What would happen if only gravitons existed?

[twistor]

This is a somewhat weird question, but here it goes:
What would happen if there were only gravitons? Would some other kind of particle appear in the Universe sooner or later? Would there be any chance for mass to appear in such a Universe (assuming that the graviton is a massless particle)? Could black holes form in the absence of mass (perhaps by some kugelblitz-like mechanism)? Sorry if my question is too crazy / stupid and thank you in advance for your answers.

 

[jfizzix]

This is similar to he hypothetical situation of if there were a universe containing only photons.

At this point, no one has a tested theory of quantum gravity, so there are many possibilities, but no consensus (as yet)

If there was a universe containing only photons, energy would be very nearly uniformly distributed, as even in the presence of gravity created by some blob of higher-than-background energy density, the photons would be deflected, but always traveling at the speed of light.
Since Light cannot slow down, it cannot form clumps steadily growing over time like matter can.
That being said, black holes could form if the energy density in one location were high enough, but this would be an incredibly unlikely occurrence.

On the other hand, if we allow the possibility of photon-photon collisions happening, producing matter-antimatter pairs, then there could be matter in a photon-universe, but individually only for a short time, as the pairs would recombine and annihilate. If for whatever reason, more matter is created than antimatter (however slightly), then eventually the universe would have a significant amount of long-living matter that could form galaxies and stuff.

As far as a graviton-universe goes, I expect the situation is very similar, but we simply don't know if anything gets created from colliding gravitons (if they exist at all).

 

[ohwilleke]

Gravitons couple to every other kind of particle and to each other, if they exist. (In contrast, photons don't couple to each other dramatically reducing the possibility of clumping and essentially ruling out collisions). So there is no fundamental reason that a graviton only universe couldn't produce ordinary fermions and bosons.

On the other hand, this may be a fundamentally counterfactual scenario because in a graviton based quantum gravity theory gravitons come into being in the first instance because they are emitted by other fermions and bosons and there is no scenario in which you can generate of huge number of gravitons in a universe with no other form of matter or energy in it.