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It has been said that humans are capable of reaching up to a billion light years away before expansion has taken those galaxies permanently out of reach without FTL methods. I aim to have a civilization use stellar engines from their home galaxy to reach these far out galaxies, build stellar engines on those stars and return them back to their home galaxy, red dwarfs will have the longevity of trillions of years for the long and slow journey back from the furthest reaches.

The largest known galaxy ic1101 has 100 trillion stars, which is massive compared to the trillion stars in Andromeda and around 400 or less billion stars in the Milky Way. In 4 billion years we will have the Andromeda and Triangulum galaxy merger with the Milky Way and by 100 billion years the local group will have merged into a super galaxy of under 2 trillion stars, at this time without out intervention there will be no new matter that can enter this super galaxy as all other galaxy groups and cluster would be out of reach and sight.

One of the major issues with bringing stars home with stellar engines is that the dark matter which is the bulk of the binding mass of the galaxy does not come with the star, neither does the black hole or inter stellar gas unless other methods are used to move or collect them.

At an estimate there are 500,000 trillion stars within one billion light years from Earth and if that many stars were tried to be contained within one galaxy it would be thousands to hundreds of thousands times larger than the largest known galaxies, And with a tiny fraction of the necessary dark matter to hold the super galaxy together it might not be possible to have so many stars as part of one system

We do have some ultra diffuse galaxies which are low or free from dark matter, they are the remnant of a galactic merger and clusters of stars that were thrown further out gravitate together without dark matter but these clusters or dwarf galaxies are small in comparison housing around 10 billion stars, similar to the size of the Triangulum galaxy

There are a few possibilities i had thought of to achieve this from lifting material to vastly increase the galactic centres black hole or even some sort of megastructure on immense proportions, possibly a simple arrangement like a globular cluster would work, there are proposed systems of matrioshka brains on clusters of red dwarfs but could that exist without lots of collisions on the massive scale i am after?

What methods that are within the laws of physics could an advanced species with stellar engine/megastructure and star lifting capabilities use to support a galaxy of 500,000 trillion stars (mostly red dwarfs) that has only a tiny fraction of the necessary amount of dark matter to bind it? Could it possibly exist as super giant globular cluster through a specific arrangement?

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    $\begingroup$ Are you seriously asking a bunch of monkeys freshly descended from the trees to provide insight into the knowledge of physics of a billion years old civilization? You do understand that five thousand years ago we didn't even have writing? (That is a time span two hundred thousand times shorter than a billion years.) (And a billion years ago our ancestors were unicellular amoebas crawling about in the slime.) $\endgroup$ – AlexP Jul 8 '20 at 13:39
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    $\begingroup$ @AlexP Going off proposed future tech or megastructures, star manipulating methods in science fiction that are within the laws of physics but many might think are possible because their an engineering nightmare but technically possible. $\endgroup$ – user69935 Jul 8 '20 at 13:52
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    $\begingroup$ @AlexP Isn’t that the appeal of this website? $\endgroup$ – Franklin Pezzuti Dyer Jul 8 '20 at 14:00
  • $\begingroup$ Another thing is my physics knowledge is limited so there might be an answer in the arrangment of the stars or something that allows them to remain together. $\endgroup$ – user69935 Jul 8 '20 at 14:11
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    $\begingroup$ Why would dark matter not come with the rest of the matter? Gravity is the force that dark matter does interact with. $\endgroup$ – Michael Richardson Jul 8 '20 at 14:43
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Your stars show up under their own power. They stay under their own power.

From OP:

I aim to have a civilization..., build stellar engines on those stars and return them back to their home galaxy..

Then

What methods that are within the laws of physics could an advanced species with stellar engine/megastructure and star lifting capabilities use to support a galaxy of 500,000 trillion stars (mostly red dwarfs) that has only a tiny fraction of the necessary amount of dark matter to bind it?

Imagine an area of ocean far out at sea. We all have motorboats, we worldbuilders. I actually have a hot tub on mine. We all get together in an area of open ocean because we want to watch the Northern Lights. We motor there from our various places of origin.

Once we are there we want to stay together, for camaraderie and so on your boat you can hear the sweet band I have playing on mine. We continue to use the motors to keep us in a tight group, not drifting off randomly from waves and wind.

So too your collection of stars. They have star lifting apparatus already on them which is how you collected them in the first place. Continue to use that to keep them collected as you see fit.

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  • $\begingroup$ Surely its not that simple, i'm wondering why i didn't think of that lol, to keep the engines going or adjusting, do you think all those engines going off next to each other would make for a dangerous environment to be in? $\endgroup$ – user69935 Jul 8 '20 at 18:47
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    $\begingroup$ @RandySavage - If I understand these things right they just channel the output of the star. So probably no more dangerous than being near a star. As far as the prospects of crashing into each other yes because they will want to do that on account of gravity. I have hung bumpers on my yacht because I know when the band starts you are going to get your boat too close and scratch the paint. But I am not sure what the equivalent is for a star. Careful piloting I guess. $\endgroup$ – Willk Jul 8 '20 at 19:09
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Barring some technology that allows for violation of thermodynamics (mass creation ex nihilo), your species will also need to collect dark matter. This is tricky, because we don't know yet the exact nature of dark matter. You can't just drag it along with you via gravitation... it comprises the bulk of the mass of most galaxies, which directly implies that the stars you are moving are less than that bulk, they simply don't have enough gravity to bring it with you.

Perhaps one of the best candidates for dark matter today is the axion. It's a primordial fundamental particle with low mass (this might be the biggest difference between it and WIMPs). Among other things, axions explain CP symmetry in our universe. Axions comprise a new "field" that makes CP symmetry happen, and if that field could be manipulated then it might also be possible to manipulate axions on a scale large enough to drag some back with you.

I propose powering your axion field manipulators with the galactic mass that you're discarding, the "not long lived" stars that will burn out before your plan is complete. Perhaps even the singularities and galactic core black holes and so forth. The field manipulation will only be able to bring back some smaller fraction of the sum of axions, you won't get 100% of them or even 50%.

To be honest though, if your species can do these things, I'm not sure they'll care so much about moving stars. This is beyond Kardashev 3 tech, veering strongly into 4+ realms.

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