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Imagine that less than a million years from now, probably in the hundreds of thousands, humanity has spread throughout the galaxy. There are afew aliens here and there, but that is insignificant, they are part of the same civilization. Anyways, for some reason our civilization has decided that the local group which will be bound to us forever doesn't have enough stars, so it decides, in less than a million years time, to send out a fleet of trillions, quadrillions of ships to take over other gravitationally bound galaxy groups and build abunch of megastructures to bring as much of the universe as possible into the gravitational neighborhood of the local group so that we will ensure our galaxy has as many gravitationally bound neighbors as possible to accompany it during the heat death of the universe. What are the specific reasons that humanity does this? unimportant. The question is how many galaxies could we bring together in an artificially created, gravitationally bound ultra-cluster centered on the local group? Assume that all fleets necessary for conquering these galaxies are sent out in less than 1 million years from now, that it takes each fleet around 1 million years to build the necessary structures for moving their particular galaxy and that the fleets travel to the galaxy at basically the speed of light (99.99%+). Assume no FTL capabilities for the return journey. What would be the fastest way to move these galaxies? Would we rely on less powerful methods like Shakardov thrusters to move near neighbors like the council of Giants to us and rely on more brute-force and expensive methods to get matter from more distant regions into our civilizational sphere? Yes, there will be other alien civilizations, some of which will resist human imperialism, but none of the others will be intergalactic. Humanity is trying to gather as many galactic acorns as possible to last itself as long as possible for the long winter that is the end of time and nobody else is going to be a significant enough power to stop them, though they may try.

It should be obvious, but this is not the same as asking "how far out can humanity spread" as my definition of "conquer" isn't that "humanity inhabits it" but that "humanity makes this galaxy/star/planet artificially gravitationally bound to the local group." The distance we could travel, assuming we travelled and light-speed, and then return to the local group is a good upper bound for how much of the universe we can conquer, but practical concerns of startup, movement, construction, and most of all the top-speed these galaxies could actually reach certainly makes the actual limit of conquest much nearer to us than the round-trip limit. I'm looking for answers that provide a conservative lower-bound estimate of what humanity could conquer and an upper bound estimate that requires really high effort and tight time-scales to actually achieve so that humanity's actual ability to succeed given the parameters I laid out would reliably fall somewhere in-between the two estimates.

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    $\begingroup$ We've had a related questions about moving galaxies which you may find helpful, and which might be close enough to be a duplicate: worldbuilding.stackexchange.com/q/175338/627 $\endgroup$
    – HDE 226868
    Jun 9, 2023 at 19:20
  • $\begingroup$ @HDE226868 That goes to how you can move galaxies, but not how many galaxies you can consume assuming you can move them. $\endgroup$
    – skout
    Jun 9, 2023 at 19:44
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    $\begingroup$ "how many galaxies could we bring together in an artificially created, gravitationally bound ultra-cluster centered on the local group?" However huge you think that the Universe is... it's much, much, MUCH larger. $\endgroup$
    – RonJohn
    Jun 10, 2023 at 5:02

2 Answers 2

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This can't really be answered without knowing the FTL capabilities of the civilization, but here are a few non-FTL realities.

Right now, at this very moment, if you started moving away from Earth at the speed of light, the furthest object you could reach would be about a billion light years away. This is because the fabric of space itself is expanding faster than you can move. This is similar to the Marching Chinese conundrum, where the distance you put behind you is matched by the increase in distance in front of you due to the Big Bang induced expansion of the universe.

This has to be applied in both directions. Even if your super-high-tech humans could move entire galaxies, they'd have to move the further galaxies FTL for them to ever arrive in our part of the universe. Overall, you'd probably be better off moving our galaxy to a more densely populated part of the universe.

For a conservative lower bounds, you have to consider that the rate of expansion keeps increasing, due to "dark energy" (which is a little like the TARDIS being dimensionally transcendental). As such, you'd basically be limited to the Local Group of galaxies. Fortunately, it's starting to look like dark energy is actually a mathematical error induced by presuming that the local group is a typically dense part of space, so maybe you'd get more, but I can't imagine you'd be able to pull together more stars than exist in Laniakea.

In terms of "how would we do it," that's magic. There are no fathomable technologies that would allow entire galaxies to be moved. There are definitely no technologies that would allow you to move them and maintain their structure. You could just as well ask how we could evenly space all of the stars in the galaxy so that they created a grid.

Addendum

I attempted to respond to the comments in comments, but it's too big a topic. Everything I'm about to say needs to be qualified with a heaping pile of "by our current best guess" and "approximately" and "somewhere around," so make up your own reality from there.

The visible universe is 13.7 billion light years. Due to the expansion of the universe the stuff we can see at that distance is now 30 billion light years away. The rate of expansion of the universe is increasing. By our current estimates, the universe will eventually be expanding so fast that our very atoms will be shredded. This is called the "big rip," and is expected to precede the heat death of the universe.

Based on that, the stars that are 2 billion lys away will continue to be 2 billion lys, even if you travel towards them at the speed of light, because the fabric of the universe is expanding that fast. It's the great cosmological "you can't get there from here".

Now you have to consider (a) Accelerating/decelerating your great fleet to get there, (b) building your star-engines, and (c) accelerating/decelerating the stars themselves. You have to accelerate them enough to escape whatever gravity well they're in. The further away they are, the more acceleration you'd need to overcome the expansion of the universe. The more you accelerate them, the less star there will be left over when it gets there.

With that, I feel confident that, given sub-light limitations, you aren't going to pull in any stars that are outside of Laniakea's 500 million ly bubble.

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  • $\begingroup$ Regarding your last paragraph, would stellar engines built around every star in the galaxy not suffice? $\endgroup$
    – M S
    Jun 9, 2023 at 17:46
  • $\begingroup$ I guess I should add the assumption that there are no FTL capabilities whatsoever. However, your figure of 1 billion light years being reachable is wrong. The cosmological event horizon is 16 billion light years away and everything within that is reachable assuming near c1 capabilities (which are assumed). However, 1 billion light years (the size of Laniakaea) may be the limit of conquerability. $\endgroup$
    – skout
    Jun 9, 2023 at 17:49
  • $\begingroup$ @skout In cosmological terms 16 billion light years IS about 1 billion light years. Which is what the answer says. $\endgroup$
    – Mathaddict
    Jun 9, 2023 at 17:54
  • $\begingroup$ @Mathaddict the light may come from 1 billion years ago, but the real distance is 16 billion light years right now and the real distance that can be brought in is what's important for the sake of the question. $\endgroup$
    – skout
    Jun 9, 2023 at 17:57
  • $\begingroup$ @MS, The whole "stellar engine" thing is magic, too, but even if you could push stars around like rocket ships, they're only about 1/10th of the mass in a galaxy. $\endgroup$ Jun 9, 2023 at 20:21
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This is the premise behind Drake's equation. Suppose a civilisation felt required to expand into the rest of the universe, how far would it go before it met another one coming in the other direction. They could hop from star to star at a small fraction of the speed of light, and still traverse the galaxy in a million years. There is then a huge jump to travelling to another galaxy: bigger than going from other planets to neighbouring stars, which would require a million years to make one hop.

The best strategy would be to migrate from where we are toward the centre of the galaxy, where stars are closer, but not so close that the SMBH becomes a nuisance.

If we have not met one of these civilisations, then the conclusions are (a) we are alone (seems unlikely) or (b) if your civilisation has lasted for a million years or so, maybe you are more laid back about things, and don't see the need to fill the galaxy with copies of yourself. So my answer to the question as posed is 'as much of the galaxy as we can be bothered to invade', but the jump to other galaxies without FTL or other magic seems like a big leap for life as we know it, Jim.

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  • $\begingroup$ Intergalactic travel is a massive leap that we have to find reasons for, and conquering other galaxies is another leap after that. Travel within any given gravitationally bound cluster will become easier over time as the group contracts, so some intergalactic travel will happen eventually, but you have to have special reasons to travel beyond the group. Sending self-replicating robots out to bring back large amounts of material seems like a good way to do it, or you could be religiously motivated to spread the good word to all alien civilizations in reach. $\endgroup$
    – skout
    Jun 9, 2023 at 19:48
  • $\begingroup$ Or c) we simply don't yet have the ability to detect whether or not an alien civilization has spread throughout the galaxy $\endgroup$
    – M S
    Jun 9, 2023 at 23:18
  • $\begingroup$ Distance is not a part of the equation. the fraction of civilizations that develop the technology, and the length of time for which they release detectable signs of their existence into space. - "how far would it go before it met another one coming in the other direction" isn't the OP's question; it's how far can we get. Which is: noone can get far enough from anywhere. Best we're hoping for is a simple radio signal that we are not alone. $\endgroup$
    – Mazura
    Jun 10, 2023 at 2:42

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