Sebastian Zell (Department of Physics, Ludwig-Maximilians-Universitaet Muenchen, Germany)
We follow the corpuscular approach proposed by Dvali and Gomez, in which N universally characterizes gravitational backgrounds (with N = (Mp)^2/Λ for de Sitter spacetime). In doing so, we explicitly draw a fully quantum picture of the background metric in the limit of weak gravity. Using coherent states in each mode, we show that it can be understood as collective effect of soft, gravitationally non-interacting gravitons. Furthermore, we derive that the motion in the curved spacetime can be naturally modeled as scattering off the constituent gravitons. Consequently, the back reaction, which is inaccessible in any semi-classical approach, immediately follows from the fact that the background has gained or lost constituents. We show that this back reaction leads to quantum corrections which scale like 1/N, as expected. Finally, we investigate a possible breakdown of the classical metric description due to decoherence effects.