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Lets take a civilization that has surpassed the type III on the Kardashev scale. It has the following assumptions:

  • It harnesses the energy of entire Galaxies,
  • its populace did not transcend. If you need a reason: passed a certain point of intelligence biological life sees no reason to live and dies off so they avoid getting smart enough.
  • The civilization has access to all current materials and highly plausible hypothetical materials.
  • The civilization can create matter and energy from nothing, otherwise all natural laws are still "normal". The way they do this has no other effects than the creation of matter and energy (so no white-holes and temporal paradoxes).

The civilization expands across the galaxy and galaxies around it. As the population grows the civilization decides to fill the universe with matter to make as much living space as possible. The question is, how far can they go in creating living space?

With such powers it would be easy to fill the universe with Dyson swarms. But could they go further? I imagined a galaxy where the planets/Dyson Spheres stop rotating (or have a rotating connection at the poles if the rotation is important) and solar systems are connected with "bridges" that can house trillions upon trillions. Eventually Gravity will be the limiting factor, as mass would cause the structure to collapse in on itself to form a new planet and if too much matter is in one place it'll eventually form Black holes. On the other hand there's also the expansion of the universe to consider and what it would do to such a Galaxy...

So what would the fullest most liveable Galaxy look like, without it collapsing in on itself or breaking apart? The answer should at least weigh off:

  • Square footage of the solution versus other solutions. Example being hanging as many planets as possible in the habiteable zone of stars versus Dyson swarms.
  • Use of the space between solar systems
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closed as primarily opinion-based by JBH, Gryphon, GrandmasterB, L.Dutch Jan 9 at 3:49

Many good questions generate some degree of opinion based on expert experience, but answers to this question will tend to be almost entirely based on opinions, rather than facts, references, or specific expertise. If this question can be reworded to fit the rules in the help center, please edit the question.

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    $\begingroup$ Comments are not for extended discussion; this conversation has been moved to chat. $\endgroup$ – L.Dutch Jan 9 at 19:29
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If your civilization can create matter and energy out of nothing (violating conservation of mass and mass-energy equivalence), then your limit is immaterial. The universe's expansion is driven by energy. If you can create infinite energy, you can expand the universe to an unlimited degree, because the added mass can be counterbalanced.

Assuming your materials are as nigh-magical as they appear to be, then take the available volume of the universe ($4 \times 10^{80} m^3$), divide by the space of a luxury apartment + amenities, which for convenience we'll set to ${1000}m^3$, and you have housing for $10^{77}$ humans, plus three times that space to grow food for them, assuming you can't just conjure that out of thin air as well. Gravitational collapse is not a problem, since stresses will be identical in all directions and the entirety of the universe will remain mostly empty space. Your only real problem will be getting rid of heat, but if you can create matter and energy, that seems like a bagatelle.

Effectively an infinite apartment building. There's probably a horror movie like that.


Edit: It's worth pointing out that we're assuming your humans have solved FTL travel, because if they haven't, their magical abilities aside, they're never going to populate even a small corner of the universe.

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    $\begingroup$ You don't need space to grow food, it can be created out of nothing. +1 for forcing me to look up the word bagatelle. $\endgroup$ – JBH Jan 8 at 22:09
  • $\begingroup$ And an upvote on the comment for pointing out my misspelling! Also, if you don't need that extra space for food... I dunno, nigh-infinite amusement parks? $\endgroup$ – jdunlop Jan 8 at 22:13
  • $\begingroup$ That's the problem with this question, it really is an "angels dancing on the head of a pin" question. $\endgroup$ – JBH Jan 8 at 22:16
  • $\begingroup$ How do you build this universe then? You dont start with a full universe so it wont start pulling from all directions until you had time to fill it. And similar to the existing universe where supposedly all matter also pulls on each other, places with more dense matter pulls harder and causes it to collapse into planets, suns and Black holes. $\endgroup$ – Demigan Jan 8 at 22:37
  • $\begingroup$ @Demigan - it only collapses inward because there's no force acting against it collapsing inward. As more matter is attracted, greater force is exerted on the body as a whole, allowing for increasing density in the centre, and so forth. This cannot happen if you're building the Universe of Apartments. Using the Willis Tower (since I have values for that), the density of a skyscraper is somewhere around 800kg/m^3 - less than the density of water! $\endgroup$ – jdunlop Jan 9 at 0:38
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The civilization can create matter and energy from nothing

If they can pull out stuff and energy from their aethers impunely, they can create a singularities that expand at light-speed if they so wish.

Simply put, if they use technology to make something out of nothing, they can create ever more machines that do it. The amount of mass they can create grows exponentially. The only limiting factor to what they can do would be the speed of light, and they can go past that if they develop the means for FTL travel.

So the amount of livable space they can make is up to what they call livable. Some people in Asia find it normal to sleep in pod hotels, whereas I would move to Mars without a second thought if I knew I could be the only person there.

If your people really like each other, then as long as gravity is not enough to knock them out you can keep expanding. Food is not a problem - you can just conjure stuff out of nothing, and then turn excrements into EM energy and send it towards the edge of the observable universe.

And who the Hades builds Dyson machinery when you have energy out of nothing?

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  • $\begingroup$ I don't know what to do here. +1 just for saying I would move to Mars without a second thought if I knew I could be the only person there. Amen, brother. On the other hand, -1 for posting a comment as an answer. But, sometimes comments are simply two small to explain why there are problems. Therfore, +1 for being a sensible frame challenge. $\endgroup$ – JBH Jan 8 at 22:05
  • $\begingroup$ The question is about creating livingspace, not how that space is divided between the people of the civilization. Livingspace could be trillions of habitats, or planets, or Dysonspheres, or whatever offers the most livingspace. I would like to know which would be the best, or if something else is better. $\endgroup$ – Demigan Jan 8 at 22:59
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It would seem to stand to reason that the goal ought not to be to build one single super-sized galaxy, but a series of regular or slightly larger than normal sized galaxies that exist together (yet independently) in a super cluster.

Superclusters are so large that the galaxies continue spreading outward with the universe. To illustrate, if you imagine, say for example, ten galaxies in a supercluster represented as ten dots made by a marker on a partially blown up balloon. As the balloon (the universe) is blown up further (expands) the dots (galaxies) will spread away from each other. During this time, you could be busily drawing the dots larger (building the galaxies larger), seemingly infinitely. The larger the dots (galaxies) become on the balloon (universe), the more over all space is taken up. But you do so in such a way that the percentage of the balloon (universe) that is filled with these dots (your supercluster) never actually increases or decreases, as it keeps pace with the expansion of the balloon (universe). Follow me?

In short, your answer is probably "a super cluster, adjusted for inflation"

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You're looking for a percentage I assume? Seeing as you're talking about a hypothetical Type-IV+ civilization, you could get 100% of the universe to be livable if you had the right resources. Gravity is determined by the interaction of two sources of mass. The Earth has a certain amount of mass spread across a certain volume of space that we get the gravity we have. By making a super-light (but durable) metal or metal-alloy, this could literally be a series of sheets spread across the universe that people walk on and take escalators or lifts to travel between them. Additionally, this Civilization would already have black holes under their control and command as is a requirement of being Type-III, so to be Type-IV+ it's a given that they could make the black holes livable as well in some way, shape, or form. By your supposition that this Civilization can create matter and energy out of nothing, you are effectively confirming the existence of white holes in your universe (something which is still only hypothetical in our own world). Worst case scenario, the black holes get filled with white holes and aside from maybe some minor gravitational distortions (depending on how good the patch-job is), you have livable universe. If you can manage to live physically inside the black holes, though, you could argue that's an infinite source of livable space independent of our universe at large, meaning the answer is technically 100%+.

The important thing to consider is this: living space is about perspective. Each family could own their own galaxy in this circumstance and that is all their living space, even if they never actually use it all, the black hole at its center being used as a furnace to power their galactic mega-structure. At the same time, you could view 3 cubic meters as the extent of one's living space, and as long as everyone has these fancy matter creators, anywhere in the universe aside from black holes (and even then) becomes potential living space because you can just eliminate whatever exists that you don't want to exist. You can create anything too, so with that there are no limits. Make material that is perfect in thickness, density, and durability... and you have now created a universe of nothing but apartments.

It's worth noting that unless you're writing an erotica, this would not be a good universe to have as a basis. Ignoring the fact that by now, purely biological humanity would be non-existent... These people would suffer from incredible boredom. In a world where you can create physically everything, which technically includes other life, there is no need to work, and every game gets boring after a while... eventually these people will be reliant on fulfilling only one primal instinct if they are still biological life at all: their instinct to reproduce. Since they can also CREATE their ideal partner(s) thanks to this technology being able to create matter and the fact being able to create/manipulate neural pathways would be child's play by this point in the future... Dating won't even be a necessity anymore, not they'd feel a need to go through that process anyways since it'd all be about the act, not the symbology involved with eventual marriage or a monogamous relationship. The reproductive process is essentially all they will be doing because nothing else in the universe will have value, and even the value of reproducing in and of itself will be null, making the value solely in the act, not the outcome. (You could also argue that a terrorist cell might form out of boredom that attempts to destroy these matter creators... but anyone with a brain would know the machines would just be built again or they'd be remade faster than they could be destroyed; therefore, making the act of terrorism pointless in-and-of itself, and anything they do could just be countered by these machines anyways meaning it is doubly-pointless.)

Solution: Limit the technology's omnipotence. The fact it can create on-command is extremely overpowered and is enough to make this universe of 100% livable space. The issue is, once you reach Type-IV, it doesn't matter what you say: This civilization has domain over the universe like we have over small patches of dirt. While we can terraform local areas, they could do so to the entire universe. Push any beyond that, and you're looking at a Civilization that has succeeded in multi-universal manipulation. It's just an exponential growth system. Stick to Type-III and lower. It avoids any of these issues.

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