thegroundhog said:
Summary:: Why are gravitons expected to exist when gravity is just warped space time?
As per the summary I don't understand why physicists talk as if gravitons are inevitable, when gravity is just curved spacetime? Why would curved spacetime have a particle?
It isn't so much that they are inevitable. It is that a theory of spin-2 massless gravitons with a coupling that is a function of the mass-energy of another particle and a coupling constant functionally related to Newton's constant is essentially identical in its behavior as a theory of gravity to General Relativity in the classical limit. They wouldn't be identical, because quantum gravity would have some properties that are exclusive to quantum theories that are absent in General Relativity (e.g. quantum tunneling and the stochastic nature of the theory). They would only be identical in circumstances where those quantum theory specific properties are negligible in importance. But, the vast majority of situations where we use General Relativity are situations where the classical limit of a quantum gravity theory and General Relativity are indistinguishable with current technology.
There are deep theoretical inconsistencies between the Standard Model of Particle Physics and General Relativity that have a lot to do with the Standard Model being formulated as a quantum theory in Minkowski space (where special relativity, but not general relativity applies), while General Relativity is formulated as a classical, deterministic theory in space-time that has geometric curvature.
Naively, a graviton based theory seems like a good way to reconcile these inconsistencies (not the only way, but certainly the most obvious one). But, it turns out that making a quantum gravity theory along these lines that works is hard (among other things the most straightforward approach to doing this is "non-renormalizable" which from a practical perspective means that it is basically impossible to do lots of kinds of calculations with a theory like this in our current state of knowledge).
In sum, if there is such a thing as quantum gravity, we know lots of qualitative things about what the theory would look like and especially, what properties a graviton would have. So, while a graviton isn't really inevitable, if gravity has a quantum nature, it is by far the odds on favorite explanation for how gravity works.