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Basically, there is a human civilization controlling the milky-way and an alien civilization controlling Andromeda. They are communicating and have decided, for some reason, that, though the Milkdromeda merger would look cool in the short term and create a short burst of much needed star formation, that is oughtweighed by the fact that the merger would produce a very boring, somewhat ugly, elliptical galaxy. Because of this, the humans and Andromedans have decided they want to halt the merger and make the two galaxies near neighbors forever, but just neighbors. How might they go about achieving this? (no-FTL, no magic.) Another practical reason they came up with is that the merger would fling more stars out of the local group than it would create, but really everyone knows they're just trying to preserve the night-sky view.

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    $\begingroup$ You say no magic, but this is going to require Clarkian magic... (i.e. science indistinguishable from magic.) $\endgroup$
    – Gene
    Jun 9 at 18:18
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    $\begingroup$ I would be interested to know how they're communicating given the no-FTL restriction. The 2.5 million year travel time for messages would seem to make that difficult. Do they have any beyond-current-science technology at all? How about being able to generate gravity artificially? $\endgroup$
    – TheUndeadFish
    Jun 9 at 19:11
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    $\begingroup$ @TheUndeadFish Some humans live in Andromeda and vice versa for reasons. FTL can happen but it is inconsistent and uncontrollable and not relevant for the actual process of stopping the merger. The decision to prevent the merger could have happened when an FTL bridge formed between the two galaxies and important people were able to make the treaty to stop the merger, but it probably collapsed and is irrelevant for most of the process. $\endgroup$
    – skout
    Jun 9 at 19:52
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    $\begingroup$ @skout I absolutely love aesthetics being the reason. Territorial dispute? Done to death. Heat death. Eternal death. Aesthetic desire is a beautiful reason. $\endgroup$
    – user86462
    Jun 10 at 9:57
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    $\begingroup$ You know what I hate more than intergalactic planning meetings? Two intergalactic planning meetings 2.5 million years apart. $\endgroup$
    – Wyck
    Jun 12 at 14:28

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The Andromeda-Milky Way collision is 4.5 billions years ahead - this leaves your species a lot of time not only for planning, but also to evolve.

The most ancient known pluricellular lifeforms on Earth are roughly 1 billion year old. The entirety of human history fits about one million times in this timeframe. And our aesthetics and culture can change drastically on a single century.

But let's consider that for some mystical reason your species' stated goal will hold on this absurdly large time scale, and they found some ways to prevent the collision to happen. Billions years means time to develop absurdely powerful technology so I guess we can leave aside the 'no magic' component of your question. Most of the strategies discussed would be targeting the Milky Way because it is smaller than Andromeda, thus easier to move.

Space expansion. Accumulation of mass or energy leads to local inflation of the time-space continuum - this is what happen in black holes, naturally formed superdense objects. If your species find a way to enhance this phenomenon, they could artificially increase the distance between the galaxies - some sort of wall made of pure distance.

Dimensional jump. Our space is typically considered as a 3D + time environment, but the string theory predicts 11 dimensions. We can consider moving one of the galaxy into an other dimension or reality so it won't get in the way of the other.

Time travel. If your species cannot endure the sight of a merged galaxy, they can simply loop back in the past when the merge did not occured.

Progressive stellar migration. This will not really prevent the merge, but at least the resulting galaxy can be reshaped beautifuly.

Destroy one of the galaxies. The simplest way to a non-merge event.

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    $\begingroup$ You forgot one option: destroy both the galaxies. $\endgroup$
    – Rohit Pandey
    Jun 10 at 15:50
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    $\begingroup$ Re: Dimensional Jump: Last I heard, String Theory predicted dimensions that are so tiny and curled up (Brian Greene likened it to length of a wire vs. going around the perimeter of its cross section) that it's not relevant to this idea. Also, string theory is bunk. If you want proper higher-dimensional space, just write it. Don't shackle yourself to the turn of the millennium's equivalent to food pills. (Hugely popular in early 20th century sci-fi, now basically nonexistent since every lay person knows we need bulk/roughage in our diet.) $\endgroup$
    – ssokolow
    Jun 10 at 18:21
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    $\begingroup$ Vogon regulations pursuant to the Spiral Galaxy Preservation Act clearly states that should an imminent merger be unavoidable both galaxies must be destroyed, as @RohitPandey seems aware. $\endgroup$
    – fredsbend
    Jun 10 at 22:45
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    $\begingroup$ Uh... I feel like destroying one (and while the idea of destroying both is somewhat humourous) galaxy would be the hardest option. This is sort of like the idea of imparting enough energy to a planet to overcome its gravitation binding, times four hundred quadrillion. $\endgroup$
    – Michael
    Jun 11 at 20:03
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    $\begingroup$ @Michael What price has true beauty lol $\endgroup$
    – user86462
    Jun 12 at 7:08
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A galaxy is made of stars among other things. For your aesthetic considerations, you seem to care mainly about stars. It is possible to move stars using Shkadov thrusters. The intensity of the force and direction can be managed. The civilizations can spread to all or most of the stars in their respective galaxies and build these thrusters around them. The force on the stars can be modulated such that the two galaxies reach an equilibrium with the gravity pulling them towards each other balanced by the Shkadov thrusters pulling them away.

Energy can be extracted from the rotations of the super massive black holes in their respective centers (via the Penrose process) as well. This too can be harnessed to include forces on them that tug away from each other, balancing out the gravity between them.

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  • $\begingroup$ How would you anchor these thrusters? $\endgroup$
    – biziclop
    Jun 12 at 13:14
  • $\begingroup$ You mean the mirrors? They are either in orbit around the star or the radiation pressure balances the gravitational pull. $\endgroup$
    – Rohit Pandey
    Jun 12 at 21:36
  • $\begingroup$ Good idea, but apparently the maximum velocity of a fusion powered photon rocket is about half of a current Andromeda-Milky-Way velocity, and we have to push more than $10^{12}$ solar masses of galaxies with less than $10^{11}$ solar masses of stars, so even neglecting gravity and the (probably insurmountable) problem of how to couple the star momenta with the galaxy momenta, we're providing much less than a tenth of the momentum needed to just stop the current rate of approach. $\endgroup$
    – g s
    Jun 15 at 7:30
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Stick a black hole on each of the 'far' side of the two galaxies

Make the black holes absolutely stonking massive. They suck the galaxies away from each other. As they're black holes, they're aesthetically inoffensive during their duration. Remove once no longer needed. Optional: Remove or shrink them to achieve beautiful stasis whereby the two galaxies have zero movement towards or away from each other.

FWIW I'm assuming a black hole generator where you type in the coordinates and press the button. The size is controlled with a large silver dial.

Rule of cool version

Stick a massive white hole (1) that powerfully repels both galaxies in the middle of them.

It's comparatively gauche but might look quite good, in a confrontational way.

(1) If this term is too well defined to mean a great big repulsor, then I'm coining the term brown hole.

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    $\begingroup$ Your understanding of Black holes and white holes is completely flawed. Black holes do not "suck" matter (I think that has been quoted quintillions of time), and white holes do not REPEL. White holes have positive gravity, However due to the curvature of space time, the objects can never approach the white hole. $\endgroup$
    – Furious Arcturus
    Jun 10 at 10:16
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    $\begingroup$ @FuriousArcturus Who cares about the details? The end result satisfies the basic criterion and looks good in a painting. Btw, nothing ever truly 'sucks', including vacuums. If it wasn't obvious to you, my answer is not hard science and is not intended to be. $\endgroup$
    – user86462
    Jun 10 at 10:17
  • $\begingroup$ If white holes have normal gravity, then use a brown hole. $\endgroup$
    – user86462
    Jun 10 at 10:22
  • $\begingroup$ @AncientGiantPottedPlant Black Holes and white holes aren't black or white just for the namesake. It's their property in respect to matter and light. IDK where you are going with brown holes. $\endgroup$
    – Proscionexium
    Jun 10 at 11:20
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    $\begingroup$ @Proscionexium Is it not obvious from context that I want a universal attractor and a universal repulsor? A giant brown hole ought to be a universal repulsor, for obvious reasons. $\endgroup$
    – user86462
    Jun 10 at 19:17
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I'd say they figured out how to affect expansion of spacetime - Hubble constant. They locally (in the space including and between the galaxies) increased it to offset the gravitational pull between Milky Way and Andromeda.

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Directional Gravity Beam

I'm sure someone's done it before, but coin the term GASER. G for Gravity. It is highly coherent, directional, and able to be negative.

It glows lavender. Imagining a map of the space between the two galaxies, have the beams almost parallel or in complicated crossing over patterns that resemble macrame.

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    $\begingroup$ I can't coin "GASER" when you already have! $\endgroup$
    – skout
    Jun 10 at 16:40
  • $\begingroup$ @skout But you can steal it from a non copyrighted internet source, which is even better! $\endgroup$
    – user86462
    Jun 10 at 19:20
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    $\begingroup$ @AncientGiantPottedPlant Ackchyually, it is copyrighted under CC BY-SA 4.0. No idea if author can revoke it or not... $\endgroup$
    – Yksisarvinen
    Jun 10 at 23:08
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    $\begingroup$ @Yksisarvinen Lol ackshual-ly, you're right but that allows reuse for any purpose including commercial use, it's basically a 'copyleft' license. Ackshially! $\endgroup$
    – user86462
    Jun 11 at 1:09
  • $\begingroup$ GASER... Oh great.. now i'm thinking of Gazerbeam (from the Incredibles) and imagining what would happen if something went wrong and the GASER beam started vaporizing planets... $\endgroup$
    – Michael
    Jun 11 at 20:08
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Although a typical black hole would have been a good method to prevent the merging of the two galaxies but it would be too costly in terms of the required efforts. Concentrating so much energy in an extremely small volume would be a nasty business.

Maybe you can manipulate the dark matter that the galaxies are made up of. Scientists have put prominent faith in the existence of such a gravitationally binding matter in interstellar space within a galaxy and among galaxy clusters. Around 85% of total mass in the universe is dark matter. The density of dark matter is related to density of normal baryonic matter in the universe. Within a galaxy the density of dark matter is a bit lower because it does not take tremendous amounts of dark matter to keep a galaxy intact with the stars (although significantly larger amount in comparison to normal baryonic matter). But when we move to spaces between/among the galaxies, there is greater void for dark matter to fill and hence the density of dark matter is quite higher. This happens in keeping together galaxy clusters or supercluster. This is because you need tremendously high amounts of dark matter to keep entire galaxies together gravitationally within a cluster.

The reason I think manipulation of dark matter density would work in your case is that the intact nature of galaxies and clusters is mainly attributed to the distribution of dark matter among them. Perhaps handling just two galaxies would not be that big a deal. For a detailed study of dark matter see Wikipedia or Encyclopaedia Britannica

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Dark Energy

One very important result of stargazing has been this: The universe is expanding. Despite that gravitational force has no limit, something is still able to push everything apart. We do not know how or what yet, so we've added a theoretical construct to describe it until we know more. Dark Energy.

Dark Energy somehow pushes the whole universe apart. We can use this to push two mere galaxies to such an extent that they hover perfectly parallel to each other. They add in a huge layer of not uniformly thick whatever expells Dark Energy between the galaxies. As it's not uniform, parts are being pushed more than others, allowing a gentle flip of the galaxies and settling them perfectly aesthetically from each other. The Dark Energy is reduced in parts until the galaxies will stay stable and aesthetically pleasing for as long as needed.

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  • $\begingroup$ Perhaps you are talking about dark energy when you said pushing. Dark matter and dark energy are kinda competing to maintain the large scale structure of the universe. $\endgroup$
    – Proscionexium
    Jun 14 at 4:52
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    $\begingroup$ @Proscionexium yes I mix them up sometimes. Thanks. Fixed it. $\endgroup$
    – Trioxidane
    Jun 14 at 12:36
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That is not dead which can eternal lie: the aestivation hypothesis for resolving Fermi's paradox addresses this in section 6.1.4: screenshot of section 1/2 screenshot of section 2/2

Sounds like it's expensive!

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