In General Relativity, the expansion of the universe is modeled using the Friedmann–Lemaître–Robertson–Walker metric, and the expansion itself is a metric expansion by which the scale of space itself changes over time. Using the Einstein field equations you can derive the Friedmann equations then solve for the scale factor, $a(t)$.
However in recent history there have be many attempts to model a quantum theory of gravity in terms of a graviton which mediates the force of gravity, often in background-dependent theories such as string theory.
My question is, how could a theory like this, especially a background dependant one, model the expansion of the universe? If gravity is modeled as a result of a spin-2 particle and not the curvature of spacetime, how can the metric expansion of space occur?
Any answers, especially pertaining to Quantum Gravity as an Effective Field Theory, or String Theory, would be much appreciated.
EDIT: I have found an article that states that gravitons are fluctuations of the geometry around flat space-time. Perhaps my mistake was assuming that gravitons and space-time curvature are mutually exclusive. As i do not have a great understanding of QFT, this would not be surprising.