If gravity is curvature of space as defined by Einstein, then how can standard model of QM have a particle called graviton?
This article explains it.
In short: standard model has to have a particle as it's supported by QM theory and also supported by GR through the existence of gravitational waves.
But gravitons haven't been observed yet. Therefore one can't certainly claim gravitons exist.
http://sciencetechworld.com/hard-science-edge-physics-gravitons-really-exist/
http://sciencetechworld.com/hard-science-edge-physics-gravitons-really-exist/