Gravity in our universe is an attractive force between bodies with mass . 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.
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.