Humans have become the leader of the Virgo Cluster faction, 800 billion years in the future. Our enemy factions are, the Fornax, Centaurus, and the Coma clusters, as well as a few dozen lesser factions.

Recently, intergalactic astrophysicists and astronomers have discovered that all of our neighboring galaxies and clusters will disappear behind the cosmic light horizon in approximately 6.9 billion years.

Now, every faction is making a united, concerted effort to move every one of the 1 trillion galaxies within 10 billion light years of the factions into an sphere of radius 2.5 billion light years to prevent them from disappearing.

The astrophysicists have determined, due to physical and space-time limitations, the maximum speed that each galaxy can travel is $10c$, to prevent the galaxy from disintegrating. How can the factions accomplish this within a reasonable timescale, assuming that everyone is very interested in saving billions of galaxies from an unknown demise?

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    $\begingroup$ If you can move at $10c$ then the cosmic event horizon is meaningless: you can still visit galaxies after they've crossed it. $\endgroup$ – benrg Mar 23 at 0:44
  • $\begingroup$ @benrg The farther away the object, the farther the recessional rate. At a distance of about ~120 Gly, things disappear behind the light horizon. $\endgroup$ – fasterthanlight Mar 23 at 0:49
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    $\begingroup$ If you're talking about real-world cosmological parameters, as suggested by the mention of real-world galaxy clusters, then the distance to the event horizon in the far future is around 16 Gly. I don't remember hearing the phrase "light horizon" but it's not an unreasonable alternate name for the event horizon. There's nothing at 120 Gly. Anyway, my point is that the horizon is not really a horizon if you can exceed $c$. If there's a hard speed limit of $10c$ then maybe you could define a true horizon based on that, which might be 10 times farther away. $\endgroup$ – benrg Mar 23 at 1:12
  • $\begingroup$ 800 billion years in the future, and they are just discovering what you know now? If we know it now, why would they not know it 800 billion years from now? $\endgroup$ – Justin Thyme the Second Mar 23 at 2:44
  • $\begingroup$ Why does it matter if a galaxy disintegrates? You're only interested in the pieces. If you can move galaxies around at 10c, rearranging a bunch of stars and miscellaneous matter into whatever form you prefer shouldn't be difficult. $\endgroup$ – Christopher James Huff Mar 23 at 3:37

You have so many options

L.Dutch pointed out some flaws in the problem. I'm going to overlook these for the sake of an interesting answer. You need to move 1 trillion galaxies within 6.9 billion years or they cease to exist for plot reasons. (That's one every other day.)

  • Giant FTL window. Your civilisations obviously have a lot of power between them, and FTL. I'm assuming stargate-style FTL windows where you can increase the power to capture a bigger area, then travel at same cost per distance as normal FTL, then release with zero cost. Hook up all the Dyson spheres together, and open a galaxy-sized ftl window to the other side of the known universe in front of the galaxy. Now the galaxy has to do the whole journey again.
  • Create some well placed black holes. Surely your civilisations have this power too. Create some massive black holes strategically placed so that passing galaxies paths are tweaked such that they dont cross the barrier.
  • Low on energy? Just take the good bits. Theres a lot of useless junk in the universe, and it's all spread so far apart. There may be 100 billion stars in each galaxy, but theres only 1 million populated worlds. If you triage the contents of each galaxy, and only take populated worlds, (and maybe a planet that's pure helium3 or platnium or something valuable - take that too.), you can use the same FTL window technique as before but with lower energy requirements. Launch them into a galaxy that's safer. You'll have plenty of spare stars elsewhere in the universe to rehome the orphan planets.
  • Divide the work of litteral evacuation. Each planet going through the barrier has 6.9 billion years to evacuate themselves out of the galaxy. This is a long time, and with everg civilisation working in parallel (and with support from your government with say donations of parts and opening the galaxy tech library royalty free), it should be doable.
    • If current 2021 earth got this message, that we had 6.9 billion years to leave the planet and galaxy, and here's 1 million advanced space ship parts (like hyper drives and reaction less thrust drives and fusion reactors and inertial dampeners), and here's instructions on how to build everything sci fi we've dreamed of, wed have ships out of the solar system within the year. Earths civilisation would survive the coming of the light horizon.

You don't move galaxies - you reverse cosmic expansion

  1. There is no single "cosmic light horizon" that applies everywhere. It depends where the observer happens to be.

  2. You cannot move galaxies at 10c

  3. The only way to get them closer together is to reverse cosmic expansion. Good luck with that and remember to stop before you get a Big Crunch.

  4. The "easiest" way to do this is by time-reversal. Either you must do it for the whole universe, in which case you have a paradox because you will reverse yourself. Or you must locally reverse time. If you do this, the member states won't be happy because they will get more and more primitive relative to everyone else while this is happening.

  5. If you can find a way of locally reversing cosmic expansion without time-reversal then you should write a paper on it and you will become more famous than Einstein.

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    $\begingroup$ I don't get it. If galaxies are traveling away from each other faster than light can catch up to them, then they must be traveling faster then c. If this is so, why can't they travel at 10c? $\endgroup$ – Justin Thyme the Second Mar 23 at 2:38
  • $\begingroup$ @Justin Thyme the Second: Because they're not traveling away from each other. Space is expanding, increasing the distance between them. Over long distances, the total rate that space expands increases the distance faster than light travels. Think of putting two marks on a rubber balloon, and then inflating the balloon $\endgroup$ – user1167758 Mar 23 at 6:10
  • $\begingroup$ @user1167758 Oh, right. Space is expanding. A meter is getting longer and longer. One is still going 10 km. per hour, it's just that the kilometre is getting longer and longer. Soon a light year will be the length of the universe. Einstein's Relativity fudge factor variable, that makes the theory work. The planets actually travel in ellipses inside of ellipses. The earth is still the center of everything. $\endgroup$ – Justin Thyme the Second Mar 23 at 11:53
  • $\begingroup$ "Space is expanding." Yes. "A meter is getting longer and longer." No. $\endgroup$ – user1167758 Mar 24 at 6:03

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