  We summarize and expand our investigations concerning the soft graviton
effects on microscopic matter dynamics in de Sitter space. The physical
couplings receive IR logarithmic corrections which are sensitive to the IR
cut-off at the one-loop level. The scale invariant spectrum in the
gravitational propagator at the super-horizon scale is the source of the de
Sitter symmetry breaking. The quartic scalar, Yukawa and gauge couplings become
time dependent and diminish with time. In contrast, the Newton's constant
increases with time. We clarify the physical mechanism behind these effects in
terms of the conformal mode dynamics in analogy with 2d quantum gravity. We
show that they are the inevitable consequence of the general covariance and
lead to gauge invariant predictions. We construct a simple model in which the
cosmological constant is self-tuned to vanish due to UV-IR mixing effect. We
also discuss phenomenological implications such as decaying Dark Energy and
SUSY breaking at the Inflation era. The quantum effect alters the classical
slow roll picture in general if the tensor-to-scalar ratio $r$ is as small as
$0.01$.
