Exact RG Flow in an Expanding Universe and Screening of the Cosmological
  Constant
  We obtain the exact renormalization group (RG) flow equation for a self
interacting real scalar field in an expanding cosmological background. The beta
functional for the potential in the local potential approximation is determined
in terms of the mode functions defining the vacuum of the theory. For a scalar
in an inflating universe, which is released in the Bunch-Davies vacuum, the
flow equation is analyzed in detail using the optimized cutoff function for the
quartically truncated potential. In that case, the constant term in the
potential, which can in fact be viewed as the cosmological constant, decreases
as the IR cutoff is removed, supporting the existence of a quantum mechanical
screening mechanism. On the other hand, the running of the quartic coupling
constant at IR is altered by the expansion, which seems to affect the quantum
triviality of the theory.
