Explaining Universe Expansion with Gravitons in String Theory/Supergravity

[nuclearhead]

In a theory like string theory or supergravity, gravity is described by gravitons on (usually) Minkowski background.

But I don't see how this works in terms of the expansion of the Universe. For example, two galaxies far apart can be moving away from each other at more than the speed of light. This is OK in General Relativity due to the expansion of space by the metric.

But in quantum gravity where all matter and forces are described by particles or strings this would mean some particles in Minkowski space are traveling faster than the speed of light. Thereby, (presumably), disobeying the rules of quantum field theory?

How is this possible?

Also, is this anything to do with propagators having non-zero values for space-like separation?

 

[ShayanJ]

Any theory of Quantum Gravity should contain, as a limit, General Relativity and, I think, GR inevitably means that gravity is spacetime's curvature.
But the more important point is, when things get quantum, you can't have a nice and smooth fabric as spacetime and it should be replaced by a rough and jiggling thing which is completely different from what we call spacetime in GR. It may have a very different structure. So actually the nice and smooth spacetime of GR is only an emergent phenomenon coming from the more basic structure that the long awaiting theory of QG must explain. So QG explains GR and GR explains expansion.
Another important point I should discuss here, is background independence. GR explains the spacetime itself and in fact is background independent because it doesn't assume a stage for the play(except about some boundary conditions!). Any theory of QG should be background independent too. In fact it should be so by definition because otherwise its not explaining spacetime in quantum realm which is the very reason of its existence. It may introduce some other things as the background which it doesn't attempt to explain but that means we still need to go deeper which again isn't what we expect from such a theory. So non of those theories assume any kind of background, Minkowski spacetime being no exception.
(Although I remember some forms of string theory are actually background dependent which I think they should be ruled out or modified. I hope M-theory is background independent though!)

 

[PeterDonis]

in quantum gravity where all matter and forces are described by particles or strings this would mean some particles in Minkowski space are traveling faster than the speed of light. Thereby, (presumably), disobeying the rules of quantum field theory?

The rules of quantum theory don't say that no quantum particles can travel faster than light. You give the reason why later in your post:

is this anything to do with propagators having non-zero values for space-like separation?

Yes, it does. QFT does not say that amplitudes are zero at spacelike separations; all it says is that operators must commute at spacelike separations.

As far as how "expansion of space", as in FRW spacetime, is modeled using the QFT method you refer to, I'm not sure "expansion of space" is actually the problem. The problem is that the model of gravity as a spin-2 field in flat Minkowski spacetime can only model spacetimes with the same topology as flat Minkowski spacetime. FRW spacetime does not have that topology, so AFAIK it can't actually be modeled using the QFT method.