Boltzmann equation in de Sitter space
  In a time dependent background like de Sitter space, Feynman-Dyson
perturbation theory breaks down due to infra-red divergences. We investigate an
interacting scalar field theory in Schwinger-Keldysh formalism. We derive a
Boltzmann equation from a Schwinger-Dyson equation inside the cosmological
horizon. Our solution shows that the particle production is compensated by the
reduction of the on-shell states due to unitarity. Although the degrees of
freedom inside the horizon leads to a small and diminishing screening effect of
the cosmological constant, there is a growing screening effect from those
outside the horizon.
