  We analyze, in the paradigm of open quantum systems, the reduced dynamics of
a freely-falling two-level detector in de Sitter space-time in weak interaction
with a reservoir of fluctuating quantized conformal scalar fields in the de
Sitter invariant vacuum. We find that the detector is asymptotically driven to
a thermal state at the Gibbons-Hawking temperature, regardless of its initial
state. Our discussion therefore shows that the Gibbons-Hawking effect of de
Sitter space-time can be understood as a manifestation of thermalization
phenomena that involves decoherence and dissipation in open quantum systems.
