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In the world I describe there is not much time left before the expansion of space takes its toll. Galaxies are already accelerating away from each other on a negatively curved spacetime. The expansion threatens the galaxy in which an advanced civilization happily lives. Until now. They realize that for their future children to live happily too they must create a larger volume of positive curvature (contracting space, giving rise to gravity as we know it).

What will they need to pull galaxies and collect them together in a giant cluster, thereby creating a local pool of positive curvature and thus postponing the ultimate faith of a big rip, at least locally?

Will this be possible in principle or is there just not enough energy for this to be accomplished? The nearest galaxies are millions of lightyears away and they consist of 100 billion stars each, but could this energetically be achieved?

I assume them to have some future device which can convert mass to energy or locally turn of the Higgs field (which would mean that all mass turned into pure energy). How long would it take? Say they could direct traversable black holes to bridge the distance. It is the energy that bothers me. Would there in principle not enough energy be present in the universe to accomplish that feat?

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    $\begingroup$ If the civilization has the technology to move entire galaxies couldn’t they use the same technology to prevent their galaxy from disintegrating directly? $\endgroup$ Aug 4, 2021 at 7:40
  • $\begingroup$ @RomanReiner Now thats a good point! But then the energy content of the galaxy stays the same. So also the contracting effect of the galaxy on the space they are in. You could gather the stars in the center though. Indeed. So the intensity of the contraction will get stronger. As a first step, not a bad idea! Maybe it should the last step. I havent done the calculation. $\endgroup$ Aug 4, 2021 at 8:12
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    $\begingroup$ There is some useful information in this nice popular science video: youtube.com/watch?v=uzkD5SeuwzM - basically there's a lot of galaxies you could never travel to, let alone be able to move, no matter how much energy you have. What's not in the video is that it's possible, given enough time, to move a whole galaxy, by moving each star individually using a stellar engine, whose energy source is the star itself. So maybe you could gather a few galaxies together, but there's a fundamental limit on how many. $\endgroup$
    – N. Virgo
    Aug 4, 2021 at 12:13

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I'll first note that any gravitationally bound system will not be torn apart by the expansion of the universe; individual galaxies will certainly be fine, as will bound galaxy groups like the Local Group. I'll still make the assumption that you want to retain unbound galaxies from a large region of intergalactic space.

The energy required to oppose the expansion of the universe is obscenely large. I would argue$^{\dagger}$ that in a universe with positive cosmological constant $\Lambda$, the total mass of matter you'd need to introduce to oppose expansion at a distance $r$ is on the order of $$M\sim\frac{\Lambda c^2}{G}r^3=6.63\times10^{11}\left(\frac{r}{\text{Mpc}}\right)^3\;M_{\odot}$$ In other words, to stop a galaxy 1 megaparsec away from escaping, you'd need a mass roughly comparable to that of the Milky Way. This actually seems reasonable, as this happens to be about the size of the Local Group, which is gravitationally bound!

Unfortunately, that $M\propto r^3$ relation gets really unfortunate really quickly. The IC 342/Maffei Group is our nearest neighboring galaxy group, lying around 3 Mpc away. All of a sudden, to keep it from leaving, we'd need something like 27 times the mass of the Milky Way. Once we consider distances of tens of Mpc, the situation becomes truly untenable on any reasonable timescale.


$^{\dagger}$One way you could do this would be to model the universe as de Sitter space where Newton's law of gravity balances out the force term from the cosmological constant. You could also note that the density this yields is similar to the critical density of the universe.

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Reversing Expansion means destroying the universe.

You demand that your aliens basically go and invert the expansion of the universe. It started in one spot more than 14 billion years ago. Everything in the universe drifts apart at more than 67 km/s/MPc. This means, first you have to cancel the expansion. That is a mind gobbling amount of energy needed to make the universe static... And just the same amount of energy would be needed to make it start to collapse back together in 14 billion years.

But how much energy is that? Well, the observable universe is some 28.5 gigaparsecs in diameter. So the edge "only" is 14250 MPc away from the center. The shell would need to expand at 954750 km/s (yes, that is correct!). That shell of observable matter weighs $1.5 \times 10^{53}$ kg. That's a kinetic energy of $E=mv^2=1.37\times 10^{71}\text{ J}$

Even if the aliens would manage to somehow generate those 2.74 times 10 by the 71st power Joules to reverse the universe's expansion... wait, that IS already about the total energy contained in the observable universe's matter! So to try to collapse it back within the time it took to get to its current size would mean destroying the universe with everything that is within it. Too bad.

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  • $\begingroup$ Cant they postpone the final rip at least locally? Or by mining a parallel universe with a wormhole? $\endgroup$ Aug 4, 2021 at 0:55
  • $\begingroup$ No. You need all the mass as energy and the collapse will take 14 billion years $\endgroup$
    – Trish
    Aug 4, 2021 at 9:23
  • $\begingroup$ But what if they use transversable wormholes to transport the mass immediately from far away places to the place needed? $\endgroup$ Aug 4, 2021 at 9:36
  • $\begingroup$ No, that does not work. You'd convert half the universe to stop expansion, then you need to destroy the rest to get it back together - Wormholes cost tremendous amounts of energy to move tiny amounts of matter. $\endgroup$
    – Trish
    Aug 4, 2021 at 9:53
  • $\begingroup$ If we convert half the mass of the universe to energy wouldnt we be able to push the remaining stars towards our region to locally make space contract more than before? $\endgroup$ Aug 4, 2021 at 10:09

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