a redshift $z$, the relation was $d_\mathrm{phys} = d_\mathrm{com} / (1+z)$. If you view a cosmological simulation in comoving coordinates (which you almost always do), you don't see the expansion, but only see the dynamics, and then you are right that the voids expand and compress the clusters. But if you view it in physical coordinates (which measure the "true" distances), the clusters also expand. Only on such small scales at groups is gravity strong enough to prevent them from receding from each other. $\endgroup$– pelaAug 23, 2016 at 7:20
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$\begingroup$ The article, which I think was in SciAm, is about larger scale structures than were known previously. $\endgroup$– JDługoszAug 23, 2016 at 11:52
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The galaxies and stars are not all moving away from each other. The space between them is expanding, however this does not affect their trajectories. For instance the Andromeda Galaxy is set to cross paths with the Milky Way, after which we will join together as the unimaginatively named "Milkdromeda". As space expands it grows faster and faster, this will eventually lead to an "island effect", where the space between galaxies will expand faster than light, at which point everything outside our galaxy will go dark forever.
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Simple, and direct. Objects moving away from each other can still cross paths. "Expansion is a general direction, not a specific flight path.
