On quantum corrections to geodesics in de-Sitter spacetime

We find a coordinate-independent wave-packet solution of the massive Klein-Gordon equation with the conformal coupling to gravity in the de-Sitter universe. This solution can locally be represented through the superposition of positive-frequency plane waves at any space-time point, assuming that the scalar-field mass $M$ is much bigger than the de-Sitter Hubble constant $H$. The solution is also shown to be related to the two-point function in the de-Sitter quantum vacuum. Moreover, we study the wave-packet propagation over cosmological times, depending on the ratio of $M$ and $H$. In doing so, we find that this wave packet propagates like a point-like particle of the same mass if $M \ggg H$, but, if otherwise, the wave packet behaves highly non-classically.