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Gravity in our universe is an attractive force between bodies with mass [citation needed]. This allows matter to coalesce and form all the large scale structures we observe - planets, stars, galaxies, etc. At small scales, however, electromagnetic and nuclear interactions dominate the formation of matter (chemical bonds, quark-quark bonds etc.)

For the purposes of this question (and simplicity), we will ignore implications for the very early stages of a universe (first few seconds), where even with our friendly, attractive gravity our knowledge is somewhat shaky and repulsive gravity may very well have been a real thing.

Could a universe evolve with repulsive gravity?

ie. The Gravitational constant has the same magnitude as in our universe, but regions of mass-energy repel each other, rather than attract.

Since 'evolve' is quite broad in this context, I'll narrow the the definition to some particular features. Some specific features of an evolving universe in rough order of necessity:

  • Existence - It's no use if the universe spontaneously collapses early on and destroys itself in some way.
  • Transparency - Being able to look around in this universe would be handy, preferably at (human) visible wavelength.
  • Stability - Could local or global equilibria occur in this universe? Or would it exist in a state of perpetual high-energy chaos?
  • Any small scale structures - Could atoms, molecules, crystals or rocks form? Could chemical reactions take place? How similar would they be to ours?
  • Any large scale structures - A transparent universe is no use if there's nothing to look at!
  • Hospitable large scale structures that some trans-dimensional traveler could visit and survive with reasonable life-support (ship, EVA suit).
  • Possibility to evolve life - If large scale structure can form, could they harbor anything that we might call life?

On the face of it, it seems as though repulsive gravity would preclude the formation of of large scale structures like planets and inevitably lead to a Big-Rip scenario. However, the gravitational force is relatively very weak, which could allow the formation of something in the first few million/billion years.

I realise I've asked about quite a few specific points, as I'd like as much insight as possible. However, general answers that look at the big picture are also appreciated!

Since this is a fair way outside the realms of theory and into the backwaters of speculation, I'm not looking for hard-science.

As a side note, presumably a universe could be designed with the other fundamental forces adjusted to allow for repulsive gravity. For the purposes of this question however, I'd like to keep the other forces fixed and change only gravitation.

EDIT:

FWIW, I threw together a very crude simulation with repulsive gravity and and short-range attraction to simulate chemical interactions (I said it was crude!). This revealed something quite interesting - If I made the system closed, initially the particles demonstrated something similar to Brownian motion, but after leaving it running for a few minutes something strange occurred. Filaments of high-energy particles emerged with large spherical regions of relative calmness between them. The filaments almost looked like highways for the high-energy particles to move along.

Now, in all likelihood the simplifications I made for the simulation have rendered it completely unrepresentative of the universe I'm suggesting, especially if the universe expands indefinitely. However, I thought the structures that formed were worth mentioning.

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    $\begingroup$ Interesting question, although I've a feeling the answer will be "a cold dark space containing a finely spread gas". $\endgroup$ – Tim B Jan 11 '16 at 15:16
  • $\begingroup$ Incidentally, in this case I think it's just about ok as all your bullet points really are a single scale but be careful when asking a worldbuilding question to make sure it's a single question with a single answer :) $\endgroup$ – Tim B Jan 11 '16 at 15:17
  • $\begingroup$ @TimB Not so cold, think about it as a rigid gas tank. The heat goes up along with the pressure of the gas. $\endgroup$ – JordiVilaplana Jan 11 '16 at 15:19
  • $\begingroup$ @JordiVilaplana It's not rigid though, as space expands the gas will drop in pressure, and as pressure drops so will temperature $\endgroup$ – Tim B Jan 11 '16 at 15:24
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    $\begingroup$ The simple answer is "NO". The force that holds the universe together is gravity. $\endgroup$ – mg30rg Jan 11 '16 at 15:31
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I guess atoms would still form (hydrogen, some helium, traces of lithium). However there won't be galaxies or stars, as it's exactly the attractive nature of the gravitation that creates that type of structure. And it's exactly because attractive gravitation does not lead to an equilibrium that we get stars: The gas collapses under its own weight until the nuclear fusion ignites and generates a pressure which counteracts gravitation and gives a temporary equilibrium (until the fusion fuel is spent). With repulsive gravitation, this doesn't happen; the gas will simply distribute equally in space, as any denser spot will be repulsive, thus reducing its own density.

Note that no stars also means no higher elements, so your universe will have no oxygen, no carbon, no nitrogen, no silicon … so no material to build planets of (not that they would form anyway, given the repulsive nature of your gravitation), and no material to form life from.

In short, your universe would be uniformly filled with hydrogen and helium, and little else.

Note that gravitation is unique in its all-attractive nature; all other forces tend to neutralize thanks to attraction of different charges and repulsion of equal ones. Thus if you remove attractivity of gravitation, all attractivity you are left with is through effective forces, more exactly, van der Waals forces.

Note that while van der Waals forces are attractive, they are not only weak, but also short range. So even if some structures (condensed hydrogen and/or helium) managed to form based on van der Waals forces, those structures cannot grow large, since otherwise the repulsive gravitation will win over. And due to the short-range nature of van der Waals forces, those structures could not develop any relevant pressure; definitely nothing that could cause fusion.

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Ohkay. I had to answer this: because I think this will be a lot more fun than you think. I think this is going to get messy.

The reason that this is going to get messy is because this proposed universe won't have any stable states. If the big bang occurs the same way (with varying degrees of asymmetry), then particles are going to be flying everywhich way in an attempt to get away from each other. Sadly: They aren't going to be able to. Particles will repel particles. Some particles will be repelled away fast enough (thanks to the same principle that makes dust storms a problem) that they will hit other particles, leading to heavier elements (only up to iron, don't get excited) forming as the gravitational force is overcome by the nuclear forces (lets face it, they're just more buff). The entire universe is going to turn into a hot, messy soup of light elements.

The important thing to note here is that there is no incentive for these atoms to reach uniformity. If you imagine a set of perfectly repelling particles with any form of damping then they quickly hit an equilibrium state where they're all equally distant. Here there is no damping. There is no equilibrium.

Even in smaller clusters they're going to keep oscillating and flowing in chaotic patterns that have no reason to stop oscillating (like the hypothetical ball in a vaccuum tube running through the centre of the earth). Periodic structures will emerge as atoms get pushed into clusters, repel, and then get pushed back again. Rogue high energy alpha radiation will batter said periodic structures apart again. Matter near the edge of the universe gets weird, no matter what view you take on cosmology.

EDIT to add a simple example

Consider our pocket universe. It's tiny, and consists of three particles and one dimension. The particles are arranged along our dimension in the order A, B, C, where asymmetry is introduced by A being further away from B than C is from B.

At the start of the universe A and B are repelled by gravity outwards, where they run into either the edges of the universe (where weirdness happens) or each other's gravitational fields (if the universe wraps around). For now let's just assume they get 'stuck' at either end of the universe and don't move any more. Now particle B is in a situation where it's being repelled by both A and C with a slight asymmetry, and thus it moves back and forth between the two other particles. As it's in a vacuum there isn't anything to slow it down, so it will oscillate back and forth pretty much forever.

Using this model we can see that our three particle system can be modelled as an oscillator. But what if we only have two particles? If the universe is infinite, they just run away from each other. If it's finite, they hit the edges and get 'stuck'. If it wraps around then we can take one of our particles as a reference point, and show that it's the same situation as having the A, B, C arrangement, but A and C are the same particle! (You also have to worry about the gravity of your own particle in this situation, but don't worry, your pull both ways around the universe cancels out). That means that the simplest universe we can have that isn't trivial is an endless oscillator.

If you scale that up to more dimensions you get the universe described above. More than a little crazy.

End example

Eventually the universe might settle down (though it will take.. erm.. the age of the universe?) and there will be no more usable energy. An analogous state to the heat death of our own universe, and a somewhat sad end for what would otherwise be a terrifying fun place to exist.

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  • $\begingroup$ So the problem would be a lack any stable equilibria that last long enough for other force interactions to take over? $\endgroup$ – user15242 Jan 11 '16 at 16:57
  • $\begingroup$ I really like this idea - what would this messy universe look like? $\endgroup$ – user15242 Jan 11 '16 at 16:58
  • $\begingroup$ Can the universe be said to have an "edge"? I'd expect it to just wrap back on itself as though laid out on a hypersphere. $\endgroup$ – Tim B Jan 11 '16 at 17:00
  • $\begingroup$ This answer is very dependent on the matter to space ratio. If the matter is forced close together by the bounds of the universe you get this. If on the other hand it can expand then basically you get every molecule in the universe accelerating away from each other towards lightspeed. $\endgroup$ – Tim B Jan 11 '16 at 17:01
  • $\begingroup$ @TimB: Like I said: It gets messy no matter what view you take on cosmology. The infinite universe 'everything accelerates away at lightspeed' is an answer so trivial as to be boring (not that I mean it's bad, I just prefer crazy chaotic universes :-). Also: That problem is the same for all the answers we could give. Infinite universe = Very diffuse (but really fast) gas. $\endgroup$ – Joe Bloggs Jan 11 '16 at 17:03
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Even if the weak and strong nuclear and the electromagnetic forces are strong enough to avoid a "quark soup" state of the universe and reach to have atoms, the electrons on the atoms' orbit will repel each atom from each other, so now you have two repulsive forces.

Maybe you can manage to have little molecules due to ionic unions if your universe expands very slow or nothing at all, but forget all about having stellar structures (stars, planets, asteroids, comets...).

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    $\begingroup$ The only molecule you'll get is H2. Without gravity to form stars, there will be no creation of elements heavier than helium. $\endgroup$ – Mike Scott Jan 11 '16 at 16:04
  • $\begingroup$ @MikeScott: I dunno. It would take a while (a really, really long time), but eventually you'd get at least a few molecules of iron through freak interactions. $\endgroup$ – Joe Bloggs Jan 11 '16 at 16:18
  • $\begingroup$ Even so, such a universe wouldn't be interesting. It would be a roughly uniform, gaseous darkness. $\endgroup$ – Draco18s Jan 11 '16 at 16:27
  • $\begingroup$ @JoeBloggs So hard to get even a single atom of iron. You need fusion to get heavy atoms, and you need a lot of pressure for that, like a small-sized star above you to make pressure enough to fusion two hydrogen cores. And fusioning hydrogen to get helium is easy. The heavier the atom cores you are trying to fusion, way much more pressure you'll need. $\endgroup$ – JordiVilaplana Jan 11 '16 at 16:29
  • $\begingroup$ @JordiVilaplana: You don't need pressure (though it's really handy), you just need the two atoms to bash into each other hard enough. Luckily for me I've got a load of free hydrogen atoms repelling each other like it's a cosmic game of bumper cars. It'll take a while, but as long as the universe isn't expanding too fast the odds are pretty good that one or three atoms are going to get boosted into each other with fairly staggering amounts of energy. $\endgroup$ – Joe Bloggs Jan 11 '16 at 16:37

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